A review of the reprocessability of uranium nitride based fuels

Abstract

This article presents a review of the behaviour of uranium nitride (UN) fuels during thermal reactor fuel-based reprocessing. UN is one of the leading candidate materials for use in Advanced Technology Fuels (ATFs) due to it having several superior properties over UO2 or MOx based fuels, including: good thermal conductivity; high melting point; low thermal expansion; high fissile density; and good compatibility with metallic or gaseous coolants currently under consideration for Generation IV reactors. However, it also has a number of drawbacks in fuel-based applications including expected higher fabrication costs and oxidative instability in water. The former is mainly due to the expected need to enrich the nitrogen component in 15N to increase the neutron economy and avoid formation of 14C from the n,p reaction of 14N. The latter property may be advantageous for UN's post-irradiation reprocessing but must be addressed if UN is to be deployed safely in the near term in existing and under-construction LWR reactors. One means by which UN's hydrolytic instability may be addressed is by the introduction of suitable protective dopants such as Al, Cr or Zr to greatly increase oxidative resistance. However, this may also impact on the reprocessability of spent UN post-irradiation. Thus, in this review, we will focus on the management and recycle options for UN fuels after irradiation. Whilst 15N enrichment is an important potential economic driver of the reprocessing of spent UN, it is not the primary concern of this review. Rather, we focus on issues that may arise during the dissolution and head-end treatment of UN fuels, and the subsequent expected behaviour of the dissolved fuel in existing and proposed solvent extraction processes. Where they exist, similarities with the reprocessing of thermal UO2 fuels will be highlighted, as will the effect of protective dopant materials on the chemistry of UN reprocessing.