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Serial | Major Trends in Energy Policy and Nuclear Power Research in Support of European Radio 198 isotope Power System Development at the European Commission’s Joint Research Centre in Karlsruhe Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings

1 Introduction The urge to discover the unknown, to explore the unexplored and to broaden our knowledge beyond the limits of the present is inherent to human nature. One of the most interesting and fascinating fields of science is the exploration of the cosmos, either from Earth using telescopes or by sending automated probes to other planets and into the vastness of space.

scientific instruments on the Moon enables long-lasting missions [20]. (Figure 1), and they were used for Unfortunately, there is a global many of the most famous and exiting shortage of this isotope, the efforts exploratory endeavours, such as associated with its production are the Pioneer missions to Saturn and high [21] and there are currently no Jupiter [9,10], the Viking missions to facilities for its synthesis in Europe. Mars [11], and the Voyager 1 & 2 An alternative is the americium spacecraft, which travelled beyond isotope Am-241, which is more easily the boundaries of our solar system available, since it is produced through and are still delivering scientific decay from Pu-241 and can be results from the interstellar medium, extracted isotopically pure from more than 40 years after their launch existing stocks of civil plutonium in [12,13]. More recently, the radio France or the United Kingdom via isotope-powered missions Galileo, chemical extraction [22]. Therefore, Ulysses and Cassini-Huygens were ESA has decided to study the use of exploring Jupiter, the Sun and Am-241 for its RPS development Saturn/Titan, respectively. These [5,16,17]. However, Am-241 has some more recent missions were performed disadvantages compared to Pu-238, in collaboration between the National such as a lower power density of | Fig. 1. Aeronautics and Space Administra- 0.114 W/g and slightly higher radia- Apollo astronaut photo of a SNAP-27 RTG on the Moon. Photo: NASA. tion (NASA) and the European Space tion levels. In addition, the oxide Agency (ESA) [14]. Historically, this shows chemical instability at high A basic requirement to operate auto- was the only way for Europe to gain temperatures, the experience with mated spacecraft successfully over the access to space nuclear power systems. Am241 is limited and additional course of an exploratory mission, is In 2005 a European Working Group research and safety assessment is the reliable supply of long-lasting on Nuclear Power Sources for Space needed before it can be used for space power. If independence of solar radia- identified RPS as a “key enabling applications. tion is required, e.g. when travelling technology for future European activi- Within the ESA research pro- SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER into deep space or to the dark side of ties in space” [15], and suggested the gramme, the UK’s National Nuclear planetary bodies, nuclear energy establishment of an European safety Laboratory (NNL) is exploring the becomes advantageous compared to framework for space nuclear power cost effective production of Am-241 other potential sources of energy. The sources and the development of and the University of Leicester (UoL) utilisation of nuclear power for appli- the technical capabilities to perform is developing a European Radio cations in space had been considered nuclear powered missions indendently isotope Heater Unit (RHU) and Radio- since the early beginnings of space [16,17]. As a consequence, a research isotope Thermoelectric Generator flight in the late 1940s, and the first and development programme was (RTG) [16,23,24]. To support these Radioisotope Power System (RPS) in launched by ESA for the production of efforts, the European Commission’s space was already launched by the European RPS to satisfy thermal Joint Research Centre (JRC) in Karls- U.S. Navy in 1961, onboard the Transit management and electrical power ruhe is investigating methods to 4A navigational satellite [1,2]. Since needs for spacecraft [16,17,18,19]. stabilize americium in the oxide then RPS have enabled some of the In the past, most RPS for space form and to establish a safe and most spectacular missions in the missions were based on the plutonium reliable pelletizing process [20,25]. history of space exploration [1-7], isotope Pu-238 [1-5], a radionuclide In collaboration with UoL, safety mostly performed by the USA but also which is superior to other isotopes, relevant properties and behaviour of by the former Soviet Union, China and because of its high specific power of Americium oxide are assessed under Europe (via cooperation with the 0.567 W/g, low radiation, compa representative conditions for storage USA). RPS were used on satellites for tibility with cladding materials and on Earth, operations in space as navigation, meteorology and commu- chemical stability as oxide. Pu-238 well as hypothetical accident and nication [8], they have powered has a half-life of 87.7 years which post-accident environments [16,26],

Serial | Major Trends in Energy Policy and Nuclear Power Research in Support of European Radioisotope Power System Development at the European Commission’s Joint Research Centre in Karlsruhe ı Daniel Freis, Jean-François Vigier, Karin Popa and Rudy J.M. Konings atw Vol. 65 (2020) | Issue 4 ı April

and the compatibility with the clad- used when onboard power is needed Energy source Energy density, MJ/kg ding material is tested. Complementa- for no more than a few weeks, or as 2 H + O 13.33 ry work is performed to develop a rechargeable energy buffer to supply 2 2 199 qualified welding methodology of the peak loads or to bridge periods with- N2H4* + O2 9.75

safety encapsulation. out sunlight. For missions, which 2 Li + O2 12.2 require continuous power supply for Li-ion battery 0.9 2 Energy Supply in Space an extended time, only solar or In order to operate spacecraft, a nuclear energy sources are feasible, Fission of U-235** 8.2 · 106 reliable source of power in the form of since the payload associated with Decay of Pu-238*** 3.3 · 105 electricity and heat is required. chemical fuels or batteries would Decay of Am-241*** 7.1 · 104 Electricity is needed to power onboard simply become too high. electronic systems such as navigation Solar energy is principally un | Tab. 1. and manoeuvring systems, onboard limited, as long as the solar cells, used Energy densities of typical energy sources. computers, lighting, robotics, scien to convert the radiation energy into *Hydrazine, **total fission, ***over 20 years mission time tific instruments and communication electricity, are not degrading, and as systems. In some cases, spacecraft are long as they can be adjusted in the controlled fission of fissile isotopes, equipped with electric propulsion direction of the sunlight and the such as U235 or Pu-239, and Radio systems, and electricity for life support spacecraft is not in the shadow of isotope Power Sources or Systems is needed if the mission is manned. planetary bodies or too far away from (RPS), which obtain their energy from Heat is needed in cold environments the sun. However, their effective area, the spontaneous decay of radioactive to keep sensitive spacecraft com the conversion efficiency and the isotopes. Both types can generate ponents at minimum operational or intensity of the solar radiation deter- heat for temperature control and/or survival temperature, e.g. during mine the power density of solar cells. electricity via additional energy lunar nights. This intensity decreases inversely with conversion systems. Primary energy sources can be the square of the distance from the While nuclear reactors are chemical, solar or nuclear. Some of sun, as shown in Figure 2, and if the generally suited for applications, these are limited with respect to their distance becomes too large solar which need significant power levels power or energy density, and the energy becomes unpractical. For above 10 kW, RPS are employed profile of each individual mission example, while the solar constant is whenever a limited amount of solar- determines which energy sources can 1.367 kW/m2 at the semi-major axis of independent power, up to 5 kW, is be utilized. Table 1 shows typical Earth, at one Astronomical Unit (AU) needed for a longer time period. RPS energy densities for different energy distance to the sun, it decreases are compact, long-lived, reliable, sources. to only 51 W/m2 or 3.7 % at the semi- robust, radiation resistant, solar- Chemical energy sources in the major axis of Jupiter (5.2 AU). There- independent, maintenance-free, and form of solid or liquid fuels can release fore, solar arrays are not an option for they have energy densities, which are a large quantity of energy in a very all research missions to the outer solar several orders of magnitude above short time, but they are limited with system, but also not if a system is to be chemical power sources (Table 1) respect to total energy density. For in- operated during long periods of dark- [1,2,3]. Figure 3 shows qualitatively stance, chemical fuels for propulsion ness, for example during the lunar the different regimes of power levels can be used whenever high thrust is nights, which last 14 days. and durations, where different energy needed for a short time, e.g. as rocket Nuclear energy has the highest sources are applicable. fuel to overcome the gravity field of power densities of all possible on- Space power systems, which are earth, but they have shortcomings if board energy sources, and can deliver based on the decay heat of radio long-lasting power or long accelera- reliable power over very long time isotopes, can be distinguished into tion times are needed, e.g. to reach periods. Most importantly, it is inde- systems which make direct use of the high velocities necessary for inter- pendent of sunlight. There are two the thermal energy, and systems SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER planetary or even interstellar travel. types of space nuclear power systems; which convert heat into electricity Chemical energy in the form of Reactor power systems (small nuclear (Figure 4). In both cases the em- batteries or fuel for fuel cells can be reactors), which generate power by ployed radioisotope is the power

| Fig. 2. | Fig. 3. Intensity of solar radiation. Application of different energy sources (reproduced from references 1 and 3).

| Fig. 4. | Fig. 5. Elements of radioisotope power systems. Thermoelectric circuit.

source while the heat sink is provided thermoelectric couple or thermo by space. Systems which make direct couple. In addition, static heat con use of the thermal energy are called version is also possible by thermionic Radioisotope Heater Units (RHU). conversion, where a flow of electrons They provide heat to the space craft is induced from a hot to a cool surface to keep sensitive electronics warm via thermionic emission. without using heavy and complicated Thermoelectric conversion is not heat distribution systems, and with- very efficient and practical RTG out creating electromagnetic inter systems using SiGe or PbTe/TAGS ference. thermocouples show typical power Heat-to-electricity conversion sys- conversion efficiencies of 6 % to 7 % | Fig. 6. tems can be classified into dynamic [5]. To improve efficiency, develop- Radioisotope power source. and static systems. The dynamic ment of high temperature thermo systems employ moving parts and use electric materials including skutter highly refractory noble metal like a thermodynamic cycle to convert udites and Zintl-based systems is iridium or platinum-rhodium alloys heat into electricity, e.g. a Stirling ongoing [28,29]. However, if operated (Figure 6), which can withstand the engine [27], and show principally the at low temperatures or under a protec- most extreme conditions (e.g. launch highest conversion efficiencies. The tive gas cover, existing RTG are very pad explosion, Earth re-entry acci- earliest efforts were focussed on the reliable, show low degradation and dents). The cladding is surrounded by development of dynamic conversion can provide power over many decades. thermal insulation, usually made of systems, and the first RPS SNAP-1 Because of the importance of reliable pyrolytic graphite, which shall protect (SNAP stands for Systems for Nuclear and maintenance-free systems for the cladding from reaching peak Auxiliary Power) in 1959 was based automated space probes, relatively temperatures during aerodynamic on a Ce-144 powered mercury low conversion efficiencies are usually heating. Finally, the thermal insula- Rankine cycle [1]. But despite their accepted. tion is surrounded by an aeroshell high conversion efficiency, dynamic The most important design made of carbon-carbon composite conversion systems have not yet criterion for any space nuclear power (fine-weaved pierced fabrice), which reached the reliability required for the system, including all forms of RPS, is provides additional protection from operation of a space probe, which safety. If an RPS shall be employed on postulated launch vehicle explosions might be on a mission for several a space application, it must comply or against impacts on hard surfaces at SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER decades without the possibility for with resolution 47/68 of the United terminal velocity [1,5,14]. maintenance or repair, and no Nations General Assembly on the dynamic conversion system was ever Principles Relevant to the Use of 3 Radioisotopes for RPS used in space. Nuclear Power Sources in Outer The selection of suitable radioisotopes The static heat-to-electricity con- Space. The resolution defines that the for application in space RPS is based version systems are called Radio use of RPS shall be restricted to those on a number of criteria; among them isotope Thermal Generators (RTG). space missions, which cannot be are a long half-life, high isotopic power RTG have no moving parts and use the performed by non-nuclear energy and a low level of penetrating radia- thermoelectric principle, also known sources in a reasonable way. It also tion. In addition, a chemically stable as the Seebeck effect, to generate states that the design and use of the compound with high density should electricity. This phenomenon was RPS shall ensure that the hazards exist, which can serve as stable host for discovered in 1794 by the Italian during operation and foreseeable the decay products and is compatible scientist Alessandro Volta and, inde- accidents are kept below acceptance to the encapsulation materials and the pendently, in 1821 by the German levels and that radioactive material potential operating or post-accident physicist Thomas Johann Seebeck. does not cause a significant contami- environments. Furthermore, the com- If two dissimilar materials are con- nation of the biosphere and outer pound should resist high temperatures nected in a closed circuit and a space [30]. and should not disperse into inhalable temperature difference is applied over In order to comply with these small particles, in case of an accident. the two junctions, a voltage can be requirements, RPS are designed to the A low solubility in the environment measured and electricity is generated meet highest safety standards. The (water) and the human body are also (Figure 5). Such a device is called a fuel is encapsulated in a cladding of a advantageous [4,5].

Isotope Half-life (y) Isotopic power (W/g) Principal decay mechanism Comment Shielding (typically)

Am-241 432.8 0.114 alpha Soft gamma radiation 2 mm lead equivalent

Cs-137 30.04 0.417 beta Gamma emitter Heavy 201 Ce-144 285 2.08 beta Gamma emitter Heavy Cm-242 0.45 122 alpha Strong neutron emitter Heavy Cm-244 18 2.84 alpha Strong neutron emitter Heavy Po-208 2.93 18.1 alpha Short half-life None Po-210 0.38 144 alpha Short half-life None Pu-238 87.7 0.567 alpha Very soft gamma radiation None Sr-90 28.79 0.907 beta Bremsstrahlung Significant

| Tab. 2. Radioisotopes for space applications [31,32].

Since the specific power correlates which is artificially created in nuclear yield of circa 13 400 n/(s g) in pluto inversely to the half-life, a compro- reactors. It was the first isotope of nium oxide [34]. The (α-n) reactions mise has to be found between specific plutonium, which was discovered by can only occur in the low abundancy weight and volume of the heat source Glenn T. Seaborg in 1940 [33], and it isotopes O-17 and O-18, while the on one side, and a long-lasting stable is the most important and most widely threshold energy of O-16 is too high power output during the required mis- adopted radioisotope for space power (15.2 MeV) [34]. The neutron yield in sion time on the other side. Therefore, applications, due to its high power pure Pu-238 oxide can be reduced to the selection of a suitable radioisotope density, long half-life, chemical sta circa 2700 n/s-g [5] if the oxygen is always depends on the concrete bility as oxide, and its low neutron and depleted in O-17 and O-18 by 98 % application. Alpha emitters tend to be soft gamma emissions. So far, Pu-238 [21]. better suited than beta emitters are, is the only radioisotope used for RPS Metallic plutonium has a density because the alpha decay energy is in space by the USA and China, while of 19.77 g/cm3 (Pu-238, α-phase at typically in the range between 5 MeV the former Soviet Union also utilized room temperature and atmospheric and 6 MeV per decay event, for Po-210. pressure), which is the highest density example compared to 0.546 MeV for Pu-238 has a half-life of 87.7 years form and would allow a volumetric the beta decay of Sr-90, and the alpha and an isotopic power of 0.567 W/g. power density of up to 11.21 W/cm3. particles do not generate Brems The isotope decays primarily via alpha However, the metal shows six different strahlung when stopped in the sur- decay to U-234, with a decay energy structural modifications at ambient rounding matter. of 5.59 MeV. The main radiation emis- pressure in the temperature range Other important criteria are the sions of Pu-238 are alpha particles from 0 K (-273.15 °C) to 640 °C (melt- availability of the selected isotope, as with an average energy of 5.49 MeV. ing point), and the phase transitions well as the production costs and In addition, the isotope emits soft cause significant dimensional changes necessary infrastructure and effort gamma radiation (main energies: as well as alterations of the mechani- to process the radioactive material. 43.5 keV & 99.85 keV) with low emis- cal and thermal properties (Figure 7). Table 2 gives an overview over the sion probability, and Auger electrons, In addition, plutonium metal is burn- most common radioisotopes for space which in turn generate X-rays at able, and the powder is extremely RPS, of which Pu-238 is by far the 17.11 keV and 13.61 keV (main lines), pyrophoric. The first RTGs in the most significant. If not mentioned also with relatively low emission 1960s were still fuelled with metallic otherwise, all nuclear data were taken probabilities. For a sintered and en- Pu-238, and designed to burn-up into from the JEFF-3.1 nuclear data library capsulated pellet, the self-shielding fine particles below 30 nm and- dis SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER [31] via Nucleonica.com [32]. effect and encapsulation are more perse into the atmosphere in case of than sufficient to shield these radia- an re-entry accident1. This safety 4 Properties of Plutonium- tions. Spontaneous fission occurs with philosophy had to prove itself, when 238 & Americium-241 a negligible probability of 1.86E-09, the Transit satellite 5BN-3 failed The development of European RPS is and the spontaneous fission reaction to achieve orbit in 1964, and the based on the strategic decision of ESA creates a neutron yield of circa SNAP-9A RTG re-entered the atmo to utilize Am-241 and to take advan- 2300 n/(s g) (oxide). However, alpha- sphere, carrying about 1 kg Pu-238 tage of the existing nuclear infrastruc- neutron (α-n) reactions in natural metal. As predicted, the metallic ture related to the civil reprocessing of oxygen cause an additional neutron fuel completely burned-up and was spent nuclear fuel in Europe, rather

than to establish an expensive pro Pu-Metal PuO2 Am-Metal AmO2 duction capability for Pu-238 [16,17]. Half-life: 87.7 y 432.8 y In order to understand the impact of this choice on the RPS design charac- Density: 19.85 g/cm3 11.46 g/cm3 13.67 g/cm3 11.68 g/cm3 teristics and the production process, a 0.500 W/g 0.101 W/g Power density: 0.567 W/g 0.114 W/g comparison of both isotopes has to be (0.418 W/g)* (0.088 W/g)** made. Table 3 summarizes the most Thermal conductivity: 6 – 16 W/m K35 3 – 10 W/m K37 10 W/m K ~ 2.0 W/m K38*** important properties of Am-241 com- Melting point: 640 °C 2744 °C36 1176 °C35 2113 °C42 pared to Pu-238. Pu-238 is an isotope of the chemi- | Tab. 3. cal element plutonium, an actinide Properties of Pu-238 and Am-241 (Metal and Oxide). *Typical isotopic composition, **Stabilized with 12 % U, ***Sub-stoichiometric

good host for U-234, the decay pro emits additional neutrons from the duct of Pu-238, which is also crystal alpha-neutron (α-n) reaction in natu- lizing in the fcc structure. ral oxygen. This reaction causes an 202 Am-241 is an isotope of the radio- additional neutron yield of circa active element americium. It belongs 2700 n/(s g)34. While this is signifi- also to the actinides and has the cantly lower compared to Pu-238 by a atomic number 95. Like plutonium, factor of six, it has to be considered americium is an artificial element and that for the same thermal power about was discovered by the group of Glenn five times more Am-241 is needed. T. Seaborg in 1944. Americium is Therefore, also in the case of Am-241 usually created in nuclear reactors oxide O-17 and O-18 depletion is by neutron capture and radioactive needed to reduce the neutron yield to decay (Figure 8). However, during acceptable levels [5]. irradiation not only Am-241 but also Metallic americium has similar other americium isotopes are created, disadvantages as metallic plutonium | Fig. 7. which are unwanted in RPS. Isotopi- for the use in RPS. In addition, its Thermal conductivity of metallic plutonium structural modifications[35] . cally pure Am-241 is produced con density is relatively low compared to tinuously in the stocks of civil plutoni- the oxide (13.67 g/cm3 α-phase at um through beta decay of Pu-241 room temperature). Elemental ameri- (t1/2=14.33 y), from where it can be cium is a soft metal, which oxidises separated using chemical methods. quickly in the atmosphere forming A cost efficient separation and purifi- a protective oxide layer. At room cation process (AMPEX) was devel- temperature americium forms a oped and demonstrated by the UK’s stable hexagonal α-phase (space National Nuclear Laboratory (NNL) group P63/mmc) [35], at 769 °C it to separate ingrown Am-241 from changes into the cubic β-phase (space plutonium [22,39]. group Fm3¯m), and at 1077 °C it Am-241 has a half-life of 432.8 converts to the γ-phase showing a years and an isotopic power of body-centered cubic structure [35]. 0.114 W/g. The isotope decays The phase transitions cause dimen- primarily via alpha decay to Np-237. sional changes, however not as signifi- In a final repository Am-241 is one of cant as in the case of plutonium. The the most important drivers for the melting point of metallic americium is medium-term heat load, and there- at 1176 °C. Americium metal powder fore the required gallery space, and is also very pyrophoric and metallic 6 | Fig. 8. Np-237 (t1/2=2.14∙10 y) is one of the americium would definitively burn-up Production of Am-241 by neutron capture and significant isotopes driving long-term in case of an uncontained re-entry β-decay. radiotoxicity [40,41]. The main radia- accident. tion of Am-241 are alpha particles The preferred form of Am-241 for dispersed into the atmosphere where with an average energy of 5.47 MeV. space applications is americium oxide, it was diluted to low concentrations Unlike Pu-238, Am-241 emits signifi- a ceramic material with a density of and did not cause any unacceptable cant gamma radiation at 59.54 keV 11.68 g/cm3 which can be sintered health hazard [1,8,14]. (main line) with a probability of 36 % into pellets [6,16,20,25]. However, After the Transit 5BN-3 accident and also Auger electrons, which cause there are two major compounds which and with the upcoming of larger RPS X-rays at 14.44 keV (probability are relevant for applications in space, with higher radioactive inventory the 33 %). Even though the radiation is the cubic dioxide (AmO ) ( -phase, SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER 2 α dispersion approach was no longer still relatively soft, shielding and space group Fm3¯m) and the accepted and a new fuel form, which remote handling tools are required if sesquioxide (Am2O3), which exists in would stay intact at re-entry was larger amounts are to be processed, the hexagonal form (Aphase, space needed. Since then, the preferred and especially when the material is in group P3¯m1) and the cubic form chemical form of Pu-238 for RPS is solution and self-shielding is not effec- (C-phase, space group Ia3¯). Unfor plutonium dioxide (PuO2), safely tive. Once the pellets have been tunately, the americium-oxygen sys- encapsulated in a cladding of high sintered and encapsulated the radia- tem shows a complex behaviour. The refractory material which can safely tion decreases. However, it remains melting point of AmO2 is at 2113 °C contain the radioactivity under all still significant and adequate radia- [42], but at high temperatures and credible circumstances [8]. PuO2 is a tion protection measures for workers in the vacuum of space americium ceramic material with a high melting need to be in place. A dose rate of dioxide loses oxygen. Due to this point of 2744 °C [36], which can be circa 150 µSv/h in 10 cm distance is process, it transforms into sub- sintered into stable pellets. The com- estimated for a typical full scale RHU stoichiometric AmO2-x, thereby slowly pound crystallizes in the face centred containing 26 g AmO2 encapsulated in increasing its volume, until it finally cubic (fcc) structure (space group 1.8 mm Pt30Rh. converts into the hexagonal sesqui Fm3¯m) [37], has a high chemical Am-241 has a very low probability oxide [37,43,44]. This process causes stability, a low solubility in water for spontaneous fission of 4.3E-12, significant dimensional changes and and does not react with the typical and the spontaneous fission reaction can lead to decomposition of the pel- cladding materials, such as iridium or creates a negligible neutron yield of lets [25,45]. In addition, the release of platinum alloys (e.g. Pt20Rh or circa 1.2 n/s-g (oxide). However, as in oxygen can cause pressure build-up Pt30Rh). In addition, it serves as a the case of Pu-238, Am-241 oxide and potentially corrode surrounding

structures. On the other higher, methods to stabilize americium in the actinide containing samples from the americium sesquioxide is prone to oxide form and to establish a safe and base material to the fully encapsu oxidation at lower temperatures and reliable pelletizing process [20,25]. lated sample, and the MA-Lab repre- will slowly convert into the dioxide in In collaboration with UoL, safety sents an ideal infrastructure for prepa- 203 air, again undergoing significant relevant properties and behaviour ration of highly radiating americium structural modifications. After several of americium oxide are assessed pellets and fully qualified fuel pins. weeks of storage, even under low [16,26]. The synthesis of the base material oxygen partial pressures, this effect, Am-241 emits a significant amount (powder) is performed in the glove in combination with self-irradiation of gamma radiation, and working box named “infiltration”. The process from α-decay, will cause Am2O3 with Am-241 can result in high dose is dust-free, based on the so-called gel pellets to disintegrate into black rates for the operating personnel. supported precipitation [47] and the dioxide powder. Both phenomena, the Remote operated and shielded porous bead infiltration technique reduction of AmO2 at elevated tem- equipment is advantageous or even [48]. This process is highly flexible peratures and the oxidation of Am2O3 necessary in order to prepare and easily adapted to the require- at low temperatures, are problematic americium-based pellets for RPS. The ments and specifications of new with respect to the integrity of the fuel JRC in Karlsruhe has a unique infra- sample compositions. The next glove pellets and can cause increased dis- structure for handling of highly box contains a calcination furnace and persion of radioactive material in case radiative actinide materials, the so- other equipment for powder prepara- of certain accident scenarios [25]. called Minor Actinide Laboratory tion. The prepared powders are dust (MA-Lab) [46]. It is of high relevance free, the individual beads typically 5 The Minor Actinide Labo- for safety research on fuels for trans- show heterogeneous size distributions ratory at the Joint Re- mutation in Europe, as it is one of the between 30 µm to 120 µm and are search Centre Karlsruhe only dedicated facilities for the ideal for pressing pellets. The ready to The Joint Research Centre is the synthesis of minor actinide containing press powder can be transferred via European Commission’s science and materials, either for property an automated channel to the next knowledge service. Its mission is to measurements or for the preparation glove box, where it can be pressed to support EU policies with independent of irradiation experiments. pellets. After sintering in reducing evidence throughout the whole policy The MA-lab consists of seven or oxidizing atmosphere (glove box cycle. Its work has a direct impact on glove-boxes with protection walls “Sintering”) the pellets are fully the lives of citizens by contributing forming two separate chains. A characterized and inserted into a with its research outcomes to a schematic lay-out of the Ma-Lab is cladding. Finally, pin welding and healthy and safe environment, secure shown in Figure 9. The glove boxes non-destructive weld examination are energy supplies, sustainable mobility are shielded by 50 cm neutron performed in the two last, alpha free and consumer health and safety. The shielding and 5 cm of lead. Based on glove boxes. JRC hosts specialist laboratories and the thickness of the water and lead unique research facilities and is home wall, the mass limits have been 6 Stabilisation of to thousands of scientists working to calculated to 150 g of Am-241 or 5 g of Americium Oxide & support EU policy (https://ec.europa. Cm-244. The glove boxes can be RHU-Size Prototype Pellet eu/jrc/en). accessed manually from the back if Production The JRC in Karlsruhe belongs to radiation levels are low enough to Unlike plutonium oxide, which is the Directorate for Nuclear Safety and perform experiment preparation or stable in a broad range of tem Security (Directorate G), where JRC’s maintenance. In addition, tele-mani peratures and oxygen potentials, nuclear work programme, funded by pulators and remote operated auto- americium oxide is prone to phase the EURATOM Research and Training mated equipment can be used for changes and disintegration in chang- Programme, is carried out. The operation at high dose rates. ing environments [25]. If americium Directorate contributes to the scien The glove boxes of the minor oxide is sintered under oxidizing SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER tific foundation for the protection of actinide laboratory are configured as conditions into AmO2, it releases the European citizen against risks complete preparation chain for minor oxygen at elevated temperatures in associated with the handling and storage of highly radioactive material, and scientific and technical support for the conception, development, implementation and monitoring of community policies related to nuclear energy. Research and policy support activities of Directorate G contribute towards achieving effective safety and safeguards systems for the nuclear fuel cycle, to enhance nuclear security then contributing to achieving the goal of low carbon energy production. The JRC supports the ESA research programme on developing a European Radioisotope Heater Unit (RHU) and Radioisotope Thermo electric Generator (RTG). These acti | Fig. 9. vities are focussed on investigating Minor Actinide Laboratory at JRC-Karlsruhe [46].

was observed, only low crystallographic swelling occurred due to self-irradiation, which saturated after 204 circa 60 days. Overall the pellet showed good long-term structural and dimensional stability under self- irradiation conditions (Figure 11) [25].

7 Development of Welding Methodology In order to be able to meet the launch safety requirements and safely ship | Fig. 10. RPS sealed sources to sites, where they Small (left side) and large (right side) scale prototype pellet. can be assembled into RHUs or RTGs and subsequently be installed onto a spacecraft, it is necessary to develop containment technologies that meet these requirements. The first layer of containment immediately surrounding the fuel pellets is the cladding, which must ensure the enclosure of radio activity during storage, normal operation and accident scenarios. The encapsulation has to be performed in a nuclear installation, ideally the manu- facturing site, and it has to be ensured that the fueled clads are free of external contamination. In order to test the feasibility of our welding equipment and to gain experience in the welding of Pt30Rh capsules, two types of Pt30Rh-en capsulation were constructed and | Fig. 11. welding tests were performed. The Crystallographic swelling of (Am0.80U0.12Np0.06Pu0.02)O1.8 mixed oxide under alpha self-irradiation. first capsule design had similar Values for PuO2 and AmO2 swelling are reported for comparison [25]. dimensions as the US LWRHU (Figure 12) and the second capsule design the vacuum of space and changes into reducing atmospheres. A solution was was made according to input by UoL sesquioxide (Am2O3), thereby under- found by inserting 12 % of uranium to host (Am,U)O2 pellets of 15 mm going strong structural reorgani into the americium oxide (in addition diameter and 20 mm height. zation, density changes and dis to 6 % Np and 2 % Pu already present). The capsules were welded using integration. If americium oxide is Thereby, it was possible to stabilize established Tungsten Inert Gas sintered under reductive conditions the cubic phase and to sinter a number welding equipment, which is also into Am2O3, it will transform into the of discs and pellets, including a used in the frame of qualified welding dioxide under accident conditions, prototype pellet in the dimensions of of fuel rodlets for irradiation experi- SERIAL | MAJORSERIAL POLICY IN ENERGY TRENDS AND NUCLEAR POWER but also under the influence of self- the US LWRHU [49] under mois ments [41]. Non-destructive as well as irradiation even at low oxygen poten- turized Ar/H2 atmosphere and a destructive weld examinations were tials, which leads to its total dis larger disc representative for a future performed and showed that good integration. European RHU [16] (Figure 10). welding results were achieved; JRC has investigated possibilities Due to the absence of phase indicating that future welding quality to stabilize americium dioxide in its changes, the stabilized material criteria can be met (Figure 13). cubic form under a broad range of showed a good sintering behaviour temperatures, in oxidizing as well as and it was possible to sinter a number 8 Conclusions of discs and pellets with good quality Radioisotope power sources are a key and without cracking. After sintering enabling technology for exploratory oxidation testing was performed and missions into deep space or to the dark the material proved stable up to side of planetary bodies, and the 1000 °C. This result represents a European Space Agency is sponsoring significant improvement with respect the development of Am-241 based to safety of the material against radio- radioisotope power systems. The active material dispersion in case of development of a new RPS based on accidental conditions. In addition, the americium is a challenging task. The macroscopic and crystallographic optimization of the fuel is a key issue, swelling was assessed on the small as the oxide of americium has signifi- | Fig. 12. scale protype pellet (Figure 10) over cantly different properties compared Small scale prototype Pt30Rh encapsulation. time. While no macroscopic swelling to that of Pu-238, both in terms of

| Fig. 13. Large scale prototype Pt30Rh encapsulation (left side) and dye penetrant test of weld (right side).

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