Study of Thermal Hydraulic Behaviors during Multiple Steam Generator Tube Rupture Events in PWR

Since the occurrence probability of multiple steam generator tube rupture (MSGTR) in a PWR is considered to be low, analytical or experimental investigation to prevent such accidents has not been performed explicitly. As new Japanese regulations require continuous effort to enhance the safety of nuclear power plants, low-probability but high-consequence events such as MSGTR should be taken into account to increase safety. In this study, a thermal-hydraulic analysis of multiple tube ruptures in a steam generator (SG) or all the SGs in a station blackout (SBO) situation was performed using the RETRAN-3D code in order to clarify the plant behavior during an MSGTR event and to contribute to risk reduction. The results show that a water supply function to SGs is important to cope with accidents involving MSGTR+SBO to prevent core damage. Furthermore, if the auxiliary feed water system loses its function when 10 tubes rupture in an SG, the time to core exposure is within 1 h and much shorter than that for SGTR, i.e., single tube rupture, in an SG. Therefore, in order to prevent core damage during MSGTR, it is desirable to have an alternative water injection system to quickly replace the auxiliary feed water system.