Evaluation of the PRT Rupture Disk Flow during an Inadvertent Safety Injection Event at Kori Unit 3

During a planned preventive maintenance on May 25 2008 at Kori Nuclear Unit 3, a functional test for the COPS (Cold Overpressurization Protection System) as a part of the ESF (Engineered Safety Features) test had been performed in the hot standby condition at Kori no. 3 Unit. Meanwhile, an operator extracted the RCS (Reactor Coolant System) temperature card without checking the state of the COPS switch and as a result, the RCS wide temperature fell to zero degrees Celsius, the COPS opened the pressurizer PORV (Power Operated Relief Valve) to prevent the RCS from being over-pressurized and the RCS pressure decreased drastically. The safety injection system was actuated when the RCS pressure reached the set-point of the RCS low-low pressure. The operator blocked the pressurizer isolation valve promptly as soon as he recognized that the cause of the RCS pressure loss was the pressurizer PORV open. Both that of the PORV isolation and the safety injection recovered the RCS pressure, however, the CIS (Containment Isolation Signal) related with the safety injection signal blocked the normal letdown flow path to the outside of the containment, so that the letdown flow was released to the PRT (Pressurizer Relief Tank). The inflow from the letdown line increased the PRT water level and over-pressurized the PRT. Eventually, the rupture disk was broken at its set-point and a small amount of water overflowed into the containment. The transient simulation for the RCS including the pressurizer (PZR) PORV open has been performed by using RETRAN-3D and the simulation for the PRT over-pressure has been performed by using MARS since RETRAN-3D has some limitations to handle the system with non-condensable gas, steam and cold liquid. In this paper, a detail description of the event and the simulation results along with the plant data will be introduced and the accumulated water flow from the PRT will be estimated by the MARS calculation