Thermal hydraulic behavior under Station Blackout for CANDU6

• CNADU 6 nuclear power plant model is constructed by using SCDAP/RELAP5 code.
• Severe accident sequence is analyzed under the severe accident induced by SBO.
• Thermal-hydraulic behavior of PHTS, CV and calandria vault is investigated.
• Model discussions have made to give some support to improve the simulation.

To evaluate thermal hydraulic behavior under typical severe accident sequences for the CANada Deuterium Uranium (CANDU) in China, the model of CANDU6 Nuclear Power Plant (NPP) is built with the code of RELAP5/SCDAP. The plant model includes Primary Heat Transport System (PHTS) with a simplified model for balance pipes, Steam Generator (SG) secondary system, calandria vessel (CV) and moderator system, and calandria vault. The PHTS model consists of the inlet headers, feeders and end fittings, fuel channels, outlet headers, pressurizer, reactor coolant pumps, and SGs. Each PHTS has liquid relief valves (LRVs) at the reactor outlet header to relief the pressure. The core is modeled by a simplified nodalization with four channels representing the 380 fuel channels arranged in 22 rows and 22 columns. The fuel bundles are modeled as 12 axial nodes and the 37 fuel elements of each fuel bundle, PT, CT and gas-filled annulus are modeled with the SCDAP core components. When the fuel channels collapse onto the bottom of the CV, the debris and CV are modeled by the COUPLE module which generates two-dimensional finite element mesh. Station Blackout caused by the failure of off-site AC Power with subsequent loss of all on-site standby and emergency electric power including Class III and IV is selected and analyzed thermal hydraulic behavior during the accident sequence until the CV failure, including the PHTS and SG response, CV response and calandrial vault response. The results show that most phenomena, such as fuel channel dry-out, fuel channel failure, moderator boiling, core collapsing to CV, and CV failure, can be simulated. Model discussions provide some suggestions to pay attention on and model modification should be made in the near future. This work can give some technical support to severe accident management measures and to develop severe accident management guidelines.