Overview of applications of the pyrochemical process developed by the CEA for the treatment of transmutation fuel targets

Abstract : Pyrochemical technology using high-temperature molten salts and molten metal media presents a potential interest for an overall separation and transmutation strategy for long-lived radionuclides. A pyroprocess was developed at the CEA Marcoule, based on a two- step liquid-liquid extraction. This process aims to demonstrate the feasibility of a group separation of actinides with sufficient decontamination from fission products.Taking advantage of chemical properties of cryolite fluoride salts, the first step consists in a selective reductive extraction of actinides coming from direct dissolution of oxide fuel in the molten fluoride salt, into liquid aluminium metal. The second step is the actinides back-extraction, which consists in a liquid/liquid oxidative stripping of the An from aluminium matrix into molten chloride media. The DOS process has been successfully demonstrated for treatment of oxide type fuels within the last years the core of the process has been already assessed and the studies have shown high selectivity and a quantitative recovery of actinides. Within the framework of the SACSESS European research program, the pyrochemical activities focused on applications of this process for treatment of CERCER and CERMET transmutation targets. These particular type of fuels consist of a mixture of minor actinides (MA) oxides diluted in an inert matrices (MgO) for CERCER and metallic Mo for CERMET. In the present work the behaviour of matrices material was first investigated regarding the solubility in the fluoride salt, starting from both oxide powders (considering a prior oxidation step Mo into oxide form for reprocessing of CERMET) or sintered pellets. Further investigations focused on the reductive extraction behaviour of Mg or Mo in metallic Al. The impact of Mg and Mo on the efficiency of An reductive extraction was also studied using uranium simulating the behaviour of MA oxides.