Zirconia inert matrix for plutonium utilisation and minor actinides disposition in reactors

The radiotoxicity of the UO2 spent fuel is dominated by plutonium and minor actinides (MA): Np, Am and Cm, after decay of the short life fission products. Zirconia ceramics containing Pu and MA in the form of an inert matrix fuel (IMF) could be used to burn these actinides in light water reactors or in high temperature reactors. Optimisation of the fuel designs dictated by properties such as thermal, mechanical, chemical and physical must be performed with attention for their behaviour under irradiation. Zirconia must be stabilised by yttria to form a solid solution such as MAzYyPuxZr1−yO2−ζ where minor actinide oxides are also soluble. MA may act as a burnable poison reducing the reactivity at the beginning of life and yielding fertile nuclides improving the reactivity at the end of life. These zirconia cubic solid solutions are stable under heavy ion irradiation. The retention of fission products in zirconia, under similar thermodynamic conditions, is a priori stronger, compared to UO2, the lattice parameter being larger for UO2 than for (Y,Zr)O2−x. (Er,Y,Pu,Zr)O2−x in which Pu contains 5% Am was successfully irradiated in the Proteus reactor at PSI, in the HFR facility, Petten as well as in the Halden reactor. These tests support potential irradiations of such IMF in a commercial reactor. This would allow later a commercial deployment of such a zirconia fuel for Pu and MA utilisation in a last cycle. The fuel forms namely pellet-fuel, cercer, cermet or coated particle fuel are discussed considering the once through strategy. For this strategy, low solubility of the inert matrix is required for geological disposal. As spent fuels these IMFs must be excellent materials from the solubility point of view, this parameter was studied in detail for a range of solutions corresponding to groundwater under near field conditions. Under these conditions the IMF solubility is about 106 times smaller than glass, which makes the zirconia material very attractive for deep geological disposal. The desired objective would be to use IMF to produce energy in reactors, opting for an economical and ecological solution.