The goals of these developmental efforts were (and still are) to improve efficiency and maximize electrical power output while minimizing power system weight, all the while striving for the highest level of safety.

40 Advanced Isotope Power Systems 3 Expanding RPS Boundaries

s RTG technology continued its evolution through the 1970s and 1980s, development of advanced isotope power systems remained an important goal within DOE and NASA. e goals Aof these developmental efforts were (and still are) to improve efficiency while minimizing power system mass, all the while striving for the highest level of safety. With these goals in mind, the agencies sought to take advantage of advancements in materials, power conversion technolo- gies, and fabrication and production techniques. In so doing, their efforts served to expand the knowledge base of RPS technology and provided a platform from which future development efforts might build.

A Modular RTG

roughout the 1980s, development and production of the new GPHS- RTG for the Galileo and Ulysses missions were the primary focus for DOE. With its modular heat source and improved power conversion system, the GPHS-RTG was a marked improvement over the MHW and SNAP-19 RTGs, especially in terms of specific power. Soon, however, RTG visionaries began to evaluate the viability of a variation of the design that would bring modularity to the converter level as well as the heat source level.

In 1980, DOE contracted with Fairchild Space and Electronics Company to develop and analyze a new RTG design based on advanced materials and fabrication techniques, as well as the new GPHS that was under development by LANL, Fairchild, and GE. e RTG concept subsequently developed by Fairchild was called the modular isotopic thermoelectric generator (MITG). As conceived, the MITG concept included a single GPHS module surrounded axially by eight multicouples, as well as a standardized section of thermal insulation, housing, radiator fins, and electrical circuit. With a projected electrical power output of approximately 20 watts, individual MITG units could be combined to create an RTG that would be scalable over a range of power levels. Power-level increments of less than 20 watts would be accommodated by adjusting the size of the heat rejection fins located on the outside of the generator housing.1

A composite of the Jovian system, including the edge of Jupiter with its Great Red Spot, and Jupiter’s four largest moons, known as the Galilean satellites. From left to right, the moons shown are Io, Europa, Ganymede, and Callisto. The Jupiter, Io, Europa, and Ganymede images were obtained by the Galileo spacecraft and the Callisto portrait was obtained by the Voyager spacecraft. (Photo: NASA/JPL/DLR)

Photo Credit 41 Advanced Isotope Power Systems Expanding RPS Boundaries

e cornerstone of the MITG the gallium-phosphide additive weight, the 12-module MITG was a thermoelectric concept would improve the efficiency of the would weigh roughly half of the called the multicouple. Developed thermoelectric material by reducing GPHS-RTG. With such promising by the Syncal Corporation, the its thermal conductivity. e performance, fabrication and multicouple concept consisted anticipated benefit of the MITG testing of the new thermoelectric of an array of p- and n-type was improved power conversion materials and multicouples were thermoelectric legs that were efficiency over the unicouple design performed during 1981 through connected to a common hot shoe, employed in the GPHS-RTG. 1983 to determine performance or heat collector, and cold-shoe and viability under conditions that mounting stub. In addition to the Based on an initial concept, simulated operational temperatures multicouple concept, Syncal had an MITG consisting of 12 of the GPHS-RTG.1 Among other also developed a modified silicon- power modules would generate things, test engineers sought to germanium thermoelectric alloy approximately 280 We, which determine the performance of that included a small amount of was comparable to the electrical the gallium-phosphide-doped gallium phosphide. Early testing output of the GPHS-RTG and its 18 silicon-germanium thermoelectric indicated that the presence of GPHS modules. Relative to system material relative to the standard silicon-germanium used in the GPHS-RTG and to estimate module lifetime through measurement of the degradation of individual multicouples. e testing would also serve to confirm adequacy of fabrication and manufacturing techniques associated with the thermoelectric materials and multicouples.

Two test assemblies, each consisting of eight MITG multicouples positioned inside a prototypic section of the generator housing, were subsequently fabricated and tested. Multicouples made from the modified silicon- germanium were used in one test module, while standard silicon- germanium multicouples were used in the second test module. e test

MITG converter unit concept. (Graphic developed by GE; provided by INL RPS Program)

42 Atomic Power in Space II Chapter 3

assemblies were heated electrically MOD-RTG the gallium phosphide, and using a heater that was enclosed in thermoelectric degradation faster an insulated graphite box having Managed for DOE by GE, the goal of than expected. In its attempt to the same outer dimensions as a the MOD-RTG program was to address the thermoelectric issues, GPHS module. develop a ground demonstration DOE suspended fabrication of the system to test the modular RTG ground demonstration system in e tests served their purpose, concept. Beginning in October 1983, mid-1986. Efforts to address the as a number of issues were a three-year plan was developed to multicouple performance issues subsequently identified. For fabricate and test an electrically stretched into a multi-year task and example, crack formation was heated MOD-RTG demonstration were the primary focus of project observed in the multicouples. unit based on six GPHS converter efforts through 1992 when the Subsequent investigation revealed units.5 By mid-1985, GE (with the project was finally terminated. In the cause of the cracking to be help of Fairchild) had completed spite of the difficulties encountered thermal stresses occurring at the a reference flight design and a and technical challenges that joints of the thermocouple array ground demonstration system remained at the time of project and the hot and cold shoes. e design. With a single thermoelectric termination, the nine-year MOD- multicouple was subsequently multicouple device designed to RTG effort did see some significant redesigned to incorporate stress- produce 19 We from one GPHS accomplishments, including the relief features at the problem module, the reference design development of reproducible areas.2, 3 In addition to the stress- consisted of 18 GPHS modules and manufacturing processes for cracking problem, independent 144 thermoelectric multicouples multicouple fabrication, resolution testing of the gallium-phosphide- to produce 340 We.6 With a of mechanical and electrical- doped silicon-germanium projected specific power of shorting problems that had been thermoelectric material, performed approximately 7.9 We/kilogram identified early in the project, at DOE’s request, revealed material (based on a weight of approximately and an understanding of the efficiencies that were lower than 42 kilograms [92 pounds]), the degradation mechanisms associated those previously reported (although MOD-RTG reference flight design with the multicouples. Although they were higher than those of was significantly higher than the follow-on work was recommended, the standard silicon germanium 5.4 We/kilogram GPHS-RTG. DOE priorities shifted to alloy). Lessons learned from the Fabrication of a ground production of the GPHS-RTGs for testing were incorporated into a demonstration system began in the Cassini mission slated for flight revised MITG design as well as the summer of 1985. in 1997.7 fabrication and manufacturing processes. With improvements Although initially optimistic, the underway, DOE decided to test MOD-RTG project soon found the MITG technology in a ground itself facing problems similar to the demonstration system as part of a MITG effort. Multicouple testing follow-on modular (MOD) RTG revealed continued performance development program (MOD- issues, including mechanical and RTG). e MITG program ended in electrical shorting problems, September 1983.4 material issues associated with

43 Advanced Isotope Power Systems Expanding RPS Boundaries

Taking Energy From Heat

Thermodynamic cycles are the basis for many common technologies, including refrigeration, car engines, and aircraft jet engines. A thermodynamic cycle is a process that manipulates the temperature, pressure, and volume of a working uid to either convert heat into energy or use energy to remove heat. Each cycle has its own intricacies but shares four common processes: compression, heat addition, expansion, and heat removal (cooling). Three thermodynamic cycles are of key interest to the space program due to their high eciency and compatibility with various energy sources (e.g., solar and nuclear).

Rankine—Often used in steam Brayton—Jet engines and power- Stirling—As with other cycles, the power plants where water is boiled producing gas turbines often use this working gas is compressed, heat is to produce superheated steam, cycle. A working gas is compressed, added, the gas is expanded, and heat which is expanded through a turbine heated to increase its pressure, is removed to restart the process. A to produce electricity; the steam and expanded through a turbine variety of Stirling-engine types exist. is condensed back into water and to produce electricity. The turbine The ones most recently investigated pumped to the boiler to restart is attached to the compressor to for potential space use are known as the cycle. For space applications, power the pressurization step. In a jet free-piston Stirling engines. In this dierent working uids (e.g., mercury, engine, the hot air is rejected to the conguration, the engine is a cylinder potassium, toluene) are considered atmosphere and fresh air is taken in. with one end exposed to a heat based on design criteria that include In a power plant, the waste heat can source and the other kept at a lower weight and system operating be used in a boiler to create steam temperature using a heat exchanger. temperatures and pressures. The for use in a Rankine cycle; such use A displacer piston inside the cylinder fact that Rankine uses a two-phase is referred to as a combined cycle. moves gas between hot and cold (liquid and gas) operation creates Brayton engines in space must use spaces and is thermodynamically engineering challenges that must a closed system and recycle their coupled to a power piston. The be addressed in the low and zero working gas (typically helium and/or pressure changes caused by the gravity of space. However, because xenon), which must be cooled before addition and removal of heat at the phase changes are an ecient way re-entering the compressor. two ends of the cylinder cause the to transfer heat, Rankine cycles are two pistons to oscillate. The power typically the lowest mass option for piston can be used to drive a linear high-power space applications. alternator to generate electricity from this motion. For the small Stirling engines that have been most recently developed for potential RPS use, these oscillations are extremely fast, on the order of 50 to 100 cycles per second, and with a piston stroke of just a few millimeters.

44 Atomic Power in Space II Chapter 3

Dynamic Isotope Power metal mercury eventually resulted alternator mounted directly to the Systems in the substitution of organic fluids turbine shaft was used to generate in the Rankine-based systems.10 electrical power. After the vapor Unlike static systems, such as the Early development work on closed exited the turbine, it would pass RTG, dynamic RPSs include a power Brayton cycle power conversion through a regenerator where a conversion technology that uses systems included a unit designed portion of the remaining heat in moving parts to convert heat into for a power output of two to 10 the vapor was used to preheat the useable electricity. Such systems kWe developed at the NASA Lewis working fluid that was entering the typically employ the Brayton, Research Center (now known as heat source assembly. e vapor Rankine, or Stirling thermodynamic the John H. Glenn Research Center then passed through a condenser cycles. Whereas the power at Lewis Field, or more commonly where it was liquefied and pumped conversion efficiency of an RTG referred to as the Glenn Research to the point necessary to complete may be on the order of five to seven Center [GRC]).11 the cycle. e heat rejection percent, the conversion efficiency system consisted of a barrel- of a dynamic isotope power system Recognizing that dynamic conversion shaped radiator through which (DIPS) may be 25 percent or higher, was the next logical progression the condensed liquid passed and thereby producing three to four in space nuclear power system excess heat would be rejected to times the amount of electrical power technology, DOE and NASA initiated space. An overall system efficiency per unit mass of radioisotope fuel. a program in 1975 to develop a of approximately 18 percent was system capable of producing 1.3 anticipated from a 475-pound Research into dynamic conversion kWe from a system weighing 450 (216-kilogram) unit based on an systems for space applications pounds (204 kilograms). Two electrical output of 1.3 kWe from began in the mid-1950s under technologies were selected for the heat input of three 2.4-kilowatt the SNAP program conducted development, testing, and evaluation. (7.2 kilowatts total) MHW heat by AEC. For example, under the e Sundstrand Corporation sources.12 SNAP-1 program, an electrical heat developed an organic Rankine cycle, source was used with a mercury- referred to as the Kilowatt Isotope e BIPS concept utilized two based Rankine power conversion Power System (KIPS), while the MHW heat sources to heat a unit to produce 470 We from the Garrett Corporation developed helium-xenon working gas which, in 13-pound (6-kilogram) system.8 a closed Brayton cycle, referred turn, drove a mini-Brayton rotating Development of mercury-based to as the Brayton Isotope Power unit—the rotating unit included a Rankine power conversion systems System (BIPS). Both contractors turbine, alternator, and compressor. continued under the SNAP-2 incorporated the MHW heat source Similar to the KIPS, the alternator and SNAP-8 reactor programs, into their design; however, testing mounted directly to the turbine in which the power conversion was performed using electric heaters. shaft was used to generate electrical system was coupled with a metal- power. After the working gas exited hydride nuclear reactor as the heat e KIPS concept consisted of the turbine, it passed through a source to produce 3 and 30 kWe, three MHW heat source assemblies recuperator where a portion of the respectively.9 In this configuration, that would heat and boil an organic remaining heat in the working gas material and corrosion problems working fluid, thereby generating was used to preheat the gas that was associated with the use of the liquid a vapor that drove a turbine. An entering the heat source assembly.

45 Advanced Isotope Power Systems Expanding RPS Boundaries

Strategic Defense Initiative e gas was then passed through a compressor and routed back to In March 1983, the Reagan Administration proposed the SDI and directed the the system to complete the cycle. Secretary of Defense to engage in space-based nuclear deterrence. The SDI was Based on system studies, an overall conceived to intercept and destroy strategic ballistic system efficiency of approximately missiles and was to be implemented by a new SDI 27 percent was anticipated from organization. The mission of the new defense agency a 460-pound (208-kilogram) unit was to research sophisticated surveillance, sensing, based on an electrical output of orbital transfer vehicles (to move satellites between 1.3 kWe from the heat input of two orbits), and intercept systems and weapons platforms 2.4 kilowatts (4.8 kilowatts total) with electrical power requirements ranging from MHW heat sources.13 hundreds of kilowattsFigure to 5.—Mini-BRUhundreds of megawatts. Recuperator. Although both contractors successfully developed a flight system conceptual design and a prototypic ground demonstration unit for testing, the Sundstrand organic Rankine system (KIPS) was selected for further development.Figure 7.—Solar Dynamic Ground Test Demonstration When the program was discontinued in September (SD1980 GTD) View from Solar Simulator Window. due to the absence of a near-term mission, the Sundstrand system had operated for over 11,000The systemhours at a was designed to produce 36 kWe at the alterna- full output designtor power with of 1.3a kWeturbine inlet temperature of 1034 K, compressor and an overall inletsystem efficiencytemperature of of 338 K, and maximum pressure of 18.5 percent.10 560 kPa. The 32000 rpm turboalternator was a scaled version The DIPS Programof BRU and Mini-BRU, designed for helium-xenon working

Against the backdropfluid of(MW the 1983 40). The system included a 94% effective recu- Strategic Defenseperator Initiative and(SDI), a separate n-heptane gas cooler. Final designs a DIPS technologywere demonstration completed by Allied Signal (formerly AiResearch), but program was initiated in 1987 as a joint DOE/DoDno effort Brayton to develop hardware was fabricated. Mass estimates were a power system104 for the kg Boost for the turboalternator, 162 kg for the recuperator, and Surveillance and Tracking System. e tracking system85 kg was for conceived the gas cooler. While the SSF SD system was never completed, NASA Brayton Isotope Power System workhorse loop. (Image: NASA) Figure 6.—Brayton Isotope Power System (BIPS) was able to demonstrate the technology via the Solar Dynamic Workhorse Loop. Ground Test Demonstration (SD GTD). The SD GTD Project 46 (1994 to 1998) assembled a 2-kWe end-to-end SD power The Mini-BRU system was designed for a turbine inlet system in a NASA Lewis thermal-vacuum facility with solar temperature of 1144 K, compressor inlet temperature of simulation,11 as shown in figure 7. The system utilized the 300 K, and maximum pressure of 738 kPa. The higher Mini-BRU components and added an Air Force gas cooler pressure allowed a smaller rotating assembly and higher shaft coupled to a pumped n-heptane radiator. The concentrator and speed (52000 rpm). The mass of the Mini-BRU and Mini- receiver were scaled versions of the SSF designs, and the BRU Recuperator were 17 and 59 kg, respectively. The Mini- receiver included integral LiF-CaF2 thermal energy storage for BRU components formed the basis of the Department of continuous sun-eclipse power generation via the Brayton. The Energy’s (DOE) 1.3-kWe Brayton Isotope Power System GTD Project compiled over 800 hr of operation and 372 (BIPS) utilizing the Modular Isotope Heat Source. A Work- simulated orbit cycles during 33 separate tests.12 horse Loop test, as shown in figure 6, was conducted that 9 A flight version of the system was developed for the Joint included a 1000 hr endurance test. U.S./Russian SD Flight Demonstration on Mir. However, the planned Shuttle delivery mission was redirected for Mir II.B. Solar Dynamic Brayton logistical resupply and the system was never flown. However, the flight development Brayton assembly from the Mir system In the mid-1980’s space Brayton technology was revived was installed in the GTD test system and operated successfully. for NASA’s Space Station Freedom (SSF) Project (1986 to 1991). A 25-kWe Solar Dynamic (SD) Power Module was II.C. Jupiter Icy Moons Orbiter planned as part of a hybrid Photovoltaic/Solar Dynamic power architecture.10 The SSF SD Brayton system included a faceted In the early 2000’s NASA began the Nuclear Systems Ini- mirror concentrator and solar heat receiver with integral tiative which led to the Prometheus Program and the Jupiter thermal energy storage that eliminated the need for recharge- Icy Moons Orbiter (JIMO) mission. Prior to JIMO, Brayton able batteries for orbital eclipse power. conversion had been considered for a number of reactor-based

NASA/TM—2007-214976 3 Atomic Power in Space II Chapter 3

to provide early detection of enemy unit to demonstrate reliability of How a Dynamic Isotope ballistic-missile launches during the seven to 10 years, and resolution of Power System Works first few minutes following launch, a all significant technological issues. A DIPS consists of a radioisotope time referred to as the boost phase.11 e planned DIPS was also to be heat source, a power conversion configured to use the latest heat system consisting of turbine e Boost Surveillance and Tracking source technology—the GPHS.14 For alternator compressor on a System was expected to require an comparison, a 6-kWe DIPS would be common shaft, a heat exchanger electrical load of 6 kW, a seven-year equivalent to 20 GPHS-RTGs. and gas cooler, a heat rejection lifetime, and 98 percent reliability. system, and associated piping, Following an evaluation of three Under contract to DOE, valves, and control systems. During candidate space nuclear power Rocketdyne (then a division of system operation, a working uid system technologies (thermionic, Rockwell International) was (e.g., xenon-helium gas mixture for reactor, and DIPS), DIPS was selected to lead the effort to a Brayton cycle and organic liquid selected for further development. design, build, and test a prototypic for a Rankine cycle) exits the heat e overall objectives of the power system through the point source assembly at a very high DIPS technology demonstration of flight readiness. Following an temperature and is piped to the program included fabrication and evaluation of Brayton and Rankine power conversion system. As the demonstration of a dynamic power technologies, the closed Brayton hot working uid ows through system that could be scalable over a cycle power system previously the turbine, the turbine-alternator- range of one to 10 kWe, life testing under development by the Garrett compressor shaft rotates, resulting of an electrically heated qualification Corporation was recommended to in the generation of electricity by the alternator. After passing through the turbine, the working uid is Compressor Alternator Turbine routed through a heat exchanger and cooler, after which it is pumped Gas flow back to the heat source via the compressor. Excess heat from the gas cooler is transferred to a heat Isotope Gas Heat rejection system via a cooling loop. cooler Gas Source The electricity generated by a flow Gas flow DIPS is then “conditioned” for use in powering on-board electrical equipment or an ion Gas flow propulsion system. Regenerator

Graphic depicting a generic DIPS. (Image adapted from “The Dynamic Isotope Power System: Technology Status and Demonstration Program,” Gary L. Bennett and James J. Lombardo, 1988)

47 Advanced Isotope Power Systems Expanding RPS Boundaries

DOE for development. Following DOE concurrence, a Rockwell While development of DIPS and other potential SEI Garrett team designed a modular closed Brayton system with an power technology concepts began to grow, they soon anticipated power output of 2.5 came face-to-face with the reality that SEI lacked kWe and an operating life of over 10 years.11 Although ground Congressional support and funding, largely due to its testing of the Brayton system never materialized (the program immense 20- to 30-year, $500-billion price tag. developing the Boost Surveillance and Tracking System decided against use of DIPS in favor of a power system concepts were the lofty human-exploration goals non-isotope technology),15 the DIPS developed to meet new missions of SEI were soon abandoned and technology was soon connected to and power levels for the exploration the DIPS Demonstration Program other space applications. of space. For example, the closed was brought to a close.19 Brayton cycle system was identified In 1989, President George H. for possible use in planetary surface Stirling Radioisotope W. Bush announced his Space applications requiring 0.2 to 20 Generators Exploration Initiative (SEI). e SEI kWe.17 For a mission conceived to had ambitious goals of returning establish a manned outpost on the By the mid-1990s, the need for humans to the moon within a moon, a 2.5-We Brayton design was an advanced radioisotope power decade and sending a manned crew compared to other dynamic power system was becoming increasingly to Mars by 2019. Within a year system technologies, both nuclear 18 important to DOE and NASA. of the announcement, DOE and and non-nuclear. In both concepts, e importance lay in the fact NASA had penned a memorandum planners assumed the use of a fueled that the agencies had been faced of understanding that established GPHS module, the mainstay of DOE with a limited inventory of a general framework by which the heat sources since its development plutonium-238 for over a decade two agencies would cooperate on for the Galileo and Ulysses missions. following the shutdown of the matters concerning information K-Reactor at SRS in 1988.c Soon, exchange and research and While development of DIPS 16 interest in another dynamic power development activities under SEI. and other potential SEI power conversion system based on Stirling technology concepts began to technology began to gain ground With the new NASA-DOE grow, they soon came face-to-face within DOE and NASA. at agreement, potential applications with the reality that SEI lacked interest had been fostered by years for a Brayton system soon shifted Congressional support and funding, of Stirling technology research, from the SDI effort to space-based largely due to its immense 20- to including experience gained during exploration. Under the DIPS 30-year, $500-billion price tag. In development of the SP-100 space Demonstration Program, dynamic the absence of the needed support, reactor power system. c. See Chapter 10, Infrastructure Inroads. e K-Reactor had provided for production of plutonium-238 for over 30 years. With its shut down, DOE lost its sole production capability for the heat source isotope.

48 Atomic Power in Space II Chapter 3

e Stirling Technology Company, With the favorable results of the Stirling Engine Origins later named Infinia, was based in 55-We TDC assessment, DOE soon Invention of the Stirling engine Kennewick, Washington, and had turned its efforts to development is generally attributed to Robert been working under contract to of a Stirling radioisotope generator Stirling, a Scottish minister who DOE to develop a 55-We Stirling (SRG). Following development invented the rst practical closed- engine called the technology of conceptual designs during cycle air engine in 1816. Initially demonstration convertor (TDC).d a contract downselect phase, developed as a competitor for the Following initial development development of an SRG formally steam engine in the 1800s, kinematic and fabrication of multiple commenced in May 2002, when Stirling systems developed in the demonstration engines, the TDC DOE selected Lockheed-Martin to early- to mid-1900s were used in was subjected to an extensive three- serve as system integrator under portable and marine generators month evaluation that included a new project to develop an SRG and in various automotive and testing for dynamic launch load capable of producing 110 We. locomotive applications. In 1974, capabilities, characterization Lockheed-Martin was responsible William Beale invented the free- of electromagnetic fields, and for the overall design, integration, piston Stirling engine, which found performance tests that measured and qualification of the planned subsequent application in RPS parameters such as power Stirling power system, eventually concepts developed by DOE and output, system efficiency, and dubbed the SRG-110. e SRG- NASA beginning in the mid-1990s. temperature. e purpose of 110 concept included use of the the DOE-sponsored evaluation, 55-We TDC under development by which began in late 1999, was to Infinia, which was responsible for assess the technology readiness convertor development, including of the Stirling convertor relative design, fabrication, and testing. materials studies, alternator testing, to viability for a mission with a Technical expertise and support for and structural-dynamics testing, December 2004 launch date and its development of the Stirling power was performed at the GRC.22 Such readiness for flight development. system were provided by GRC. With testing provided opportunities To support the evaluation, DOE a contract and project team in place, to address technical issues and tapped into the space nuclear plans were laid to bring a flight- refine the convertor design and power system expertise of NASA, qualified Stirling RPS to fruition.21 supported overall integration with Lockheed-Martin, Orbital Sciences the Lockheed SRG design.23 Corporation, and others. At the As design of the Stirling generator conclusion of the three-month progressed, fabrication and testing By the end of 2005, Lockheed had evaluation, the 55-We TDC won of TDCs continued in an effort designed a Stirling power system that the support of the evaluation team to address manufacturability, could operate in the vacuum of deep as well as technology decision- performance, life, and reliability space and on the surface of Mars. makers within NASA and DOE, criteria. Much of the testing to e SRG-110 design consisted of a and follow-on development soon support technology development, beryllium housing that contained commenced.20 including convertor performance two free-piston Stirling engines tests, thermal vacuum tests, d. e NASA Stirling technology community uses the term “convertor” rather than “converter” when referring to Stirling power conversion. at convention is reflected throughout this document as appropriate.

49 Advanced Isotope Power Systems Expanding RPS Boundaries

(i.e., convertors), two GPHS While the specific power of a convertor-specific power of modules, thermal insulation, and approximately 3.5 We/kg for the greater than 90 We/kilogram. various support components. An SRG-110 was consistent with the In the context of a Stirling electronic controller and other objective to use plutonium-238 more generator system, the specific miscellaneous components were efficiently, the 5.4-We/kilogram power was projected by GRC to mounted on the outside of the specific power of the GPHS-RTG be approximately 8 We/kilogram, housing, as were several fins that suggested there might also be room more than double that provided by served to reject residual heat for improvement to the specific the TDC-based SRG-110 system that wasn’t converted to useable power. To continue the advancement and much better than the GPHS- electricity. Each closed cycle free- of RPSs, NASA and GRC issued a RTG. An early ASC test model had piston Stirling engine would convert the heat from the GPHS module into reciprocating motion, which was To continue the advancement of RPSs, NASA and subsequently converted to useable electricity through use of a linear GRC had issued a research announcement in alternator. Each TDC was designed to produce approximately 60 watts of 2002, the focus of which was radioisotope power (alternating current) electrical power, conversion technology. which was converted into a direct current power level of approximately 55 watts. e SRG-110 design, using the Infinia convertors, resulted research announcement in 2002, also successfully passed vibration in a system specific power of the focus of which was radioisotope testing without power degradation approximately 3.5 We/kilogram and power conversion technology. One or convertor failure.27 a system-efficiency of approximately of the technologies subsequently 23 percent.21, 24 selected for a three-year In light of such potential, NASA development and demonstration requested that DOE complete With the SRG design in place, project included a free-piston fabrication (already in progress) and fabrication of an engineering unit Stirling engine concept developed by testing of the SRG-110 engineering generator, a complete system Sunpower, Incorporated.26 unit, utilizing early generation prototype built to test the ability ASCs in place of TDCs to better to meet flight requirements, Under a 2003 NASA contract, understand the potential of the new was nearing completion in 2005. a Sunpower-led team pursued technology. Completed in 2008, Design, fabrication, and testing of development of the advanced the effort was originally planned to a qualification unit had also begun Stirling convertor (ASC). Over the be the end of the project; however, and was scheduled to be complete course of three years, Sunpower renewed interest in Stirling systems by the end of 2006. However, cost developed a convertor design with combined with favorable generator overruns and the lack of a specific an estimated electrical power test results led to a new flight mission resulted in a decision to output of 80 We (alternating development effort called the cancel further development of the current), a conversion efficiency Advanced Stirling Radioisotope SRG-110 system in 2006.25 of greater than 30 percent, and Generator (ASRG) project.24, 25

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Under the new ASRG project, the development and testing of environments (e.g., the Jovian Lockheed-Martin continued the ASC included development system) but also originating from to serve as system integrator, of heater heads fabricated of the plutonium oxide fuel. ese under contract to DOE, and held Inconel 718 and MarM-247, two and other key technical questions the responsibility for design, superalloys selected for operation had to be resolved as they arose fabrication, and testing of the at temperatures of 650 degrees to demonstrate the feasibility, ASRG. Sunpower was responsible Celsius (°C) and 850°C, respectively. longevity, and reliability of the for design, fabrication, and Although operating at the higher conversion system for space use.28 testing of the Stirling convertor. temperature would offer improved GRC provided technical support conversion efficiency, testing Lockheed-Martin, developers of the and testing capabilities for the revealed ongoing materials issues Stirling generator system, faced a Sunpower convertors, just as they at the higher temperature. For similar set of questions and challenges had for the Infinia technology instance, convertor designers in their effort to develop, qualify, and demonstration convertors. had to revisit the possibility of integrate the yet-to-be-demonstrated the permeation of helium, the convertor into the new ASRG. Although initial development convertor working fluid, through Between 2008 and 2010, Sunpower and testing of the Sunpower ASC this new heater-head material fabricated numerous convertors of was encouraging, its readiness operating at a higher temperature. varying materials for a series of long- for flight use remained a distant If helium losses due to permeation life reliability tests performed at the target as technical questions and were too high, operational GRC Stirling Research Laboratory. challenges remained to be resolved. performance of the Stirling System-level testing of the ASRG For example, designers needed convertor would be reduced, engineering unit, including vibration, to demonstrate a 17-year life for thereby lowering the power output shock, and thermal vacuum tests the convertor heater head, the of the system. For this reason, that simulated launch and space portion of the Stirling convertor a special permeability testing environments, was completed by that interfaced directly with the apparatus had to be designed Lockheed in 2008. e engineering GPHS module and had to be able and fabricated to address the unit was subsequently transferred to withstand prolonged exposure permeability question. Another to GRC and placed under long-term to high operating temperatures. area that had to be revisited was operation.29 By 2011, the ASRG had To increase the temperature organic materials, which were projected performance capabilities of ratio of the system and its overall present in the convertor for uses approximately 130 We using a little conversion efficiency, developers such as electrical insulation and more than 2.2 pounds (one kilogram) of Stirling convertors sought structural bonding in the linear of plutonium oxide fuel. e resulting to maximize the temperature alternator. e materials selected system power conversion efficiency difference between the hot and cold for use in the ASC were different of approximately 27 percent would ends of the convertor. than those used in the TDC, and be achieved from a unit expected to it was necessary to understand weigh no more than 70 pounds Technical issues that had been how they would perform under the (32 kilograms).30 under investigation and/or closed planned operating temperatures as for the Infinia design had to be well as in the presence of radiation, revisited for the ASC. In addition, primarily from possible space

51 Advanced Isotope Power Systems Expanding RPS Boundaries

How an ASRG Works As development of the ASRG progressed, NASA decided in An ASRG produces electricity by converting heat to motion inside the 2011 that the ASRG development engine, and then converting the motion into electricity that is useable by schedule should be consistent with the spacecraft. Inside the ASRG is a device known as the Advanced Stirling supporting a future mission to be Convertor (ASC), which contains an oscillating piston and a companion launched as early as January 2016; displacer sealed inside a closed cylinder and suspended in helium gas. The the decision added substantial displacer and piston move back and forth in response to pressure changes schedule risk to the project.31 Due between the hot side, heated by the plutonium fuel, and a passive cooler at to the cost limits associated with the other end. The steady alternating expansion and contraction of gas within Discovery missions, NASA also this Stirling heat cycle drives the magnetized piston back and forth through intended to provide the ASRG a coil of wire, with the magnet and coil forming a device known as a linear to the mission as government- alternator (also inside the ASC), as movement occurs approximately 100 times furnished equipment. per second to generate an alternating current of electricity in accordance with Faraday’s Law (a property of physics). In 2011, the ASRG design was Each ASRG contains two ASCs. The ASCs are aligned end-to-end in the subjected to a final design review middle of the generator, which serves to cancel out their vibration when their that served to confirm system motion is synchronized. The helium gas inside each convertor functions as a adequacy relative to specified hydrostatic bearing, which prevents the displacer and piston from rubbing performance and operational 32 against the walls of the cylinder to minimize the potential for physical wear. requirements. e review led to

The ASRG also includes a End dome (2x) controller, connected to the ASRG housing by electrical cables, Shunt dissipator unit that is designed to synchronize the two pistons, provide ASRG- Heat source pre- General purpose load assembly (2x) related data to the spacecraft, heat source (2x) and transform the alternating- Pressure relief current power produced by the Pressure relief device device generator into approximately 130 Gas watts of direct-current power at a management voltage useable by the spacecraft valve Interconnect tube (Adapted from NASA Fact Sheet - Advanced Stirling Radioisotope Advanced Stirling Cold side convertor (2x) Generator, 2013). adapter ange (2x) Radiator n (12x)

Mounting interface

52 Atomic Power in Space II Chapter 3

technical questions that required continued to impact the project In November 2013, any glimmer of additional investigation and reviews cost and schedule. While ASRG hope for a near-term ASRG flight that continued into 2012. Although supporters remained hopeful was lost when NASA announced that many of the technical questions that a near-term mission was still it had directed DOE to discontinue were addressed during this period, viable, those hopes began to fade in further work on ASRG flight units— the time needed for their resolution August 2012 when NASA selected citing budgetary constraints and a raised concerns relative to the a solar-powered Mars lander over favorable plutonium-238 inventory ability to provide a flight-qualified two ASRG-powered missions for outlook resulting from a new project unit in the 2016 timeframe.33 At a 2017 Discovery-class planetary approved to restart production of the the same time, the remaining science mission. ASRG developers heat source isotope.35 After nearly unresolved technical challenges, immediately began looking to the 14 years of development, use of an such as material-properties issues next Discovery-class planetary SRG in space would have to wait. with critical components and mission as an opportunity to nuclear launch safety concerns demonstrate the new RPS in a space Looking to the Future related to the housing design, application.34 Over the years, DOE and NASA have invested substantial time and money to advance RPS technology, particularly in the area of dynamic power systems. Although the focus was largely on Brayton, Rankine, and Stirling dynamic systems, other efforts have been undertaken to develop technologies, such as the Alkali Metal ermal-to Electric-Converter.36 More recently, research into new thermoelectric materials (i.e., skutterudites) is showing promise for application in power conversion technology. All of these efforts have contributed to the space nuclear power system body of knowledge, providing an ever-larger base from which future development efforts can build.

Testing of an unfueled ASRG at NASA’s GRC. (Photo: NASA)

53 Until the day when new missions rekindled interest in space nuclear reactor technology, much of the 1970s was devoted to simply keeping the technology alive.

54 Reactors Redux 4 Space Nuclear Reactor Interlude

ollowing termination of the NERVA nuclear rocket program and other space reactor research in 1973, the remainder of the decade was a dry time for space nuclear reactor technology development. FChanging national priorities and reduced Federal budgets hampered further research through much of the decade. At the same time, there were still strong incentives for use of space nuclear reactors. Apollo-era projects provided a solid foundation to build upon, and reactors had capabilities unmatched by competing technologies like solar power. e greater power, compactness, and robustness of the technology could help the United States keep tabs on potential enemies, enable more civilian uses of satellites, and dramatically accelerate exploration of the outer solar system. Until the day when new missions rekindled interest in space nuclear reactor technology, much of the 1970s was devoted to simply keeping the technology alive.

Space Reactor Revival

Although space reactor research in the United States was defunded in 1973, a smaller space nuclear power program continued to operate with the majority of funding directed toward RPS development. During a hearing before the Joint Committee on Atomic Energy in March 1973, David Gabriel, Director of the Space Nuclear Systems Division at AEC, noted that space reactor research was terminated because of budget priorities and the lack of near term NASA missions requiring the power levels afforded by space nuclear reactors: “ ese projected [mission] delays, along with the budget priorities, led to the decision that the distant payoffs did not warrant continued funding of high-powered nuclear propulsion or reactor power systems.”1

Despite the end of large-scale space reactor development, the years that followed saw a small but ongoing effort to maintain the viability of space reactor technology.2 In addition, some space power energy conversion technologies found new life in ground-based power and transportation programs only to be resurrected years later in new space reactor programs. One example was the ermionic Energy Conversion for Applied Research and Technology program, which researched the use of thermionic conversion to produce electricity using heat recovered

Apollo astronaut on the moon. Apollo-era projects provided a solid foundation to build upon, and reactors had capabilities unmatched by competing technologies like solar power. (Photo: NASA)

Photo Credit 55 Reactors Redux Space Nuclear Reactor Interlude

from coal-fired central early 1976. e working Force, the case for a renewed space power stations. In 1975, group was tasked to reactor development program NASA broadened the study future DoD space continued to gain traction, as program to include high power requirements noted by the Steering Group in temperature out-of- to determine which January 1977: core nuclear thermionic applications would best power systems for future be served by nuclear “Although the Steering Group space applications.3 power systems and to has been unable to identify any recommend a space power approved and budgeted DoD In 1973, AEC, DoD, and NASA technology development program.5 missions (requiring greater than formed an ad hoc group “…to 3 kWe)… a reactor power supply evaluate the future DoD needs In August 1976, DoD Steering is presently the only candidate in space power and to indicate Group Chairman A.E. Vossberg spacecraft power option for future the possibility of meeting those sent a letter to Richard W. Roberts, high power applications. is fact, needs with space [nuclear] power ERDA’s Assistant Administrator combined with data on space systems.”2 e group’s final for Nuclear Energy, stating, “In our reactor power capabilities outside report, issued in March 1974, continuing effort to ensure that the U.S., the enhanced military recommended preserving the future space power requirements capability provided by having reactor technology developed under of the DoD can be met on a timely sufficient power to operate on-orbit the SNAP program and stimulating basis, I wish to call your attention equipment such as radar, and “a focused space power program for to the growing likelihood of need future threats to our space defense earlier payoffs on DoD missions.”2 for space nuclear reactor systems in posture afforded by similar high the 10 to 100 kW electric range in power capabilities in the hands of e focused space power program the late 1980s and beyond.”2 adversaries, has led the Steering began to take shape in 1975 Group to recommend that a reactor when DoD and the newly-formed By 1977, the Steering Group had power development program be ERDA, the successor agency to identified several DoD missions initiated by the U.S. following AEC, established a Space Nuclear with power requirements up to intensive preparatory studies to Applications Steering Group. 100 kWe. Comparing reactors define the reactor power system and Chaired by George P. Dix, former to their main space competition, its requirements.”2 head of the AEC space nuclear solar-battery power, the group safety program,4 the group was found that for military missions, Additional support for a renewed tasked to establish effective solar panels coupled with batteries space reactor development management and communication were competitive with nuclear program was provided by the channels between the agencies in the range of 25 to 50 kWe; space power system working group in order “to encourage a proper nuclear power was judged to be when it recommended a “modest development program for space superior above power levels of technology and experimental nuclear energy systems.”2 In concert 50 kWe. With several potential program to provide a solid basis with the steering group, DoD and DoD missions needing 25 kWe or from which to develop space ERDA also established a space more, particularly a space-based reactors.”5 e recommendations nuclear power working group in radar system planned by the Air soon bore fruit.

56 Atomic Power in Space II Chapter 4

Space Electric Power Dix became the Director of Safety delivered over the mission lifetime. Supply Program and Environmental Operations e reactor was also expected to within the new Federal agency4 and meet all regulations of NASA, In 1977, DoD and ERDA initiated a Bernard Rock became the Director DoD, DOE, and the National Range joint technology-screening study to of the Office of Space Nuclear Commanders in charge of the evaluate existing space reactor power Projects. e Space Electric Power sites where the system would be system technologies and develop a Supply program continued under launched.5 space reactor power system concept the Nuclear Energy Programs group for further development. e study within the Assistant Secretary of In addition to the potential DoD was performed by LASL under a Energy Technology organization in missions, studies by Grumman new Space Electric Power Supply the new DOE.7 and McDonnell Douglas identified program.6,7 With the advent of the commercial industrial-scale low- planned Space Transportation e screening study included earth-orbit missions that were likely System, or space shuttle (under the identification of several DoD to require a space nuclear power development since 1972), a new era missions that could require power system. One mission envisioned of space use and exploration was levels up to 100 kWe, such as a construction site in space to expected to open up and, along with satellites and space-based radar, build solar- or nuclear-powered it, larger space-based systems that many of which were expected to satellites that would send energy to would require higher power, thereby be needed by the early 1990s. DoD Earth. Another was a low-gravity giving impetus to the new space required a seven-year lifetime and manufacturing facility. e studies reactor efforts.8 95 percent reliability, preferring also proposed a civilian version of the military’s new GPS and scientific missions focused toward With the advent of the planned Space Transportation the stars and planets.5 For NASA, potential applications included System, or space shuttle (under development since communication and surveillance 1972), a new era of space use and exploration was systems, electronic mail, and advanced television antenna systems expected to open up. for which five to 220 kWe was expected, and planetary exploration missions requiring even higher Although the technology screening designs that would degrade only power levels. study was initiated under ERDA, gradually and avoid the potential it would be completed under for a single fault to cause the whole Based on a target power level of 10 a new Federal agency. Only 20 system to fail (such a failure is to 100 kWe, LASL developed 135 months after the formation of often referred to as a single-point reactor power plant combinations ERDA, its almost 9,000 employees failure). e reactor had to be that reflected a suite of reactor were consolidated into the newly able to operate in Earth’s natural designs, electric-power-conversion created DOE, which combined radiation fields, and radiation technologies, and heat rejection ERDA with other energy-related created by the system had to be systems. e reactor designs Federal organizations. George limited both in rate and amount included heat pipe, gas-cooled, and

57 Reactors Redux Space Nuclear Reactor Interlude

liquid-metal concepts, while fuel included weight, size, reliability, importance of incorporating safety types included uranium carbide, safety, and development cost into all aspects of space nuclear uranium oxide, and uranium and time, LASL recommended a power system design. In January nitride. Several power conversion technology development program 1978, a malfunction aboard a technologies were evaluated, based on a concept consisting of a Russian satellite (Cosmos 954), including static (thermoelectric and heat pipe solid-core reactor with which was powered by a nuclear thermionic) and dynamic (Brayton, thermoelectric power conversion, reactor, resulted in its failure to potassium Rankine, and Stirling) and a heat pipe radiator.5 boost into a higher orbit. Upon systems. Heat rejection options re-entry, the reactor disintegrated included heat pipes, pumped fluid As the LASL technology study in the upper atmosphere (per its with fin radiators, and pumped progressed, an accident involving a design) resulting in radioactive fluid with heat pipe radiators. After Russian space reactor power system debris being scattered over a consideration against criteria that provided a somber reminder of the 48,000-square mile (124,000-square kilometers) area of northern Canada. A joint response by Canada and the United States managed to find approximately 0.1 percent of the reactor core.9, 10 e event raised international policy questions regarding the use of nuclear reactors in space and led to the creation of a United Nations working group to address the topic. It also motivated President Jimmy Carter to propose a joint U.S.-Soviet ban on nuclear reactors in Earth’s orbit if a fail-safe mechanism to prevent radioactive material from entering the atmosphere could not be implemented; however, the ban was not accepted by the Soviet Union.11

While the Cosmos 954 accident had broad visibility, it also provided a context for discussion of safety as it pertained to the LASL space A joint response team from the United States and Canada, dressed in specially designed arctic clothing, search for Cosmos 954 radioactive debris with hand-held radiation detectors. (Photo: DOE/NV1198)

58 Atomic Power in Space II Chapter 4

reactor technology assessment Heat Pipe Technology effort then underway. David Buden, A heat pipe is a highly ecient way of transferring heat a key member of the LASL space from one location to another. It is a sealed tube containing reactor technology assessment a low-pressure working uid (e.g., sodium or lithium) team, noted that “safety has been matched to the preferred system-operating temperature. and continues to be a major concern The uid evaporates at the heated end of the tube and of U.S. scientists involved in using condenses at the cooler end, releasing its heat and wicking reactors in space.”12 Safety was built back toward the hot end of the tube by capillary action. into space reactor designs through a For the SPAR reactor concept, the heat pipes, integral to the combination of engineering design, reactor core, would extend beyond the core and traverse analysis, and testing. For example, the reactor shielding, where they would then connect with the use of reactor safety features the thermoelectric power conversion system. Because such as backup control rods, there are no moving parts (only the working uid moves), where only one would be unlocked heat pipes are highly reliable. The heat pipe was developed at a time, served to prevent in 1963 by LANL physicist George Grover, and it was rst inadvertent operation. Reactor implemented in the NERVA program. NASA continued design also included measures to develop heat pipe technology through the 1960s. to prevent inadvertent criticality Today heat pipes are routinely used to cool electronics on when immersed in water. e geostationary communication satellites.13 long-standing emphasis on safety (Image adapted from “Space Nuclear Power,” Joseph A. was, and would continue to be, a Angelo, Jr. and David Buden, Orbit Book Company, 1985)

Heat input Heat output Wall Wick

Heat flow Vapor flow Liquid flow

Liquid Vapor

Evaporator Adiabatic section Condenser

59 Reactors Redux Space Nuclear Reactor Interlude

fundamental aspect of U.S. space to 100 kWe. e LASL heat pipe By 1981, an initial design for reactor power system development, reactor power system concept was SPAR had been developed. e including the space reactor work subsequently named SPAR.14,15 reactor was being designed to being performed at LASL. Concurrent with the DOE produce a nominal 1,200 kW of activities, NASA also began thermal power while operating at Space Power Advanced funding work on heat pipe and 1,500 Kelvin. e reactor design Reactor power conversion development, incorporated a core of both at LASL and their own 90 uranium oxide sodium-filled In late 1979, DOE initiated a facilities. In early 1980, DOE and heat pipe fuel element modules. five-year program to develop NASA joined with DoD to create e heat pipes would remove the technology base of the heat- a steering committee and space thermal energy from the reactor pipe reactor power system reactor working group to bring core and transfer it to the recommended by LASL. With some unity to the DOE-funded thermoelectric power conversion funding of $2 million per year, the SPAR effort and NASA’s own space system. For compatibility with reactor work (the groups worked the space shuttle power system, goal was to develop a space reactor 2 system capable of producing 10 together until 1981). the mass would be less than 4,210 pounds (1,910 kilograms).16

e core was to be surrounded by a neutron reflector of beryllium or beryllium oxide, which would control reactor operation. As with the NERVA program, rotating drums were to be used for power control; each drum would contain a boron carbide sector that could be rotated in and out of the reactor to control reactivity. Redundant instrumentation and electronics would increase reliability, which was considered as important as safety, and the reactor had to keep operating even if some components failed.

For the thermoelectric power conversion system, LASL planned to use an improved version of the Concept of a heat pipe nuclear reactor coupled directly to thermoelectric converters. The heat pipes extend from the reactor core (bottom left of image) and carry heat to the thermoelectric converter. Excess heat not converted to electricity is transferred to the radiator via a second set of heat pipes. (Image: LANL)

60 Atomic Power in Space II Chapter 4

silicon-germanium thermoelectric also performed to demonstrate the Reagan National materials used in the MHW-RTGs reactor would remain safely sub- Space Policy that powered the Voyagers 1 and 2 critical in the event of immersion In July 1982, President Ronald spacecraft; the improved silicon- in water. Fuel development focused Reagan announced his National germanium material, then under on production processes for the Space Policy, which was intended development by DOE, contained uranium oxide fuel and in-reactor to strengthen U.S. security, expand gallium phosphide and offered the testing to verify fuel performance private-sector investment, and potential for higher conversion and heat transfer characteristics. increase exploitation of resources efficiency. Excess heat would be Development of the molybdenum and international cooperation. radiated to space through the use heat pipes included materials The 1982 policy established the of a heat pipe radiator system. A testing, wick design development, space shuttle as a major factor shadow radiation shield design was development of processes to bend in the U.S. program and called also drawn from the earlier SNAP the heat pipes, and performance on NASA to continue exploring and Rover reactors. Because the testing for compatibility with the “requirements, operational reactor was to be used in space working fluids. For the power concepts, and technology” needed where there is no air to deflect conversion system, activities to support permanent space neutrons and gamma rays around focused on development of facilities – a space station. 20 the shield, weight could be reduced silicon-germanium thermoelectric by placing the reactor and payload modules (i.e., panels) that would at opposite ends of the spacecraft interface with the heat rejection with shielding in between, instead system heat pipes.17,18 configuration, the available room in of shielding the whole reactor. the shuttle could be reduced to 42 feet (12.8 meters) long and 14 feet (4.3 meters) in diameter.19 Excess heat would be radiated to space through the use of a heat pipe radiator system. Defining Roles and Goals: Establishing Cooperation

As the technology effort progressed, the future of its funding By 1982, the SPAR technology Because the space shuttle was to be soon came into question. During development program had the primary method of launching the formulation of its fiscal year evolved into a broad testing, systems into space, reactor power 1982 budget, DOE was directed experimental, and analytical system designers also had to ensure by the Office of Management and program centered on the reactor, that the spacecraft and its reactor Budget to reduce its funding for heat pipes, thermoelectric power system would fit in the space reactor development to materials, and shielding. For shuttle cargo bay, a cylinder 60 $1 million, thereby putting DoD example, predictions of neutron feet (18.3 meters) long and 15 feet and NASA on the hook to fund behavior in the reactor core were (4.6 meters) in diameter. When the shortfall. Although DoD experimentally checked using a an upper stage launch vehicle opted out of funding, NASA critical assembly. Analyses were was factored into the spacecraft

61 Reactors Redux Space Nuclear Reactor Interlude

agreed to support the project and work with DOE toward a Although the technology development program joint technology verification phase; NASA mission models had shifted to support NASA, groups within DoD had indicated that 100 kWe was continued to maintain an interest in a space suitable for both outer-planetary and earth-orbital missions. Because reactor power system. of the budgetary constraints at DOE, NASA also assumed responsibility for development of Although the technology With the continued interest the power conversion subsystem development program had in space power reactors, DOE while DOE retained responsibility shifted to support NASA, separated its Office of Space for development of the reactor groups within DoD continued Nuclear Projects into an Office of subsystem, with funding support to maintain an interest in a Special Applications focused on from NASA.15 e arrangement space reactor power system. In RPS technology and an Office of marked a change from previous 22 addition to its attractiveness for Space Reactor Projects. joint NASA-DOE approaches space-based radar, surveillance, under which DOE was solely communications, electric The National Research responsible for funding reactor 3 propulsion, and jammers, such Council Lends a Hand technology development. systems offered other benefits, as noted by Gordon L. Chipman, Shortly after NASA became a co- In the months that followed the DOE Deputy Assistant Secretary sponsor of the SPAR technology conception of the DOE NASA for Breeder Reactor Programs (and development program, it was SP-100 reactor, the agencies oversaw its Office of Space Reactor named the Space Nuclear Reactor began working with the Defense Projects): Power Systems Technology Advanced Research Projects Program and the SPAR reactor was Agency (DARPA) to establish a “[N]uclear power enhances renamed SP-100 (for Space Power joint program for development of survivability against nuclear attack, 100 kWe).2, 15 e SPAR reactor a 100-kWe space reactor system. laser attack, and antisatellite design was also refined to ensure its Disagreements over management, attack. It also makes it practical compatibility with the space shuttle organization, and program goals to provide the payload with high and to raise its temperature and soon led to tension. For a short power, which enhances survivability energy density.21 e new program time in late 1982, NASA began by permitting higher orbits, more goals were similar to those outlined working with DARPA under a ground links, harder electronics, for the original SPAR design and project called the Technology for smaller antennas, and mobile included full-power operation at Advanced Space Power program, ground receivers. Nuclear power 100 kWe for seven years, with an also provides the spacecraft with overall system life of 10 years, and an improved field of view and no single-point failures.2 improved pointing accuracy and permits undegraded operation in the Van Allen radiation belts.”15

62 Atomic Power in Space II Chapter 4

leaving DOE to continue work on development efforts. To accomplish In its final report, the space nuclear technology for the SP-100 reactor.21 its task, the committee responsible power assessment committee noted for the assessment organized a that a government-wide joint space As the three agencies struggled symposium on advanced reactor reactor power system program to find common ground, the concepts in November 1982. e was appropriate because both the Departments of the Army, Navy, symposium offered an opportunity military and civilian agencies had and Air Force; DARPA; and NASA for experts throughout the space future power needs that could only sponsored the National Research nuclear power community to be met with reactors. Failure to act Council in October 1982 to assess discuss space power technology would mean a higher bill later for a the state-of-the-art advanced concepts, safety, research and crash program or simply not having nuclear power systems with development issues, and mission the needed technology at all. e possible aerospace applications in requirements for both space reactor report also included assessments of the area of propulsion, including power and propulsion systems. several items that had been cause shielding and safety problems. It also provided the basis upon for earlier frustration and tension e Council was also asked to which the committee developed its among the agencies, including describe research gaps and areas assessment and recommendations funding and research program of uncertainty in space nuclear for future space nuclear power goals, and provided an assessment power system technology and to technology development efforts.23 of the LASL heat pipe reactor.24 make recommendations for future e report accurately described a chicken-and-egg dilemma that Failure to act would mean a higher bill later for a DoD, NASA, and DOE had been crash program or simply not having the needed facing in deciding whether and how to proceed: technology at all. “Most research and development managers would like to be in a situation in which a user (with Space Nuclear Power Symposiums resources) can specify with precision a requirement that can serve as the In the fall of 1982, a small group of government, industry, and academic target for a technical development representatives, including University of New Mexico professors Dr. effort. However, experienced Mohamed EI-Genk and Dr. David Woodall, decided to hold an annual technical managers recognize symposium on space nuclear power systems due to growing interest in that such a linear situation such systems within the Federal government. The rst symposium, held in rarely obtains [sic], especially in 1983, was enthusiastically received by the space nuclear power community. circumstances in which long lead Though initially small, the annual symposium brie y rose to prominence times and expensive development several years later. After a decade of obscurity, limited funding, and slow efforts are required. Prudent development, space nuclear reactors were again on the front burner of U.S. program managers are reluctant to space power research. risk or expose large scale resources

63 Reactors Redux Space Nuclear Reactor Interlude

to achieve stated requirements On the subject of the DOE-NASA warning: “ e major lesson from until the viability of the technology SP-100 reactor, the report noted this history is the importance is sufficiently well established to that the LASL design was of of approximately matching the provide a reasonable prospect that high quality but “not sufficiently research and development effort the requirement can be met at unique or demonstrably superior to the process of emergence of a estimated costs. On the other hand, to alternative concepts to justify firm requirement. e committee major resources for research and selection of this approach.”27 e seeks to avoid a massive research development programs cannot be report identified several areas and development program that easily justified to those who control that still required significant never meets the needs or resource funds unless a firm requirement development before a full availability of military or civil exists. e inevitable result of such ground test could be pursued, space users.”28 a situation is no action unless most notably in the heat pipes research and development programs (fabrication, performance, and In February 1983, DOE, NASA, and can be launched and pursued longevity), reactor (fuel behavior DARPA finally came together and with a realistic acceptance of the and actuator performance), and signed a tri-agency memorandum uncertainty…”.25 high-temperature thermoelectric of agreement to take action along performance. For these reasons, the the lines of the National Research e report also weighed in on the committee urged that alternative Council recommendations.29 e question of which agencies should concepts “be brought to a stage in agreement called for the three pay for the needed research and which they can be evaluated relative agencies to assess and advance the development: to the SP-100 on a similar basis.” technology for 100-kWe and multi- megawatt (MMW) space nuclear “Potential Air Force and NASA In light of its assessment, the final power systems, provide engineering users are loath to adopt a report recommended a research development and production requirement prior to demonstration and development program be systems for users, and ensure of the technology from a concern funded at a level of $10 to $15 nuclear safety. Under the new about… a large development bill, million per year to develop a agreement, the agencies carried perhaps in the range of $500 million 100-kWe space power reactor as the DOE-NASA SP-100 name into to $1 billion. Yet most managers of a generic multi-use development, a new space reactor development space systems programs recognize not tied to a specific mission. e program that would be the largest that the future… points toward recommendation came with a since the days of Rover/NERVA. nuclear power… e military users should recognize that someone will need to bear the research and development cost for the operational “The committee seeks to avoid a massive research capability they will require. and development program that never meets the Accordingly, these users should recognize that the desired capability needs or resource availability of military or civil will not be forthcoming unless they space users.”28 are more supportive of these initial research and development efforts.”26

64 Atomic Power in Space II Chapter 4

The Interlude Gives Way

e 10-year period that followed the termination of the Rover/ NERVA program seemingly served as an interlude for U.S. space reactor development. Efforts were aimed at keeping the technology moving forward. Although seemingly buried, the prospects of power and other benefits afforded by a space reactor power system brought about a renewed development effort focused on a heat pipe reactor that served to expand the base on which future space reactor work could build. e desire for space reactor power also provided the impetus by which the broader space nuclear power system technical community was brought together to share technology status, concepts, and information. at gathering gave rise to what would become a decade-long annual event that eventually expanded to include international partners. At its conclusion in 1983, the interlude had given way to the SP-100 program, a new movement in the concerto of space reactor power system development (discussed in Chapter 5). As for the LASL heat pipe reactor system concept, it was carried into the technology assessment phase of the new SP-100 program but eventually was set aside in favor of other technologies.

65 Intro“The successfultext test flights of the Space Shuttle mark the start of a new era – an era of routine manned access into cislunar space.” –Buden and Angelo, 1983

66 The SP-100 Program 05 A 100-KWe Space Reactor

n the 10 years following termination of the Rover/NERVA program, the domestic space reactor program had maintained a tepid pulse through occasional funding for technology reviews and limited development Iefforts. As the country turned the corner on the 1980s, that pulse began to quicken as talk of missions requiring higher-power systems became more common within the walls of DoD and NASA. In 1981, talk turned to optimism as DOE, DoD, and NASA sought common ground on plans to undertake a new space reactor development program. at optimism became reality when the agencies signed a tri-party agreement in February 1983 (as discussed in Chapter 4) to jointly pursue development of technology for a space nuclear reactor power system capable of producing electrical power in the range of tens of kilowatts to 1,000 kilowatts. e new SP-100 program, as it was called, was a successor to the late 1970s space reactor development effort undertaken at LANL under the SPAR/SP-100 moniker and opened a new chapter in the history of U.S. space reactor development.

Gearing up for Success

e SP-100 program was planned as a three-phase effort to be conducted over a period of 10 years. Phase I (1983 through 1985) would involve technology assessment and advancement, and would culminate in a ground-test-phase decision. If warranted, Phase II (1986 through 1989) would involve development and ground testing of a reactor power system prototype, while Phase III (1990-1993) would involve flight qualification of the power system.1

e 1983 tri-party agreement provided the general framework under which DOE, NASA, and DARPA worked together during Phase I to select a space reactor power system concept. Overall programmatic direction and policy were provided by a tri-agency senior-level steering committee. Technical direction and integration of project activities were provided by a project office established at the JPL and led by Vincent Truscello, with assistance from LANL and the NASA Lewis Research Center.2

Artist’s concept of a space nuclear power reactor orbiting above Earth. (Image: NASA)

Photo Credit 67 The SP-100 Program 100-KWe Space Reactor

To support the technology performed support test-facility safety-evaluation program was assessment and development work.3 e Lewis Research Center established to ensure the reactor activities during Phase I, a generic provided support in areas of system concepts and the technology set of performance criteria were mission analysis, with particular supporting those concepts would established for the planned SP-100 emphasis on space shuttle missions not result in designs that would system. e criteria included a and development of technologies lead to unacceptable nuclear safety power output of 100 kWe; a design such as energy conversion, thermal risks. Phase I also included an effort lifetime of 10 years, with seven management, and space power to evaluate candidate DOE sites years at full power; a maximum materials and structures under an for reactor-power-system ground system mass of 6,600 pounds (3,000 advanced technology program.4 testing suitability.2 kilograms); and a maximum length of 20 feet (6.1 meters). e length criterion was associated with the Linking the SP-100 power system to a specific space shuttle cargo bay, which was to be used to launch the space mission was crucial for justifying the program and reactor into orbit. e power system establishing design goals. Such linkage was also a would also need to be scalable to higher or lower power levels without necessity to ensure long-term funding and support. major design changes.2 Although generic in nature, the power system criteria provided broad targets for Missions, Power Systems, Linking the SP-100 power system evaluation and design of candidate and Technology to a specific mission was crucial SP-100 power system concepts. for justifying the program and With funding of approximately establishing design goals. Such With the assistance of three $15 million per year, Phase I of linkage was also a necessity to contractors (GA Technologies, the SP-100 program consisted of ensure long-term funding and Rockwell, and GE), JPL was three core tasks: (1) definition of support. Indeed, history had responsible to review candidate potential DoD and NASA missions shown that while previous space power system concepts and that might require nuclear power, reactor development efforts, such recommend one concept that could (2) evaluation of reactor power as the Rover/NERVA program, meet expected civilian and military system concepts that could meet had demonstrated a high measure mission requirements. Research to mission requirements, and (3) of technical success, the absence advance nuclear technology, such of a definitive mission could technology advancement (including 5 as fuels and materials research, was testing and analyses) to address preclude ever going operational. performed at DOE laboratories, areas of technical uncertainty. Of To that end, review groups were including LANL, ORNL, and primary concern from the outset established by DoD and NASA Argonne National Laboratory- was the need to ensure nuclear to perform mission analyses and West (ANL-W). e DOE Energy safety was properly addressed requirement studies early in the Technology Engineering Center, throughout the entire program, first phase. Workshops provided located in Los Angeles, California, including Phase I. erefore, a an avenue to discuss mission needs

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in the context of technology and In the area of technology initiated. LANL re-established a power requirements.6 Several development, Phase I activities production capability for uranium generic missions were eventually focused on a broad range of testing nitride fuel elements, lost since the identified. DoD anticipated power and experiments to advance SNAP-50 program in the 1960s, and demands up to 100 kWe for robust technology that was common to continued efforts to demonstrate a surveillance systems, survivable multiple reactor systems or had fabrication capability for refractory communications with anti-jamming applications beyond the SP-100 metal fuel pins.2 In the area of capabilities, and electric propulsion program. For example, in-core power-conversion technology, the systems for orbital transfer and reactor-life testing of thermionic Lewis Research Center initiated space-based weapons applications. diodes was initiated to demonstrate a Space Power Demonstrator NASA anticipated nuclear electric the potential for a seven-year Engine project to demonstrate the propulsion and power needs life. Research was initiated in feasibility of a 25-kWe free piston up to 50 kWe for uses such as high-temperature thermoelectric Stirling engine for possible use with interplanetary missions, an Earth- materials. Compatibility testing of the SP-100 system.8 orbiting tug, and manned space reactor construction materials and stations and planetary bases.7 reactor coolants was conducted. As technology development and Fabrication and life testing of mission analysis progressed, so did refractory metal heat pipes was also the development and evaluation

Cold War Concerns

Throughout much of the 1980s, Russian Ocean.9 Although not as serious as the Argentina] according to Defense Daily. space nuclear activities continued to be re-entry of Cosmos 954 over Canada in My people were involved in two Soviet a source of military and international 1978, at least in terms of response and space emergencies: the Cosmos 954 concern, which provided impetus for cleanup, the event highlighted concerns reactor emergency in Canada in 1978, the SP-100 program. Soviet surveillance about Russian space reactor technology, and the recent Cosmos 1402 reactor re- satellites were designed to detach from as noted by Mr. Herman Roser of DOE, entry this January. It requires only a little their reactor power sources at the end during an address to the National imagination to be concerned with what of their missions, after which the reactor Research Council on June 15, 1983: the Soviets are doing in space today and was to be boosted into a higher orbit, what they will be capable of doing in the delaying re-entry for hundreds of years “While the nuclear Navy is an future, with their advantages in space while ssion products decayed to a safe outstanding example of the use of nuclear power, given their possession level. In early 1983, the reactor from nuclear reactors to achieve defense of hardened multi-hundred kilowatt or the Russian Cosmos 1402 spy satellite, energy security, we have not fared as megawatt nuclear reactors in orbit.”10 launched in August 1982, separated well in our [space] reactor programs… from the satellite but the booster On space reactors, we lag the Soviets by rocket failed to re. As a result, the 10 years… Two Soviet nuclear-powered reactor reentered Earth’s atmosphere in satellites were over the Falklands [during February 1983 over the South Atlantic the war between Great Britain and

69 The SP-100 Program 100-KWe Space Reactor

Mission Requirements of reactor power system concepts. By early 1984, preliminary Preliminary • Spacecraft operations technical assessment • Launch vehicle assessments had been completed • Lifetime that provided a broad evaluation System Goals of the suitability and performance of reactor technologies in the • Power • Mass areas of nuclear fuels, refractory Detailed technical • Reliability alloys, and other materials for assessment • Configuration constraints high-temperature applications; fast • Radiation • Safety and moderated cores and gas- and liquid-metal-cooled reactor types; Technical Assessments System Concepts Trade Studies heat pipe, thermoelectric, and thermionic static power conversion; Select System as well as dynamic conversion systems, nuclear safety, and nuclear Ground Demonstration System radiation and shielding. ree promising reactor-power-system concepts were selected for further Flight Evaluation System evaluation: (1) a high-temperature liquid-metal-cooled pin-element Basic steps in development of a space nuclear power system. (Adapted from fast reactor with out-of-core “Outlook for Space Nuclear Power Development,” G. L. Chipman, Jr., 1982) thermoelectric conversion, (2) an

Technology Pros and Cons

Several factors made space reactor elds, reduced drag in orbit, a smaller of photovoltaic panels available at power systems attractive for both detectable cross section, and enhanced the time also degraded rapidly in so- civilian and military applications in the maneuverability and hardenability. called low-intensity, low-temperature early 1980s (several of which still apply Nuclear power sources are also conditions found far from the sun. From today). For example, at electrical power hardened against radiation by design a military perspective, solar panels levels above approximately 25 kWe, the to protect the power processing and had been shown to be vulnerable to power-to-mass ratio of a space reactor control systems. the after-eects of nuclear explosions, system could be considerably higher which send charged particles into orbit than its solar/battery system cousins. A Conversely, solar cells suer radiation in Earth’s Van Allen radiation belt.11, 12 space reactor power system could also damage over a period of years, making be used for deep-space applications them unreliable for long-term missions without orientation to the sun. The where radiation is high (Jupiter’s relatively low cross-section conguration radiation belts hold the same threat for oered enhanced survival in radiation NASA’s spacecraft). The performance

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in-core thermionic fast reactor that drove the decision was that Bill e design power level was power system, and (3) a low- Wright [of DARPA] in particular subsequently returned to 100 kWe temperature pin-element reactor was adamant… [that] mass was the approximately one year later.15 with Stirling power conversion. In key. And in order to find an actual August 1985, following a detailed application, the unit had to be While the power-level decision systems study of the power relatively lightweight.”13 After three introduced questions of technical system concepts, the fast reactor years and a cost of approximately feasibility, the thermoelectric thermoelectric power conversion $51 million, Phase I was completed choice was not unanimously system was selected for follow-on in September 1985.14 supported and divisions began development during Phase II.7

Although the thermoelectric The Air Force people, for the most part, were technology offered lower power- conversion efficiency than adamant that thermionics was the right answer. the thermionic and Stirling technologies, it represented the lowest technical risk of the three Separate Directions options due to the technology forming in the program. “ e Air having been successfully used Force people, for the most part, in RTGs for several decades. Concurrent with the decision to were adamant that thermionics For this reason, the Interagency proceed with the thermoelectric- was the right answer. ey were Steering Committee selected the based reactor power system, the very unhappy about the decision thermoelectric reactor power Interagency Steering Committee to go thermoelectric…and so some system for further engineering decided to increase the ground-test were actually contemplating filing a development and ground testing power level from 100 kWe to 300 formal dissent to their decision but kWe to meet evolving DoD needs, they opted not to do that…” noted based, in part, on the judgment 13 that it was the only technology that based primarily on an Air Force Wiley. After much discussion 12 could be ready for flight system recommendation. e decision with the thermionics advocates, development by the end of fiscal introduced a small problem for DOE and SDIO initiated an in- year 1991 at a cost of less than developers of the reactor power core thermionics development $500 million. Other factors that system—the higher power level was program called the ermionic technically incompatible with the Fuel Element (TFE) Verification influenced the decision included e operating life and weight. Robert capabilities of the thermoelectric Program. NASA also started Wiley, who worked on the SP-100 system. In spite of the technical another program to develop a high- for the Strategic Defense Initiative incompatibility, program inertia temperature Stirling engine that Organization (SDIO), which and the political risk associated would be able to produce five times replaced DARPA in directing the with going back to another round the output of the thermoelectric SP-100 program beginning in Phase of technology selection kept converter with the SP-100 reactor. II, recalled, “One of the key things the program moving forward. Despite the original intent to focus e. e TFE Verification Program is discussed in detail in Chapter 7, ermionics Revisited.

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on a single technology in Phase II, agreement that identified agency- responsibilities. DOE provided development would proceed down specific roles and responsibilities, much of the funding and was independent technology branches.16 an overarching management responsible for development and structure, and a six-year funding testing of the reactor power system, Ground Testing Plans plan totaling approximately $500 including selection and preparation Take Root million, with ground testing to be of a reactor test facility. NASA and completed by the end of fiscal year DoD continued mission analysis; With selection of a reactor power 1991. With a planned launch date however, most of the potential of 1996, optimism in the SP-100 mission emphasis and planned system completed, the SP-100 17 program turned its focus to the program was riding high. user agency program funding rigorous engineering and testing remained with DoD. NASA also activities needed to develop the While overall program direction continued development of non- system for flight qualification. e remained with the Tri-Agency nuclear systems, such as power general framework and plan for Steering Committee during the conversion and power conditioning, the second phase of the program ground-engineering and test phase, but at relatively modest funding was defined in a new tri-agency each agency now took on specific levels under its SP-100 advanced

What Makes a Good Space Power Plant (Adapted from “Nuclear Reactors for Space Power”)18

Factors that must be considered by designers of space nuclear power plants vary but always include those shown in the table below. The factors are all interdependent and often one can be improved most eectively only at the expense of the others. For example, system weight can be signicantly reduced by raising the operating temperature of the reactor power system; however, power system equipment might deteriorate more quickly at higher temperatures. At this point, the designer may step in with trade-os, such as how much weight-saving must be traded for one additional month of operational life? Ideally, this balancing act would result in a low-weight, low-cost, ultra-safe, and highly reliable power plant. In a practical world, however, compromises are usually needed in the process of power system optimization.

Desirable factor What it means

Low Weight The power plant’s specic mass (mass per unit of power) should be as low as possible.

Reliability The probability should be high that the power plant will run for the specied length of time (usually several years), with little or no human attention, in the presence of meteoroids, high vacuum, and the other hazards of space.

Nuclear Safety Under no predictable circumstances should the crew or Earth’s populace be endangered by radioactivity.

Compatibility Power plant characteristics must not require unreasonable restrictions on spacecraft design or operation.

Availability The power plant must be ready when the rocket and/or payload are ready for launching.

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technology program. Project conversion system utilized silicon- Engineering Development management functions remained at germanium/gallium-phosphide Laboratory (later renamed the JPL/LANL.17 thermoelectric materials assembled Hanford site) near Richland, Washington. DOE selected Hanford, which was managed In the absence of a specific mission, a reference by Westinghouse, in November 1985 based on an evaluation that flight system design was developed that could be included five candidate sites. scaled upward or downward. e selection of Hanford was due in part to the availability of the decommissioned Plutonium Recycle Test Reactor (PRTR) In the absence of a specific mission, in thermoelectric modules to facility. Having been defueled after a reference flight system (RFS) produce a nominal 100 kWe. A its decommissioning in 1969, the design was developed that could portion of the excess heat that remaining containment building be scaled upward or downward to wasn’t converted to useable and other facilities and equipment accommodate the broad range of electricity would be removed provided an ideal location power levels for the SP-100 system. from the system using a series of for the planned operational, e flight system was designed to heat pipes (through which the performance, and reliability tests support an earth-orbit military lithium reactor coolant flowed) on the SP-100 reactor system mission and provided the basis connected to radiators. After and its major components. In for design and development of the passage through the radiator pipes, 1986, DOE initiated the safety ground-based systems and facilities the cooled lithium was returned and environmental evaluations that would be needed to test the to the reactor core. e overall needed to modify and upgrade reactor power system. length was approximately 40 feet the containment structure and (12 meters), including the reactor other supporting facilities for the At the heart of the RFS was a system, the energy conversion planned nuclear assembly test. lithium-cooled pin-element fast assembly, and the heat rejection Planned modifications included reactor with uranium nitride fuel. system. A radiation shield would installation of a large vacuum e reactor, which was about minimize the dose at the payload chamber for testing the reactor the size of a five-gallon bucket, (approximately 82 feet [25 meters] system components in a near-space would generate 2.4 MWt at a from the reactor), while a heat environment. Phase II testing temperature of approximately shield would protect the reactor in would culminate in a “nuclear 1,350 Kelvin. e heat generated the event of re-entry. An auxiliary assembly test” designed to check in the reactor core would be cooling loop was designed to operation of the SP-100 reactor, transferred to the thermoelectric remove heat in case the primary primary heat transport (cooling) power conversion system via a system lost its coolant.7, 19, 20, 21 loop, and the radiation shield.19 series of pipes through which the lithium-metal coolant was Ground testing of the SP-100 In addition to Hanford, DOE had moved using an electromagnetic reactor power system was planned at its fingertips in the mid-1980s pumping system. e power to be conducted at the Hanford a nuclear infrastructure that had

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been developed over a period of several decades, dating back to the Manhattan project. Second to none, the infrastructure included a cadre of national laboratories such as LANL, SNL, ORNL, and ANL-W. Engineering and reactor expertise had been developed at locations such as the Idaho National Laboratory (INL; formerly the National Reactor Testing Station) and the Hanford reservation. Private industry partners in nuclear research and development included GE, Westinghouse, Rockwell, and Aerojet. At the heart of this unique national resource was a very Artist’s concept of the SP-100 reactor power system and space craft. (Image: Smithsonian Institute)

Academic vs. Practical Reactors

Admiral Hyman Rickover, known … a practical reactor plant can designer must live with these same as the father of America’s nuclear be distinguished by the following technical details. Although recalcitrant Navy, described two types of characteristics: (1) It is being built and awkward, they must be solved reactors, which he divided into now. (2) It is behind schedule. (3) It and cannot be put o until tomorrow. academic and practical: is requiring an immense amount of Their solutions require manpower, time, development on apparently trivial and money… For a large part, those “… An academic reactor or reactor items. (4) It is very expensive. (5) It involved with the academic reactors plant almost always has the following takes a long time to build because of its have more inclination and time to basic characteristics: (1) It is simple. (2) engineering development problems. present their ideas… Since they are It is small. (3) It is cheap. (4) It is light. (6) It is large. (7) It is heavy. (8) It is innocently unaware of the… diculties (5) It can be built very quickly. (6) It is complicated… of their plans, they speak with great very exible in purpose. (7) Very little facility and condence. Those involved development is required. It will use ‘o- The academic-reactor designer is … with practical reactors, humbled by the-shelf’ components. (8) The reactor free to luxuriate in elegant ideas, the their experiences, speak less and worry is in the study phase. It is not being practical shortcomings of which can more…”. 22 built now. be relegated to the category of mere technical details. The practical-reactor

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capable workforce described as “a SP-100 Technology mechanical and structural systems. large and diversified population of Moves Forward Each subsystem performed a technical experts with interest and specific function. For example, the experience in advanced nuclear To facilitate engineering reactor subsystem provided the power systems…waiting to be development, the reactor power source of heat from which the power reengaged and redirected in a new system design and development conversion subsystem, consisting effort to put nuclear power to work work was separated into a logical of the thermoelectric cells and in space.”23 is resource, which had set of individual subsystems. related components, converted been primed by the preliminary Major subsystems included the the heat into useable electricity. feasibility work of Phase I, was reactor system, power conversion e instrumentation and control ready for the challenge posed by system, heat rejection system, subsystem ensured proper operation the second phase of the new space instrumentation and control and safety of the reactor. e shield reactor program. systems, the shield system, and subsystem protected the spacecraft

PRTR at Hanford circa 1964. Following its decommissioning in 1969, the containment structure would later be considered as a test structure for the SP-100 space reactor. (Photo: DOE Flickr)

75 The SP-100 Program 100-KWe Space Reactor

payload from the undesirable the fuel pellet inside its metal radiation levels for several months effects of the intense neutron and cladding cocoon, which consisted to several years. Such irradiation gamma radiation emanating from of an inner metal liner encased in testing provided fuel designers with the reactor once it was started in the outer metal cladding. Inspection information pertaining to material orbit. Each subsystem had to be and measurement of the clad pellets degradation and other criteria, integrated and work together for ensured they met dimensional which was needed to verify that proper operation of the reactor requirements for placement inside the fuel would last the required power system. Designers also had a fuel element, the structural seven-year lifetime at the expected to ensure that each subsystem and component that held multiple reactor operating temperature. e their respective components would fuel pellets and formed the basic uranium nitride fuel and niobium- meet requirements associated with building block of the reactor core. alloy fuel pin developed by LANL launch and operation in space, such e fuel pellet was but a microcosm was eventually demonstrated at fuel as temperature limits, pressure of the overall set of fabrication and burnups equivalent to a life of seven limits, and shock and vibration production processes that were years at cladding temperatures limits, as well as applicable safety developed for the SP-100 space that exceeded the system design requirements for launch and reactor program. temperature. operation of the nuclear power system. Designs were verified In addition to the design and Relative to the power conversion through analysis, testing, and/or fabrication processes, a rigorous system, GE focused its efforts on experiments. testing program was established developing a thermoelectric cell that served to verify the design with a power density 16 times As the reactor power system of the components, subsystems, greater than the power units design progressed, another and overall reactor power system. successfully used in the RTGs that equally important task focused on Experiments that verified nuclear powered the Galileo and Ulysses development of the fabrication and physics calculations and other spacecraft. Integral to the GE effort manufacturing processes needed related parameters for the SP- was the use of coating materials to build, assemble, and test the 100 reactor core were set up and on the external surface of the various components and parts of performed using the Zero Power thermoelectric cell to help improve the SP-100 system. In some cases, Physics Reactor (ZPPR) located overall structural integrity in several different processes were at ANL-W.24 Nuclear testing of support of a seven-year operating needed for a single component. For fuel components (i.e., fuel pellets life at full power. example, production of the reactor and cladding materials) was fuel pellet required a specification conducted using several facilities e major effort associated with that identified the exact chemical within the DOE infrastructure. e the heat transport system was makeup of the uranium nitride Experimental Breeder Reactor–II development of a thermoelectric feedstock that would subsequently (EBR-II), operated by ANL-W, and electromagnetic pump by which the be pressed into a fuel pellet. A the Fast Flux Test Facility (FFTF), liquid lithium would be pumped process for producing the fuel pellet located at Hanford, provided unique through the primary and secondary from the feed material had to be testing capabilities in which fuel reactor coolant loops. e pump developed. Another production pellets and cladding materials were was self-actuating, receiving its process was established for encasing subjected to high temperatures and power from the current produced by

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thermoelectric cells located between non-nuclear heat source similar to or contributed only $260 million the primary and secondary coolant what was done with the GPHS-RTG. of the approximately $450 million loops that passed through the pump. planned per the Phase II tri-agency Tensions Mount agreement. At the agency level, Closely related to pump the funding levels equated to $160 development was the need to While significant technical progress million of $210 million planned for ensure the lithium coolant (in solid was made during the first several DOE (a 25 percent reduction), $82 form before system operation) years of Phase II, financial clouds million of $220 million planned was thawed in a manner that began to form over the SP-100 by SDIO (a 60 percent reduction), would allow the pump to operate program in 1986. Spurred by and $19.9 million of the $16 million as designed. In addition to the Graham-Rudman-Hollings planned for NASA. e situation demonstrating assembly techniques, Deficit Reduction Act of 1985, the didn’t improve in 1990 or 1991, as the project team validated its Federal government began a broad appropriations continued to lag the hydraulic and electromagnetic tightening of its fiscal belt. e funding plan. In addition to reduced performance through a series of fiscal tightening translated into funding levels for the agencies, pump tests. Testing of the pump reduced funding for all Federal the SP-100 program experienced and other major SP-100 subsystems, agencies, and the SP-100 program significant cost growth due to such as the thermoelectric power was hit particularly hard. During ongoing technical issues, thereby conversion and heat rejection worsening the fiscal outlook for the the first four years of Phase II (1986- 25 systems, was performed using a 1989), the agencies received and/ program.

In the wake of the eroding financial picture, major program changes soon followed. e ground engineering system and technology development activities were delayed, shifting the planned completion date from 1992 to 1994, and then later from 1994 to 2002. e tri-agency agreement was updated more than once to reflect the changing funding and schedule realities.25 As a result of the funding problems and schedule delays, frustration began to mount, particularly within SDIO, the primary mission organization and source of the major SP-100 program funding reductions.

SP-100 core mockup in the ZPPR at ANL-W. (Photo: INL)

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As the reality of funding cuts and design contracts to identify key technology in its space reactor schedule delays were taking their technology issues for several 40- program over a period of several toll on the SP-100 program, efforts kWe reactor designs: (1) STAR-C, decades. One particular thermionic to tie the SP-100 power system to (2) Heat Pipe ermionics, (3) reactor concept, dubbed TOPAZ- a specific mission continued. In the Small Externally-Fueled Heat II by the United States, caught the 1989, potential Air Force missions Pipe ermionic Reactor, (4) the attention of SDIO in 1989. With resulted in the development of Moderated Heat Pipe ermionic significant interest in the Russian various “hardened” designs capable Reactor, and (5) the Space Power technology and hopes of gaining of providing 10 to 40 kWe and Advanced Core Element Reactor, decades of Russian development at a meeting military reactor goals that a derivative of a Soviet design that fraction of the cost for a comparable included hostile threat survival.26 had become available as a result of domestic technology development the economic and political decline program, a deal was subsequently DoD interest in lower-power that transpired in the former Soviet brokered that eventually brought systems continued into 1990, when Union in the late 1980s.27 e Soviet TOPAZ-II technology to the United the Air Force signed five one-year Union had developed thermionic States (at least temporarily).f

At NASA, interest in the SP-100 system was strengthened in light of the SEI announced by President George Bush in July 1989. SEI brought a renewed vision for the future of space exploration that included manned missions to the moon as well as to Mars by 2019. Mission planners at NASA and DOE soon began looking anew at nuclear propulsion concepts for the out- year manned mission to Mars and nuclear electric power for a planned lunar outpost.

The FFTF was a 400-megawatt thermal, liquid metal (sodium) cooled reactor. The white dome in the background is the containment building that holds the reactor vessel. (Photo: LANL Flickr) f. e TOPAZ-II reactor is discussed more fully in Chapter 7, ermionics Revisited.

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Changes on the Horizon reactor power system in 2004 at a time, and the desire to buy a TOPAZ cost of approximately $1.8 billion, reactor from the Russians… is Although interest in an SP-100 representing a 10-year schedule slip program is in serious trouble…”. 29 power system continued, by 1991 and $1.3 billion overrun. Although the SP-100 program was facing a number of possible missions had Testimony was given by a mounting wall of uncertainty been identified, the review found no representatives from all three regarding its future. Inadequate firm civilian mission requirements agencies, including William Young funding continued to adversely for the space reactor system. To from the DOE Office of Nuclear affect testing and development address the growing schedule Energy (DOE-NE); Dr. Robert Rosen, plans. Mission requirements and cost for the SP-100 system, Deputy Associate Administrator for remained a moving target, shifting the agencies were subsequently Aeronautics and Space Technology frequently within DoD and then asked to develop planning options at NASA; and Col. Simon “Pete” NASA. And tensions were high to facilitate a more-competitive, Worden of SDIO. Testimony was between the agencies, and even lower-cost, faster-paced, and also provided by the General higher within some agencies. flexible program for developing Accounting Office and Steven space reactor power systems for Aftergood of the Federation of In November 1991, SDIO potential DoD/NASA use in the American Scientists. rough the announced it would no longer early to mid-2000s.28 course of the hearing, the merits of support the SP-100 program the SP-100 program were discussed in order to pursue the Russian Shortly after the Office of and debated. Agency, management, thermionic technology. e Management and Budget review, the and organizational issues, tensions, announcement raised new concerns program came under further scrutiny and frustrations were aired. e that acquisition of the Russian during a Congressional hearing in lack of a specific mission was noted, reactors would undermine the March 1992. e hearing chairman, raising questions as to whether the SP-100 program by redirecting Representative Howard Wolpe, SP-100 program had become an government funding for space started the hearing on a dire note: ongoing research and development reactor power system development program. Questions also arose as to to SDIO. It also left the Air Force as “ is is a program in crisis… After who should pay for such an effort. the only DoD entity with a stake in 10 years and the expenditure of Since DOE and DoD had provided the SP-100 program.28 $400 million, the SP-100 has yet the vast majority of funding for to be chosen for a firm mission, the SP-100 program, NASA was With the pullout by SDIO, the by either DoD or NASA. And no chided for trying to get something question of mission purpose for firm missions appear to be on the for nothing (or at least very little). the SP-100 program resurfaced. horizon. e original program cost On the topic of technology, SDIO At the request of Secretary of estimate has soared, and the project presented its case for pulling out Energy James Watkins, the Office schedule has dramatically slipped. of the SP-100 program in favor of of Management and Budget One of the SP-100’s sponsors, the the Russian thermionic reactor subsequently reviewed the Department of Defense, recently technology. e SP-100 program was beleaguered SP-100 program. By withdrew financial support, citing at times pitted against the new SDIO the time of the review, the SP-100 dissatisfaction with program program in spite of many unknowns team was looking to ground test the management, high costs, long lead regarding the foreign technology.

79 The SP-100 Program 100-KWe Space Reactor

When all was said and done, significantly less expensive SP-100 define options for a small (5 to skepticism regarding efforts to system that could be launched in 20 kWe) space nuclear reactor reduce the cost and shorten the the 1990s. e agencies focused program for space-science and program schedule remained. e their efforts on technology to planetary-surface applications and hearing was closed with the same support systems in the 5 to 15 kWe a high-performance propulsion tone with which it had opened, power range. Cost and schedule system for piloted and cargo “SDIO… has pulled out of the savings could also be achieved by missions to Mars. e DOE/ SP-100 program. If current-year using the qualification system as NASA team put forth a plan funding were to continue, the the flight system. Seven conceptual centered on a 1998 launch using program would have an annual design options and three launch existing infrastructure to fulfill budget of about $50 million to fund date opportunities were developed. NASA scientific and exploration a $1.5 billion program… it will take Four options used prototypic objectives that could not be met about 50 years to complete the ground-flight system components by other power systems. e 1998 program at that rate. at clearly for a 15-kWe system for launch flight was based on development is not an option… I think it is time, in either 1997 or 1999, depending of a 500-kWt SP-100 reactor very frankly, to terminate this on system details. e remaining coupled with a 20-kWe closed project.”30 options used RTG thermoelectrics Brayton cycle power conversion subsystem. In response to this recommendation, DOE redirected When all was said and done, skepticism regarding the program and initiated a system design activity. A design review efforts to reduce the cost and shorten the confirmed that the closed Brayton program schedule remained. cycle design approach was feasible for an early mission. No major closed Brayton cycle development issues were identified, although Regardless of the issues raised to accommodate a 1996 launch normal engineering development during the hearing, the SP-100 date. All of the options proposed would be required.26 program continued to move elimination of the full-scale ground forward. In response to the Office test to reduce costs. Previously In the area of technology of Management and Budget considered a radical approach, development, the generic flight request, DOE, NASA, and DoD elimination of full-power ground system design was also updated developed a planning options testing began to make sense based in 1992. e updated design study that recommended a space on economics and engineering demonstrated that a 100-kWe reactor power system program benefits, which included the system with a mass of no more that would launch a prototype powerful analytical capabilities of than 10,000 pounds (4,600 reactor by 2000.27 In conjunction the day.26 kilograms) was achievable. with development of the planning Researchers identified a thaw option study, DOE and NASA In late 1992, DOE and NASA concept that utilized an auxiliary began evaluating options for a undertook another effort to cooling loop to allow the reactor

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to restart after shutdown, and interplanetary or asteroid mission.26 demonstrated power densities engineers completed development e final system redesign, however, approximately 16 times greater of fabrication techniques for received no more support than than GPHS-RTG technology. e the reactor, fuel, and fuel pins. any of the earlier efforts. As reactor actuator assembly, the GE continued development of discussed in Chapter 3, SEI lacked only SP-100 device with moving the assembly process for the Congressional support and funding, parts, had been successfully thermoelectric cells. Two test and the hoped for missions never developed and tested at prototypic loops containing high-temperature materialized. temperatures (800 Kelvin) in a lithium coolant demonstrated hard vacuum. A prototype self- welding and fabrication techniques Orderly Shutdown powered electromagnetic pump for refractory niobium alloys. Tests capable of pumping two separate showed that the reactor design Despite efforts to accommodate liquid metal loops simultaneously underwent lithium thaw with had been produced, deriving its 31 Congressional concerns, the minimal stress. Clinton Administration showed power from the hot liquid. Low- no inclination to support the SP- cost heat pipes were life-tested at By 1993, the agencies were 100 space reactor program, and it prototypic temperature, showing working under the presidential was scheduled for termination in long-term stability. As part of the administration of William fiscal year 1994; funding of $16.9 closeout activities, equipment (Bill) Clinton. e Clinton million was provided for closeout was distributed to government Administration had a new set of activities. Including the amount for laboratories, universities, priorities that didn’t include nuclear closeout activities, approximately and industry. In addition, key power research and development. $520 million had been spent on the fabrication-process documentation, e SP-100 program had been SP-100 program. work records, and other program- attempting to modify its testing related documents were placed in 32 and development plans to better Although the SP-100 space reactor government repository storage. match anticipated missions. In power system was never fully early fiscal year 1993, however, developed, the program achieved During the course of the program, it became increasingly clear several accomplishments and took a number of technologies had that an early closed Brayton notable efforts to preserve the also been successfully developed, cycle-based SP-100 mission was technology for future researchers. including those with additional unlikely, which led to a decision to e reactor fuel had successfully applications outside of the space generate a 20-kWe thermoelectric demonstrated low swelling and program. e requirements for design with the rationale that lifetimes exceeding the seven- the SP-100 to operate at high with additional development year requirement. LANL had temperatures in a vacuum for 10 time, the design would be more fabricated, inspected, and accepted years had resulted in technology competitive in terms of mass sufficient uranium nitride fuel developments in the areas of and lifetime capability. As with pellets for a 100-kWe space reactor. high-conductivity heat transfer, the closed Brayton cycle design, ermoelectric-cell and power- self-lubricating bearings, stress- requirements were based on a five- converter technology overcame relieving components, self- year nuclear electric propulsion major technical hurdles, with energized pumps, compact heat exchangers, bonding of ceramics

81 The SP-100 Program 100-KWe Space Reactor

to metals, high-temperature electric coils, electrical insulators, During the course of the program, a number of thermometers, high-temperature motors, and generators.32 technologies had also been successfully developed, including those with additional applications outside Because many of these components had potential of the space program. commercial uses, with permission from DOE, the SP-100 project office at JPL began an aggressive Over 100 companies expressed of the gas-separator concept to project late in 1993 to find interest in the technology transfer remove gases from liquids in the commercial applications for prospects that included self- manufacture of syrup for soft fabrication processes, devices, and lubricating ball bearings for the drinks.32, 33 components developed during the space shuttle, electric motors for SP-100 program. aircraft activators, and the use Looking Back

With the end of the SP-100 program, the latest chapter in U.S. space reactor history came to a close. Although the proposed reactor system was never fully developed, many advancements were made in space reactor system technology and other supporting areas. e vision of repeating the success of the SNAP-10A launch 30 years earlier finally faded, succumbing to the weight of inadequate funding, lack of missions, and a changing political landscape that questioned the very need for ongoing nuclear research and development.

e tentacles of change reached far beyond the SP-100 program. e EBR-II, one of the nation’s

Experimental Breeder Reactor-II at Idaho National Laboratory. (Photo: Idaho National Laboratory Flickr)

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only fast-reactor test facilities, was shut down in 1994. e ZPPR facility was shut down in 199038 and the Hanford FFTF was shut down in 1992 (the last fuel was removed from the reactor years later, in 2008). Other facilities, such as the Plutonium Recycle and Test Reactor Complex, would eventually succumb to the massive cleanup effort conducted under the DOE Environmental Management Program. For some, such shutdowns and dismantlement may have signified progress relative to the nation’s Cold War cleanup legacy. For others, the shutdowns meant the loss of a livelihood. Regardless, the nation lost a piece of its nuclear heritage and a significant amount of its nuclear infrastructure.

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