Neptunium mononitride as a target material for Pu-238 production

Deep space exploration requires specialized sources for both thermal and power applications. Radioactive decay heat of plutonium-238 (²³⁸Pu) provides these sources in the form of radioisotope thermoelectric generators (RTGs). The ²³⁸Pu is produced via neutron capture reaction involving neptunium-237 (²³⁷Np) target material. Continual optimization of ²³⁷Np target materials and evaluation of potential alternative targets for production of ²³⁸Pu RTGs are advantageous for meeting ongoing space power system resource requirements. Current production of ²³⁸Pu for RTGs for the United States space program utilizes neptunium dioxide (²³⁷NpO₂) targets; however, the use of neptunium mononitride (²³⁷NpN) presents an opportunity to increase the mass of ²³⁷Np per target compared to the dioxide form, as well as increase the thermal conductivity of the target. To assess the viability of a ²³⁷NpN target material, the material chemistry must be thoroughly evaluated, including synthesis methods and dissolution and reprocessing schemes. This review presents a summary of synthesis pathways for ²³⁷NpN based on published literature on actinide mononitrides. Specific literature on ²³⁷NpN is limited, necessitating evaluation of other actinide systems to gather parallels. This suggests a need for additional experimental studies on ²³⁷NpN. A particular limitation in the existing literature is a lack of information on the differences in material characteristics, such as morphology, particle size, and trace chemical impurities, as a function of synthesis method. These parameters may affect subsequent reactor performance or dissolution of irradiated targets. The evaluation of existing literature is presented with a focus on the efficacy of ²³⁷NpN targets for ²³⁸Pu production.