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In some engineered systems, the interface thermal conductance is a key parameter that governs the heat transfer behaviour of components in solid-solid contact. For example, in certain postulated accident scenarios for CANDU reactors, the pressure tube (PT) may deform into contact with the calandria tube (CT) to form a more direct path for heat transfer from the fuel to the moderator. There have been no direct measurements of interface thermal conductance in integrated “Contact boiling” experiments designed to mimic this Loss-of-coolant accident (LOCA) scenario due to the geometrical limits of the test components and the cumbersome nature of the instrumentation required to extract contact conductance data. It has been noted that the modelling of the contact conductance is one of the main sources of uncertainty in predicting the outcome of the contact boiling experiments that mimic LOCA scenarios. The present study demonstrated an analogy between the electrical and thermal contact conductance for PT/CT interfaces. The range of interface pressure and interface temperatures studies are selected to match the expected range of conditions during a CANDU LOCA scenario. The experiment setup consists of two sets of specimen representing PT and CT material. The specimen are instrumented with four K-type thermocouples in sequence to capture the temperature gradient imposed via a three-chamber oven. Within a range of interface pressures from 2 to 7 MPa and a temperature range from 510 to 720°C, the analogy is independent of the interface pressure or the load applied. This demonstrates the measurement of the electrical conductance between the PT and CT in contact boiling as a promising technique for obtaining in-situ information on the thermal contact conductance during integrated experiments.

The boiling behaviour on the surface of large diameter tubes is known to be strongly dependent on the local orientation of the surface around the circumference. Understanding such local variations in boiling behaviour is of particular interest in the CANDU® nuclear industry, where part of the heat removal path for cooling the fuel under postulated accident conditions is via pool-boiling on the surface of the 132 mm diameter calandria tube that is immersed in a pool of the heavy water moderator. While the average pool boiling behaviour of the calandria tubes has been well studied with integrated experiments, local boiling correlations have not been developed for prototypical diameter calandria tubes. Local boiling correlations will allow for more detailed modelling of postulated accident scenarios and better quantification of safety margins. In this study, the nucleate and film pool-boiling characteristics of a large diameter Zircaloy-2 CANDU® calandria tube were tested in a pool of subcooled water. A novel technique is developed to derive the local boiling curve along the circumference of the tube, involving the local heating, measurement of the wall temperature, and heat flux. The adequacy of the technique is determined by comparing the local boiling curves with previous experiments in which averaged pool boiling characteristics were obtained.