Thermomechanical Behavior of Pressure Tube Under Small Break Loss of Coolant Accident for PHWR

Some postulated events for pressurized heavy water reactor (PHWR) small break loss of coolant accident (SBLOCA) may lead to flow stratification in the reactor channels. Such stratified flow causes a circumferential temperature gradient in the fuel bundle as well as in the surrounding pressure tube (PT). The present investigation has been performed to study the thermomechanical behavior of a PT under an asymmetric heat-up condition arising from flow stratification in a 19 pin fuel element simulator. A series of experiments has been carried out at various stratification levels and PT internal pressures. The asymmetrical heat-up creates a temperature difference of 400 °C across the diameter of the PT. At high temperature the internal pressure causes ballooning of the PT. With the stratification, ballooning is found to get initiated at top hot side of PT and further propagates unevenly over its periphery. Axially ballooning is found to get initiated from center and then propagates toward both the ends of the PT. This results in an axial temperature gradient on the PT in addition of circumferential gradient. For a pressure higher than 4.0 MPa, the integrity of PT is found to be lost due to the combined effect of circumferential and axial temperature gradient generated under uneven strain distribution.