238Pu production: a State-of-the-Art review of NpO2 target fabrication technologies and processing of irradiated targets

Abstract Space mission beyond the solar system cannot rely on photovoltaic (PV) cells as primary power source, and combinations of PV cells and batteries. For such purpose, Radioisotope Power Systems powered with 238 Pu have served well for all flagship space exploration missions since the early Apollo missions until today’s Mars Exploration Program and continue to be the preferred primary energy source for future missions. 238 Pu production proceeds via neutron irradiation of 237 Np, which is created as a by-product in nuclear fission reactors. Neutron irradiation of 237 Np to produce 238 Pu is conceptually very simple, but the production of sizeable quantities of 238 Pu with acceptable isotopic purity, and the further separation and processing stages pose formidable technological challenges. 238 Pu is a highly radiotoxic alpha emitter with an elevated specific activity and high decay energy with high risk for the workers, and its precursor, 237 Np, is a radiotoxic alpha emitter whose daughter 233 Pa decays to 233 U with a strong gamma emission. Furthermore, the very rich and weakly explored chemistry of Np is also a challenge. The historical 238 Pu inventory of US-DOE has been highly consumed and supplies of available 238 Pu to support new missions have diminished. NASA and US-DOE have started an extensive project to re-establish 238 Pu production for US space missions. In Europe, ESA has also shown some interest in an European production of 238 Pu for the European space missions. In the present review, the manufacture of Np targets for 238 Pu production by irradiation, and the target processing are discussed and assessed from an European production perspective by comparing mainly US state-of-the-art with the European know-how and the current facilities. Two principal options for target fabrication stand out: aluminium-clad NpO 2 –Al CERMET and zircaloy-clad full-ceramic NpO 2 targets. The principal advantages of the well documented CERMET route can be found in the irradiation stage. These targets allow a high flexibility in heavy metal loading fraction and have high thermal performance. But, they have significant drawbacks in terms of the amount and type of nuclear waste that is generated at the back-end. Instead, the full-ceramic targets are conceptually very similar to conventional nuclear fuels, and would be preferred from back-end perspective, although a demonstration of large scale production remains yet to be done. Presently, there are no operational large scale Np target production, irradiation, processing or 238 Pu handling facilities in Europe, but the nuclear infrastructure exists and is operational to study each of the steps at least at small scale and all steps have similarities with well-established industrial capabilities in Europe.