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4TH EUROPEAN CONFERENCE FOR AEROSPACE SCIENCES (EUCASS)

THE NTER: A PROPOSED INNOVATIVE CONCEPT FOR MANNED INTERPLANETARY MISSIONS

C. Dujarric European Space Agency, HQ, 8-10 rue Mario Nikis, 75738 Paris Cedex 15, FRANCE

A. Santovincenzo, L. Summerer European Space Agency, ESTEC, Noordwijck, Netherlands

Abstract proposed at an IAF conference in 1999.[1][7] Following a slow maturation process, a new concept was defined which Conventional propulsion technology was submitted to a concurrent design (chemical and electric) currently limits the exercise in ESTEC in 2007. The pre- possibilities for human to definition work made clear that, based on the neighbourhood of the . If farther conservative technology assumptions, a destinations (such as ) are to be specific of 920s could be obtained reached with humans on board, a more with a of 110kN. Despite the heavy capable interplanetary transfer engine dry mass, a preliminary mission featuring high thrust, high analysis using conservative assumptions is required. showed that the concept was reducing the required Initial Mass in Low Earth Orbit The source of energy, which could in compared to conventional nuclear thermal principle best meet these engine for a . A requirements is nuclear thermal. However patent was filed on the concept. Because of the nuclear thermal technology is not the operating parameters of the nuclear core, yet ready for application. The which are very specific to this type of development of new materials, which is concept, it seems possible to test on ground necessary for the nuclear core will require this kind of engine at full-scale in close loop further testing on ground of full-scale using a reasonable size test facility with safe nuclear rocket . Such testing is a and clean conditions. Such tests can be powerful inhibitor to the nuclear rocket conducted within fully confined enclosure, development, as the risks of nuclear which would substantially increase the contamination of the environment cannot be associated inherent nuclear safety levels. entirely avoided with current concepts. This breakthrough removes a showstopper for nuclear rocket engines development. Alongside already further matured activities in the field of space sources The present paper will disclose the NTER for generating on-board power, a low level (Nuclear Thermal Electric Rocket) engine investigation on has been concept, will present some of the results of running since long within ESA, and the ESTEC concurrent engineering exercise, innovative concepts have already been and will explain the concept for the NTER

Copyright © 2011 by C. Dujarric, A. Santovincenzo and L. Summerer. Published by the EUCASS association with permission. Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration on-ground testing facility. Regulations and and without effective means to influence safety issues related to the development and decisively their fate, especially when facing implementation of the NTER concept will non-nominal situations. It is mandatory to be addressed as well. give them a propulsion mean on which they can rely to implement their decisions to Introduction restore their nominal mission or initiate a safeguard alternative and monitor the effect The present paper focuses on inter-orbital of their action within short delay. propulsion, i.e. the propulsion needed to escape earth orbit and conduct space Nuclear propulsion state of the art exploration, including return to Earth. For such missions the optimisation of the inter- Nucleo- propulsion was orbital propulsion is a key driver for the developed both in the U.S. and in Russia overall mission cost, as the slightest aiming to satisfy the simultaneous need for performance gains on the interplanetary high thrust and high specific impulse. transfer engine has huge impacts on the size of the required Earth departure means. The nuclear propulsion development efforts lasted in the U.S. from 1955 until 1972. The typical advantage of chemical Twenty rockets and furnaces were propulsion is to provide a high successfully ground tested in the frame of acceleration for a low engine mass, while the KIWI and NERVA programs. The electric propulsion may be preferred for its NERVA-NRX/XE, which featured higher specific impulse whenever thrust can carbide core coated with be applied for a long time without carbide, was the ground-qualified model the detrimental effect on the payload. R&D closest to a flight model. strives to mitigate the shortcomings and pushes the limits of each type of propulsion. However even taking into account the anticipated progresses of these technologies, a part of the long-term space exploration needs may lie beyond the physical limits of those conventional propulsion means.

Human exploration of space beyond the Figure 1: the NERVA NTR engine Moon orbit generates mission constraints The nuclear thermal propulsion which neither chemical nor electric development efforts started also in 1955 in propulsion can satisfy. The time of the Russia but lasted until 1989. Several engines interplanetary travel must remain (RD-0140, RD-0411 and RD-410) were sufficiently short to keep the space designed in CIS and their effects to an acceptable level. The crew elements based on ternary carbide twisted endures not only a cumulated radiation dose ribbons were successfully tested, however proportional to the duration of their travel at not as part of an engine system. first order approximation, but also takes a proportionally cumulated risk of being hit More recent conceptual studies were by a solar flare or meteorite. Furthermore, conducted in the U.S., which are not yet on a long interplanetary ballistic trajectory, tested at engine system level. The core the crew cannot be left in a waiting mode technology investigations led to the Page 2 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS

-metallic () fuel in a fast ESA’s initial proposals to upgrade the reactor. This solution offers better nucleothermal propulsion performance resistance to corrosion and a lighter design, but further work is still Several ideas were already investigated in needed to reduce fission gas swelling effects the 1990’s, in particular by Aerojet in the in the fuel. U.S., to take advantage of the unlimited power available from the nuclear core to At system level, bimodal engines were increase the specific impulse of the engine investigated in the U.S. and in Russia to as compared to what is obtained by a simple deliver electric energy to the spacecraft heat transfer to the hydrogen. These studies during cruise for example to keep the were stopped in view of the complexity of hydrogen in state, and also to mitigate the thermal machines involved, and their the hydrogen consumption during the consequent mass. nuclear core start-up/shutdown transients. Along these lines, ESA proposed its first Some conceptual design work on nuclear ideas in 1999, which consisted in using the propulsion also started in Europe with the electric power obtainable from a bimodal French MAPS program in 1995, but this engine to heat the hydrogen plasma effort did not result in any hardware supersonic exhaust by electric induction. fabrication or testing. [1][2] This nuclear inductive concept is recalled in Figure 2:

25% parahydrogen, 75 % orthohydrogen 75 b, 700 K

100 b, 563 K

Caesium 68 MW seeding 150 MW catalysised para/ortho 2000 K 2200 K 95 b conversion Pc = 34 MJ/kg 1 bar, 75 K 60 b H=404 kJ/kg 20% ortho 90 b 53K 70 b Alternator 82 MW Inductive heating O2 (shown Impedance & Electric circuitry 100 bar operating loop : 70 MW into gas frequency deep cooling 5 MW acceleration matching parameters are with no Total enthalpy 44 MJ/kg Expansion ratio 250 feed) 3 b, 725 K Theoretical performance without O2 : Isp= 930 s , Thrust = 64 kNH2 slush 7 kg/s 100% parahydrogen H= -341 kJ/kg Figure 2: the nuclear inductive concept

Page 3 Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration One particular benefit of the idea was the The NTER concept synergy and efficiency gain, which was achieved by using as a cold source for the From 2000 till 2007 the nuclear inductive the incoming cryogenic concept went through a slow maturation . Variants of this concept included process during which the inductive heating mixed chemical/ nuclear inductive of the supersonic exhaust plasma was propulsion to increase the thrust when proposed to be replaced by conductive necessary, to the detriment of the specific heating of the hydrogen in the subsonic area. impulse. This change removed all uncertainties and most potential inefficiencies due to the non- The nuclear inductive concept offered some well mastered plasma physical behaviour. advantages, such as a high expected specific impulse (930s) despite a moderate core A major goal of this evolution was also to (hydrogen temperature at the reduce the engine dry mass; therefore the nuclear core exit of 2200K). It produced transmission of energy from the Brayton large thrust (64kN) at the highest Isp, while cycle to the hydrogen had to be simplified. offering the possibility to further multiply In this respect, an innovative device called the thrust level by a factor 3 at the expense turbo-inductor, which is the object of a of a large specific impulse decrease (480s). European patent has been described.[3] This Since the additional energy was introduced new device reshapes the whole engine in the supersonic zone of the exhaust flow architecture, whose name becomes now the where the flow is already cooling down, no NTER (Nuclear Thermal Electric Rocket) part of the engine had to sustain a engine. The description in this paper of the temperature equal to the stagnation concept will not detail the nuclear core temperature of the exhaust flow. design, as there is presently no on-going activity in this domain at ESA, but will However the concept also presented a cover only with the thermo-mechanical number of weaknesses: mainly its device, which is installed around the core, complexity and anticipated dry mass, but which is meant to increase the engine also the development risks which resulted performance. from the non-well mastered physics of the electric induction in the supersonic Figure 3 shows the NTER concept around a hydrogen plasma. Induction heating CERMET-type nuclear core. Indeed, the experience does exist at relevant pressure remaining fuel swelling problems of this with various gases at VKI in Belgium, but type of core must be resolved before this not in supersonic conditions. While further type of core can eventually be selected. In fundamental research is worthwhile to case the CERMET-type nuclear core in the consolidate and identify the limits of the end cannot be retained, another variant of supersonic plasma induction idea (in the NTER is described in Figure 5, which particular with respect of the plasma thermal builds on the older but already proven non equilibrium effects), it is prudent to NERVA-derived nuclear core and proposes look for a variant of this concept, which a work-around solution to the now well would suppress the biggest identified understood hydrogen corrosion problem. drawbacks, and mitigate the remaining weaknesses. Two main fluid circuits, hydrogen and , are shown on Fig. 3:

Page 4 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS

Liquid H2 H2 62K

H2 He 75 K He 690 K He 430 K

H2 2550 Cryogenic Small space H2 540K heat He 1900K K exchanger (OPTIONAL) He excitation 430 The Turbo-inductor K

H2 3250 K

Figure 3: the NTER concept

The cryogenic hydrogen is first will be described in Figure 4 below. pumped at high pressure, and then Its temperature is increased to flows through a heat exchanger to be 3250K by convection. Finally, the used as cold source for the Brayton hydrogen is exhausted through a cycle. At the exit of the exchanger, to produce the thrust. the warm hydrogen can optionally A Brayton cycle using Helium is provide some cooling to the throat implemented. The cold source is the area to protect the throat from heat exchanger with the cryogenic excessive heating. At this point the hydrogen, where the Helium is hydrogen enters the nuclear core at a cooled down to 75K (this very low temperature beyond 540K. It is temperature yields an excellent heated by the nuclear core up to an Brayton cycle thermodynamic assumed temperature 2550K, which efficiency, but condensation prevents takes into account ample margin using additional Xenon). Helium is with respect to the CERMET core then pumped at high pressure and technology. Then the hydrogen then heated by the nuclear core flows through the longitudinal flowing through bimodal-like channels of a turbo-inductor, which dedicated channels. The heat

Page 5 Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration exchange through these channels is The turbo-inductor, which is the innovative tuned such that the helium part of the concept, operates as shown on temperature remains with margins Fig. 4: below the temperature limit for the turbine stages, which are installed in Several successive turbine stages are the turbo-inductor placed installed on individual bearings and are downstream. At the exit of the turbo- freely rotating without any mechanical inductor, the helium is further power transmission. These turbine stages are expanded through two turbines, powered by the Helium expansion. Two which drive the two pumps, and consecutive turbine stages are contra- returns to the hydrogen heat rotating. No stator is therefore needed, exchanger. which avoids the energy losses due to stator Two auxiliary circuits are also shown on stages. At the tip of the turbine blades a ring Figure 3: is installed which bears coreless coils An Helium bleed circuit which picks creating radial fields. One coil can be Helium at the exit of the pump installed between two consecutive blades. around 430K and feeds the cooling Two consecutive coils on a ring present circuits of the turbo inductor to opposing poles. The ring constitutes maintain its bearings and electric basically the rotor of an alternator, but no subsystems at acceptable magnetic core is installed due to the high temperature. local temperature (which would disable any Taking advantage of the already ferromagnetic property) and to the high implemented bimodal architecture of local acceleration. the nuclear core, an optional bimodal circuit can be installed to provide Longitudinal channels are installed around electric power when the propulsion the rings, which duct the hot hydrogen from is off. This circuit also could help to the nuclear core towards the nozzle. These manage the thermal transients of the channels are coated internally with . core during the propulsion start-up At any channel location a longitudinally and and shut-down. radially variable magnetic field is generated by the coils installed on the nearby contra- Hot hydrogen flow Gas tight shell rotating stages. As a consequence, Foucault Tungsten currents are created in the tungsten coating coating Helium flow along transverse and orthoradial planes.

Turbine Excitation These currents heat the tungsten by ohmic stage coils effect, which in turn heat the hydrogen by Rotation direction convection. Turbine stage Rotation direction Given the excess of power available from Turbine stage the Brayton cycle, the 3250K limit to the hydrogen stagnation temperature as shown on Figure 3 is imposed by the melting of the Rotation direction Helium flow tungsten. This technology limit sets the specific impulse of the engine. Figure 4 : the turbo-inductor stages Even higher performance could be obtained, subject to confirmation by additional R&D,

Page 6 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS

if no tungsten is installed in front of the last hydrogen, needs to be resolved. The turbine stages; then induction and heating NERVA-derived core design features a would occur directly in the hydrogen core matrix protected from the subsonic plasma), with an energy transfer hydrogen flow by a efficiency which remains to be determined. coating. This coating is applied by chemical vapour deposition at 1500 K. Due to the In the case the CERMET-type fast neutron thermal expansion coefficient mismatch nuclear core is not finally retained due to with the graphite, the coating cracks after technological hurdles the NERVA-derived fabrication during its cooling. During the epithermal neutron core remains a valid NTR operation, the degradation is moderate candidate as it has been tested on a rocket in the region of the hydrogen entrance into model on ground in the sixties. Then the the core because cold hydrogen does not problem discovered during these tests, spontaneously react chemically with the which is the problem of core erosion by

Liquid H2 H2 62K

H2 1500K He 75 K He 690 K He 430 K

H2 2550 Cryogenic Small space He 2550K K H2 540K heat heat exchanger exchanger

He 1900K The Turbo-inductor He excitation 430 K

H2 3250 K

Figure 5: The NTER concept for a NERVA-derived nuclear core

Page 7 Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration graphite matrix. However, in the core region The main design issue was to find an above 1000K, hydrocarbons form which get optimal sharing of the fixed mixed with uranium particles into the power between the one used to heat up exhaust flow. In the region above 1500K, directly the propellant flow and the one used due to the thermal expansion of the to heat up a fluid working within a zirconium carbide, the cracks close again, that generates and material creeping makes the coating induction power later transferred to the gas-tight. propellant fluid at a given point of the engine nozzle. The NTER architecture offers a unique opportunity to work around this problem, as shown on Figure 5: Unlike the NERVA engine, the hydrogen does not enter the nuclear engine at low temperature, since the NTER cryogenic heat exchanger already heats the hydrogen above 500K. On this NERVA technology variant, an additional heat exchanger with Helium pre-heats the hydrogen beyond 1500K before the entrance of the nuclear core channels, i.e. beyond the temperature range where cracks can appear in the ZrC coating. For this purpose, the Helium comes out of the nuclear core at a temperature of 2550K, the same as for hydrogen. In addition, this heat exchanger is dimensioned such as to bring the Helium exit temperature down below the technological temperature limit for the turbine entry; if necessary, further heat is provided to hydrogen by heat exchanger with the nuclear core structure and internal tubing as was already achieved in NERVA.

The NTER predesign at ESTEC CDF

The ESTEC Concurrent Design Facility has performed in 2007 a feasibility assessment Figure 6: CDF design of the NTER engine of the NTER concept. Reference Performance requirements were defined based on a possible application for a The power handled by the thermodynamic Human Mission to Mars. Those are: cycle needs to be commensurate with the physical and technological limits of its Specific impulse ~900 s components (heat exchanger and turbo- Thrust ~ 100 kN machinery), with the total mass and volume Restartability > 3 times of the system and with the characteristics Engine dry mass < 30 tons and constraints of electromagnetic induction (inductor design). Above a certain power

Page 8 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS

threshold either the efficiency of the thermodynamic cycle (and thus system The test facility concept is the following: performance) decreases dramatically, or the The whole engine (except its nozzle) is dimension of the engine (including the tested in closed loop under a confinement inductor) become unrealistic. Below a wall. certain power threshold the additional complexity of the thermodynamic cycle Downstream the sonic nozzle throat, a shock does not pay off. brings back the flow to subsonic conditions Figure 6 shows the configuration of the and the extremely hot hydrogen flow is sent NTER engine as from the CDF study. towards a heat exchanger which evacuates All components are arranged axially with all the heat through the confinement wall to the the thermodynamic cycle elements on the aft outside environment, e.g. water flowing part and the nuclear reactor, the inductor and naturally from an altitude lake (The facility the nozzle at the end. This is the simplest shall be resistant to natural disasters such as configuration albeit not the most compact. earthquakes; therefore a natural altitude lake Possibilities exist to come to a more seems preferable to an artificial water dam). parallelised configuration or to reduce the Water ducts would be redundant and feature total length by for instance using a over-dimensioned bellows centrifugal H2 compressor instead of an axial one. Hydrogen is then pumped by a circulation pump. This pump is a part of the test System, fluido-dynamics and thermo- equipment, not part of the engine tested. It dynamic analysis was performed leading to uses external power only when the test is the following performance results: going on. The power is tuned such that the hydrogen pressure reaches the value it Dry Mass excl Margins 23,500 kg should have on an isolated engine at the Nozzle Inlet Temperature 3250 K entry of the heat exchanger with helium. Nozzle Inlet Pressure 42.15 bar Propellant Mass Flow 12.0 kg/s The external water flow is tuned such that Rate the temperature of the hydrogen at the exit Thrust in 111.2 kN Isp in vacuum 921 s (with exp of the circulation pump equals the ratio =100) temperature it should have on an isolated Table 1: NTER rough performance estimate engine at the entry of the heat exchanger with helium. The testing of the NTER on ground The equality of hydrogen pressure and Another benefit of the NTER concept is that temperature at the circuit exit and at the its architecture offers the unique opportunity intermediate entry of the engine enables to test the engine system on ground in fully closing the loop for the part of the hydrogen, confined closed loop conditions. This which comes in contact with the nuclear opportunity results again from the warm to core. The helium circuit is anyhow already high temperature of entry of the hydrogen nominally working in close loop. In case into the nuclear core. The test facility products are released in the concept is presented on Figure 7 for a hydrogen or helium by erosion or even as NERVA-derived core engine. the consequence of an engine malfunction,

Page 9 Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration those products remain confined under the rate in the closed hydrogen circuit. The wall. hydrogen at the exit of the heat exchanger can be either burnt as shown on the figure, On the cold side of the hydrogen circuit, the or stored to be re-liquefied later at a slower heat exchanger is fed with cryogenic rate using a reasonably dimensioned hydrogen coming from the outside, with a liquefaction plant for subsequent reuse. mass flow rate tuned equal to the mass flow

Figure 7: ground test facility concept for the NTER engine

Since the purpose of this facility is to generates and stores its own electricity qualify an engine which will operate in independent from the grid. The cold source space for a relatively short time (of the order is provided by water flowing naturally from of one hour), the testing duration in the an altitude lake, without any need for facility will be in total only a few tens of external mechanical or electric power nor hour. If such facility is installed in a region for any other fluid to be continuously with low seismic activity and out of reach of provided to the facility. The capacity of the natural disaster like tsunamis, a situation lake might be considered unlimited if it is requiring stopping an on-going test is fed by a river with sufficient flow rate. unlikely to happen. Should such situation still happen, the facility is designed to stop Of course the same kind of test facility can the thrust simulation and operate be designed with a CERMET-type core autonomously its cool-down using its design. auxiliary Brayton cycle, which by the way

Page 10 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS

Advantages and drawbacks of the NTER test conditions, robust to catastrophic events from outside the containment wall. As an outcome of the studies performed in ESTEC CDF, the following advantages and It cannot be envisaged to skip future nuclear shortcomings of the NTER concept have engine system tests on ground, whatever the been identified: engine design chosen. The only technology, which is already extensively tested is the Advantages compared to other concepts: NERVA technology, but the test results High specific impulse, high thrust show it needs anyhow to be modified and No necessity to operate the nuclear re-tested. All other concepts relying on core to its technological temperature different core technologies must be verified limit for getting a good performance at the engine system level. (operating at lower temperature increases the core life duration and Several solutions have been proposed to the safety margins) perform the engine level tests, but none Less thermal stresses in the nuclear provides fully confined gas protection. core Scrubbing the exhaust gases cannot be Availability for NERVA technology efficient 100%, especially in case an engine of a workaround solution to avoid malfunction occurs, as it did occur with the hydrogen corrosion snag NERVA. Dumping the exhaust gases in Strong synergy with bi-modal deep ground cavities can help to dilute and functions. filter the radioactive particles, but this solution creates unbounded quantities of Advantages over the nuclear inductive polluted soil, with no certainty that the concept previously proposed by ESA: pollution will never migrate back to the Plasma physics uncertainties avoided surface.

Mass gains due to the suppression of a large amount of intermediate The sensitivity of the population to subsystems: turbine stator, environmental protection is growing, and mechanical shaft, alternator, the fear related to nuclear activities in impedance adapters, lines, coils, general has not vanished. , offset by very few additions (turbo-inductor rings and Therefore after a successful development tungsten channels) and qualification of all the necessary subsystems, the engine system testing issue Reliability gain also due to may remain in the end the only real intermediate subsystems suppression showstopper for future nuclear thermal Remaining shortcomings: rocket engines, unless the ground facility

Engine system mass and complexity concept linked to the NTER engine is used. Engine development costs (huge!) Safety and regulatory aspects However the most decisive advantage is perhaps the possibility offered by the After the early experimental phases of NTER to test the engine in fully confined and engineering, when the safety of these activities played only a

Page 11 Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration secondary role which was mainly dealt with at laboratory level, the defence sector For the purpose of this paper, the concept is developed some early nuclear safety briefly analysed with respect to the requirements that were subsequently serving provisions in these documents. For such an as a basis for the general nuclear safety analysis, it is useful to keep the separation framework developed in parallel to the between nuclear safety aspects related to the large-scale development of civilian nuclear purely terrestrial activities and those related energy applications. to in-space activities.

Together with the Atoms for Peace Given the scope of the paper, regulatory and programme of the US government starting safety aspects related to the engineering, in the early 1953 and which led to the laboratory work as well as transportation of creation in 1957 of the main international associated are not covered nuclear organisation also dealing with since these are relatively straight forward nuclear safety, the Vienna-based and not different from many other nuclear International Atomic Energy Agency activities. For terrestrial, pre-flight (IAEA), nuclear safety has early on been activities, the focus therefore will be on the recognised as being of international, trans- regulatory and safety aspects related to the border and eventually global concern. As ground testing of the NTER concept. part of its core mandate, the IAEA establishes international standards and Concerning the space activities, one could guides covering nuclear safety, radiation argue that the 1992 principles do not or only protection, management, partially apply to the NTER concept due to the transport of radioactive materials, the the affirmation in the preamble that the set safety of facilities and the of principles “applies to nuclear power associated quality assurance. Under the sources in outer space devoted to the umbrella of a recently agreed high-level generation of electric power on board space publication, the so-called “Safety objects for non-propulsive purposes, which Fundamentals”,[4] the IAEA safety standard have characteristics generally comparable series include generic standards as well as to those of systems used and missions specific ones for performed at the time of the adoption of the Nuclear Power Plants, Principles”. The NTER concept clearly has Fuel Cycle Facilities, some characteristics that are different to the Research Reactors, systems used at time of adoption of the Radioactive Waste Disposal principles and the nuclear energy is used for Facilities, propulsive purposes in this concept. Mining and Milling, Application of Radiation Sources, One might still argue that the electric power generation aspects of the NTER, which are Transport of Radioactive Material. not directly linked to propulsive purposes In parallel, the specifics of space but for other energy needs of e.g. human applications of nuclear power sources have missions would fall under the scope of the triggered the development of dedicated, principles. In this case, the most relevant separate recommendations for the safety of technical provisions of the principles would nuclear power source applications in be those in Article 3, e.g. related to the space.[5][6] prevention of potential contamination of

Page 12 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS

outer space and the restriction to using only and early launch phases. The reactor most highly 235 as fuel. likely would have undergone only 0-power testing and would thus not contain any The international Safety Framework for significant amount of radioisotopes. The Nuclear Power Source Applications in main focus therefore would be on the Outer Space, adopted end 2009 as part of prevention of any accidental criticality in the UN COPUOS report to the UN General case of launch accidents and situations Assembly follows a different approach than created by launch accidents. These might the NPS Principles from 1992. The safety include scenarios such as intact or partially framework intends to be a model safety intact accidental landing of the reactor core framework for implementation at national or in water, wet sand or other media, which international level. It provides guidelines for could provide conditions for accidental governments, management as well as chain reactions. technical guidelines to achieve its objective of “protecting people and the environment One of the main advantages of the proposed in Earth biosphere from potential hazards design over other nuclear propulsion options associated with relevant launch, operation is the option to test the nuclear reactor, the and end-of-service phases of space nuclear engine and the associated conversion system power source applications”. Contrary to the in a fully confined environment with an principles, the framework specifically essentially closed-loop system. Since such includes all applications of nuclear power development and operational tests are to be sources, thus also those for propulsion, but it conducted on ground, terrestrial regulations also excludes some aspects included in the related to nuclear installations will principles, such as the protection of outer determine the details of the safety related space and of humans involved in missions aspects of such an installation. The general that use space NPS applications. approach to nuclear safety is handled slightly differently in different countries. It is useful to make a further distinction For example, the safety of French nuclear between the type of use of the NTER reactors is based essentially on a concept: in case the system is used in Earth deterministic approach, while others such as orbits, which include the option of collisions e.g. the UK and the US rely more on with other objects or space debris and the probabilistic safety assessments. At this option of re-entry, the safety assessment stage it is of little added value to speculate would be slightly different than if the on the location of an eventual development would only be started once and testing facility. Therefore the on an interplanetary trajectory with no considerations made so far are referring to possibility of any re-entry into Earth and the basic safety principles adopted by all thus also excluding Earth swing-by national regulations and consensually manoeuvres. For the following assessment, expressed within the IAEA safety standards the later is assumed as a baseline. series.

Chapter 5 of the framework deals with Given the novel nature of such an technical recommendations. The prime installation as well as the one-off type, the provisions therefore would be related to the most suitable IAEA safety series documents state of the nuclear reactor during launch seem to be those related to research reactors.

Page 13 Symposium on Propulsion Physics, Session on Nuclear Propulsion for Exploration Research reactors are defined by the IAEA The key principle that the intended as “nuclear reactors used mainly for the terrestrial NTER testing installation will generation and utilization of radiation for have to conform with is the general concept research and other purposes, such as the of defence in depth, applied to all safety production of radioisotopes. This definition related activities: organizational, excludes nuclear reactors used for the behavioural or design related. Application of production of electricity, naval propulsion, the concept of defence in depth throughout desalination or district heating.” [8] While design and operation provides a graded the type of ground testing installation for the protection against a wide variety of NTER concept is not specifically included, transients, anticipated operational it is also not excluded and given the strong occurrences and accidents. R&D aspect of it, these regulations seem most appropriate to take as a working The key mechanism for the assessment of baseline. Paragraph 1.9 of the IAEA Safety the safety of such installation is a safety Requirements documents for research analysis which needs to include all planned reactors confirms this approach by normal operational modes of the nuclear providing specifically for similar cases as installation and its performance in the one discussed that “Research reactors anticipated operational occurrences, design with power levels in excess of several tens basis accident conditions and event of megawatts, fast reactors, and reactors sequences that may to beyond design using experimental devices such as high base accidents. These analyses usually need pressure and temperature loops, cold to be independently assessed by the neutron sources and hot neutron sources operating organization and by the regulatory may require the application of standards for body. power reactors and/or additional safety measures.[…] For facilities of these kinds, Technically, the defence in depth approach the standards to be applied, the extent of includes also the three basic safety functions their application and any additional safety mentioned, which can be summarized as 1. measures that may need to be taken are shutting down the reactor, 2. cooling, in required to be proposed by the operating particular the reactor core, and 3. confining organization and to be subject to approval radioactive material. These are usually by the regulatory body.” [8] fulfilled by incorporating into the design an appropriate combination of inherent and The IAEA Safety Requirements [8] intends passive safety features, safety systems and to establish requirements for all important engineered safety features, and by applying areas of the safety of research reactors. In administrative procedures over the lifetime addition to requirements for design and of the reactor (e.g. the appropriate choice of operation, it also includes requirements on materials and geometries to provide prompt regulatory control, management, verification negative coefficients of reactivity). Even of safety, quality assurance and site though NTER firing times are planned to be evaluation. While some of these relatively short in time, these requirements requirements are the same as or similar to will still likely shape some of the designs of those for nuclear power reactors, they are the ground testing as well as possibly the applied in accordance with the potential reactor design itself. hazards associated with the reactor by means of a graded approach.

Page 14 C. DUJARRIC, A. SANTOVINCENZO, L. SUMMERER: THE NTER: A PROPOSED INNOVATIVE PROPULSION CONCEPT FOR MANNED INTERPLANETARY MISSIONS

Conclusions [5] United Nations, “Principles Relevant to the Use of Nuclear Power Sources Human far space exploration constitutes a in Outer Space,” United Nations challenge, which is commensurate only with General Assembly Resolution, vol. global cooperation. The Nuclear Thermal 47/68, Dec. 1992. Electric proposed in this perspective may potentially be a technical [6] Safety Framework for Nuclear Power enabler for a manned exploration mission to Source Applications in Outer Space, Mars as well as other farther exploration United Nations Committee on the missions Peaceful Uses of Outer Space and IAEA, 2009. This propulsion concept is offered to be [7] L. Summerer and K. Stephenson. deeper investigated at worldwide Agencies Nuclear power sources: a key level in order to assess its benefits as enabling technology for planetary compared to other propulsion options. exploration. Proceedings of the Institution of Mechanical Engineers, Worldwide know-how and technologies will Part G: Journal of Aerospace be needed to contribute to the development Engineering, 225(2):129–143, 2011. of a manned interplanetary propulsion system, but except for the nuclear core [8] Safety of Research Reactors - Safety itself, many of the technologies relevant to Requirements, IAEA Safety Series the NTER concept are readily available in No. NS-R-4, July 28, 2005 Europe.

References:

[1] C. Dujarric, “An innovative hybrid rocket propulsion concept for take-off from planets and interplanetary missions,” Amsterdam: IAF, 1999. [2] C. Dujarric, “A new propulsion concept for interplanetary missions,” ESA Bulletin, vol. 108, 2001, p. 66– 71. [3] C. Dujarric, A. Santovincenzo “European Patent Application number 10290333.3 filed on 21/06/2010,” . [4] IAEA, Fundamental Safety Principles, Vienna, Austria: IAEA, 2006.

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