Large Break Loss of Coolant Accident Analysis of VVER-1000 reactor using CATHARE Code

摘要

In the safety analysis of the nuclear power plants Large Break Loss of Coolant Accident continue to be one of the major issues. In this study the last version of the French best estimate computer code CATHARE2 V2.5_1 Mod 6.1 was used in order to predict the thermal-hydraulic phenomena in the VVER-1000 reactor during LB LOCA. This type of reactors are in operation and construction in several countries like Russia, Ukraine, Bulgaria, Czech Republic, China, India. The paper first presents the CATHARE modeling of VVER-1000 reactor, including the core, the vessel, the primary and secondary circuits with the pressurizer, main circulation pumps, horizontal steam generators and steam lines. The ECCS is presented also with hydroaccumulators, high pressure and low-pressure safety injection pumps. The break, located in the cold leg, close to the reactor, is represented by the RUPTURE module of CATHARE, which modelizes a double ended guillotine type of break. In the current calculations the Bottom-up and Top-down reflooding play a very important role, compared to some western PWR, because of the ECCS injection into the downcomer and upper plenum of the VVER-1000. That is why special attention is paid to the validation of the CATHARE reflooding model, based on REWET-II experiment, devoted to VVER. In CATHARE a special 2D reflooding module has been developed, which takes into account the radial and axial conduction in the vicinity of the quench front. Then, the paper presents the results of the LB LOCA analysis for VVER-1000 (reference case and sensitivity study), describing the blowdown, refill and reflood phase of the accident. Problems, related to the counter current flow limitation (CCFL) phenomena at the core outlet are considered also. Basic parameters, important to the safety of the plant, are illustrated: break flows, pressures, temperatures, void fractions and especially quench front propagation and maximal fuel cladding temperatures.