Modelling Thermalhydraulic/Thermal-Mechanical Behaviour of a Fuel Channel with Stratified Two-Phase Flow Using CATHENA

Abstract

Under some postulated accident scenarios in a CANDU (CANada Deuterium Uranium) reactor, some fuel channels may experience periods of stratified flow in which the top portion of the pressure tube and fuel elements are exposed to superheated steam while the bottom portion is cooled with water. As a result, the circumferential temperature gradient that develops on the pressure tube could result in nonuniform deformation and potential failure of the pressure tube. The ability to model the transient thermalhydraulic and thermal-mechanical behaviour of the fuel channel during such a scenario is an important part of the licensing analysis for CANDU reactors during a postulated loss-of coolant accident (LOCA). Four pressure tube circumferential temperature distribution experiments were simulated using the thermalhydraulic code CATHENA MOD-3.5dRev 0 to demonstrate its ability to model the combined thermalhydraulic and thermal-mechanical behaviour of a fuel channel subjected to stratified two-phase flow. The experiments simulated involved boil-off of the coolant in a horizontal channel with or without make-up water or steam. These experiments had channel absolute pressures ranging from 1.1 to 5.6 MPa and pressure-tube heating rates up to 5.4 degrees C/s. CATHENA accurately simulated boil-off rates, test-section temperatures, and nonuniform pressure-tube deformation during coolant boil-of/ without make-up water or steam. When make-up water or steam was provided, the simulated fuel channel behaviour agreed with experimental results, provided the cross-sectional steam temperature gradients in the horizontal channel and the flows near the test section inlet were correctly modelled. The work described in this paper was funded by the CANDU Owners Group (COG).