Theoretical analysis of two-stage fission gas release processes: grain lattice and grain boundary diffusion

A theoretical analysis of two-stage fission gas release processes, grain lattice and grain boundary diffusion, coupled with bubble trapping and resolution, is carried out. Final results show that the conventional single-stage fractional release solution can be expanded for the two-stage release case with the introduction of a new dimensionless parameter a/(α+a) where α is the relative diffusivity ratio defined as Dveff/Dgbeff. In fact, α is a temperature- and burn-up-dependent gas transport property. Recent evaluations of α demonstrate that grain boundaries can play significant roles in fission gas release, depending on the fuel power history, and the high burn-up behavior of fission gasses can be directly reflected in the two-stage parameter. Inter-granular bubble linkages effectively increase the two-stage parameter, which leads to apparent burn-up enhancement of the fractional fission gas release.