Models for numerical simulation of burst FGR in fuel rods under the conditions of RIA

Two models are developed and implemented in the FALCON code. One is dedicated to simulation of the pellet-cladding bonding assisted trapping of released fission gases during base irradiation. The other describes burst release of fission gases retained by the grain boundaries and trapped by bonding during thermal transients, such as RIA. The model for bonding assisted trapping of the released fission gas is shown to be able to significantly alter the calculated FGR into the free volume of the BWR rods. The prediction capability of the code for FGR during the base irradiation is essentially improved after the model implementation. Significantly better agreement of the code calculation with the measured FGR in the CABRI RIA test CIP0-1 is obtained after implementation of the model for burst FGR. No burst FGR is predicted for the NSRR RIA tests LS-1, LS-2 and LS-3, while a good agreement with the data is obtained due to models for fuel swelling and thermal FGR, including the earlier developed model for fast-transient specific behaviour of the intra-granular fission gases. Prediction on absent grain-boundary fracture in the NSRR tests LS-1, LS-2 and LS-3 is consistent with the data of destructive PIE. Possible further extension and verification of the new models are discussed.