Numerical Simulation of Pressure Transients in a PWR Main Steam Line System Due to Quick Operations of Pressure Relief Valve Available to Purchase

At a PWR (pressurized water reactor) nuclear power plant in Korea which was under being retrieved from the zero reactor power after a periodic overhaul in 2005, manual operation for opening the PORV (power operated pressure relief valve) was tried at a low speed to reduce pressure in the MSL (main steam line). Initially, one MSIV (main steam isolation valve) and five MSSVs (main steam safety valves) installed to the MSL remained all closed. To speed up the pressure reduction, the reactor operator in charge abruptly switched the operation mode from “Manual” to “Auto.” Just after the valve was opened fully in a very short time, rapid reduction of pressure in the MSL occurred. A little later on, the operator closed the valve quickly to mitigate the unexpected transient occurrence. In consequence of the result of the sudden discharge of high pressure steam to the atmosphere through the PORV, a curved pipe spool in the downstream was separated from both connections and blasted away into air. Then it collided with the outer wall of the RWST (refueling water storage tank) located about 50 m away, resulting in mechanical damage on it with the maximum permanent deformation of about 60 mm in the radial direction at a local part of wall. This paper describes a numerical simulation of the transient pressure responses to the quick opening and subsequent closing operations of the PORV on the PWR MSL initially maintained as a closed system at a high pressure, which has been performed to illustrate the incident scenario as well as to demonstrate its root cause. For computational efficiency, the pressure field to be solved has been modeled with a two-dimensional mathematical formulation. As the result, the simulation has shown realistically how the incident was initiated and developed.