Research in Support of European Radioisotope Power System Development at the European Commission’s Joint Research Centre

Radioisotope  Power  Systems (RPS) represent  a Key  Enabling  Technology  for  European  autonomy  in  space exploration.  The  European  Commission’s  Joint  Research Centre (JRC) is supporting the European Space Agency (ESA) in the development of a European RPS by performing research on Am-241 and Pu-238 based fuel forms. The research activities on  Am-241,  which  is  the  current  technology  basis  for  the  ESA programme,   are   based   on   three   pillars.   The   first   is   the optimization of the chemical stabilization of AmO2in its cubic form over a wide range of conditions. JRC research has shown that  this  can  be  achieved  by  addition  of  a  small  amount  of Uranium,  which  will  allow  the  pelletisation  of  AmO2with  a suitable  microstructure.   The   second   research   pillar   is   the determinationof  safety  relevant  thermophysical  properties  of stabilized  AmO2,  as  well  as  safety  testing  of  AmO2under relevant  operational  and  accidental  conditions,  with  emphasis on the pellet integrity and compatibility with the cladding. These activities are performed in close collaboration with ESA and its partner organizations, the National Nuclear Laboratory (NNL) and   the   University   of   Leicester,   both   UK.   Recently,   a Collaborative Research Agreement between ESA and JRC has been  concluded  to  streamline  the  common activities  and  to provide  a  framework  for  further  development  of  the  research agenda. The third pillar of JRC’s research on Am-241  based RPS  is  a  more  basic  research-oriented  approach  to  look  into other   compounds   of   Am,   and   to   perform   a   systematic assessment to potentially find alternative chemical forms other than the oxide, which should be stable and have a high specific Am  density.  So  far,  five  different  Am-compounds  have  been synthesized,    were    characterized    for    their    chemical    and thermophysical  properties,  and  were  tested  for  their  stability under  relevant  conditions,  including  accident  situations  and post-accident environments. In addition to the research on Am-241 based RPS, JRC has recently  partnered  the  H-EURATOM  collaborative  research project  PULSAR  on  the  establishment  of  a  European  supply chain  of  Pu-238  for  space  exploration.  In  the  frame  of  this project, JRC is investigating the synthesis of stable PuO2pellets with suitable microstructure that is targeted for Pu-238 sources. This work is complemented by an assessment of handling large quantities  of  Pu-238  with  high  specific  power  in  a  nuclear laboratory, and the development of a Laser welding technique to     perform     qualified     close-welds     of     Iridium     safety encapsulation. In this contribution, we will give an overview of the ongoing work  in  support  of  a  European  RPS  development  at  the  Joint  Research Centre in Karlsruhe, Germany, as well as an overview of recent research results and an outlook into future activities.</p

Optimization of Uranium-Doped Americium Oxide Synthesis for Space Application

Americium 241 is a potential alternative to plutonium 238 as an energy source for missions into deep space or to the dark side of planetary bodies. In order to use the ²⁴¹Am isotope for radioisotope thermoelectric generator or radioisotope heating unit (RHU) production, americium materials need to be developed. This study focuses on the stabilization of a cubic americium oxide phase using uranium as the dopant. After optimization of the material preparation, (Am_0.80U_0.12Np_0.06Pu_0.02)­O_1.8 has been successfully synthesized to prepare a 2.96 g pellet containing 2.13 g of ²⁴¹Am for fabrication of a small scale RHU prototype. Compared to the use of pure americium oxide, the use of uranium-doped americium oxide leads to a number of improvements from a material properties and safety point of view, such as good behavior under sintering conditions or under alpha self-irradiation. The mixed oxide is a good host for neptunium (i.e., the ²⁴¹Am daughter element), and it has improved safety against radioactive material dispersion in the case of accidental conditions

Thermal Properties and Behaviour of Am-Bearing Fuel in European Space Radioisotope Power Systems

The European Space Agency is funding the research and development of 241Am-bearing oxide-fuelled radioisotope power systems (RPSs) including radioisotope thermoelectric generators (RTGs) and European Large Heat Sources (ELHSs). The RPSs’ requirements include that the fuel’s maximum temperature, Tmax, must remain below its melting temperature. The current prospected fuel is (Am0.80U0.12Np0.06Pu0.02)O1.8. The fuel’s experimental heat capacity, Cp, is determined between 20 K and 1786 K based on direct low temperature heat capacity measurements and high temperature drop calorimetry measurements. The recommended high temperature equation is Cp(T/K) = 55.1189 + 3.46216 × 102 T − 4.58312 × 105 T−2 (valid up to 1786 K). The RTG/ELHS Tmax is estimated as a function of the fuel thermal conductivity, k, and the clad’s inner surface temperature, Ti cl, using a new analytical thermal model. Estimated bounds, based on conduction-only and radiation-only conditions between the fuel and clad, are established. Estimates for k (80–100% T.D.) are made using Cp, and estimates of thermal diffusivity and thermal expansion estimates of americium/uranium oxides. The lowest melting temperature of americium/uranium oxides is assumed. The lowest k estimates are assumed (80% T.D.). The highest estimated Tmax for a ‘standard operating’ RTG is 1120 K. A hypothetical scenario is investigated: an ELHS Ti cl = 1973K-the RPSs’ requirements’ maximum permitted temperature. Fuel melting will not occur