Safety assessment of severe accident mitigating system “Passive Residual Heat Removal System of secondary side” in K-2/K-3 NPP

Abstract

The presented analysis carried out for the safety assessment of the Passive Residual Heat Removal System of secondary side (PRS), one of the Severe Accident Mitigating System of K-2/K-3 Nuclear power plants. Preliminary analysis of the PRS is carried out using computer code MELCOR, which is a thermal hydraulic code that models the progression of severe accidents in light water nuclear power plants. The system was analyzed under SBO without the main feed-water, auxiliary feed-water and the safety injection systems. The system is activated after the SBO and failure of auxiliary feed-water pumps and the turbine driven pumps by opening the isolation valves and closing the main steam isolation valve to avoid the steam loss. Three trains of PRS are modeled and connected with secondary side of each Steam Generators along with a make-up tank, each trains is connected with the same cooling tower (Heat Sink). The system can maintain the reactor in safe condition for 72 hours, if it is actuated before the large loss of primary coolant inventory or fuel damage in the core. The three independent trains for each steam generator are consider for heat removal of secondary side. If one of the train is considered unavailable that will lead to fuel failure and RPV failure within 72 hours. Although the failure time increases in comparison to the SBO without the PRS. The reactor remains in safe conditions if the PRS system operate at or above 80% of its design thermal load removing capability. The safety assessment was made with and without using this severe accident countermeasure introduced in K-2/K-3 NPP.