Influence of coolant phase separation on event timing during a severe core damage accident in a generic Candu 6 plant

The Modular Accident Analysis Program for CANDUR (MAAP4-CANDU) was developed to assess the consequences of a severe core damage progression in a CANDU nuclear power station. The results from MAAP4-CANDU analyses support Level 2 PSA activities being performed for CANDU stations. During the early stages of a severe core damage accident in a CANDU plant, the primary heat transport system (PHTS) coolant is likely to be a two-phase homogeneous mixture. Because of the PHTS depressurization after a postulated initial event such as a large break, combined with a loss of coolant circulation and loss of any coolant makeup, the PHTS coolant void continuously forms. Eventually, the steam and liquid phases of the PHTS coolant will separate. Once phase separation occurs, the upper fuel channels in the reactor core could void. Fuel in the voided fuel channels would heat up from the decay heat and low cooling capacity of the PHTS coolant. The onset of phase separation is an important parameter for severe core damage accident progression in a CANDU reactor, because it affects the overall timing of events, such as fuel heat-up initiation, debris formation inside the fuel channel, and fuel channel failure. MAAP4-CANDU has a simple thermalhydraulic model of the PHTS, and the PHTS coolant phases separate when the coolant void fraction exceeds a user-defined value (VFSEP). This paper reports the results of a scoping study, conducted to investigate the influence of the input parameter VFSEP on the dryout (void) time of the upper level fuel channels in a generic CANDU 6 reactor core, during a postulated Large Loss-of Coolant Accident with complete loss of emergency core cooling (LLOCA+LOECC). A code-to-code comparison was performed to evaluate the values of the parameter VFSEP appropriate for a given LLOCA case. CATHENA (a detailed thermalhydraulic code used for design basis accident analyses of a CANDU plant) was used for the code-to-code comparison with MAAP4-CANDU. Code version MAAP-CANDU v4.0.5A and generic CANDU 6 plant data were used. Results of the scoping analyses demonstrate that the input parameter for phase separation onset in the PHTS is important for predicting the timing of events of accident progression. (authors)