Equivalent Moderator Subcooling Methodology to Determine Fuel Channel Integrity Upon Pressure Tube and Calandria Tube Contact

Abstract

During a postulated event of large LOCA in CANDU reactors, the pressure tube may balloon to contact with its surrounding calandria tube to transfer heat to the moderator. To confirm the integrity of the fuel channel after the contact with a given moderator subcooling, many experiments have been performed in the last three decades by applying different pressure tube heatup rates, different pressure tube pressures and different moderator subcoolings. In this work, the available pressure tube/calandria tube contact experiments data were collected to analyse the impacts of the pressure tube pressure, the pressure tube heatup rates and the calandria tube surface conditions on the requirements of the moderator subcooling to ensure fuel channel integrity. A new methodology is put forward by using the concept of equivalent moderator subcooling (EMS) to determine the integrity of fuel channels after pressure tube/calandria contact. The EMS is an artificial term combining all the parameters that impact the dryout area and the maximum calandria tube temperature. This EMS value may help to determine experiment and analysis matrices before applying more complicated methodologies for a detailed analysis. An empirical equation is recommended to determine the maximum calandria tube temperature based on the obtained EMS values for a reference state and the safety boundary for fuel channel integrity upon PT/CT contact is also provided. The equation can be used to estimate the maximum calandria tube temperature in a conservative manner. In the mean time, it is determined that with the application of glass-peened calandria tubes, the moderator subcooling can be reduced by 9ºC in comparison with the smooth calandria tubes to ensure fuel channel integrity upon the PT/CT contact.