Effects of irradiation on the insolubility of sodium fast reactor (U,Pu)O2 mixed oxide with a very high amount of plutonium

•Dissolution tests were carried out with irradiated and uniradiated MOX pellets containing 45% of plutonium.•As expected, the pellets exhibit a high insolubility due to the amount of plutonium.•Despite the lack of data at such high plutonium content, the insolubility and dissolution rates seem consistent with the literature.•A beneficial effect of irradiation on the insolubility is observed and discussed. Sodium fast nuclear reactors might operate with MOX fuels containing up to 45% of plutonium, nevertheless, the reprocessability of spent nuclear fuel is sparsely investigated in the literature for plutonium contents higher than 30%. The dissolution step is a critical stage for MOX fuels since plutonium oxide exhibits a much slower dissolution kinetics as compared to uranium oxide. In particular, many authors suggested a significant change in the dissolution behavior of mixed oxides for plutonium contents between 30% and 50%. In this paper, the dissolution tests of unirradiated and irradiated CAPRA pellets with an initial plutonium content of 45% is described. Up to our knowledge, this is the first dissolution test reported in the literature with such high plutonium content in irradiated nuclear fuel. The fresh unirradiated pellets appears almost insoluble due to a very slow dissolution kinetics. Although the insolubility remains high after irradiation, a beneficial effect of irradiation on the dissolution behavior is observed with a depletion factor of five. The dissolution rates observed are very consistent with previous studies reported in the literature. The results show that complementary steps such as fuel pre-treatment or depletion steps should be considered in order to recycle spent nuclear fuel highly enriched in plutonium.