Study of various options for final disposal of HTR coated particles

In the frame of the RAPHAEL European project, The back end of the fuel cycle is addressed concerning mainly open cycles: direct disposal of spent fuel or of separated constituents (fuel matrix, particles, compacts or pebbles). One of the objectives is study and compare potential matrixes and their fabrication routes for incorporation separated particles. The direct disposal of used prismatic fuel blocks (or graphite pebbles of PBMR's) is not considered due to of the large volume of fuel to dispose of. While the immobilization of reprocessing residues in glass may follow largely existing processing schemes, employed for LWR fuel waste, the direct embedding of particles in glass without reprocessing seems also be feasible if reprocessing of certain spent fuel kernel types would not be beneficial, due to the high burn-up. However, other matrix SiC may be considered as potential matrix material as well. The embedding of fuel particles would allow for a strong volume reduction compared with the direct disposal of fuel pebbles or compacts. Concerning encapsulation in glass, large scale radioactive production experience and favorable disposal properties make glass a primary choice as confinement matrix. The product is capable to withstand groundwater attack for many hundreds of thousands of years. Alteration of by water will lead to the transformation of the glass into a dense gel like alteration phase which most likely will maintain strong confinement properties of fuel particles. Vitrification of fuel particles was achieved via two methods: glass melting and sintering. Inert fuel particles and a borosilicate glass were used. With oxidizing lass melting at 1200°C-1300°C floatation and decomposition of carbon and silicon carbide occurred, leading to CO₂ gas formation. Thermal pre-treatment of the particles lead by oxidation to the removal of the outer pyrocarbon layer, but glass/SiC contact remained weak. The sintered glass at 700°C showed better embedding properties of the fuel particles despite higher porosity compared to glass made by melting. Aqueous leaching properties of sintered glass were similar to those of melted glass. The lower operation temperature makes the use of sintered glass an interesting option to conserve as much as possible the strong confinement properties of the fuel particles. Embedding in Sic is investigated as alternative method. This material would be also capable to withstand a groundwater attack for a long time. Conventional methods for SiC production, such as liquid sintering or hot pressing are not applicable when processing spent coated particle fuel. These methods hold the danger of damaging the particles and release fission products, either due to the high temperature( 1800°C-2000°C) or the high pressure applied. In order to maintain integrity of the coated particles, a mild processing method for the production of the composite material is required. The route tested is the production of green bodies from well-dispersed aqueous SiC and graphite powder slips to which particles were added. These green bodies are subsequently infiltrated and reaction- bonded with molten silicon. The samples produced show no fractures and the new SiC matrix is grown together with the outer pyrocarbon layer of the fuel particle. The properties of this matrix are under investigation to compare them with the glass matrix. (author)