Thermal-Elastic Deformation of CANDU Pressure Tube Following Fuel Element to Pressure Tube Contact

Abstract

Thermal strain deformation is a pressure tube failure mechanism. The main objective of this paper is to develop mechanistic models to evaluate local thermal-mechanical deformation of a pressure tube in a CANDU reactor and to investigate fuel channel integrity under localized contact between fuel elements and pressure tube. The consequence of concern is potential creep strain failure of a pressure tube and calandria tube. The initial focus will be on the case where a fuel rod contacts the pressure tube at full power with highly cooling condition. Such an event could occur if a fuel element was to become detached from a bundle. Calculations are performed using the finite element method in which the heat and thermal mechanical strain equations are solved simultaneously. The heat conduction from fuel sheath to the inner surface of the pressure tube with appropriate convective and radiation boundary conditions has been simulated. Furthermore, the thermal-elastic stresses have been obtained for when the pressure tube is under full operational condition. The contact boundary could be a spot or a small arc contact between the fuel sheath and pressure tube. The vapor pockets are considered in the areas beside the contact region where the convective cooling is drastically decreased. Subsequently, modeling has been extended to the contact of number of fuel elements where several fuel rods are postulated to contact the pressure tube under fully cooling conditions. It is observed that the pressure tube thermal strain will occur if sufficiently high temperature is reached. Sensitivity analysis is performed in order to evaluate the contact conductance, the extension of vapor region and the contact width. The pressure tube local strain is very sensitive to these parameters where any local strain will act to reduce the contact width, contact conductance and pressure tube thermal strain and therefore will be self-limiting.