Improvement of RELAP5/MOD3.2.2 flow regime and development of fuel heat transfer model for thermal hydraulic safety analysis of CANDU plant

Thermal-hydraulic models of RELAP5/MOD3, current auditing tool for LWR licensing, have been improved to develop an auditing code for CANDU. It was attempted to extend the auditing area to CANDU LOCA event by improving RELAP5/MOD3/CANDU version, which had been already developed from RELAP5/MOD3.2.2 in order to audit non-LOCA events in CANDU plant. Major improving areas are flow regime model and heatup model of fuel elements. Horizontal flow regime map was improved by modifying the horizontal stratification criteria in CANDU fuel channel. Improved regime map was verified through the assessment of AECL experiment. Fuel heatup model was also developed by considering the fuel element elevations in CANDU channel if horizontal stratification occurs. Different thermal hydraulic conditions are applied for each fuel element in the estimation the heat transfer coefficient. RELAP5/MOD3/CANDU+ version have been developed by the application of models. The developed code extends the model capabilities of exiting RELAP5/MOD3/CANDU version which was based on RELAP5/MOD3.2.2 gamma version, and the code can be used for more realistic estimations of thermal hydraulic behavior in CANDU channel during a LOCA