Intra- and intergranular fission gas transport on large irregular hexagonal grain networks by an included phase model

Abstract

The evolution of intergranular fission gas bubbles, including growth, percolation through the fuel matrix, venting and tunnel collapse is simulated using the Included Phase Model (IPM). The model accounts for the effects of vacancy and fission gas species on bubble growth and morphology, which ultimately lead to tunnel formation, venting of fission gas, and subsequent tunnel collapse. A simplified two-dimension bubble model is implemented on a randomly generated irregular, equiangular hexagonal network of grain boundaries and coupled to an intragranular diffusion model. A set of 28 simulations were conducted on a 20 by 15 grid of hexagons with an open surface at one end to permit venting and ensuing tunnel collapse. Results are post-processed and considered statistically to investigate macroscopic fission gas release parameters and the percolation of the network. The behaviors of individual grains and edges are further investigated as a function of edge length, equivalent grain size, and proximity to the free surface. This analysis revealed discrete growth modes corresponding to the number of bubbles on each edge. These modes emerge because edge lengths vary continuously but only an integer number of bubbles precipitate on them, leading to an unequal number of bubbles per unit length for different grains, and corresponding difference in vacancy potential. This result may improve edge percolation conditions utilized in other models. Limitations resulting from the two-dimensional model and avenues future development of more quantitative models are also discussed.