Application of the FALCON code to PCI induced cladding failure and the effects of missing pellet surface

摘要

A methodology for the analysis of cladding failures caused by Pellet-Cladding Interaction (PCI) that may result in the Stress Corrosion Cracking (SCC) during power ascension at a PWR reactor start-up is presented. The proposed approach is based on the capabilities of EPRI's FALCON MOD01 code - as developed by ANATECH Corp. - with the PSI in-house model GRSW-A for the micro-structural processes occurring in the fuel. The methodology allows for analysis of the impact of missing pellet surface (MPS) on the failure-related characteristics of the cladding, particularly the peak local hoop stress, along with the accounting for the transient gaseous fuel swelling and FGR. The application of the developed methodology to the ramp tests with PWR fuel samples from the SUPER-RAMP project, carried out in Studsvik (Sweden) in 1980s, is presented. This analysis has been conducted in the framework of the PSI participation in Fuel Modeling Programme FUMEX III, recently carried out by IAEA. As a result, the capability of the new methodology to differentiate between the power ramps with failure and without failure of the claddings of non-defect fuel rods is shown, and the appropriate failure thresholds for the selected criteria are determined. The results of calculation for the stress-concentration factors caused by MPS, as a function of the angular size of the MPS defect, are in good agreement with previous similar studies, specifically with the one undertaken by the principal FALCON MOD01 code developer - ANATECH Corp. Furthermore, the predicted effects of MPS defect size is compared with the effects related to the power ascension rate, with the power ramp level being kept the same. A reduced power ascension rate is determined, which is capable of 'neutralizing' the detrimental effect on the local stress concentration in the cladding caused by the MPS defect under consideration.