Westinghouse developed a pressure suppression containment, which uses ice to reduce the pressure. The loop pipework is in the lower part of the containment but the operating floor effectively separates the lower levels of the containment from the open main dome area. The two volumes are linked by a structure surrounding the NSSS, which contains refrigerated baskets of ice. The steam/ water mixture, released from the breach, flows through these and loses energy.

The use of pressure suppression containments is much more common in BWRs. In the US GE designed the containments and the NSSS and developed a series of containments as illustrated in Fig. 10.18. Each consists of a suppression pool and a main containment volume. The geometry is such that discharges from the RCS pass through the suppression pool to absorb energy. The suppression pool also provides the heat sink to quench discharges from the vessel safety relief valves and the ADS.

Systems for severe accident mitigation

LWR containments cover a wide range of strengths and volumes, which is illustrated in Fig. 10.19. Not surprisingly the pressure suppression containments have either smaller volumes or lower design pressures. Following the accident at Three Mile Island Unit 2, the need to look at the mitigation of potential beyond design basis accidents was highlighted. In particular the need to manage hydrogen production during severe accidents was identified as an issue to be addressed.

The existing plants have systems to manage the production of hydrogen post LOCA by radiolysis; however, this cannot deal with the rate of production, which

10.18 Schematic of GE BWR containments.

10.19 Typical containment volumes and design pressures of US plants (Hessheimer and Dameron, 2006).

occurs from steam zirconium reactions if the fuel clad secondary temperature limits (~1200 °C) are exceeded. This leads to the possibility that hydrogen concentrations may build up in the containment to the level where if ignited the resultant hydrogen explosion may exceed the containment ultimate failure pressure. The ultimate failure pressure typically exceeds the design pressure by a factor of 2 or more (Hessheimer and Dameron, 2006) and so large dry containments can withstand a large hydrogen burn, but the pressure suppression containments do not have a large enough margin.

The issue is the rate of release of energy since the pressure suppression systems can cope with the integrated energy release but not the instantaneous pressure increase. The solution was therefore to add passive autocatalytic recombiners to combine the hydrogen with oxygen at below the lower flammability limits or to install hydrogen igniters which would ensure combustion occurs at close to the lower flammability limit. Igniters were fitted to ice condenser and BWR Mk III containments but it was decided to inert the atmosphere of the smaller BWR Mk I and II containments. Subsequently passive autocatalytic recombiners have been installed on a number of large dry containments in Europe to provide additional defence in depth.

Severe accident management guidelines were developed for existing plants and this led to the introduction of additional systems. The prevention of containment failure following a severe accident was seen as the main focus for these additional accident management procedures. One means of achieving this was to provide
additional (usually mobile) pumps to inject water to cool the containments. In the case of BWRs water could also be injected directly into the primary circuit because dissolved boron is not used as a primary means of reactivity control so dilution by the injection of unborated water is not an issue. In other plants (e. g. Sizewell B) alternative means of providing or reinforcing existing containment cooling systems were provided.

Addition of water to the containment increases the heat sink available but does not provide a heat removal route. One means of removing energy from the containments is by periodically venting the steam generated. A number of plants back fitted filtered venting systems, which allowed the containment to be vented whilst reducing the activity discharged. The most common filters used were sand/gravel beds and water scrubbers. In some BWRs pressure-retaining vent lines were added, which allowed the venting of the smaller containments at an early stage, before significant activity is present in the atmosphere, to prevent failure.

The use of venting as a means of controlling containment failure is particularly important for steel containments. The research carried out at Sandia Laboratories (Hessheimer and Dameron, 2006) showed that there was a large margin between the design pressure and the ultimate failure pressure for both steel and concrete containments, but the failure modes tended to be different. The failure of the steel containment tended to be associated with a rapidly propagating ductile fracture rapidly releasing the stored energy. On the other hand the concrete containments failed by liner tearing and gross leakage rather than by the failure of the reinforced/ pre-stressed concrete structure. Thus the provision of venting systems on steel containments ensures a more benign failure mode in addition to reducing the release.