Production and Characterization of ZrN and PuN Materials for Nuclear Fuel Applications

Abstract: At the heart of a nuclear reactor resides its fuel. The chemical composition of the nuclear fuel affects the performance of several key properties important for the overall operation and safety performance of the reactor. The energy-producing reactors presently operational in the world utilize oxide-based nuclear fuel, which has been the dominating fuel form throughout the history of commercial nuclear power. The concept of nitride-based nuclear fuel is not new and development in the field, together with studies on actinide nitride properties, has been conducted for 50-60 years, although in limited scale compared to oxides. Nitride-based nuclear fuel possess properties making it an interesting fuel form in light water reactors as well as liquid metal cooled reactors envisioned for fourth generation nuclear power systems. In generation four reactor systems nitride-based fuel is an interesting fuel option both in UN-based liquid metal cooled breeding reactors as well as in reactors dedicated to the burning of transuranic actinides present in the used fuel of current light water reactors. This thesis focuses on the studies of inert matrix fuels mainly for non-breeding reactor purposes. In this work preparation and characterization of nitride fuel matrices of ZrN, ZrPuN and PuN have been studied. The internal gelation method has been applied for formation of oxide microsphere precursor material and carbothermal reduction has been applied for nitride production. Nitride fuel pellet purity and final density have been studied through cold pressing and sintering as well as ZrCN pellet formation from microspheres through spark plasma sintering. Investigations of nitride purity estimations through powder X-ray diffraction data has been performed. Am losses during the production route of PuN pellets by both internal gelation and powder synthesis has been investigated. Production of reasonably pure PuN through carbothermal reduction is possible and purities up to PuN0.99C0.01 have been reached. Carbide formed in ZrN was more stable and in the Zr system nitride purities of about ZrN0.85C0.15 were achieved. Conventional pressing and sintering of ZrCN microspheres did not reach densities above about 50% of theoretical density while spark plasma sintering of ZrCN of similar purity reached about 85% TD. PuN powder could be conventionally pressed and sintered to almost 80% of TD. It has been shown that ZrCN materials can be accurately composition estimated by XRD data while quaternary (Zr,Pu)(C,N) material estimations overestimated nitride purity somewhat. During PuN synthesis about 4% Am was lost during carbothermal reduction. Sintering in N2 atmosphere resulted in total losses of about 11% Am while sintering in Ar atmosphere yielded losses of about 50% Am.