Simulation of pressure tube deformation during high temperature transients

Abstract

In the case of a loss of coolant accident (LOCA) with/without loss of emergency core cooling system (ECCS), in a PHWR, the pressure tube temperature could rise significantly depending on the scenario due to fuel heat up, with the system internal pressure also varying from 9 to 0.1 MPa during the event. This transient can cause metallurgical and geometrical changes in the pressure tube. Depending on the temperature reached, its structural integrity needs to be ensured during such events with the possibility of pressure tube–Calandria Tube contact. At high temperature the pressure tube deforms plastically due to internal pressure and fuel weight. Both ballooning and sagging behaviour are observed in the pressure tube. However, the relative contribution of each deformation is dictated by the pressure and temperature transient. Pressure tube sagging would always lead to an initial bottom contact. In such cases, studies have shown that the temperature difference between the top and bottom of the pressure tube could be as large as 500 °C. Along with the internal pressure, the thermal stress developed may threaten the integrity of the pressure tube. Modelling has been carried out to simulate the simultaneous sagging and ballooning deformation behaviour expected during the channel heat up condition. This paper describes the prediction of creep deformation in the context of the pressure tube configuration in Indian PHWR's and the study of the simultaneous ballooning and sagging behaviour of a pressure tube. The paper also presents the application of this model for a specific postulated LOCA in Indian PHWRs.