Evaluation of VVER-1200/V-491 Reactor Pressure Vessel integrity during large break LOCA along with SBO using MELCOR 1.8.6

Abstract

After Fukushima accident and stress test recommended by IAEA for existing reactors, higher safety requirements are enforced upon nuclear power plants during design extension and severe accident conditions. Based on those arguments, Vietnam Government requests a lot of effective safety solutions, in designs proposed for the nuclear power plants in Ninh Thuan province of Vietnam, which can prevent the accident progression toward severe accidents and mitigate severe accident consequences. One of safety requirements is related to delay time of core melt during design extension condition. Especially, if the worst case of accidents occurs, the reactor vessel integrity must be maintained at least 24 hours from the beginning of the accident. With the aim at investigation of Reactor Pressure Vessel (RPV) integrity, in this study, MELCOR 1.8.6 code is used to evaluate the integrity of RPV lower head for VVER-1200/V-491 reactor during a Large Break Loss of Coolant Accident (LBLOCA) in combination with Station Blackout (SBO) event. The study figures out several parameters related to melt down progress such as: rupture position and rupture timing, the amount of hydrogen generated. Availability of the second stage hydro-accumulators (HA2) in the VVER-1200/V-491 is assumed as an additional improvement to delay the timing of core melt as well as to maintain the vessel integrity for long-term.

Transient Analysis of LBLOCA with Subsequent Loss of Offsite Power and Loss of All AC Power in VVER-1200 Reactor

“…On the other hand, the hydro accumulator second stage (HA-2) began to run sooner for LB LOCA with LOACP and the hydro accumulators handled all of the core cooling. It was discovered that the hydro accumulator findings in the scenario of LB LOCA with LOACP were in good agreement with those provided by the MELCORE 1.8.6 code [18].…”

“…Additionally, [7] indicates how the break size spectrum of LOCAs affects the time of vessel rupture as well as the quantity of generated hydrogen, corium, and fission products released for VVER-1000/V320. The lower head integrity of the RPV for the VVER-1200 (V-491) reactor is assessed in a study conducted by [18] using MELCOR 1.8.6 considering LB LOCAs and SBO occur at the same time. Moreover, [9] analyzed the thermal hydraulic parameters of the VVER-1200 reactor during LBLOCA concurrent with LOOP using PCTRAN.…”