The toxicity of the UO2 spent fuel is dominated by plutonium and minor actinides (MA): Np, Am and Cm, after
decay of the short live 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. 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 AnzYyPuxZr1-yO2-y where minor actinide oxides are also soluble.
Burnable poison may be added if necessary such as Gd, Ho, Er, Eu or Np, Am them-self. These 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 irradiations make the Swiss scientists confident to irradiate such IMF in a commercial reactor that would
allow later a commercial deployment of such a fuel for Pu and MA utilisation in a last cycle. The fuel forms namely pellet of
solid solution, cercer or cermet 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 IMF’s are demanding 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 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.
