Thermomechanical analysis of selected Kozloduy NPP, WWER-440 fuel assemblies to estimate the possible causes for loss of hermeticity

The paper gives a brief review of the fuel rod failure mechanisms and root causes as a necessity to clarify the reasons for fuel assembly loss of hermeticity. Fretting wear of fuel rods in contact points with grids and debris; pellet-cladding interaction and stress corrosion cracking; excessive cladding corrosion, crud effects and water chemistry; power-cooling mismatch and cladding dryout are found to be the main reasons for fuel failure. Fuel behavior predictions and analyses by means of modeling and computer codes are also made and calculated results for selected assemblies from Kozloduy NPP Unit 2 (16-th fuel cycle) are presented in the paper. Some assumptions about the probable reason for failure of fuel assembly untightness are given as final conclusions