Three-dimensional simulation of threshold porosity for fission gas release in the rim region of LWR UO2 fuel

The threshold porosity above which fission gas release channels would be formed extensively in the rim region of high burnup UO2 fuel was estimated by the Monte Carlo method and Hoshen–Kopelman algorithm. With the assumption that both rim pore and rim grain can be represented by cubes, the pore distribution in the rim was simulated three-dimensionally by the Monte Carlo method according to rim porosity and pore size distribution. Using the Hoshen–Kopelman algorithm, the fraction of open rim pores interlinked to the outer surface of a fuel pellet was derived as a function of the rim porosity. The simulation revealed that it is the rim porosity rather than the pore size distribution or rim thickness that determines the fraction of open pores connected to the pellet surface. The analysis also indicated that the threshold porosity is around 24%, above which the number of rim pores forming release channels increases very rapidly.