Containment pressurisation
Challenges
Containment pressurisation occurs upon adding mass or energy, or both, to the containment. The core damage process yields addition of steam, hydrogen, CO and CO2 to the containment, as is discussed above. Steam may already have been released to the containment previously (i.e. before core damage) by the initiating event, such as a large break LOCA, or by the discharge of safety and relief valves in a transient. Notably for smaller containments the pressure rise may be fast (hours) if safety relief valves discharge into their atmosphere, with a temporary holdup in the suppression pool (e.g., BWR Mark I and II).
Part of these materials may disappear through bypass or leakage from the containment or even released directly from the RCS (ISLOCA).
The pressure rise, hence, is a combination of mass and energy, discharged into the containment. Hydrogen ignition or recombination brings also additional energy into the containment. From recombiners, the exhaust gases may be released close to the containment wall, and hence bring considerable local thermal loads to the containment.
The severe accident management itself may also contribute to the pressure rise. Cavity flooding was already mentioned, but also long term measures may load the containment. Such as in the final state the containment is flooded to the top of active fuel to cover all debris, including that which has remained inside the vessel. This may severely decrease the free volume of the containment, notably in BWRs with a high vessel location, and also bring the static load of the water to the containment bottom and lower walls.
Threats should also be identified for seals and penetrations. These suffer also from local high temperatures resulting from combustion or recombination. An example of a vulnerable location is the seal of the drywell hood of BWRs. Real world experience from the accidents at Fukushima shows that all three reactor accidents encountered containment failures in the vicinity of the upper drywell head, presumably due to containment overpressure and excessive strain in the drywell head bolts. This is evident from the high radiation in the drywell head flange region and by analytical assessment of the accidents. Read more →
Finally, threats should also be identified for instrumentation inside the containment. Where instruments will deviate due to the harsh environmental conditions, such deviations should be specified and, where possible, quantified. An example is the level indication in the steam generator, which deviates if the containment pressure rises. It may be useful to determine a containment fragility curve, so that both strengths and weaknesses of the containment can be quantified.
Strategies
Various tools can be used to depressurise the containment:
  - Fan coolers or other heat sinks;
  - Sprays;
  - Ventilation systems;
  - Dedicated containment coolers;
  - Containment vent systems.
The use of ventilation systems may overload available filters and cause a large release outside. Dedicated filtered containment vent systems are available at NPPs in many countries.
Using ventilation systems requires caution for H2 combustion, as the steam may condense in the filter, whereupon pure hydrogen will be released, likely resulting in combustion downstream of the filter.
Using sprays should be done with care, so as not to de-inert an initially inert containment, if hydrogen burns should be avoided.
In determining at which pressure the containment should be vented, it should be considered that the existing pressure may be augmented by a pressure spike from hydrogen/CO combustion. This may lead to a lower pressure at which venting becomes necessary than originally anticipated.
In manual operation of the vent valves, one should consider that the environment of the valves is extremely radioactive, which may hamper local operation. In addition, operation without the need for external power (AC, DC) is preferable, as containment failure by overpressure must be avoided under all circumstances.
Apart from plant sprays, also spraying a containment building from outside (e.g., by the fire brigade) may be considered. Venting a BWR containment should be done via the suppression pool, to scrub fission products. Venting from the drywell should only be considered as a last resort.
It should be noted that containment venting may already be applied at the beginning of the accident, before there is any (substantial) release of fission products, to create additional margin for any pressure build-up during the severe phase of the accident, i.e. when hydrogen, CO, CO2 are released together with many fission products. In addition, time will be gained for repairs and restoration actions.
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