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 238Pu 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. 238Pu production proceeds via neutron irradiation of 237Np, which is created as a by-product in nuclear fission reactors. Neutron irradiation of 237Np to produce 238Pu is conceptually very simple, but the production of sizeable quantities of 238Pu with acceptable isotopic purity, and the further separation and processing stages pose formidable technological challenges. 238Pu is a highly radiotoxic alpha emitter with an elevated specific activity and high decay energy with high risk for the workers, and its precursor, 237Np, is a radiotoxic alpha emitter whose daughter 233Pa decays to 233U with a strong gamma emission. Furthermore, the very rich and weakly explored chemistry of Np is also a challenge. The historical 238Pu inventory of US-DOE has been highly consumed and supplies of available 238Pu to support new missions have diminished. NASA and US-DOE have started an extensive project to re-establish 238Pu production for US space missions. In Europe, ESA has also shown some interest in an European production of 238Pu for the European space missions. In the present review, the manufacture of Np targets for 238Pu 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 NpO2–Al CERMET and zircaloy-clad full-ceramic NpO2 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 238Pu 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.