New Insights Into The Head-End Treatment Process Of Spent Fast Neutron Reactor Mox Fuels For Improved Plutonium Dissolution
Résumé
Fast Reactors (FR) are considered in the future to fully exploit the uranium resource and enlarge the solutions to decrease the volume and impact of the final wastes. Initial Pu (recycled from LWR spent fuels) is needed to produce the fuel for the first FR (e.g. mixed uranium and plutonium oxide MOX). Sustainability is then ensured by progressively deploying a FR reactor fleet and repeatedly recycling plutonium together with uranium (multi-recycling). Far higher Pu concentrations in the spent FR MOX than for existing spent Light Water Reactor (LWR) fuels requires a specific head-end treatment process in order to dissolve the Pu-rich oxide and, possibly, complementary Pu recovery steps from the solid residues left after the primary fuel dissolution process. Locally higher Pu content in the (U,Pu)O2 oxide may indeed lead to the existence of less soluble portions in the fuel, particularly when dealing with fuels with relatively high initial Pu enrichment. Higher platinum-group metal content in the fuel induces also the formation of refractory intermetallic alloys in the spent fuel material, which could combine with part of plutonium. This is exacerbated in FR fuel due to higher temperatures reached in the core and relatively higher burnup. In order to ensure quantitative plutonium recovery for recycling with minimal residual plutonium traces in the waste, coupling of the primary dissolution with an additional digestion step is currently assessed and optimized for advanced FR fuel treatment. It consists in chemically attacking persisting Pu-containing particles in order to retrieve the residual Pu in solution, after separating them from the large volume of solution produced in the batch dissolver. To achieve high separation and Pu recovery yields, knowledge of the particle characteristics is essential. However, these characteristics are very specific to the initial fuel. The current RetD program in this area is hence based on the use of experimental irradiated Phenix fuels chosen specifically to sweep various key parameters such as Pu content, burnup ratio, cladding material, etc. Targeted investigations into those fuels in the CEA ATALANTE facility are presented in this communication and aim at completing databases and optimizing the overall head-end treatment process for FR fuel cycle.