Fitness-for-Service Assessment of Calandria Tube to Liquid Injection Shutdown System Nozzle Contact in a CANDU Reactor

Abstract

In CANDU1 reactors, calandria tubes are used to separate the fuel channels from the moderator. The Liquid Injection Shutdown System (LISS) nozzles, which contain holes for discharging neutron-absorbing liquid into the moderator, are arranged perpendicularly in the gap between adjacent rows of calandria tubes. Both calandria tubes and LISS nozzles sag during service due to creep, with the more heavily loaded calandria tubes sagging relatively more. When contact between a calandria tube and a LISS nozzle has been detected by in-service inspection, or is predicted to occur, a fitness-for-service assessment is permitted by CSA Standard N285.4 to demonstrate acceptability of continued operation until the end of the next periodic inspection interval, provided that the fitness-for-service assessment is acceptable to the Regulatory Authority. A fitness-for-service assessment has been recently performed for a calandria tube and a LISS nozzle that were predicted to contact at a future time. The assessment has demonstrated that for an evaluation period of three years after the predicted contact time, the structural integrity of the calandria tube and the LISS nozzle is maintained, both components will continue to function in accordance with their design requirements, and their contact will not result in contact between the calandria tube and a pressure tube.