Root Cause Analysis of the Failure of PORV Downstream Steam Discharging Pipe System Connected to the Main Steam Line

On April 16, 2005, the Kori Unit 1 plant was being retrieved from the zero reactor power at the final stage of the 23rd periodic overhaul when manual operation for opening the PORV (Power Operated Relief Valve) was performed at a low speed to reduce the pressure of main steam line with an MSIV (Main Steam Isolation Valve) and 5 MSSVs (Main Steam Safety valves), which remained closed. When the pressure in the main steam line reached about 7.115 MPa and a reactor operator switched the PORV operation mode from 'Manual' to 'Auto' to speed up the pressure reduction process. Immediately after that the PORV opened fully in a short time due to higher pressure beyond the PORV setpoint, which resulted in excessively rapid reduction of pressure in the main steam line system. A little later on, the operator closed the valve manually to mitigate the unexpected transient response of system. Due to the abrupt pressure reduction of steam generator and main steam line caused by the sudden opening of PORV, the reactor shut down by the low pressure signal of safety injection and some main steam separation valves seemed to be actuated. As the result of the rapid release of high pressure steam to the atmosphere through the PORV, the supports for the curved pipe spool of about 3.3 m long and 300 kg weight, which had been connecting the straight pipe line in the downstream of the PORV and a silencer for reducing noise generated by the steam discharging to the atmosphere, were broken away. At the same time, the pipe spool was separated and projected from the line. Finally it flew and collided with the outer wall of the RWST 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 the local part of wall. Although the release of radiation did not occur, it will be meaningful to find out the root-cause of such incident by calculating the transient hydraulic loads resulting in the unwelcome failure of pipe line in the downstream of the PORV for the purpose of establishing the appropriate regulatory action. Considering this as a safety-related issue, a comprehensive analysis was performed to find the root cause and evolution process of this event out