Parametric study of iodine-129 releases from nuclear fuel to fuel-clad gap & primary coolant in PWRs

Abstract

A numerical methodology is developed to compute volatile fission fragment release from fuel to gap and then to the primary coolant under changing reactor conditions. Based on diffusion of fission fragments and mass balances in gap and in coolant, a FORTRAN-77 based program DIFFP (Diffusion of Fission Products) has been developed to solve numerically the time & space dependent equations using source release rate respectively. For power shutdown effect of spiking have been modeled for given release rate. The predicted gas fractional release as a function of time using DIFFP program was in good agreement with the reported results. Then parametric studies for empirical diffusion constant (D′), coolant purification rate (βP), and gas escape rate coefficient (ν) respectively were done to find the sensitivity of parameters. For operation time >150 days, the sensitivity increases from 4.5% to 20.4% with an increase D′ from 4.1 × 10−10 s−1 to 6.9 × 10−10 s−1 respectively and sensitivity is a nonlinear function of D′: sensitivity=(-62.20±0.78)+(16.83±0.37)D′Do′+(-0.703±0.0398)D′Do′2where, 0.69<D′/D0′<1.17 and D′ is the reference value (5.9 × 10−10 s−1) for the defective fuel rods. Sensitivity for gas escape rate coefficient (ν) was also determined by increasing it from 5% to 50% from its reference value (1.4 × 10−4 s−1) and results show that the coolants activity becomes insensitive to changes in the gas escape rate coefficient and sensitivity value becomes less than 1% when effective full power days are larger than 150. If coolant purification rate constant (βP) is parametrically changed from 5% to 30%, then the absolute sensitivity value increases from 4.8% to 23% respectively and for all cases, the absolute sensitivity remains independent of time (in terms of effective full power days) for a given value of βP and rises linearly with an increase in the value.