Operator actions evaluation for MSGTR accident mitigation

Abstract

The safety assessment of nuclear power plants for multiple failure accidents such as Multiple Steam Generator Tube Rupture (MSGTR) and the verification of the suitability of accident management measures have gained prominence in the nuclear engineering field after the Fukushima accident. Compared to Steam Generator Tube Rupture (SGTR), the MSGTR accident progresses rapidly due to the high break flow rate, and thus appropriate operator actions are essential to prevent the release of radioactive materials into the environment. It is crucial for the operators to identify the MSGTR early and perform each operator action within the allotted time in order to mitigate the accident successfully. In this study, a reference analysis model for MSGTR is developed, differences from SGTR are analyzed, and the required time for major operator actions are derived through various sensitivity analyses. In conclusion, the operator can confirm that there is a high possibility of MSGTR occurring through the changes in the subcooling degree at the hot-leg and the Pressurizer (PZR) water level. The appropriate time to be initiated for major operator actions are as follows: (1) Reactor Coolant Pump (RCP) manual trip within 10 min after the reactor trip, (2) Reactor Coolant System (RCS) temporary-cooldown operation through Main Steam Isolation Bypass Valve (MSIBV) and Steam Bypass Control System (SBCS) within 15 min after the reactor trip, (3) PZR spray operation – RCS temporary-cooldown operation in progress, and (4) RCS controlled-cooldown operation using Atmospheric Dump Valve (ADV) within 15 min after the end of the RCS temporary-cooldown operation. Additional possible action is the restart of RCP, but the execution of this action depends on the operator’s judgment. The results of this investigation have significant implications which can be utilized for improving the accident mitigation strategies and the operator training for MSGTR.