A Calculation on the Migration of Fission Gas in Material Exhibiting Precipitation and Re-solution of Gas Atoms Under Irradiation

Multiscale Modelling for the Fission Gas Behaviour in the Transuranus Code

“…The formulation of Speight [10] describes intra-granular diffusion of fission gas during irradiation of UO2 assuming that (i) gas atoms diffuse through the UO2 lattice with a single atom diffusion coefficient Ds, (ii) gas bubbles are immobile, (iii) gas atoms are absorbed into bubbles at a rate g (trapping parameter), (iv) gas atoms are knocked back from bubbles into the lattice at a rate b (irradiationinduced resolution parameter), and (v) bubbles are effectively saturated (quasi-stationary approach). The parameters g and b are taken to be spatially independent within the grain and slowly varying in time [10,19].…”

“…The parameters g and b are taken to be spatially independent within the grain and slowly varying in time [10,19]. Under the above assumptions, the intra-granular gas diffusion in presence of trapping and irradiation-induced resolution may be evaluated by solving a single diffusion equation, instead of a diffusion equation coupled with an equation for the gas balance in the bubbles.…”

“…Diffusion models used in fuel rod analysis (e.g., [5,8,9]) make use of an effective gas atom diffusion coefficient that is generally based on the formulation of Speight [10]. This concept implies a level of simplicity of the mathematicalnumerical treatment that allows application to integral fuel performance codes [8].…”

“…In this work, an extension of the formulation of Speight [10] is proposed, which includes an estimation of the contribution of bubble motion to fission gas diffusion, while retaining the advantages in terms of simplicity of the mathematical-numerical treatment, thus offering great advantages for implementation in fuel performance codes. Bubble movement can provide an explanation for the sudden increase of the effective diffusion coefficient during transients, although there is an alternative explanation [16] based on the intersection of growing bubbles with the grain boundaries, which is consistent with the sudden increase of the release fraction from the grains as proposed by Verwerft [17] and implemented in the COSMOS code [14,15].…”

“…In the next section, the original model of Speight [10] is outlined, along with the necessary assumptions and the corresponding equations for the new model. The formulation for the irradiation induced resolution term is inferred from the recent molecular dynamics studies.…”