CHAPTER 2: SEVERE ACCIDENT PHENOMENA AND MITIGATION STRATEGIES

Containment sub-atmospheric pressure

Challenges

The evolution of a severe accident causes the containment pressure to rise, due to the various gases that are transported to the containment (steam, hydrogen, CO, CO2) and the deposit of stored and decay heat in the containment, as has been discussed.

The pressure rise can lead to the need to vent the containment. In most cases, the vent valves are closed again once a lower pressure has been obtained (in some cases, there are rupture discs which after rupture stay open permanently). They are opened up again if pressure again will rise above the venting set point.

In the longer term, when also containment cooling has been restored and the steam is condensed, the containment pressure will decrease to atmospheric pressure and below, because during the venting, part of the air in the original containment atmosphere has been expelled. This may even occur without venting, because the containment always leaks somewhat, so containment contents are lost. Late spraying, for example to scrub fission products, may then also bring the containment below ambient pressure.

If the containment fails due to overpressure in the form of cracks and crevices (i.e. no catastrophic failure), steam condensation may also lead to sub-atmospheric pressure and cause further failures.

Removal of hydrogen by combustion or recombination will absorb the oxygen, and condensing the steam then may also lead to sub-atmospheric pressure, even without venting.

Containments are strong against internal pressure, but are usually weak against sub-atmospheric pressure. Appropriate consideration and mitigation of this challenge is therefore needed.

Strategies

Many containments have vacuum breakers, which may effectively mitigate a sub-atmospheric pressure. It should be checked whether their design would be capable to mitigate this severe accident challenge, as they have not been designed for this function.

Other strategies may be to stop mechanisms that extract heat from the containment (e.g. stop sprays, fan coolers), and strategies that bring additional mass and /or energy into the containment (e.g. by opening pressuriser PORVs, or RCS vent valves (on pressuriser, reactor head). Note: valves must be opened and kept open for the duration of the mitigation process, which is notably relevant if PORV need the system pressure to open.