Investigation of primary to secondary leakage with loss of primary coolant to the environment through SG SV

• We investigated operator actions during Steam Generator Tube Rupture.
• It has been involved failure of SG safety valve in open position.
• We compare the single pipe break with break of two pipes.
• RELAP5/MOD3.2 computer code has been used to simulate the SGTR.

The paper presents the results of an investigation of Steam Generator Tube Rupture (SGTR) accident. In the paper are discussed Nuclear Power Plant (NPP) behaviour and the operator actions during primary to secondary (PRISE) leakage. During the transient, it is assumed opening and stuck in open position a Steam Generator Safety Valve (SG SV) of damaged steam generator. Opening of a SG SV will be possible to happen only in case of earlier isolation of damaged SG from a Main Steam Header (MSH). In this way the investigated accident progression will jeopardize reactor system to lose all of primary coolant, including and all emergency coolant water on the NPP Unit from one side and from the other side there is a direct by passing of confinement and an earlier releases of radionuclides.Thus, if the operator actions are not appropriate, it is possible a transition from a design accident to a severe accident and after some time the NPP will be without any emergency water.The purpose of the analysis is to demonstrate the ability to terminate primary to secondary leakage and indicate an effective strategy of operator actions for preventing secondary leakage to the environment and this way to prevent radiological release to the environment.A double-ended single pipe break and double-ended break of two pipes in SG #6 have been chosen as representative. The reason for investigation of both cases is to understand influence of increasing of break flow rate from primary to secondary side on progression of accident scenario. In addition, it has been performed to simulate two different strategies to operator emergency procedure to stability the plant without any uncover of the core.The used model (Fletcher and Schultz, 1995 and Groudev et al., 2011) has been developed at the Institute for Nuclear Research and Nuclear Energy, Bulgarian Academy of Sciences (INRNE-BAS), Sofia, for analyses of operational occurrences, abnormal events, and design bases scenarios. The model provides a significant analytical capability for specialists working in the field of NPP safety.