ANSYS Thermal Analysis of Bearing Pad/Pressure Tube Interface

Abstract

This paper describes the use of the ANSYS finite element program to estimate the heat transfer coefficient at the interface between contacting fuel-element bearing pads and the pressure tube of a CANDU reactor. An understanding of the thermal behaviour of this interface is needed to determine whether local strain failure of the pressure tube could occur under contacting bearing pads following a hypothetical postulated large-break stagnation LOCA. Estimates of the behaviour of the bearing pad to pressure tube (BP/PT) heat transfer coefficient were obtained by adjusting the value of the BP/PT conductance so that pressure tube temperatures predicted by ANSYS agreed with measured values obtained from a series of experiments performed using small sections of fuel element and pressure tube.

Results of the thermal analysis of the experiments conducted in an argon/oxygen atmosphere show that the BP/PT heat tran3fer coefficient rises from a nominal value of around 0.5 kW/(m²-K) to 2 kW/(²-K) when the bearing pad surface temperature remains between 600 to 1000°C. Above 1000°C, the conductance value rises to about 4 kW/(m²-K). Thus, the BP/PT contact conductance can be.defined as a "three-step" conductance depending on the bearing pad surface temperature.

Correcting for the difference in gas environments between in-reactor conditions and the experiments (ie. D₂0 vapour versus Ar/O₂), leads t o a predicted maximum in-reactor BP/PT conductance of about 3 kW/(m²-K) for bearing ptd surface temperatures between 600 to 1000°C and about 6 kW/(m²-K) above 1000°C. These inferred values are appropriate to the case of pure thermal interaction where no straining of the pressure tube occurs. Analysis of additional tests where straining occurs is currently in progress.