Bearing-Pad/Pressure-Tube Rupture Experiments

Abstract

The behaviour of a CANDU pressurized heavy water reactor fuel channel during a postulated loss-of-coolant accident with coincident loss-of-emergency-core cooling is being studied at AECL-Whiteshell Laboratories. In some of these postulated accidents, the pressure tube is predicted to become completely dry in a matter of seconds after flow stagnation occurs. As the pressure-tube depressurizes, the fuel cladding temperatures in the bundle can easily exceed 1000 degrees C. Most of the pressure-tube circumference will be heated by thermal radiation, except at spots where the bearing pads are in contact with the pressure tube. Here, conduction and thermal radiation are the dominant modes of heat transfer. Therefore, local hot spots can develop on the pressure tube under the bearing pads. Whether the pressure tube would fail at these hot spots depends on the localized temperature and pressure transients it experiences. Experimental results to date, using electrically heated fuel element simulator bundles, have shown that localized hot spots created by bearing-pad contact do not result in significant local strain on the pressure tube during ballooning. Tests were done in steam and simulated steam atmospheres using "as-received," "worn" and the modified "T-pad" bearing pads with similar results. Hence, the probability of pressure-tube failure during ballooning does not appear to be influenced by localized hot spots caused by bearing pads of overheated bundles for the range of experimental conditions studied.