The Multimegawatt Program Taking Space Reactors 6 to the Next Level

s development of a 100-kilowatt electric space reactor power sys- tem progressed under the SP-100 program, space-based weapon and sensor designs continued to evolve under SDI. As the vision Afor global defensive systems capable of protecting against the threat of a Cold War Soviet attack expanded, energy-intensive space-based weapons system concepts—such as electromagnetic rail guns, free electron lasers, and neutral particle and charged-particle beam systems—began to emerge. And with the emergence came a need for advanced power systems capable of feeding the energy-hungry weapons.

From Kilowatts to Megawatts

SDI space-based weapons concepts were categorized into three operational modes (housekeeping, alert, and burst) with general power groupings. A housekeeping mode, applicable to operational baseloads such as communication and surveillance systems, required power levels of several kilowatts to tens of kilowatts over an operating life of 10 or more years. An alert mode, applicable to placement of a system in a state of readiness in the event of a hostile threat, required power levels of 100 kilowatts to 10 megawatts. A burst mode applied to weapon systems during battle scenarios and required power levels from tens to hundreds of megawatts for a period of hundreds of seconds. These high-power space-based concepts soon gave rise to the need for advanced multi-megawatt (MMW) power systems.1

Development of MMW power systems fell under the auspices of an SDIO MMW space power program, through which overall programmatic direction and guidance for power development efforts were given. The program had three principal elements: (1) military-mission analyses and requirements definition, (2) non-nuclear concepts and technology, and (3) nuclear concepts and technology. While responsibility for the first two elements was assigned to the Air Force, the nuclear concepts element was addressed in a joint initiative between SDIO and DOE.

An artist’s concept of a ground/space-based hybrid laser weapon, 1984. (Image: U.S. Air Force)

85 The Multimegawatt Program Taking Space Reactors to the Next Level

The MMW Space Reactor ground system testing of the reactor component selection, and Program power system concept.2 resolution of feasibility issues. If desired, Phase III would consist The joint SDIO-DOE initiative, or The program was planned to of ground-engineering system MMW Space Reactor Program, consist of four phases, with development for a single reactor began in 1985 as part of a DoD/ technical feasibility work concept during the mid-to-late DOE Interagency Agreement comprising the first two phases. 1990s. Flight demonstration under which DOE supported SDI During Phase I, several reactor work was planned for the last efforts. The objective of the new power system concepts would be phase, Phase IV, and expected MMW program was to establish selected for concept evaluation, to commence in the late 1990s, the technical feasibility of at least analysis and tradeoff studies, and with completion in the early 21st one space reactor system concept identification of issues that might century.3 that could meet applicable SDIO adversely affect system feasibility. performance requirements. The Phase II was planned for detailed To support development of the goal was to demonstrate technical analysis of the two or three power- reactor power system concepts feasibility by 1991. Based on system concepts that showed the during the first two phases, the outcome of the feasibility most promise for meeting SDI DOE initiated a technology work, SDIO would subsequently application requirements. Phase development program through decide whether to proceed with II was also to include preparation which the expertise and resources engineering development and of preliminary safety assessments, of its national laboratories could be accessed to address reactor technology issues. Information learned during the technology development process would also support decisions regarding concept feasibility. Pacific Northwest Laboratory had the lead for reactor-fuel development while materials work was completed at ORNL. SNL led development efforts associated with instrumentation and controls. LANL led heat pipe and thermal management development efforts. Finally, the Idaho National Engineering Laboratory (later the INL) was responsible for system and technical integration among the various laboratories, while

Strategic Defense Initiative space-based weapon concept. (Image: U.S. Air Force)

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coordination of nuclear safety was the responsibility of LANL.2, 4 Through the efforts of several national laboratories

Although the new space reactor and private companies, development of a broad program was a joint DOE-SDIO spectrum of preliminary reactor system concepts initiative, implementation was the responsibility of DOE. The DOE began in 1986 . management structure included DOE Headquarters and their Idaho Operations Office (DOE-ID). presented themselves as a moving consisted of short-duration burst- Overall program responsibility target. As SDIO mission planning type systems producing tens of resided with the Assistant Secretary evolved, uncertainties soon arose megawatts with effluents permitted for Nuclear Energy. Responsibility as to when the space reactor (open system). Category II systems for program direction was assigned power system would be needed. were similar to Category I but to the Division of Defense Energy In response to the possibility of a with no effluents (closed system), Projects under the Office of timeframe earlier than originally a minimum life of one year, and Defense Energy Projects and planned, DOE modified its overall capable of meeting burst-power Special Applications (in the program strategy and developed requirements continuously or Nuclear Energy organization). three broad preliminary power recharging within a single orbit. Day-to-day program execution categories to cover a range of SDI Category III concepts were and project management was applications. The categories were intended to provide hundreds of delegated to a project integration used as a framework for subsequent megawatts of burst power and office established at DOE-ID, reactor power system concept could be open or closed systems.3, 5 and included responsibility for development. Category I concepts managing day-to-day project activities and oversight of the Idaho National Engineering Laboratory. MMW Reactor Power Categories

Through the efforts of several DOE developed MMW power system categories to address the following national laboratories and private SDI space applications:6 companies, development of a broad spectrum of preliminary reactor Category I Category II Category III system concepts began in 1986. Power Requirements As reactor power system concept 10s 10s 100s (MWe) development progressed, the young Operating Time 100s DOE program soon found itself 100s 100s face-to-face with two decades- (seconds) one-year total life old problems—a lack of funding Effluents Allowed Yes No Yes and mission requirements that

87 The Multimegawatt Program Taking Space Reactors to the Next Level

Preliminary reactor power 1988 when six contractor teams, development activities by the system concepts included open- representing six different reactor General Accounting Office (GAO) and closed-cycle systems and concepts, were awarded contracts in 1987. The audit stemmed from a thermionic systems concepts. Of to refine their respective power Congressional request in May 1986 particular interest by SDIO were system concepts. In addition to and included review of the MMW gas-cooled open-cycle reactor- the conceptual development work, and SP-100 space reactor programs. system concepts because of the contractor efforts included The review considered program potential mass advantages over identification of technical issues status and the management and reactor systems that utilized a that could affect the feasibility of coordination among the sponsoring closed-cycle design.7 Work on the the proposed power system. With organizations of the space reactor preliminary power-system concepts Phase I formally underway, initial programs. In their final report, began in 1986 and was followed concept designs were completed GAO noted that both programs by a multi-agency team evaluation by early 1989.9 Of the six concepts faced several challenges and consisting of representatives from selected for Phase I studies, three observed that: several DOE laboratories, the Lewis were for Category I systems, two Research Center, and the Air Force for Category II, and one for “The Multimegawatt program, Weapons Laboratory in 1987.8 Category III.6, 10 which is still in its infancy, faces perhaps even greater challenges After evaluation of the initial The funding shortfall and its impact than the SP-100 program…Higher reactor-system concepts, further on the program were highlighted reactor operating temperatures and concept-development work was during an audit of DOE space major technological advances in cut short due to funding shortfalls. nuclear reactor research and space power systems are needed. Development efforts restarted in However, the program’s funding levels have been reduced. As a result, DOE has adjusted the time Open-Cycle vs . Closed-Cycle Systems frames and scope of work originally planned. Program managers state Open-cycle reactor power systems are designed such that the working fluid that it will still be possible for DOE is used only once and then exhausted to space. Unique features of an open- to meet its goal of determining the cycle system include operation at a higher temperature relative to a closed- technical feasibility of providing cycle system and the need for a working fluid storage system in lieu of a heat MMW nuclear power for SDI by rejection system. While these features generally translate to advantages in the early 1990s. However, program weight and materials, an open system introduces the potential for an adverse officials stated that high risk, but reaction of the hot exhaust gas with the spacecraft weapons and sensors.1 promising, space reactor concepts may not be practical to pursue at Closed-cycle reactor power systems are designed such that the working fluid is currently forecast budget levels and contained in the system rather than being exhausted directly to space. Features time constraints.”5 of closed-cycle systems include operation at a lower temperature (relative to open-cycle systems) and use of a heat rejection system, both of which generally translate to advantages in system efficiency.1

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6 As noted in the GAO report, MMW Space Reactor System Category I Concepts funding problems for the MMW GE proposed a derivative of the 710 reactor designed for the PLUTO nuclear program started in 1986, when ramjet program conducted in the 1960s. The fast-spectrum, ceramic- the program received only $15.8 metal fuel, gas-cooled reactor concept included twin counter-rotating million of the $17.2 million open Brayton cycle turbines/generators integrated with super-conducting (combined funding from DOE generators. Testing of fuel elements for the 710 program had produced data and SDIO) requested. In fiscal on this fuel type. year 1987, the situation worsened, as the program received only Boeing developed a hydrogen-cooled open Brayton cycle system using a $14.6 million of the $40 million new reactor design with a fuel-pin core designed by Britain’s Rolls Royce. requested. With the 1987 funding The core used a two-pass flow configuration in which the hydrogen would level at only 37 percent of enter the reactor, flow through an outer ring of fuel pins to an upper plenum, request, and the future looking reverse direction, and then flow down through the center array of fuel pins. no better, it was no surprise that The system was designed to be scalable, with the objective of meeting the schedule delays ensued. Reactor Category III requirements with modifications.11 power system concept definition, originally planned to proceed until A Westinghouse team designed a NERVA-derivative hydrogen-cooled reactor August 1987, was delayed with a using an open Brayton cycle with counter-rotating turbines and generators. planned resumption date of April The design had substantial operational data from the NERVA program. 1988. By 1988, budget limitations were expected to push design MMW Space Reactor System Category II Concepts concept selection beyond 1991, and final development of a MMW General Atomics proposed a closed-cycle system consisting of a liquid-metal- reactor beyond the year 2000. In cooled in-core thermionic reactor coupled to alkaline fuel cells that could be addition to funding shortfalls, used to supply burst power. SDIO began to decrease funding Rockwell proposed a lithium-cooled, ceramic-metal-fuel fast-reactor system for development of nuclear space to drive a Rankine-cycle power conversion system. The closed-cycle reactor power technology in favor of non- system would be used to recharge sodium-sulfur batteries after a nuclear technologies. The program, power burst. barely in its infancy, was already feeling the effect of broad Federal fiscal belt-tightening that had MMW Space Reactor System Category III Concept resulted from ballooning Federal A Grumman-led team proposed a hydrogen-cooled particle bed reactor using budget deficits. Nevertheless, DOE an open Brayton cycle system with a ten-step turbine and alternator. continued to move forward with system studies.5

89 The Multimegawatt Program Taking Space Reactors to the Next Level

While GAO reviewed DOE space levels. Relative to MMW power processes using surrogate and reactor research and development systems, the committee recognized uranium-nitride fuel particles. Tests activities, a National Research that power requirements for SDI were conducted to evaluate the Council review team examined burst-mode applications could compatibility of uranium nitride advanced power systems for space significantly exceed the capacity fuels with tungsten-rhenium and missions in a broader context. of available and planned power molybdenum-rhenium alloys, Stemming from a DoD request systems and recommended that and on the fabrication, welding, made when SDIO was in its infancy “both the nuclear and non-nuclear and materials properties of high- in 1984, the Research Council SDI MMW programs should be temperature refractory alloys.2 review was initially intended to pursued.” The report provided Progress continued in 1988, with address space power systems a caveat relative to the nuclear advances in lightweight heat pipe related to SDI applications but was option, however, noting that “a and refractory reactor materials, broadened to include military space nuclear reactor power system may and in fabrication and testing of power requirements, other than prove to be the only viable option particle bed reactor materials and those of SDIO, and potential NASA for powering the SDI burst mode components, including in-core space power requirements. MMW (if effluents from chemical power reactor testing of particle bed fuel space reactor systems offered sources prove to be intolerable)...”.1 element assemblies for MMW several desirable features, including A similar caveat was provided reactor types.12 low weight, compactness, long life, relative to alert-mode power levels. potential for continuous use, benign In early 1989, the project got or no effluents, high reliability, and In light of the funding shortfalls its first taste of success with the inherent radiation hardness and in 1987, the external GAO review, submittal of six reactor-system survivability. As such, potential and the National Research Council concept packages at the conclusion civil applications included nuclear effort, the MMW program still of Phase I. The concept packages electric propulsion and nuclear made progress on the technology provided a description of the thermal propulsion to reduce front. Emphasis was placed on reactor power system concept, interplanetary transport times, those areas that were particularly provided a preliminary approach nuclear-surface-power systems for relevant to the concept-feasibility to safety, and detailed an approach manned bases on the moon or on evaluation, including reactor fuels, for follow-on development work Mars, and nuclear power systems materials, energy storage, thermal that was planned for Phase II. Of for large-scale industrial processing management, and instrumentation the six concepts evaluated, three schemes in space. and control. Relative to reactor were planned for follow-on design technology, progress included the development: (1) the Westinghouse Based on a review of advanced issuance of contracts for fabrication NERVA-derivative concept, (2) power system concepts and of depleted and enriched uranium- the Grumman particle-bed open- information in 1987, the carbide zirconium-carbide-coated cycle concept, and (3) the Rockwell final report provided several fuel particles and fuel elements for ceramic-metal-fuel closed Rankine recommendations for consideration a particle bed reactor concept, and cycle concept.8 by those involved in planning space development and demonstration missions requiring MMW power of ceramic-metal-fuel fabrication

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As the reactor-concept development Designing Reactors for Space efforts progressed, the evolution Space reactor power system design offers many technical challenges resulting of the SDIO architecture away from constraints imposed by criteria such as weight, microgravity, and high from high-power space-based temperatures. In the case of SDI applications, the following designs also platforms finally caught up benefited from the unique aspects of space-based weapons.11 with the space reactor program. SDIO system designs eventually Weight: changed, resulting in decreased With launch costs on the order of thousands of dollars per kilogram, the need power requirements. With lower to minimize weight was reflected in the use of high operating temperatures to power requirements, non-nuclear increase system efficiency; the use of high-strength, high-temperature metals power system alternatives became and composites; and the development of improved heat rejection and power more competitive. The need for conversion and power conditioning systems. an MMW space reactor program soon disappeared and, with it, the Microgravity: SDIO funding. Although NASA had The effects of microgravity on systems that rely on two-phase (gas and identified possible uses for MMW liquid) flow, such as the closed-Rankine-cycle system, require special design space reactor technologies, they had considerations. For example, vapor condensation is controlled by shear no funding for development. DOE forces since no falling film condensation occurs. Also, the absence of gravity wasn’t prepared to fund reactor introduces pumping startup issues that must be considered. development without a sponsor. Temperature: Consequently, the MMW program, The temperatures associated with high-power space reactors such as those barely in its fourth year of existence, envisioned under the MMW program generally require the use of materials and was terminated in 1990 before Phase nuclear fuels capable of operating near their melting points. Development of II began.8 The total funding provided such materials may require a proportionately larger investment of time and for the program by SDI and DOE funding. from fiscal year 1986 through fiscal year 1989 was $37.1 million. Benefit: Many SDI system concepts used liquid hydrogen to cool the weapon. Once Although the MMW program died exhausted from the weapon, the hydrogen could be used as a coolant in open- after its first phase, some elements cycle reactor concepts, such as the open Brayton cycle system. of the program continued. With the advent of the SEI in 1989, several MMW concepts and technologies were later identified as leading candidates for NASA space nuclear propulsion and power applications. Thermionic technology also continued to draw the interest of DoD.g g. Thermionic technology is discussed in Chapter 7.

91 With the announcement of SDI by President Ronald Reagan in March 1983, interest in space nuclear power systems for space-based satellites and weapons was rekindled .

92 Thermionics Revived 7 A Second Chance

evelopment of thermionic space reactor power systems in the United States had its origins in the mid-1950s. Between 1963 and 1973, thermionic reactor programs under the direction of DAEC emphasized development of in-core conversion concepts, in which the nuclear fuel and thermionic power conversion system are integrated in a thermionic fuel element. Thermionic reactor concepts developed under the early AEC programs resulted in designs for a broad range of space applications and power levels, including 5-kWe systems for unmanned satellites, 40-kWe systems for a manned space laboratory, and a 120-kWe system for nuclear electric propulsion. However, none of the concepts were ever developed to the point of flight readiness. Following termination of AEC space nuclear reactor power system development in 1973, thermionic reactor power system research shifted to development of out-of-core thermionic converter concepts, in which the power conversion function was located external to the reactor. Within a decade, the focus would return to in-core reactor concepts.1

With the announcement of SDI by President Ronald Reagan in March 1983, interest in space nuclear power systems for space-based satellites and weapons was rekindled. Although a fast-reactor thermoelectric power conversion system was selected for development under the SP-100 program, thermionic reactor technology was still considered a viable alternative. To capitalize on that viability and provide a backup technology for the SP-100 program, in-core thermionic technology development was revived by DOE under a TFE Verification Program (TFEVP) in the mid-1980s. To capitalize on foreign thermionic research and development, DoD parted ways with the domestic DOE development program in favor of technology that began to be available as the Cold War drew to a close.

Thermionic Power Systems (In-Core)

In-core thermionic space reactor power systems utilize TFE converters to produce electricity. At the heart of the thermionic reactor power system is the nuclear reactor itself. In the United States, thermionic reactor designs were centered on a multicell TFE. Other reactor designs, such as some developed in Russia, utilized single-cell TFE technology. Each approach had pros and cons regarding the testability, weight, and conversion efficiency.

Materials testing under the TFEVP was conducted in a General Atomics TRIGA reactor (pictured), EBR-II at INL, and Hanford’s FFTF. (Image: General Atomics)

Photo Credit 93 Thermionics Revived A Second Chance

Thermionic Power A typical multicell TFE contained six the element had an operating life Conversion individual TFE converters stacked of approximately 20,000 hours, inside a fuel element, much like consistent with the performance Thermionic emission is the heat- batteries are stacked in a flashlight. goals at the time. Two major issues induced flow of electrons from a With a nominal output of 0.4 We prevented longer operating life. The hotter surface (emitter) to a cooler for each converter, an individual first was deformation of the emitter. surface (collector), typically across a multicell TFE could produce Emitter deformation was caused small gas-filled gap. 2.6 We with its individual converters primarily by dimensional swelling A thermionic converter is a static connected in series. The reactor core of the fuel pellets. Swelling of the power-conversion device in which was consequently sized to produce a fuel pellet caused bulging of the electrons are boiled from the hot desired power output. For example, emitter, which resulted in contact emitter surface across a small gap, a thermionic reactor power system with the surface of the collector. typically less than 0.5 millimeter, consisting of 176 multicell TFEs The resulting emitter-collector to the cooler collector surface. The generating approximately 1.3 MWt contact created short circuits inside 1 electrons absorbed by the collector could generate 110 kWe. the fuel cell, thereby reducing the produce an electrical current as they output voltage to zero. The second return to the emitter through an When domestic space reactor power issue involved radiation induced external circuit. The space between system programs were terminated structural damage of the insulator the emitter and collector is filled in 1973, design of a multicell TFE seals. The damage consisted of small with an ionized gas (typically cesium) had advanced to a point where cracks through which fission gases that serves to neutralize the space could pass into the inter-electrode charge that would otherwise build space between the emitter and Interelectrode space up around the emitter and collector. Mixing of fission-product slow the passage Thermionically of electrons. emitted electrons Thermionic Vacuum or vapor converters have Space charge neutralizing ions efficiencies of Hot emitter - - Cold collector approximately + 1 + + 5 to 10 percent. QIN QOUT Heat in - Heat out + - + - -

Enclosure

Electrical feedthrough

Electrical load

94 Atomic Power in Space II Chapter 7

gases with the cesium gas reduced part of a broader TFEVP initiated Thermionic Converter the effectiveness of the space charge by DOE in 1986.3 (in-core) provided by the cesium.2 An in-core thermionic converter is made The TFEVP was established by up of several individual components, DOE to demonstrate the readiness TFE Verification Program including the nuclear fuel, an emitter, of a multicell TFE suitable for use a collector, an insulator sheath, in a thermionic reactor with an Efforts to address TFE lifetime and a cesium reservoir. The emitter electrical power output of 0.5 to 5.0 issues were restarted in 1984 includes the nuclear fuel and various MWe and a full power life of seven under a thermionic technology components that hold the fuel in place program conducted as part of the years. Led by General Atomics, with during launch. A fission product trap broader SP-100 program. The work support from Space Power, Rasor, provides for the collection of solid was led by General Atomics with and the ThermoTrex Corporation, and condensable fission products to support from Rasor Associates, the program included a broad set prevent their exit from the fuel cell. Space Power, Inc., and the Thermo of non-nuclear tests, component Heat shields serve to protect the upper Electron Corporation. Through tests, and integrated TFE testing. and lower parts of the emitter from the an iterative process that included Westinghouse Hanford provided high temperature of the fuel. A tri-layer design, fabrication, in-core overall coordination of fast-reactor sheath consists of an inner collector, a irradiation testing, and analysis, testing, while program-level middle insulator that provides electrical the renewed thermionic program technical oversight was provided isolation of the collector from the fuel continued the advancement of by LANL. element structure and coolant, and an multicell thermionic fuel element outer metallic structural layer. A cesium technology. For example, the reservoir provides cesium gas to the gap U02 fuel emitter deformation issue Fuel pellets between the emitter and collector. was addressed by increasing its U0 thickness, doubling the gap between 2 the emitter and collector, and Electrons from payload lowering the operating temperature of the emitter. The insulator issue Electron emission Emmiter was addressed through selection Interelectrode gap occurs when of alternative materials.2 By the (electron transport temp. ~1,500 Kelvin end of 1985, nine fueled emitters facilitated by a cesium vapor) and several insulator test articles Collector had been designed and fabricated Insulator and were being irradiated in a Sheath trilayer Outer Training, Research, and Isotopes sheath layer General Atomic (TRIGA) reactor. Electrons to payload Irradiation testing of the fueled emitter and insulator specimens Thermionic fuel element schematic. (Adapted from “Summary of Space Nuclear continued into 1986 under a Reactor Power Systems (1983-1992),” David Buden, pg. 64 in A Critical Review of separate thermionic irradiation Space Nuclear Power and Propulsion, 1984-1993, Ed. Mohamed S. El-Genk, American program, which eventually became Institute of Physics, 1994)

95 Thermionics Revived A Second Chance

Building on the TFE technology Electron-beam welding equipment fuel element that was then tested and database developed by AEC and a facility for plasma-spraying in a TRIGA reactor. Testing in and NASA in the 1960s and early sheath insulators were established. the TRIGA reactor provided an 1970s, and the more recent SP- Once the fabrication processes were environment in which the TFEs 100 thermionic development developed and operators trained were subjected to multiyear work, a baseline multicell TFE was on each process, component radiation and thermal conditions. designed for a 2-MWe conceptual production commenced, which Using several partial-length reactor. Consistent with the SP- was followed by a rigorous testing elements, a series of tests 100 program goals, the baseline program.4 were conducted to determine fuel element was designed for a parameters, such as fuel element seven-year operating lifetime and Component-level testing served performance relative to emitter provided the starting point for to verify and validate the design swelling, durability of insulator design of the various fuel element and demonstrate acceptability of materials, and adequacy of fission components. Components that the fabrication and production gas venting channels. The results required specific design for the processes. Such testing included allowed estimates to be made of power and lifetime goals included non-nuclear development and expected fuel-element performance the uranium oxide fuel, emitter and screening tests, as well as nuclear over the desired seven-year lifetime. collector, insulator, fission-product testing conducted in FFTF at A total of six TFEs were tested trap, and various alignment and the Hanford site and EBR-II at in the General Atomics reactor support items.4 ANL-W. The FFTF and EBR-II during the TFEVP: three single- cell fuel elements, two three-cell fuel elements, and one six-cell fuel Once the fabrication processes were developed element. Irradiation of the fuel elements was performed between and operators trained on each process, September 1988 and October 1993. component production commenced, which was The in-core testing time for the first four elements ranged from 14,000 followed by a rigorous testing program . hours to 20,000 hours. Problems with test instrumentation or the TRIGA test vehicle limited the In addition to the design effort, reactors provided a fast-reactor duration testing of the first four fabrication and production environment in which the elements. The last two elements processes were established for components were bombarded with were in the process of being each component. For example, neutrons and tested under reactor irradiated when the verification equipment and processes for thermal conditions. program was terminated in fiscal chemical vapor deposition year 1994; the two elements had of tungsten on emitters were Once the design of every been in the irradiation environment developed, as were procedures component was verified and for a period of 4,300 hours and and equipment for the fabrication validated, the components were 8,000 hours when testing was of uranium oxide fuel pellets. assembled into an integrated terminated.4

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Nuclear testing revealed two issues. Thermionic Space Nuclear In a dual-path down-select process During one of the early tests, a Power System Program that began in 1992, contracts were tungsten component inside the awarded by DOE to two different emitter appeared to affect emitter As the TFEVP was progressing, teams to develop a thermionic space lifetime, which was successfully DoD began to re-evaluate the nuclear power system scalable over corrected in a subsequent test. The power needs for its future missions, five to 40 kWe. One concept, other issue discovered during testing largely in response to changing developed by a Rocketdyne was related to the size of the fission- conditions as the Cold War drew Corporation consortium called gas port for channeling fission to a close. Following a series of S-Prime Thermionic Nuclear Power gases out of the emitter. No other reviews and design studies, a System, was based on multicell lifetime or materials issues affecting new set of performance goals thermionic fuel element technology. fuel element performance were was established that was lower The other concept, developed by a identified during the reactor testing. than those being worked in the Space Power, Inc. consortium called SP-100 program. In response to the Space Power Advanced Core- TFE testing in the TRIGA reactor the reduced DoD requirements, length Element Reactor Thermionic ended in October 1993, finally DOE, SDIO, and the Air Force System, was based on single-cell giving way to other thermionic initiated the thermionic space thermionic fuel element technology. program efforts. By the end of the nuclear power system design and Initial calculations indicated that TFEVP, a multicell TFE lifetime of technology demonstration program both systems had a specific power of 18 months had been demonstrated in 1991. Under a Memorandum of 18 We/kilogram for a 40-kWe and multicell TFE technology had Agreement signed in June 1991, system, and growth capabilities also been advanced in several areas, the agencies sought to build on above 100 kWe.6 The original plan including TFE fabrication processes Air Force thermionics work and was to complete preliminary designs and fuel-emitter and sheath- capitalize on the availability of and demonstrate key technologies insulator longevity. Nonetheless, Russian thermionic technology. and components by the end of 1995. issues related to the desired The new thermionic program However, funding cuts led to seven-year life still remained and encompassed the multicell TFE program termination in 1995. pointed to additional testing and testing that was being performed analysis related to fueled-emitter under the TFEVP, which had been Russian Technology Finds degradation mechanisms, TFE running on a parallel track with Its Way to Americah performance prediction, and the SP-100 program. The goal of 4 sheath-insulator lifetime. the program was to design and While DOE was working to demonstrate a 40-KWe thermionic improve its multicell thermionic power plant with a design-life goal fuel element under TFEVP, SDIO 5 of 10 years. began exploring the use of Russian h. The process by which Russian TOPAZ-II reactors were acquisitioned by SDIO was centered on a desire to minimize development costs of a space reactor power system by building upon the developmental efforts of the former Soviet Union. It provides lessons in procurement, contracting, and partnership development with foreign entities. It also provides lessons related to the requirements set forth in the Atomic Energy Act that govern the transfer of nuclear-related technology to and from the United States. The interested reader is encouraged to consult Booz-Allen & Hamilton8 and Dabrowsk.9

97 Thermionics Revived A Second Chance

thermionic reactor technology As the political and economic what was once a tightly controlled to meet its mission needs. Space environment in the former Soviet and classified technology, following nuclear reactor power systems Union changed in the late 1980s, the collapse of the former Soviet had long been used in the former the Russian space nuclear research Union.9, 10 Soviet Union. Radar ocean community faced an uncertain reconnaissance satellites, known future. During this time, officials of Testing the Russian in the United States as RORSATs, the Russian space program offered Technology were powered by a fast reactor to sell to DoD two complete, coupled with a silicon-germanium unfueled, electrically-heated Under the TSET program, non- thermoelectric power conversion TOPAZ-II reactor systems and nuclear testing of the two unfueled system. The reactor power system associated test equipment. TOPAZ-II reactors and single-cell produced power levels ranging thermionic fuel elements began in from several hundred watts to a The arrangement was viewed 6 November 1992 at the University of few thousand watts. by DoD as a means of acquiring New Mexico Engineering Research a turn-key system, including Institute in Albuquerque, New In the late 1980s, a new thermionic the two reactors, a vacuum test Mexico. The TOPAZ-II reactor reactor power system was tested stand and associated pumps, a system was designed to be ground in a series of two space tests. The fuel-element test rig, and control tested using tungsten electric new thermionic system, based hardware at a cost significantly heaters in lieu of fueled elements, on a multicell thermionic fuel less than would be required thereby allowing the entire reactor element design, came to be referred if a comparable development system to be tested at elevated to as TOPAZ-I in the United program were undertaken in the temperatures in the absence of States. The tests were successfully United States. After a lengthy nuclear fuel. The TOPAZ-II test completed when the TOPAZ-I process of negotiations, licensing, program included a series of system provided in-orbit power authorizations, and approvals electrical, mechanical, and thermal to two Cosmos satellites in 1987. that involved multiple Federal In a separate (but parallel) effort, and foreign agencies, private tests and operations that provided another thermionic reactor power companies, and consortiums, verification of baseline design and system based on a single-cell two unfueled TOPAZ-II reactors system performance, and provided thermionic fuel element design and associated testing equipment the opportunity to train American had also been developed in the were purchased and transferred operators on the Russian systems. former Soviet Union. The TOPAZ- to the United States in May Of particular importance was the II system, as the unit came to be 1992 under the auspices of the need to demonstrate the viability of called in the United States, was SDIO and the Air Force Phillips the TOPAZ-II technology against never launched by the Soviets, Laboratory in New Mexico. The DoD space flight requirements. but had undergone a significant equipment transfer and subsequent Reflecting upon the TOPAZ- development effort and was Thermionic System Evaluation Test II acquisition, Richard Verga, considered flight-ready under the (TSET) program effort represented the SDIO manager for power former Soviet system.6, 7, 8 a prominent example of technology, described the thinking international cooperation between as “not just [to] reverse engineer, the United States and Russia, in

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but to see if it was possible to make Russian Thermionic Technology a U.S. variant of the TOPAZ [II] The former Soviet Union began researching thermionic space reactor technology technology that would embody our in the 1960s. By 1967, two thermionic reactor concepts were being independently expectations of power [weight], developed in secret programs by two different teams of Soviet technical institutes. particularly safety.”11 Development occurred in a largely competitive environment between two technical institutes, akin to the competition one might see between the Lockheed-Martin and The TOPAZ-II reactor system Boeing corporations in the current U.S. aerospace industry. While the United States was designed to produce focused on development of RTG technology, space nuclear power development in approximately 6 kWe (including the former Soviet Union included a substantial investment in the two thermionic the 1 kWe needed to operate reactor technologies. the sodium-potassium pump) from a reactor thermal output The Central Design Bureau of Machine Building, in conjunction with the Kurchatov of approximately 115 kilowatts. Institute of Atomic Energy and Krasnaya Zvezda (Red Star) State Enterprise, led the The cylindrical reactor core was development of one thermionic concept that utilized a multicell TFE. The multicell relatively small, with a diameter TFE consisted of a stack of short thermionic cells that acted as a single fuel element, of approximately 10 inches (25 similar to the way batteries are stacked in a flashlight. The multicell TFE concept was cm) and a length of 15 inches (38 developed under a program called TOPAZ, a Russian acronym meaning “thermionic cm). The reactor coreconsisted experiment with conversion in active zone.”9 In 1987, the TOPAZ system was of 37 single-cell TFEs, each launched aboard two Russian satellites, Cosmos 1818 and Cosmos 1867, marking of which contained a stack of the first successful use of thermionic nuclear reactor power systems in space. annular-shaped highly enriched Cosmos 1818 operated for 142 days and Cosmos 1867 operated for 342 days.6, 7, 8 uranium oxide fuel pellets. Reactor Design life was a major difference between Russian and U.S. designs. The TOPAZ cooling was provided by a liquid- design life was only one year, limited in part by the on-board cesium supply. Unlike metal consisting of sodium and the sealed U.S. designs, the cesium in the interelectrode gap flowed through the gap potassium.6 during operation. Both units produced approximately five kWe after accounting for the approximately one kWe of electrical power needed to operate the pump for the During the approximately sodium-potassium liquid metal cooling loop.5 For safety reasons, reactor operation three-year TSET program, an began only after a nuclear-safe orbit of approximately 800 kilometers above Earth American-Russian research was attained.12 team completed facility and reactor system acceptance The second thermionic concept, developed by the Kurchatov Institute of Atomic testing, training of U.S. operators Energy and Scientific Industrial Association Luch, used a single cell TFE that had on the Russian reactors, and been developed under a program called ENISEY (pronounced Yenisee). Although testing necessary to characterize the reactor system was never launched, it had been subjected to a significant performance of the reactor ground-testing effort, both unfueled and fueled, by its developers. One advantage systems. A total of 11 thermal- it held over its multicell counterpart was a design that allowed the use of electrical vacuum tests were completed on heaters, in lieu of nuclear fuel, during testing. In an effort to distinguish the two the two reactor systems. Testing reactor systems in the United States, the single-cell reactor system, ENISEY, became of one of the reactor systems known as TOPAZ-II, and the multicell system, TOPAZ became known as TOPAZ-I.9 showed susceptibility to output-

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The End of the Soviet Union power oscillations. The other TOPAZ-II: Preparing unit, which included testing at for Flight As the Soviet Union weakened nominal operating conditions economically in the late 1980s, for up to 1,000 hours, resulted Mikhail Gorbachev pulled the Soviet Under BMDO management, in the observation of small leaks Union back from its international NEPSTP had four goals: (1) in the interelectrode gap and commitments and stopped demonstrate the feasibility of intermittent short circuiting of one participating in the Cold War arms race launching a reactor power system; of the thermionic fuel elements. with the United States. The perceived (2) demonstrate the ability to loss of prestige led to resistance within Other tests were performed that adjust orbits using nuclear electric the Soviet government that culminated assessed the electrical output of the propulsion; (3) evaluate the in in a failed coup d’état by a core group thermionic fuel element converters, orbit performance of the TOPAZ- of Soviet hardliners in August 1991. verified thermophysical properties II reactor and selected electric When the hardliners announced on of the reactor and fuel elements, thrusters; and (4) measure, analyze, state television that Gorbachev, whom and operated the systems under and model the nuclear electric 9, 13, 14 they had sequestered, was ill and mechanical and shock loads. propulsion environment. The would not be able to govern, massive intended mission called for initial protests immediately ensued across Based on the results of the tests, a launch into a circular orbit of 3,260 the country, and the military refused follow-on demonstration project miles (5,250 kilometers). Once in to obey orders to crush the protests. was planned in which a TOPAZ-II orbit, reactor startup would be After three days, the coup organizers reactor would be used as a power prompted by a ground command. surrendered, realizing that they would source for satellite-based electric With successful reactor operation, be unable to govern without the propulsion technologies in space. each of several on-board ion support of the military. The magnitude The Nuclear Electric Propulsion propulsion thrusters would then of the protests made it clear that the Space Test Program (NEPSTP) be tested for a 1,000-hour period. Soviet Union was no longer governable became part of the Ballistic Missile The thrusters would slowly raise under the old system, and the country Defense Organization (BMDO) the orbital altitude of the satellite began preparing to dissolve itself. following an organizational name to 25,000 miles (40,000 kilometers) change of SDIO in May 1993. In over a period of approximately On December 25, 1991, Mikhail late 1993, the TOPAZ-II program 27 months.15, 16 Gorbachev resigned his post as the was also renamed the TOPAZ Executive President of the Soviet Union, International Program, in a move The NEPSTP would include all having already resigned as General to better reflect the international aspects of a launch program, Secretary after the coup. On January 1, makeup of the TSET team, which including mission and spacecraft 1992, the Soviet Union officially ceased included British and French design, safety, integration and to exist, and 13 independent countries researchers in addition to the qualification, launch approval, and were formed. The decades-old long Americans and Russians.9, 15 launch operations. One key benefit post-World War II rivalry between the of performing such a mission United States and the Soviet Union was would be possibly characterizing over, leaving the United States as the the electromagnetic and plasma world’s only remaining superpower. environments generated by a

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reactor and electric propulsion in orbit, which could be used for As project managers soon discovered, the path future reactor-based electronic system designs. Another benefit leading to success, particularly when success hinges would be the identification of on foreign technology, presented many challenges . requirements associated with reactor launch, such as safety and approvals. The interest in the second benefit had its basis in and volume to the spacecraft. the fact that the only reactor ever The TOPAZ-II electronics were launched by the United States had subsequently replaced with a been the SNAP-10A unit in 1965, smaller unit based on U.S. thermal and almost 30 years had passed management philosophy that since that first launch. As project relies on conduction and radiative managers soon discovered, the heat transfer processes, thereby path leading to success, particularly minimizing mass and volume. when success hinges on foreign Other design changes centered on technology, presented many the approach to ensure the liquid- challenges.15 metal coolant didn’t freeze prior to reactor startup in space. Rather than The first major challenge presented using high-current ground-based itself in the form of technology electrical heaters to heat the coolant integration. For example, the prior to launch, as the Russians did, TOPAZ-II reactor had been NEPSTP designers incorporated an designed for integration with approach that used a combination a Soviet proton rocket; design of ground-based temperature changes were needed to support control airflow with a reduced time integration with a U.S. launch period before reactor startup.15 vehicle. Design changes also manifested themselves in the Safety considerations gave approach to thermal management rise to yet additional design for spacecraft electronics. In the changes. A preliminary nuclear TOPAZ-II system, electronics were safety assessment, performed to enclosed in a pressurized vessel, and demonstrate compliance with convective heat transfer was used guidelines for a U.S.-based launch, to remove excess heat generated by identified the need for several the electronics. Although thermal safety features to ensure nuclear management was simplified with safety of the reactor system during this approach, it added mass the planned mission. The needed TOPAZ-II reactor system. (Photo: Scott Wold)

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modeled on an earlier interagency Safety considerations gave rise to yet additional (DoD, NASA, and DOE) study conducted for the SEI.16, 17 design changes . While non-nuclear ground testing provided significant value, the safety features included a thermal landed in the ocean or other body overall demonstration effort shield to prevent breakup of the of water; and a control system couldn’t be completed without reactor system during a postulated to ensure automatic reactor testing in a fueled configuration. re-entry event; engineered controls shutdown. The safety concerns gave Each TOPAZ-II reactor was to ensure the reactor remained rise to additional design changes designed to be fueled with subcritical during flooding events, and development of a safety approximately 59 pounds such as would occur if the reactor requirements document that was (27 kilograms) of high-enriched

Unloading TOPAZ-II from its shipping container. (Photo: Scott Wold)

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uranium. After considering of the Atomic Energy Act. The TOPAZ Sputters Out its options, BMDO elected to review team concluded that the purchase Russian fuel that had information available at the time of In 1993, funding for the TOPAZ-II been specifically fabricated for the the assessment was insufficient to International Program was reduced TOPAZ-II reactor. In addition to confirm the safety of the proposed as the result of cost-cutting the fuel, four additional unfueled flight program and that it would be pressures and changing defense- TOPAZ-II reactor units were also “extremely difficult to conclusively spending priorities. To keep the made available to BMDO. The four demonstrate that inadvertent program alive, SDIO expanded additional units included two units criticality can be prevented for all its goals to include defense built to Russian flight standards (the credible accident conditions during conversion—aiding the Russians in first two units acquired by SDIO the launch or for end-of-mission converting portions of their defense were not flight-qualified). With two re-entry phases.” Alan Newhouse, industry to civilian operations—in flight-qualified units, BMDO would Director of the DOE Office of Space addition to the original technology have one unit dedicated for flight and Defense Power Systems at the transfer goal. The remaining four use and another serving as a backup time, later described one of the key TOPAZ-II reactors in the Russian flight unit. Following receipt of problems with the Russian reactor: inventory were brought to the DOE authorization for the purchase United States in March 1994. Two (import and utilization for non- “The Russians… [had] an interesting were intended for ground testing fueled ground testing only), the four design. It had good features to it. to support spacecraft integration; unfueled units were delivered to the It just had flaws… it had what the other two were planned for use United States in March 1994.16 was called a positive temperature during proposed flight tests. coefficient, which meant if it were As BMDO and the Air Force immersed in water, for example, In October 1995, the TOPAZ- moved forward with design it would go prompt critical and II International Program was changes and non-nuclear testing dissolve itself with a big boom. transferred from BMDO to the of the TOPAZ-II systems, DOE We… wouldn’t allow our space Defense Nuclear Agency (DNA). In initiated an independent safety reactors to be launched with that anticipation of the transfer, DNA assessment of the TOPAZ-II space characteristic. You want the thing formed a working group and invited nuclear power system. The pre- to be self regulating. All the Navy DOE to help guide the future authorization assessment was reactors are.”18 of its thermionic development performed in anticipation of a program. Having begun under DoD request to conduct operations The review team concluded that BMDO and its predecessor agency involving nuclear material, low-power (critical) nuclear as a demonstration program for including the purchase of nuclear experiments could be safely TOPAZ-II capabilities, followed fuel for the TOPAZ-II reactors, performed under DOE oversight, by a flight demonstration, funding ground testing involving nuclear and recommended that an cuts severely reduced the program. fuel, and the launch of the fueled additional, independent safety As a result, DNA planned to re- TOPAZ-II system, as modified to review be conducted after all orient the program for improved meet applicable U.S. requirements; analyses, experiments, and safety consistency with the broader space such authorization was (and still report preparations had been nuclear reactor technology needs of is) required under Section 91b completed.16 the country.

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Shortly before its transfer to DNA, the TOPAZ-II International In 1996, the beleaguered TOPAZ-II international Program came under the scrutiny of several investigations amidst program finally met its demise when it was allegations of mismanagement officially terminated . and contracting improprieties. Questions arose surrounding the acquisition, contracting, and Thermionic Space Power funding practices of the program. Program Reflections Concerns had also surfaced within DOE and the Air Force Phillips Laboratory regarding lack of From its new beginnings under accessibility to all the TOPAZ-II the SP-100 program, through the technology due to proprietary DOE TFEVP and thermionic space and trade-secret assertions by nuclear power system programs, Russia. The GAO questioned and then the DoD acquisition and whether the original program goal testing of the TOPAZ-II reactor of technology transfer was truly systems, thermionic space reactor accomplished.19 technology had found the favor of a contingent within the broader In 1996, the beleaguered TOPAZ-II space nuclear power system International Program was officially community. Although the nearly terminated. The termination came decade-and-a-half effort had clearly in part due to findings from the served to advance the technology GAO audit but also from the lack base of space nuclear thermionic of a defined DoD or NASA mission power conversion, it also revealed and changing priorities within divisions that had plagued the U.S. the defense agency. Following its space nuclear power community termination, the six TOPAZ-II in the past, such as which space reactors originally purchased for reactor technology held the most testing and flight demonstration promise. Although the much were returned to Russia by 1997, hoped for gains of utilizing foreign consistent with the plans conceived technology were never fully early in the TOPAZ-II negotiations. realized, the effort did provide lessons related to the pursuit of such exchanges.

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105 The two initiatives resulted in two separate nuclear propulsion programs . . providing possibly the broadest support for space nuclear thermal propulsion systems since the days of NERVA .

106 Nuclear Propulsion 8 Space Reactors Heat Up

ith the termination of the NERVA nuclear rocket program in 1973, U.S. space nuclear propulsion efforts lay dormant for more than a decade. The little amount of space nucle- War reactor research that did exist was focused largely on development of reactor power systems rather than propulsion systems. By the late 1980s, that began to change, first under the auspices of SDI, and then under the umbrella of SEI. The two initiatives resulted in two separate nuclear propulsion programs, one built around military missions, and the other built on space exploration. For a brief moment, the two initiatives overlapped, providing possibly the broadest support for space nuclear thermal propulsion systems since the days of NERVA.

Timberwind and the Particle Bed Reactor

While SDIO held a large stake in the development of the SP-100 space reactor power system, its attention soon turned to yet another space nuclear power system for possible military applications. With support from the DOE Office of Defense Programs and the national laboratories under its purview, SDIO initiated a program in 1987 to explore the feasibility of developing a new nuclear-powered rocket. Rather than building on Rover/ NERVA reactor technology, SDIO selected for its new propulsion system a particle bed reactor (PBR), a concept that had its origins in the 1960s.1

The concept of a particle bed space reactor was first investigated in the 1960s at Brookhaven National Laboratory in Upton, New York. During the 1970s and 1980s, Dr. James Powell of Brookhaven developed the particle bed concept further by designing a gas-cooled reactor that employed a fuel element consisting of small spherical fuel particles packed between two concentric porous cylinders called frits. The PBR was envisioned to consist of 19 fuel elements assembled to form the reactor core. Each fuel element could contain millions of tiny uranium fuel particles (approximately 0.01 inch [0.5 millimeter] diameter). Hydrogen gas would enter the top of the reactor core and pass through the outer cylinder walls of the fuel elements into the fuel particle bed where the heat from fission would be transferred to the hydrogen. The heated hydrogen gas would then be expelled through the inner cylinder wall of the fuel element and exit the reactor core into a nozzle chamber, from which the exhausted gas would provide thrust for

Artist’s concept of a nuclear thermal propulsion transfer vehicle and the ascent stage of a two-stage Mars lander. (Image: NASA, Pat Rawlings, SAIC)

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Nuclear Thermal the spacecraft. In theory, the small PBR Feasibility Propulsion fuel particle size provided a very high surface-area-to-volume ratio, With the initiation of Timberwind Nuclear thermal propulsion thereby enabling efficient heat in mid-1987, SDIO began a two- systems produce thrust by heating removal, high power density, and year effort to evaluate the feasibility a propellant (usually hydrogen) compactness, which resulted in a of the PBR technology. With passing through a nuclear reactor relatively small, lightweight reactor support from the Office of Defense and expanding the hot gases system.2 Programs at DOE, an industry through a nozzle. Upon exiting team led by Grumman, and two the throat of the nozzle, the hot In the early 1980s, Powell and DOE national laboratories (Sandia gas expands against its flared Brookhaven began collaborating and Brookhaven), the two years sides, thereby generating thrust, with an industry team led by were filled with design, analysis, which propels the nozzle/rocket Grumman Aerospace Corporation fabrication, and testing activities. forward. The very-high-temperature to develop the PBR concept for The major emphasis was placed capability provided by a reactor and various space applications. As on development and testing of the the use of a low-molecular-weight a compact, lightweight, high- reactor systems, including the fuel propellant offer the potential for a density power system, the particle particle, fuel element, and reactor.3 high specific impulse (a measure of bed technology soon caught the the efficiency of a space propulsion attention of SDIO as a potential Fuel designers sought to develop a system defined as the ratio of power system for a kinetic energy fuel particle that could withstand engine thrust to propellant flow weapon called the electromagnetic an extremely high temperature rate) and high levels of thrust for rail gun. Interest in kinetic energy of approximately 3,500 Kelvin a relatively low propellant and weapon applications gave way to achieve a desired hydrogen system mass. Such systems also to the concept of using a PBR- gas exhaust temperature of have the capability to produce powered nuclear rocket as a rapid approximately 3,000 Kelvin. As a very high velocities. With these intercept vehicle to destroy ballistic point of reference, the maximum benefits, SDIO hoped to develop an missiles in the early stages of their fuel temperature actually interceptor system that would more boost phase.3 It was the boost- demonstrated during the Rover/ than double the performance of phase interceptor application that NERVA projects was approximately conventional rocket engines in use led to a highly classified program 2,600 Kelvin. A baseline fuel particle at the time (e.g., specific impulse in 1987, codenamed Timberwind, design, derived from a commercial- approaching 1,000 seconds and a under which the feasibility of the scale high-temperature gas-cooled thrust-to-weight ratio of 25 to 35 PBR nuclear thermal propulsion reactor program, was developed for thrust levels of at least 20,000 system concept was first evaluated. that consisted of a uranium carbide pounds). Other applications that Until the time that the program fuel kernel surrounded by a porous have been considered for nuclear was declassified several years later, graphite buffer layer. Surrounding thermal propulsion systems include development of the new nuclear the porous graphite layer was a space exploration, such as a manned propulsion system was invisible to dense graphite layer, which was mission to Mars, and lifting heavy the public and the broader space then surrounded by an outer layer payloads into space.3 nuclear community.1 of zirconium carbide. Although

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the baseline fuel particle had an technology and equipment to tests were performed using inherent temperature limitation manufacture the coated micro- furnaces at Babcock and Wilcox of approximately 2,800 Kelvin particle fuel. The production facilities. Nuclear testing was (well below the 3,500 Kelvin process included the use of a performed using the Annular Core planned for flight-qualified fuel), fluidized-bed chemical vapor Research Reactor, a TRIGA-type its development and use served to deposition process by which the test reactor at SNL.4 Such testing, develop an experience base and graphite layers were applied. With and the inspection that followed, support the development of other the aid of LANL and General provided data on temperature components.3 Atomics, the Babcock and Wilcox limits, coatings, particle strength, fuel designers also developed a and other parameters that served Along with the design of chemical vapor deposition process to verify the fuel design, identify the baseline fuel particle, a for coating the small fuel particle potential failure modes, and production capability was needed with the zirconium carbide.3 evaluate effects of manufacturing to produce the very small (0.01 process variability. The importance inch [0.5-millimeter] diameter) Testing of fuel particles and fuel of such testing was soon evident. fuel particles. Fortunately, fuel elements included non-nuclear During testing of the early fuel developers got help from ORNL, and nuclear aspects. For example, element design, performed under from which they received the non-nuclear fuel particle heating the Pulse Irradiation of a Particle

Typical Fuel Element Hydrogen Fuel Particle propellant flow Zirconium carbide coating Neutron High-density graphite moderator material Uranium carbide kernel Porous Fuel particles graphite Cold frit ~0.5 mm Hot frit

Hot hydrogen gas

Typical PBR fuel particle and fuel element. (Adapted from Final EIS for the Space Nuclear Thermal and Propulsion Program, May 1993)

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Bed Fuel Element project, fuel As development of the fuel particle could further reduce the density of particle breakdown was observed and fuel element progressed, such the hydrogen and the amount of when carbon contamination of efforts would have to eventually heat carried away by the hydrogen the test loop was discovered. address the possibility of hydrogen flow. This cycle could continue until During a later series of tests, it flow instabilities in the core, the fuel particle failed, producing was discovered that the baseline a phenomenon largely unique particles that could block additional design fuel particles failed at a to the PBR. Because the fuel flow pathways, causing progressive temperature of approximately 2,500 element consisted of randomly failure of the system.5, 6 Kelvin rather than the theoretical packed spherical fuel particles, limit of 2,700 to 2,800 Kelvin. the pathways through which the In addition to the heating-induced As a consequence of the failure, hydrogen coolant could pass would particle failure, attention was fuel designers began developing naturally vary. Reduced hydrogen also given to other mechanisms two advanced fuel particles: an flow in one of the pathways would by which fuel particles might infiltrated kernel particle and a reduce the amount of heat being be damaged, such as corrosion, mixed-carbide particle.3 carried away from the fuel, and friction from fuel particle the resulting temperature increase movement or vibration during launch, or from the propulsion system turbomachinery. Although early evaluations by Brookhaven National Laboratory suggested that most of these particulate sources would be insignificant and not create a problem of flow instability, the long-term testing and experience needed to address plugging or local flow blockage was not performed due to program termination.3, 7

In support of the reactor design, a 19-element critical experiment reactor was also designed, built, and tested at zero power at SNL. Following approval of the critical experiment reactor by DOE, a series of critical experiments began in late 1989 that served to verify the nuclear-specific design of the reactor

Annular core research reactor at SNL. (Photo: SNL Flickr)

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and benchmark reactor design Timberwind Expansion thermal propulsion engines. The codes. Because the heterogeneity of Faces Headwinds planned location for the new the PBR was expected to produce nuclear propulsion test complex nonuniform neutron flux and power Although the Annular Core was the Saddle Mountain Test distributions, designers needed to be Research Reactor provided Site at the Nevada Test Site, which able to calculate the internal neutron excellent data on fuel particle would include several testing physics behavior to match coolant and fuel element designs, facilities, analogous to the old flow and obtain a uniform hydrogen operational and power limitations Rover/NERVA facilities. The PIPET exit temperature. Analytical methods of the research reactor limited its facility was to include testing predicted performance within 0.5 usefulness in terms of the testing systems for the fuel assemblies, percent of actual behavior, providing needed to fully qualify the fuel including a bunker for control confidence in the design.3 and other nuclear components for consoles; an assembly facility for flight use. In reality, there were no non-nuclear testing of reactor After two years of design, analysis, domestic test reactors capable of cores; the PIPET reactor test cell; fabrication, and testing, the producing the high temperatures a coolant supply system to supply feasibility of the PBR technology (3,500 Kelvin fuel particle), cryogenic hydrogen (the primary had been established to an extent power densities (40 megawatts coolant) and helium (to be used sufficient to support a follow-on per liter), and operational to purge the system); a remote development and testing phase. environment (flowing hydrogen) inspection and maintenance Existing test facilities had been put needed to qualify the PBR and its system to allow reactor evaluation to use and new test facilities were components. To address this issue, in a high-radiation environment; in the initial throes of planning a new test reactor was planned. and an effluent treatment system and design. The project team had The PBR Integral Performance to remove potential radioactive worked together for over two Element Tester (PIPET) was contaminants from the hydrogen years and many of the bumps and conceived as part of a larger new exhaust gas so it could be flared hurdles that come from bringing test complex at which the systems while keeping atmospheric a diverse team together had been and infrastructure needed for emissions within limits.3 ironed out and cleared. With the testing and qualifying an integrated feasibility of the PBR technology nuclear thermal propulsion engine The plan accommodated expansion showing promise, a new contract could be located.3 to a full-scale facility, including was initiated in 1989 to begin a building with cells for testing the next phase of development, As originally conceived by SDIO, ground-test and qualification-test testing, and validation of the PBR PIPET was going to be a small, low- articles, coolant- and effluent-system propulsion system in preparation cost, single-use facility for testing upgrades, a disassembly facility for for an eventual ground test of a PBR fuel elements and engines. post-irradiation evaluation, and a flight demonstration engine. Over time, the concept evolved non-nuclear engine integration test into a large-scale ground-test facility in which comprehensive facility for reactors and all nuclear cold flow tests (without a reactor) components, with a separate facility could be performed to characterize, for testing integrated nuclear integrate, and qualify the engine feed system, propellant management

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system, and engine components. The PIPET reactor would be sited in As SDIO worked to address Congressional a reinforced concrete cell, partially below ground. The reactor core concerns, the headwind of agency and mission would be confined in two carbon- carbon pressure vessels and a change was soon felt when the former Soviet metallic pressure vessel.3 Union was in the throes of significant political

Two other major facilities were and economic reform . . planned for testing of non-nuclear components. The first was the San Tan Hydrogen Test Facility located resources to support thermal/fluid, system, suggesting that the nation in a valley on the Gila River Indian neutronic, and other reactor system would be better served by a broad Reservation approximately 20 modeling.3 development effort.1,3 miles (32 kilometers) outside of Tempe, Arizona. The facility was As the vision for an expanded As SDIO worked to address being built to enable tests with both testing capability unfolded, progress Congressional concerns, the cryogenic and high-temperature on Timberwind soon stalled in the headwind of agency and mission (3,000 Kelvin) hydrogen. The site face of several headwinds, including change was soon felt when the had been in operation for over 30 congressional actions, global former Soviet Union was in the years to test aerospace systems and events that resulted in agency throes of significant political and components produced by Allied and mission changes, and public economic reform that brought Signal. The facility would enable awareness of the planned nuclear the Cold War to an end in 1991. design, development, verification, rocket program. In fiscal year 1990, Priorities in defense systems were and qualification of components Congress limited funding for the soon redefined, and the SDIO plans and materials exposed to hydrogen, nuclear rocket program pending for its nuclear-powered interceptor such as the engine turbopump, broader DoD endorsement, missile gave way to the use of feed valves, nozzles (subscale), including that of the Defense the PBR technology to lift heavy 3 and the hot frit. The second non- Science Board, a committee of payloads into Earth’s orbit. As an nuclear facility was located at the civilian experts that advises DoD upper-stage launch vehicle, the Grumman complex in Bethpage, on a various scientific and technical new mission focus lent itself well to New York. The Grumman System matters. Technical progress the Air Force need to launch heavy Integration and Test Laboratory slowed while the program satisfied satellites and other communication was used to develop integrated the Congressional language. systems into space. With the PBR- engine systems and was integral in By October 1990, the required based interceptor off the table, there the development, verification, and endorsements had been received. was no reason for SDIO to continue validation of operational software. As part of its endorsement, funding it. In fiscal year 1991, after The laboratory developed the however, the Defense Science an investment of $131 million, SDIO flow control system for nuclear Board had recommended a multi- abandoned the nuclear project it had element tests at SNL and included agency development for the PBR- started four years earlier. The project special-purpose computer based nuclear thermal propulsion was subsequently transferred to the

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Air Force, with management of the lifted in early 1993, at which time 930 seconds and a thrust-to-weight nuclear rocket activities assigned to only the nuclear technology portions ratio of 20:1, a capability somewhat the Air Force Phillips Laboratory in of the program under the cognizance reduced from earlier goals Albuquerque, New Mexico.8 of DOE remained classified.3 established for the boost-phase interceptor missile. Compared to In early 1991, as SDIO was preparing Timberwind Rebranded— chemical propulsion systems, such to hand the PBR technology Space Nuclear Thermal as those used with the Titan- and program off to the Air Force, the Propulsion Atlas-class launch vehicles, the existence of, and information on, design represented an improvement the still-classified program was Following transfer of the of approximately two to four times leaked to the public. Several public Timberwind program to the Air for payload lift capability. With the revelations followed, including an Force, it was rebranded as the shift in mission focus to an upper- April 1991 New York Times article Space Nuclear Thermal Propulsion stage launch vehicle or orbital that revealed the general outlines of (SNTP) Program and restructured transfer vehicle, the concern with the program. The article also cited as a technology development effort. use of the nuclear thermal rocket in Steven Aftergood of the Federation The new program was introduced Earth’s atmosphere was addressed of American Scientists as saying that to the public by Senator Pete by planning for reactor startup an analysis prepared by SNL showed Domenici and representatives only when it was 497 miles (800 that the probability of crashing into from Phillips Laboratory during kilometers) above the earth. The New Zealand in the event of the the Ninth Space Nuclear Power decision significantly improved failure of a prototype nuclear rocket the risk picture of the nuclear Symposium in January 1992. It was 3 during a projected suborbital flight announced that the Air Force had propulsion project. test over the ocean near Antarctica 9 been supporting a nuclear rocket would be one in 2,325. Other technology development program Nuclear Propulsion and the articles followed, including an article using an advanced PBR concept. Space Exploration Initiative in Scientific American magazine Although the main applications for in which representatives from the the nuclear thermal rocket were As DoD worked through its agency working groups of the Federation upper-stage launch vehicles and and mission changes, NASA of American Scientists (including orbital transfer vehicles, no specific soon entered the national nuclear Aftergood) and the Committee of mission was identified.12 propulsion venue in a much larger Soviet Scientists for Global Security role with the announcement of a put forth an argument for banning In the absence a specific mission, new SEI in July 1989. Marking the the use of nuclear power in Earth’s the Air Force established a broad 20th anniversary of the Apollo 11 orbit; the Timberwind program 10 set of performance goals for its new moon landing, President George was used to bolster their case. thermal propulsion program in H. W. Bush set forth a vision for Questions regarding the level of order to remain flexible to potential the future of U.S. space exploration classification and concerns regarding user needs and technology that included a permanent return the adequacy of technical review developments. The final baseline to the moon and a human mission and Congressional oversight of the 11 design represented a system capable to Mars. To bring focus to the new classified program soon followed. of 40,000 pounds of thrust (1,000 initiative, Bush put forth several Classification of the program was MWt) with a specific impulse of challenges, including a goal to

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place humans on Mars by 2019, a In a separate but parallel effort, development, while responsibility lofty goal that would mark the 50th NASA, DOE, and DoD also began for nuclear systems resided at anniversary of man’s first landing looking at propulsion technologies DOE-Idaho. By October 1991, the on the moon.13 In America at the to support the new space initiative. agencies had formed the foundation Threshold (a foundational report Under the leadership of Gary for a new civilian nuclear that set forth a sort of technology Bennett (NASA), Earl Wahlquist propulsion program; however, roadmap for achieving the goals of (DOE), and Roger Lenard (DoD, funding in fiscal year 1992 was only the new space initiative), nuclear Air Force Phillips Laboratory), approximately $3.5 million. thermal propulsion was identified an extensive evaluation process as “the only prudent propulsion began in 1990 to identify and Nuclear Thermal system for Mars transit,” in part evaluate both thermal and Propulsion Doubles Down since it would result in a significant electric nuclear propulsion reduction in travel time to and from technologies. Early planning By 1992, the nation was supporting the red planet, thereby minimizing evolved into a broad-based effort two separate nuclear propulsion the adverse effects of long-term in which six interagency teams, 14 programs, with funding and space travel on astronauts. including several nuclear-industry oversight provided by different participants, delved into the details congressional committees. The of nuclear propulsion technologies, NASA-led SEI effort focused mission analysis, nuclear safety on nuclear thermal propulsion policy, fuels and materials and nuclear electric propulsion technology, and the facilities that concept and technology feasibility would be needed to test and qualify 15,16 evaluations for its moon and Mars new nuclear propulsion systems. missions but had yet to transition into technology development or As nuclear thermal propulsion other hard efforts. and nuclear electric propulsion concept development and Meanwhile, SNTP continued on evaluation evolved, the agencies technology development for the eventually banded together to PBR. Efforts included development implement a national, broad- of a laboratory-scale process to based nuclear propulsion produce the advanced infiltrated program to support SEI and kernel fuel particle, including its other civilian and military graphite microspheres. Several missions that might arise. critical experiments (using Overall program direction came the baseline fuel particle) had from the headquarters of NASA been performed to support and DOE. A nuclear propulsion determination of reactor physics project office, established at the parameters.3 A nuclear element test NASA Lewis Research Center (the first of four planned) designed (now the GRC), was responsible to validate the PBR fuel element for nuclear propulsion technology concept, obtain engineering

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data, and benchmark codes was heavily redacted classified EIS Traveling to Mars performed using the Annular Core had been previously issued for the When planning for a mission to Research Reactor; failure of the earlier DoD efforts), providing an Mars and back, two space nuclear fuel element at a temperature of opportunity for full public scrutiny propulsion systems are available 1,700 Kelvin, however, showed of the agency’s nuclear propulsion for consideration—nuclear thermal ongoing issues with the fuel plans. In its EIS, the Air Force propulsion and nuclear electric element design (particularly the noted it was considering whether propulsion. In selecting a specific frits).3,17,18 In addition, the reality the SNTP should be continued system for a given task, planners of the potential cost to complete and, if so, at what location—the consider the use (e.g., lifting heavy the planned ground engineering proposed Saddle Mountain Test objects into space versus spacecraft development effort also began to Site at the Nevada Test Site or at an propulsion through space), payload show, with Phase II cost projections alternative, contained test facility weight, and mission timing relative to ranging from $500 million to over at the Idaho National Engineering launch windows. $1.2 billion for a comprehensive Laboratory in southeast Idaho.2 development and testing program.3 In the process of identifying and Nuclear electric propulsion systems use evaluating the alternative testing a nuclear reactor to generate electricity that provides power to an electric thruster system. Unlike nuclear thermal On the other side of the nuclear propulsion house, propulsion systems, nuclear reactors used in electric propulsion systems SNTP continued a forward march on technology are designed to operate at lower temperatures over a period of several development for the PBR . years. Nuclear electric propulsion systems typically generate very low vehicle thrust and acceleration levels, With two separate programs locations, competition had been but much higher specific impulse (force and two hefty price tags on the created between the two proposed per unit mass of rocket propellant), table, improved cooperation sites, as each site hoped for the which makes the most efficient use of was inevitable. For example, the promise of new facilities and new propellant over a long period of time. agencies eventually began to explore jobs in light of the DOE emphasis Conversely, nuclear thermal propulsion the possibility of using common to consolidate and cleanup its systems offer high vehicle thrust and nuclear thermal propulsion testing weapons complex. acceleration levels, which translate facilities, such as PIPET, to meet to relatively brief reactor operational 19,20 the needs of both programs. Notwithstanding efforts to times (hours) and generally lower In June 1992, responsibility for search for common ground, the specific impulse. For the planned SNTP support within DOE was two nuclear thermal propulsion SEI Mars mission, nuclear thermal also transferred from the Office programs were the topic of a propulsion solid-core concepts were of Defense Programs to DOE-NE. Congressional hearing held proposed as the baseline technology With support from DOE-NE, the in October 1992, at which for propulsion from Earth’s orbit to Air Force issued an unclassified representatives from DoD, NASA, Mars’ orbit and back.14 SNTP EIS for public review (a and DOE were in attendance. Of

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interest to Chairman Howard the propulsion program before historian Thor Hogan noted, the Wolpe and his oversight committee proceeding further.1 broad political and Congressional was the prospect of another support needed to provide any expensive space nuclear endeavor Requiem for Nuclear hope for SEI survival was lacking, (the other being SP-100) and Propulsion largely the result of a hefty price tag the need and expected benefits (upwards of $400 to $500 billion thereof. Topics included nuclear By 1993, a new presidential over a 30-year period), and “a thermal propulsion technologies, administration was in place under deeply flawed policy process that anticipated development costs, Bill Clinton. Although NASA failed to develop (or even consider) and agency roles and cooperation. continued SEI planning even as the policy options that may have been At the conclusion of the hearing, politically acceptable given the Bush Administration came to an 13 Wolpe expressed ongoing concern end, it appeared unlikely that the existing political environment.” despite agency efforts to alleviate incoming Clinton Administration them, and noted his hope that would pursue the same space Upon his inauguration, Clinton and Congress would look hard at exploration goals. As NASA his administration embarked on a new direction for the country, one that included a major emphasis on Federal deficit reduction. Clinton noted in his first State of the Union address on February 17, 1993:

“It puts in place one of the biggest deficit reductions and one of the biggest changes of Federal priorities...in the history of this country…My recommendation makes more than 150 difficult reductions to cut Federal spending... We are eliminating programs that are no longer needed, such as nuclear power research and development. We are slashing subsidies and canceling wasteful projects...We’re going to have to have no sacred cows except the fundamental abiding interest of the American people.”21 Artist’s concept of possible exploration programs. A nuclear thermal rocket fires upon arrival in the vicinity of Mars to insert the transfer vehicle into orbit. Nuclear propulsion can shorten interplanetary trip times and can reduce the mass launched from Earth. (Photo: NASA, Pat Rawlings/SAIC)

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Funding for the small NASA nuclear propulsion effort The broad political and Congressional support (approximately $3.5 million per year) didn’t materialize in fiscal year needed to provide any hope for SEI survival was 1993; the broader SEI program was lacking, largely the result of the hefty price tag . . canceled in 1996.

As for the SNTP, despite the fact that an unclassified EIS was eventually released for public review, the Air Force requested no new funding for fiscal year 1994. It also withheld further funding in fiscal year 1993 pending transfer of the technology program to another agency, presumably one that would be interested in carrying the technology forward. When a transfer failed to materialize, the SNTP program was finally terminated in January 1994.1, 2 In seven short years, the two nuclear thermal propulsion programs had come to an end.

117 When planning to utilize a nuclear electric propulsion system, in missions that will gather increasingly more data and transmit them in ever-decreasing times, one factor emerges as a recurring hurdle—power .

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ollowing the resurgence in space nuclear reactor development between 1983 and 1993, U.S. investment in space reactor research and development largely waned. Although some small study and Ftechnology efforts continued, the focus was on solar, chemical, and radioisotope systems to meet the power and propulsion demands of space and planetary exploration missions. However, when planning to utilize a nuclear electric propulsion system, in missions that will gather increasingly more data and transmit them in ever-decreasing times, one factor emerges as a recurring hurdle—power. If that hurdle could be cleared, exploration of the solar system would take on a whole new dimension.

As a new millennium began to unfold, a change in presidential administration brought renewed interest to nuclear technology. On January 20, 2001, George W. Bush was sworn in as the 43rd President of the United States. Early in his first term, Bush announced a new national energy policy. The policy included strong support for nuclear power as a key component in the nation’s energy portfolio.1

A Project Takes Flight

During the early part of the Bush presidency, the men and women of NASA were working with foreign partners to develop and operate the International Space Station and were continuing efforts to establish a robotic presence on Mars. Along with continued exploration of the solar system, NASA also worked to maintain a long-term program of remote earth sensing. Those efforts would soon be placed under the direction of Sean O’Keefe, the new NASA Administrator. With a background in public administration and financial management, O’Keefe wasn’t a renowned space guy. He was, however, a keen and determined administrator who had a distinguished career in government and academia prior to his appointment at NASA. O’Keefe was tapped for the NASA position upon the departure of Daniel Goldin. In addition to his skill in public administration, O’Keefe brought to NASA an awareness of the capabilities of nuclear propulsion. He began his appointment as the 10th Administrator of NASA on December 21, 2001.2

Small ion rocket being tested inside a vacuum test facility in 1959. Such systems were first used operationally in the Soviet Union and later employed by American commercial spacecraft and NASA space probes. (Photo: NASA)

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Early in 2002, NASA put forth a previous name, DOE-NE) had reformulated planetary exploration worked with NASA to provide program. A key element of the new RPSs and was supporting NASA in program included an investment in new space reactor technology the development of nuclear-electric efforts. During Senate hearings on propulsion technologies. Perhaps the 2003 budget for DOE, William such an investment would address D. Magwood IV, Director of the limitations to space exploration DOE Office of Nuclear Energy, imposed by solar and chemical Science and Technology, power. In the eyes of some, the acknowledged the new NASA technology supporting planetary initiative and noted that DOE exploration was stuck in the past. would continue to participate in Dr. Edward Weiler, then head of the nuclear electric propulsion space science at NASA, offered this development effort; however, the William D. Magwood IV perspective in a New York Times extent of that participation by his 4 Director of the Office of Nuclear article describing the paradigm office had not yet been defined. Energy, Science and Technology shift: “We are trying to continue at DOE. the exploration of the solar system The extent of DOE-NE involvement in covered wagons…Now it’s time in the nuclear fission reactor work to switch to the steam engine and was still evolving because O’Keefe build railroads to explore the solar had been collaborating with a system like railroads contributed separate arm of DOE, the Office of to the exploration and expansion Naval Reactors (DOE-NR) within of this country.”3 The tracks for the National Nuclear Security this cosmic railroad would be laid Administration, to garner technical by the Nuclear Systems Initiative support for the space reactor (NSI). The initiative was to be a development effort. In response five-year, $1 billion investment to questions regarding DOE-NR that would resurrect space nuclear involvement, Admiral Frank L. reactor research and development, “Skip” Bowman, DOE-NR Deputy and continue development of a new Administrator, acknowledged generation of RPSs, a technology that preliminary discussions had that had been successfully used in taken place between high-level NASA missions for decades. Finally, officials within DOE and NASA. Sean O’Keefe the power hurdle could be cleared. He also noted the purpose of the 10th NASA Administrator. discussions had been to identify As with all things nuclear, NASA issues that would need to be would need the continued support addressed to allow DOE-NR of DOE. The DOE Office of involvement in the space nuclear Nuclear Energy, Science and power effort. Bowman also offered Technology (later returned to its a caveat, reminding the Senate

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Committee that Deep-space Inroads Dawn of Enlightenment any decision In Greek mythology, Prometheus, regarding Project Prometheus had two the son of a Titan, brought fire to such overall objectives: (1) develop mankind. As a result of his actions, involvement a space vehicle that combined mankind grew in knowledge and would a nuclear reactor with electric wisdom. The story is often used reside with propulsion for robotic exploration as a metaphor for enlightenment. the president of the outer solar system; and (2) During his tenure at NASA, or Congress since DOE-NR was execute a scientific exploration O’Keefe believed that the next responsible for naval nuclear mission to Jupiter and three of its breakthough in space exploration, propulsion, and not civilian space icy moons—Callisto, Ganymede, whether human or robotic, would reactors.4 While involvement and Europa.5 require space nuclear power, of DOE-NR was not yet firmly as noted years later by John established, O’Keefe had set the The space vehicle was conceptually Casani, JPL Project Manager. That stage for a new player to come to straightforward. A nuclear reactor conviction gave rise to project the space nuclear reactor table. would generate heat from the Prometheus.7 fission of uranium. The heat from As the vision for use of nuclear the reactor would be transferred electric propulsion evolved, it to a power conversion system and was soon connected to a specific converted to useable electricity. mission. The mission would The electricity would power an the power needs of the spacecraft employ a nuclear-reactor-powered electric propulsion system and and associated systems. The largest spacecraft that would tour Jupiter other spacecraft equipment. individual power need would come and three of its moons. In late Any heat that wasn’t converted from the ion-thruster propulsion 2002, the Jupiter Icy Moons to useable electricity would be system that would operate at a Orbiter (JIMO) mission was born. transferred, or rejected, to the power level of 180 kWe and a As envisioned, JIMO would be coldness of space using a heat specific impulse in the range of part of a broader project called rejection system. 6,000 to 8,000 seconds. To meet the Prometheus, into which the RPS power demands of the propulsion and space reactor goals of the NSI While conceptually straightforward, system and other on-board systems, had been incorporated. Beginning the path to achieving a targeted the reactor-power conversion with $20 million in 2003, and a 2015 launch date was extremely system would need to generate at request for $93 million in 2004, the challenging. The spacecraft would least 200 kWe of electric power, Prometheus/JIMO project began in be designed to operate for 20 years. which corresponded to a reactor March 2003.5, 6 During those 20 years, the space thermal power output level of reactor would provide 10 years approximately one megawatt of operation at full power and 10 (1,000 kWt). years of operation at a reduced power (assumed to be 30 percent of full power). The reactor output, or power level, would be driven by

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The weight of the spacecraft and all of its systems would be From the perspective of NASA, the assignment tightly controlled; however, the 37,000 pounds (16,800 brought to the project “50-plus years of practical kilograms) envisioned for the craft experience in developing safe, rugged, reliable, approached the upper limit of launch-vehicle capabilities. Data compact, and long-lived reactor systems designed collection, storage, and transfer rates would be maximized but to operate in unforgiving environments .” also constrained, simultaneously, by the capabilities of the Deep Space Network and Planetary NE, two DOE laboratories (LANL would provide the launch vehicle Data System, the earth-based data and ORNL), and the GRC. As the and associated ground support handling and storage systems that project matured, other partners capabilities. Other components, support NASA space exploration. were brought into the fold. In such as the heat rejection and ion Finally, the technologies would March 2004, two years after propulsion systems, would be led have to be extensible to Lunar- discussions between NASA and by NASA field centers. and Mars-surface missions, DOE began, the space nuclear thereby introducing additional reactor design and development With all of the organizations, technical complexities into the effort was finally given to DOE- companies, and personnel involved project. Terms such as “aggressive,” NR. From the perspective of with the project, management and “unprecedented,” and “push the NASA, the assignment brought administration of the project would technology envelope” were used to the project “50-plus years of prove every bit as challenging as the when discussing mission goals practical experience in developing technical aspects. With the large in the context of the level of safe, rugged, reliable, compact, number of partner organizations, technological advancement that and long-lived reactor systems each with its own culture, systems, designed to operate in unforgiving and practices, challenges would would ultimately be needed for 9 mission success.5, 8 environments.” include geographical separation and communication barriers. To meet the challenges posed During project execution, DOE-NE Roles and responsibilities would by the project, NASA turned to would continue to support other need to be clearly defined, as JPL and John Casani to lead the NASA space nuclear technology would organizational interfaces. effort. Casani brought decades of efforts, such as development Reporting, document, and experience to the project, having of RPSs. Later that year, NASA management systems would all been involved with previous awarded a $400 million contract require alignment. The list seemed missions, including the Mariner to Northrup Grumman Space endless, and the experience gained missions to Mars and the Voyager, Technology and announced they during project execution provided a would be responsible for co- wealth of lessons from which others Galileo, and Cassini missions. 10 The initial project team included design of the JIMO spacecraft, could learn. members from JPL, NASA, DOE- including integration of all systems with the spacecraft. NASA itself

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The Spacecraft Takes The DOE-NR team spent several on the Prometheus project, the Shape months identifying and evaluating gas-reactor/Brayton cycle system an exhaustive set of nuclear reactor “appears capable of fulfilling the While DOE-NR had significant and power conversion technologies. mission requirements…simplifies experience with the design, All aspects of a nuclear reactor engineering development testing, operation, and maintenance system were considered, including and offers the fewest hurdles to of nuclear electric propulsion the reactor core, fuel and materials development.”12 The direct gas/ systems in the oceans of Earth, performance, reactor shielding, Brayton reactor concept was the environments of outer space primary-coolant transport and subsequently approved by or a Lunar or Martian surface materials compatibility, energy DOE-NR. brought a new set of challenges. conversion and heat rejection Where reactors on Earth include operations, and operational The reference reactor plant concept provisions for control by human concerns. Technologies were employed a single gas reactor, operators, systems in space must considered against the mission located at the forward end of the be entirely controlled remotely or established operational, power, and spacecraft. An inert gas mixture, autonomously. Where an ocean lifetime requirements. They were consisting of xenon and helium, provides an endless supply of water also evaluated from the perspective would be used to transfer heat for cooling a reactor core, cooling of developmental challenges and from the reactor core to the power in space is accomplished using a technical maturity. Hundreds of conversion system. The reactor heat rejection system such as a parametric studies were performed, core would consist of cylindrical large radiator. The large radiator including trade-off studies and highly enriched uranium ceramic would have to be designed to fit system optimization. Five candidate fuel elements arranged within an inside the rocket fairing (i.e., by reactor-plant concepts were appropriate core structure. The folding) and deployed only after the eventually developed and evaluated vessel holding the reactor core spacecraft reached orbit.11 To meet for overall capability, reliability, would be relatively small, only two these challenges, DOE-NR would deliverability, cost, and safety. feet (0.6 meter) in diameter and solicit the help of the engineers From the five candidate systems, five feet (1.5 meters) in length. A and scientists at their naval nuclear the DOE-NR team selected a combination of fixed and moveable propulsion laboratories, including gas-cooled fission reactor coupled reflectors surrounding the reactor Bettis Atomic Power Laboratory, with a Brayton cycle power vessel would provide the means Knolls Atomic Power Laboratory, conversion system. As noted in a to maintain reactor reactivity at and Bechtel Plant Machinery, Inc. report that summarized the work desired operating temperatures. The Eventually, engineers from other performed by the DOE-NR team reactor system would also include DOE national laboratories and NASA research centers would also join the DOE-NR team, bringing Five candidate reactor plant concepts were together decades of reactor and propulsion design, operation, and eventually developed and evaluated for overall safety experience. capability, reliability, deliverability, cost, and safety .

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Brayton Cycle Power at least one safety shutdown rod such a system is a direct function of Plant to preclude inadvertent criticality the mass flow rate of the propellant during operations involving ground and the velocity of the propellant A Brayton cycle system consists of a transport and launch. A shadow as it is exhausted from the system. turbine, heat exchanger, gas cooler, shield, located between the reactor The primary components of an ion compressor, and associated piping, and the remainder of the spacecraft thruster propulsion system include valves, and control systems. During systems, would reduce the adverse a propellant, a system for generating reactor operation, a xenon-helium effects of neutron and gamma electrons, a thruster chamber in gas mixture would exit the reactor radiation on electronic equipment which the electrons collide with core at a very high temperature and and other components once the the propellant, resulting in its be piped from the reactor vessel spacecraft achieved orbit and the ionization, and an electrical energy to the Brayton cycle turbine. The reactor was placed into operation. source to create a large voltage turbine and an alternator would potential across which the ionized share a common shaft. As the hot The propulsion system employed an propellant is accelerated to an gas passes through the turbine, the ion thruster technology. In an ion extremely high velocity as it exits the turbine-alternator shaft rotates, thruster system, thrust is generated thruster chamber. Thus, although resulting in the generation of by exhausting a high-speed an ion thruster has very low thrust, electricity by the alternator. After propellant from a thruster chamber. its continuous operation results in a passing through the turbine, the The amount of thrust generated by very high specific impulse. gas would be routed through a heat exchanger and gas cooler, after which it would be pumped back to Magnetic field enhances Magnet Ions electrostatically the reactor core via the compressor. ionization efficiency rings accelerated Excess heat from the gas cooler Propellant atom would be transferred to the heat Anode rejection system via a cooling loop. Discharge plasma The electricity generated by the Propellant Brayton system alternator would injection Electron then be conditioned for use in Ion beam powering an ion thruster propulsion Electrons emitted system and other on-board by hollow cathode Ion electrical equipment.12 traverse discharge and are collected by anode Electrons impact atoms to create ions Positive Negative grid (+1090v) grid (-225v) Hollow cathode plasma bridge neutralizer Electrons injected into beam for neutralization

General components of an ion thruster. (Adapted from NASA/TM-2004-213290, Electric Propulsion Technology Development for the Jupiter Icy Moons Orbiter Project)

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For the JIMO mission, xenon gas programs would increase while would be the propellant of choice, that in other areas of the Federal and electrons would be generated government would hold steady. The using a microwave source and/or a goal was to keep the growth rate hollow cathode. Although greatly of Federal spending to less than simplified, thrust is generated one percent and reduce the Federal by the following process. When deficit by 50 percent in five years. In the xenon gas and electrons are establishing a new financial reality introduced in the thruster chamber, for the nation, agencies like NASA the gas molecules collide with and DOE would begin to feel the the electrons, resulting in their constraint of flat-line budgets.15 ionization. The ions are created President George W. Bush unveils a new at a high voltage relative to the Vision for Space Exploration on January In early 2004, Senators John spacecraft. A system of two grids, 14, 2004. (Photo: NASA) McCain and Daniel Inouye, who located at the exhaust side of the were responsible for oversight thruster, are used to establish a space program on a new course and of the NASA budget and were voltage potential (or difference) gave NASA “a new focus and vision concerned with the looming that is significantly lower than the for space exploration.”13 The shuttle Federal budget constraints, called electrical charge of the ions. The fleet, grounded since the February for an audit of the Prometheus resulting voltage differential creates 2003 Columbia space shuttle project. The GAO was asked to the force by which the ions are disaster,14 would be returned to determine if NASA had established accelerated to an extremely high service to meet existing obligations justification for the investment in velocity (e.g., 65,616 to 328,083 feet connected to construction of the the Prometheus/JIMO project and [20,000 to 100,000 meters] International Space Station by 2010. how the agency planned to ensure per second) as they exit the Following completion of the space critical technologies would be at an thruster, thereby producing the station construction, the shuttle appropriate level of maturity when thrust that propels the spacecraft fleet would be retired after nearly needed. The NSI, announced in through space. 30 years of service. NASA would 2002, was targeted as a five-year, therefore begin development and $1 billion effort. Prometheus, which Changes on the Horizon testing of a new space vehicle to began in 2003, expanded upon ferry astronauts to and from the the initiative and had a five-year As the Prometheus project gained space station. Finally, the United budget of $3 billion; however, the momentum, two events in January States would return to the moon by budget didn’t reflect the cost of 2004 would have far-reaching 2020. out-year activities that would be impacts for NASA and its future needed to support the 2015 launch, missions and plans. On January However, during his State of the and a life-cycle cost estimate was 14, President Bush announced a Union address later that month, not expected until the summer new Vision for Space Exploration, Bush discussed the “war on terror” of 2005. To make matters worse, thereby establishing a new space and the future of the nation as a cost estimate developed by the policy for the nation. In announcing the country continued to move Congressional Budget Office his policy, Bush set the nation’s forward. Funding for defense indicated the project could cost

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$10 billion. The final audit report, issued in February 2005, provided Through design and development, tests and a broad discussion of the business case for the project but also made experiments, and successes and failures, the note that “NASA announced in technology base that would be used for JIMO its fiscal year 2006 budget request that it was conducting an analysis and also serve as a springboard for future space of alternatives to identify a new mission with reduced technical, reactor efforts continued to grow . schedule, and operational risk.”11

Growth Curves areas of reactor fuel and structural Solar Technology Application materials. Integrated design and Readiness (NSTAR) engine. The As the new policies and realities testing of the reactor system would test successfully demonstrated announced by President Bush pose another significant challenge. AC-to-DC conversion and fault began to take root, the engineers Also, material behavior questions tolerances for the thruster. Other and scientists working on the would require irradiation, creep, tests, performed in an inert gas Prometheus project continued their and compatibility testing to ensure environment and at the project- efforts to demonstrate the feasibility the fuel systems could meet defined operating temperatures, of getting a reactor-powered vehicle operating lifetime and temperature pressures, and speeds, evaluated to Jupiter. Through design and requirements. Challenges were not conditions related to bearing development, tests and experiments, limited to the realm of the reactor- startup, load capacity, and power and successes and failures, the propulsion system. Material supplies loss. Knowledge was gained in the technology base that would be and manufacturing capabilities area of materials behavior through used for JIMO and also serve as a would need to be re-established to long-term tests of the super- springboard for future space reactor ensure high quality and repeatable alloy materials of which system efforts continued to grow. component performance. The components would be fabricated.5 final report summarizing DOE- Also, as part of an activity initiated For the nuclear electric propulsion NR efforts on the project noted under the JIMO project by the GRC, system, development of the gas- “…in future projects, the scope a dual closed-loop Brayton power reactor/Brayton concept was largely and timescale required for conversion system, with a common a paper exercise. The project team an engineering development, gas inventory and common heat had gathered, evaluated, analyzed, manufacturing, and testing effort of source, was procured, analytically and documented an extensive this magnitude must be understood evaluated, installed, and successfully database of information in areas from the beginning….”12 performance-tested. The test such as reactor physics, thermal and demonstrated that the dual loop mechanical evaluations, reactor core In the area of power conversion, configuration could become a viable and plant arrangements, material a team lead by GRC performed a power conversion system candidate properties, and instrumentation and first-ever Brayton ion propulsion for a direct coupled, gas-cooled control development. The biggest test using a 2-kilowatt Brayton test- nuclear reactor power system.16, 17 challenges were judged to be in the bed in conjunction with a NASA

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On the electric propulsion front, 1999. Improvements included a Things continued to progress in teams led by the GRC and JPL 10-fold increase in power, a two- to other areas as well. In the area of pursued advancements in ion three-fold improvement in specific heat rejection technology, efforts thruster technology. Performance impulse, a 30 percent improvement focused on heat pipe design and testing and 2,000-hour wear tests of in overall thruster efficiency, and testing, development of brazing two candidate thruster systems, the improvements in grid voltage and techniques, and materials- and Nuclear Electric Xenon Ion System thruster lifetime. Although chemical-compatibility testing. (NEXIS) and High Power Electric development efforts for both Other teams made headway Propulsion (HiPEP), were systems were progressing well, the in the areas of high-power successfully completed. Both project team concluded that effort telecommunications, low-thrust systems met project-required would be better spent focusing on a trajectory tools, and radiation specifications for specific impulse single-thruster technology because hardening of electronics, which (6,000 to 9,000 seconds), efficiency of the similarity of many features of were necessary to protect against (greater than 65 percent), and the two-thruster systems. The team the destructive effects of neutron power levels (20 to 40 kilowatts). subsequently selected a single and gamma radiation generated These new classes of nuclear thruster design, nicknamed by the nuclear reactor and the electric propulsion thrusters offered Heracles, based on the ion thruster naturally occurring high-radiation substantial performance technology used in the HiPEP and environment in the vicinity of improvements over the electric NEXIS designs.5 Jupiter and its moons. Ground- propulsion engine used on the based systems (i.e., testing facilities, Deep Space 1 spacecraft flown in offices, laboratories, and other work spaces) would eventually need to be planned, designed, and developed. The personnel needed to conduct mission operations, the procedures under which those operations would be performed, and the ground-based software needed to conduct mission operations would also need to be put in place to support the Prometheus project and eventual launch of JIMO.5

Glenn Research Center, Cleveland, Ohio. (Photo: NASA)

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A Vision Fades space vehicle that would replace In May 2005, barely three years the shuttle. Those priorities were into the project, NASA pulled the As quickly as the Prometheus aligned with the Vision for Space plug on the JIMO project, and the project became a shining star Exploration policy presented by Prometheus project was officially within the NASA family, that star President Bush. In the scheme of shut down in October. Nearly began to fade. Questions related to nuclear initiatives, which were $465 million had been spent since project cost and space exploration largely postponed, nuclear electric the project was first announced. priorities could not be ignored. propulsion would be reprioritized However, the hoped-for flight of a The aggressive and unprecedented behind nuclear surface power and space nuclear reactor would have to nature of the nuclear aspects of nuclear thermal propulsion. wait for another day. the project resulted in formidable challenges. And the father of Reprioritization within NASA Prometheus (Administrator came on the heels of the O’Keefe) eventually removed resignation announcement by himself from the game. Sean O’Keefe in December 2004. In a letter to President Bush, In 2005, after re-evaluating its O’Keefe cited commitment to priorities in light of anticipated family for his pending departure budgets, NASA determined that its but noted he would remain at highest priorities were returning his post until a successor was the space shuttles to service, identified. O’Keefe left NASA in completing the International Space April of the following year. Station, and building the new

HiPEP thruster beam extraction test. NEXIS thruster emitting a 4,300-volt Xenon beam. (Photo: NASA GRC) (Photo: NASA GRC)

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Early concept of Jupiter Icy Moons Orbiter spacecraft exploring Jupiter and its moons. The nuclear reactor and Brayton cycle power plant are located at the front of the spacecraft. The large structure in the middle is the heat rejection system. Two ion thrusters, located at the rear of the spacecraft off of the science platform, provide propulsion. (Image: NASA)

129 To fulfill its responsibility of providing such systems, DOE maintains an infrastructure of facilities, equipment, laboratories, and a cadre of highly skilled workers that provide all facets of RPS development, including design, fabrication, testing, assembly, and delivery . 10

130 Infrastructure Inroads 10 Taking RPS Concepts to Flight

hen one views images taken by a spacecraft as it journeys through space, or collected by a rover as it traverses a plan- etary surface such as Mars, the technical accomplishment Wis awe inspiring. Constantly at work behind the scenes is a power system that allows such feats. For deep-space and planetary mis- sions, an RPS is often the only system capable of providing the reliable, long-lasting power necessary for mission success.

To fulfill its responsibility of providing such systems, DOE maintains an infrastructure of facilities, equipment, laboratories, and a cadre of highly skilled workers that provide all facets of RPS development, including design, fabrication, testing, assembly, and delivery.1 The infrastructure also includes a group of scientists and engineers who operate and maintain testing and analytical capabilities to support the rigorous safety review processes that must be completed to support the launch of an RPS into space. Although the principal activities have largely remained the same, the locations where those activities have been performed have changed over the years, giving rise to a description of key events and circumstances that formed the modern RPS infrastructure landscape.

Heat Source Production—the Early Days

As RTG technology matured through the 1950s, plutonium-238 eventually became the isotope of choice for use in U.S. RPSs. With increased demand on the horizon, AEC turned to SRS for production of the needed isotope. Operated by DuPont, SRS was home to several production reactors that were used to produce tritium and plutonium in support of the nation’s weapons program. With the new plutonium-238 production mission came the challenge of developing the requisite processes and operations needed to support its production.2

The Plutonium Fuel Form Facility at the DOE Savannah River Site. (Photo: DOE SRS)

Photo Credit 131 Infrastructure Inroads Taking RPS Concepts to Flight

How was plutonium-238 The first step on the journey to By 1971, concern had arisen and neptunium-237 producing plutonium-238 was within AEC over the possibility produced? development of new chemical of an accident during fuel form separations processes to recover production operations. Specifically, Plutonium-238 does not exist neptunium-237 from the highly the concern was centered on the naturally. It is a man-made radioactive liquid by-product left large quantity of plutonium-238 radioisotope that was produced by over from ongoing nuclear fuel oxide powder present and the irradiation of small neptunium-237 processing operations at the site. handled in the facility. Although compacts with neutrons in the SRS The recovered neptunium-237 the operations were performed K-Reactor. The capture of a neutron by was processed to create a small in gloveboxes, and building neptunium-237 forms neptunium-238 compacted slug, or compact, which and process ventilation systems which, in turn, decays with about a was then irradiated in the SRS included high-efficiency particulate two-day half-life to plutonium-238. K-Reactor. Following chemical air filtration, an AEC evaluation After irradiation in the K-Reactor, processing of the irradiated slugs, of the operations noted that “… the neptunium-237 compacts were the recovered plutonium-238 widespread release of radioactivity placed in storage to allow short- product was transferred to Mound at Mound appears very unlikely, but lived radioisotopes to decay, and Laboratory where it was processed an incident can be postulated which then dissolved using a chemical to produce the encapsulated could have serious consequences in separations process in the HB-Line heat source fuel forms used in a densely populated area...”. Concern to recover the plutonium-238. The RPSs. SRS produced its first regarding the possibility of such resulting plutonium-238 solution was plutonium-238 in 1961.4 an accident wasn’t taken lightly. then purified and converted to a solid In March 1971, AEC decided to plutonium oxide before being shipped From the early 1960s through the transfer its fuel form production off-site for fuel form production. late 1970s, Mound Laboratory activities from Mound to SRS, was home to RPS heat source fuel which had the desired benefit of The neptunium-237 was produced form production and encapsulation distance between its facilities and by neutron capture of uranium-235 in addition to its defense-related the public. AEC subsequently in nuclear reactor fuel during missions. Initially conducted in the notified the Joint Committee on operation of the production Special Materials Facility, which Atomic Energy of its decision in reactors. Following irradiation, was built in 1960, the activities August of the same year. In 1972, neptunium-237 was recovered were transferred to the Plutonium Congress authorized $8 million during chemical processing of spent Processing facility (Building-38), for the design and construction nuclear fuel in the H-Canyon fuel which was constructed in 1967.5 of a new fuel form processing reprocessing facility. The recovered Operated by the Monsanto facility at SRS.6 Completed in neptunium-237 was subsequently Research Corporation, the Mound 1977, operations at the new purified, converted into an oxide Laboratory was located in the Plutonium Fuel Form (PuFF) facility form, and then fabricated into center of Miamisburg, Ohio, and commenced in 1978.7 targets for irradiation.3 on the outskirts of the greater metropolitan area of Dayton, home Although Mound lost the fuel form to nearly 2.5 million residents. production mission, it retained responsibility for fuel encapsulation

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activities. In addition, Mound was provide support in the areas performed in gloveboxes, PuFF was also given responsibility for RTG of fuel form development and designed as a series of nine shielded assembly and testing in the late launch-related safety and accident hot cells in which the fuel form 1970s, work that had previously been analyses, respectively. As DOE production operations would be conducted at the facilities of the RTG and its infrastructure focused on performed. Five hot cells provided developers – GE and TES. The new the upcoming Galileo and Ulysses for the hazardous operations RTG assembly and testing mission missions, issues related to facility involving plutonium-238 powder, was performed in Building 50, the deterioration and reactor safety at including receiving, processing, facility in which the Mound team SRS began to emerge that, coupled hot-pressing, and high-temperature built the first-ever flight-qualified with the end of the Cold War in 1991, furnace operations. The remaining GPHS-RTGs that were used on the led to a second round of changes in four hot cells provided for fuel Galileo and Ulysses missions. the RPS infrastructure landscape. form encapsulation and operations supporting heat source shipments.8 A Decade of Change—the To make maximum use of existing The decision to use hot cells rather 1980s facilities, the operations associated than gloveboxes was related to with the new PuFF facility at SRS concerns over worker radiation By the early 1980s, the RPS were designed to fit inside an doses, which were expected infrastructure was spread largely existing facility, Building 235-F. to be relatively high due to the among Mound, SRS, and ORNL. Unlike Mound and LANL, where quantity (~30 kilograms per year) LANL and SNL continued to plutonium-238 operations were of plutonium-238 expected to be handled in the facility.

Once operational, the first two years of PuFF operations were centered on production of MHW fuel spheres to be used in the MHW-RTGs originally planned for the Galileo mission. Once the fuel sphere production campaign was completed, operations shifted to the production of GPHS fueled clads, the smaller heat source planned for use in the new GPHS- RTG for the Ulysses and, later, in the Galileo missions. Between June 1980 and December 1983, PuFF operators produced the requisite GPHS fueled clads in support of the Galileo and Ulysses missions.

Mound Laboratory in Miamisburg, Ohio circa 1990. (Photo: Mound)

133 Infrastructure Inroads Taking RPS Concepts to Flight

Following completion of fueled the hot cells began to deteriorate in- clad production, the facility was cell process equipment. One of the placed in a standby mode pending bigger issues involved the hot cell identification of a new mission remote manipulators, which were for which heat sources would be used to perform in-cell operations, required.7 decontamination, and maintenance activities. As a result of the corrosive Unfortunately, the hoped-for effects of the plutonium-238, the mission never materialized. In manipulators eventually froze in addition, hot cell and equipment place, precluding further use.7 maintenance began to wane. Funding was tight, and an argon By 1990, estimates to gas system used to maintain an decontaminate and refurbish the inert atmosphere in the hot cells hot cells approached $50 million, was eventually shut down. In the with a completion window of at presence of oxygen (i.e., air), the least two years. For DOE, this was plutonium-238 contamination in troubling news, as NASA was then RTG assembly chamber for the GPHS at planning two future missions for Mound. (Photo: DOE Flickr) the mid-1990s (Comet Rendezvous Asteroid Flyby and Cassini) that would require multiple GPHS- RTGs; the plans eventually culminated in the 1997 Cassini mission and its three GPHS-RTGs. In light of the steep price tag and at least two-year refurbishment window tied to PuFF, DOE decided (in 1990) to relocate GPHS fueled clad production activities, including fuel pellet production and encapsulation, to LANL.9 Although the relocation was initially carried out as a temporary fix pending resolution of the PuFF deterioration issues, the temporary fix eventually became permanent, and PuFF was never returned to service for heat source production.7

View of hot cell 6 inside the PuFF facility (looking through water-filled shield window), where iridium shell welding operations were performed to encapsulate GPHS fuel pellets. (Photo: SRS Flickr)

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The selection of LANL for With the shutdown of the While hopes for eventual restart of production of the GPHS fueled K-Reactor, DOE began looking the K-Reactor were initially high, clads for the Cassini mission was at other reactors within its by the early-1990s, those hopes a straightforward solution to a complex for a possible solution had vanished when the reactor potentially thorny programmatic to its plutonium-238 production shutdowns became permanent. issue. Research and development related to plutonium materials had been carried out at the Plutonium While hopes for eventual restart were initially high, Handling Facility in Technical Area 55 at LANL for many years. Specific by the early-1990s, those hopes had vanished to RPS heat source plutonium, when the reactor shutdowns became permanent . LANL experience included development of the plutonium oxide processing flowsheet for the MHW fuel spheres and the GPHS problem. That search took them Although DOE had a sufficient fuel pellets, which dated back to the other side of the country plutonium-238 inventory on hand to the early 1970s. In addition, and FFTF, located at the Hanford to meet known existing needs, that production of the LWRHUs for site. FFTF was an attractive option inventory was finite and, for the the Galileo mission between 1981 because of its proximity to the first time in nearly 30 years, DOE and 1984 provided the opportunity Fuels and Materials Examination faced an unknown future regarding for LANL scientists to refine their Facility (FMEF), another Hanford the production of the heat source heat source plutonium oxide facility then being modified to isotope.2 processing experience.9 Building on accommodate a planned transfer that experience, production of the of RTG assembly and testing Finally, to consolidate activities plutonium-238 heat sources for operations from Mound in support associated with the production of the Cassini mission eventually of a future DoD mission for a DIPS. iridium hardware, responsibility began in 1993.10 With the prospect of becoming a for production of iridium cladding national center for space nuclear and frit vents was transferred As DOE worked to address the power systems, optimism was from Mound to ORNL in the late problems at PuFF, another issue running high in the state of 1980s. The Metals and Ceramics arose that required additional Washington.11 Division at ORNL had long been attention. By 1989, environmental home to development of advanced and safety concerns had resulted That optimism, however, soon alloys for a wide range of purposes. in the temporary shutdown faded. The RTG assembly and For 20 years, ORNL had served of the production reactors at testing mission never fully as home to production of iridium SRS, including the K-Reactor. materialized because the DoD plans feedstock materials that were Without the K-Reactor, the for high-powered isotope power subsequently shipped to Mound sole plutonium-238 production systems went away when the Cold for fabrication of cladding and frit capability was lost. War ended. The use of FFTF for vents for heat sources such as the plutonium-238 production gave MHW fuel sphere assembly and way to other priorities within DOE. the GPHS fueled clad.12 Following

135 Infrastructure Inroads Taking RPS Concepts to Flight

demonstration of the capability As the 1980s drew to a close, so did address decades of nuclear material to produce flight-quality iridium the decades-long Cold War between production and the environmental hardware, ORNL was given the United States and Russia. consequences left behind. Both responsibility for future iridium With an ensuing de-emphasis on efforts led to facility closures that hardware production, beginning weapons production, DOE began left no site untouched. At Mound, with the Cassini mission. ORNL to evaluate how its complex of this led to a decision in the 1990s maintained the facilities and facilities, equipment, and personnel to shut down all weapons-related equipment needed for production might be reconfigured to decrease activities at the site, thereby of the carbon-bonded carbon-fiber its size and cost. As reconfiguration leaving RTG assembly and testing insulation sleeves used in the planning progressed, DOE also operations as its sole nuclear GPHS module.13 began a massive cleanup program to operations activity.

The K-Reactor at DOE’s SRS. (Photo: SRS)

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RPS Infrastructure and the domestic plutonium-238 production of Russian plutonium-238 fuel.14 New Millennium strategy for the long-term. The first shipment of Russian plutonium-238 purchased under By the early 1990s, the RPS On the foreign-supply front, DOE this agreement made its way to the infrastructure was poised to support found a most unusual partner, at United States in 1994. production of three GPHS-RTGs for least at the time, in the former the upcoming NASA Cassini mission. Soviet Union. For decades, the U.S. plutonium-238 production Production of iridium cladding and United States and the Soviet Union moved more slowly. For almost 15 frit vents was performed at ORNL. had been stuck in an arms race, years, DOE analyzed production LANL produced the pressed oxide each producing a nuclear arsenal to options, evaluated its facilities, fuel pellets using plutonium-238 keep up with the other. Secrecy had and prepared the necessary supplied by SRS, and encapsulated the fuel pellets in the iridium clad- vent sets provided by ORNL. The On the foreign supply front, DOE found a most encapsulated GPHS fueled clads were shipped to Mound for GPHS module unusual partner, at least at the time, in the former assembly and RTG assembly and Soviet Union . testing. SRS continued to maintain the plutonium-238 oxide inventory and operate chemical separations processes to purify the oxide for use been the norm and both countries environmental documentation in heat source fuels. took extensive measures to protect to support a plutonium-238 nuclear-related information and production decision. The early While efforts to support the Cassini material. With the fall of the former 1990s were a time of planning and mission were ongoing, a parallel Soviet Union in 1991, however, strategizing as a post-Cold War effort was underway to address how opportunities for new partnerships structure of DOE began to emerge. best to secure a future supply of between the two Cold War By 1998, a plan for production plutonium-238 beyond the Cassini enemies soon arose. From a supply of the heat source isotope had horizon. With the shutdown of the perspective, Russia maintained an been developed, and DOE SRS K-Reactor, DOE faced a limited inventory of plutonium-238 that announced its intent to prepare a number of options: either produce had been previously produced. plutonium-238 production EIS.15 new plutonium-238 using a different On the demand side, the United That plan was soon abandoned nuclear reactor or procure such States needed plutonium-238 to in favor of a broader effort to material from a foreign supplier. supplement its dwindling inventory address the future of DOE’s nuclear With the future of its weapons to meet anticipated missions. By energy infrastructure, of which complex and defense production the end of 1992, a partnership plutonium-238 production was but reactors in a state of flux, DOE was consummated when DOE one piece. Development of a nuclear ultimately pursued a dual path signed a contract with the Russian energy infrastructure programmatic strategy—supplement the existing Federation-Mayak Production EIS ensued. Just months after DOE inventory with foreign material in Association for the purchase of released a Record of Decision the short term while pursuing a up to 88 pounds (40 kilograms) announcing future plans for

137 Infrastructure Inroads Taking RPS Concepts to Flight

Ensuring Safety that infrastructure,16 the nation Before Flight experienced the worst terrorist attacks that had ever occurred on While heat source production and U.S. soil. The September 11, 2001 RTG assembly and testing play a attacks set in motion a series of major role in the RPS infrastructure, far-reaching initiatives as America those activities are supported by the sought to bolster and strengthen efforts of scientists at SNL, LANL, and the security of the nation. JHU-APL, where the safety testing and analysis needed to demonstrate For DOE, the new homeland protection of the public and the security paradigm translated into environment during mission use of a need for improved security and RPSs are performed. Safety testing safeguards measures to protect its may be performed at numerous special nuclear material. That need facilities, both internal and external soon reached Mound, where the to the DOE Complex. Such facilities RTG assembly and testing facility provide the capability to simulate needed upgrades to meet the new events such as high-velocity impacts, security criteria. As DOE considered fires and re-entry, and explosion overpressures, and include a rocket sled at SNL and the Isotope Fuels Impact Tester at LANL.

The information gathered during safety testing is used to support the Safety testing for the GPHS-RTG nuclear risk assessment and safety included use of the SNL rocket sled. analysis required by DOE for every Simulated GPHS modules were heated mission in which a nuclear power and placed in the RTG housing (orange/ source is planned for launch, as white item in pictures). The housing discussed in Chapter 2. Historically, was positioned on the rocket sled in such analyses were performed by a the vertical orientation (top and center systems integration contractor (e.g., pictures), then rotated to a horizontal Lockheed-Martin) and reviewed orientation and accelerated to a speed by a DOE-led team that included of approximately 57 meters per second Tetra Tech NUS, Orbital Sciences, (128 miles per hour) for impact against and JPU-APL. In 2005, responsibility a concrete target. The post-test RTG for the analyses was transferred is shown in the bottom picture; the to SNL, where such capabilities dented section, shown on the right remain an integral part of the DOE side of the picture, hit the concrete infrastructure needed to support target first. (Photos: SNL) nuclear missions.

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Looking Ahead For DOE, the new homeland security paradigm As the first decade of the new translated into a need for improved security millennium came to an end, and safeguards measures to protect its special the future of plutonium-238 production remained largely nuclear material . unknown. The largest blip on the RPS radar had come in the middle of the decade. Along with the the costs of upgrades against the the end of Mound’s role in RTG- other changes driven by security backdrop of other assets, it appeared related inventions, development, needs following 9/11, DOE for a that the RTG operations might be and operations. Despite that end, time considered a plutonium-238 more economically performed in the Mound story would continue production approach that was part an existing secure facility located to live through the efforts of a small of a larger initiative to consolidate elsewhere in the DOE Complex. group of former employees who RPS operations to INL. The concept Elsewhere ended up being established the Mound Museum. included use of the INL Advanced ANL-W in Idaho. Just as RTG assembly and testing Test Reactor and construction of a new production facility for operations were moved to ANL-W, 19 Established in the early 1960s, in a similar fashion, DOE decided plutonium-238. Ultimately, ANL-W was home to a variety of in 2004 to relocate its stockpile the plan was tabled when DOE nuclear reactor development and of neptunium-237 from SRS to decided to revisit the costs and testing operations, including fuel ANL-W.18 other criteria associated with their development and actinide research. planned approach. With a host of secure facilities for Along with the new RTG assembly its own plutonium and uranium and testing mission, ANL-W Finally, in 2012, a project was fuels, and having been in the also received the equipment initiated to produce plutonium-238 DOE-NE family for decades, and shipping packages used to using existing facilities at ORNL ANL-W was clearly a logical choice. transport assembled RTGs and and INL, with an average RTG assembly and testing operations related nuclear material over production goal of 3.3 pounds were subsequently relocated from public highways. With the two (1.5 kilograms) per year. With the Mound to ANL-W beginning in new missions, ANL-W had been project underway, the last piece of 2003.17 Following a year-long effort to adopted into the DOE RPS family. the RPS infrastructure puzzle had transfer equipment, tools, and other been put in place for continued material from the Mound location to support of future NASA missions. ANL-W, Mound ceased to be part of the RTG family.i In a bittersweet twist of irony, the Golden Anniversary of the invention of the RTG marked

i . See Chapter 12 for more information about the transfer of activities from Mound to ANL-W.

139 At NASA, hopes for a new planetary mission to Saturn had been in the works since the early 1980s . Scientists had long sought to visit the second-largest planet in the solar system, with its fascinating system of rings, numerous moons, and unique magnetic field . 11

140