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

Triclinic–Cubic Phase Transition and Negative Expansion in the Actinide IV (Th, U, Np, Pu) Diphosphates

The <i>An</i>P₂O₇ diphosphates (<i>An</i> = Th, U, Np, Pu) have been synthesized by various routes depending on the stability of the <i>An</i>^IV cation and its suitability for the unusual octahedral environment. Synchrotron and X-ray diffraction, thermal analysis, Raman spectroscopy, and ³¹P nuclear magnetic resonance reveal them as a new family of diphosphates which probably includes the recently studied CeP₂O₇. Although they adopt at high temperature the same cubic archetypal cell as the other known MP₂O₇ diphosphates, they differ by a very faint triclinic distortion at room temperature that results from an ordering of the P₂O₇ units, as shown using high-resolution synchrotron diffraction for UP₂O₇. The uncommon triclinic–cubic phase transition is first order, and its temperature is very sensitive to the ionic radius of <i>An</i>^IV. The conflicting effects which control the thermal variations of the P–O–P angle are responsible for a strong expansion of the cell followed by a contraction at higher temperature. This inversion of expansion occurs at a temperature significantly higher than the phase transition, at variance with the parent compounds with smaller M^IV cations in which the two phenomena coincide. As shown by various approaches, the P–O_b–P linkage remains bent in the cubic form