CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

By-pass of the containment

Challenges

Depending on the initiating event, the containment could be bypassed or have failed already before the onset of core damage or could fail as a consequence of events occurring during the progression of the accident. For example, external events may cause damage to the containment before there is any fuel damage. Alternatively, a leakage path outside the containment can develop if there is a failure in the containment isolation system.

Initiating events such as Interfacing System Loss-of-Coolant Accidents (ISLOCAs) or a steam generator tube rupture will provide a direct path for fission products to escape from the reactor coolant system outside the containment.

Events during the course of an accident, such as a consequential steam generator tube rupture, notably an induced steam generator tube creep rupture, can lead to containment bypasses. The induced steam generator tube creep rupture is one of the major threats, as it would occur early in the accident, at transients where the RCS pressure remains high, such as an unmitigated station black-out. See EPRI TBR, Vol. 2, Appendix I Read more →

Strategies

In the preceding section - strategies to mitigate large releases - strategies are mentioned to isolate or reduce leakages from all release locations. Similar strategies could be used to mitigate containment bypass challenges.

One such strategy is aimed at preventing a containment bypass situation that might result from an induced steam generator tube creep rupture. This can be done by flooding the steam generator secondary side and/or by depressurising the RCS. Depressurisation of the RCS is possible through feeding one or more steam generators, if feed water is available, or through the primary power operated relief valves (PORVs) or similar valves (dedicated valves, as some NPPs have). Although the actual value is plant specific, often the risk of steam generator tube creep rupture can be reduced if the pressure difference between the RCS and the steam generator is smaller than 35 bar (actual value is plant specific). Note that maintaining steam generator secondary side water is a key mitigating strategy in maintaining decay heat removal and therefore RCS coolant inventory for the PWR, thereby avoiding core damage in the first place.

Flooding the secondary side of the steam generators will also scrub any fission products that escape through ruptured tubes. A second and important objective is to reduce the RCS pressure so as to enhance the capability to restore injection into the RCS.