Gradient percolation of fission gases in nuclear fuel pellet

Abstract

Fission gas release (FGR) is one of the most complex multi-scale phenomena affecting safety performance of nuclear fuel. To improve mechanistic understandings of FGR phenomenon, especially the third stage of gas venting process through connected grain boundaries in a fuel pellet, explicit grain boundary network (GBN) percolation modelling of FGR was carried out in combination with the traditional 2-stage Booth model. Besides the fluctuating and reduction features of FGR accompanied with percolating process, the percolation front and its fractal feature were investigated in-depth in the study. The results predicted existence of an enrichment region of gas contents around the medium radius of the pellet, which coincide with previous published experimental data, and may be directly related with the augmentation of FGR as well as formation of micro-grains at medium region of pellet under high burn-up conditions. Therefore, possible explanations for these phenomena have been proposed from the aspect of percolation process. Moreover, in analogy with the percolation front or hull, we defined “FGR front” as the external perimeter of vented clusters in the GBN. Fractal features as well as its position and width of the front were investigated quantitatively based on scaling laws using gradient percolation theory, the incipient point of gas releasing can be predicted directly using scaling laws related with the conductance probability of grain edges. Results and methodologies in this study may have practical significance for not only understanding mechanisms of FGR but also safety promotion of nuclear fuel pellet pertaining to containment of fission products (FPs).