Mechanical behaviour and diffusion of gas during neutron irradiation of actinides in ceramic inert matrices

Abstract

Fission of actinides from nuclear waste in inert matrices (materials without uranium) can reduce the period in time that nuclear waste is more radiotoxic than uranium ore that is the rock from which ordinary reactor fuel is made. A pioneering study is performed with the inert matrices: MgO, MgAl2O4, Y3Al5O12, Y2O3 and CeO2-x. These inert matrices contain inclusions of actinide-oxides with a diameter of about 200 μm. UO2 is used as a phase of actinides to simulate the implantation of fission products in inert matrix fuels. After neutron irradiation, inter-particle fracture was present in fuels with matrices that do not form a solid solution with UO2. These cracks are caused by swelling of the actinide phase and insufficient creep of the matrix. The diffusion length of fission gases is significantly reduced by inter-particle fracture and thereby fission gas releases till 50% were measured. This inter-particle fracture may be prevented when inert matrix fuels are fabricated with porous actinide inclusions embedded in inert matrices that are polluted by actinides. The amount of helium generated during neutron irradiation of a few actinides from nuclear waste can be far larger than the amount of fission gases. Parameters and diffusion mechanisms to simulate diffusion of helium in MgAl2O4 were derived by thermal evolution of He concentration profiles, atomistic calculations and numerical analysis using original definitions of diffusion coefficients for inert gas in materials with defects for single crystal as well as polycrystal media. This study shows that, amongst others, the material degradation of MgAl2O4, caused by the generated amount of defects and He during irradiation conditions, is acceptable. But the pioneering study with UO2 as a phase of actinides, shows that the solid state swelling by fission products in MgAl2O4 is about ten times larger than this swelling in 238UO2 that is the matrix in fuel for commercial reactors. The commercial feasibility of reduction of actinides from nuclear waste, by neutron irradiation in inert matrices, requires more research.