Advanced plants have a number of design features, active systems and attributes relying on natural processes to reduce the source term in the event of vessel failure.

Clearly corium emanating from the vessel should be appropriately quenched (in such a way to avoid a steam explosion). Ways in which this can be achieved have been discussed above.

MCCIs result in the emission of a large quantity of aerosols that carry fission products into the containment atmosphere. Possible measures to reduce MCCIs, e. g. using ‘core catchers’ have also been discussed above.

Large surface areas are useful for the plate out of aerosols. There are many natural processes, agglomeration, sedimentation, diffusiophoresis, thermophoresis and hygro — scopicity that promote deposition onto surfaces.

Internal containment sprays provide a means of entraining or dissolving air-borne fission products in water which can then be retained in the containment sump. There are chemicals such as sodium hydroxide, sodium thiosulphate or hydroxine that can be put into the water in the spray systems to enhance the removal of some fission products, especially iodine and caesium.

Fission products can be scrubbed in large pools of water. Similarly, water flooding of debris also provides a potential for scrubbing.

Elemental iodine resuspension can be reduced by the maintenance of a pH > 7 in water pools.

In the SBWR design (Naitoh et al., 1992), steam released to the drywell is channelled through a condenser. It is then condensed and then returned to the gravity-driven cooling system pools. This provides a mechanism for aerosol deposition and fission product removal.

The source term can be mitigated in some designs by introducing ventilation systems for cleaning exhaust air. The SPWR, which is a variant of the AP600, developed by Westinghouse and Mitsubishi, includes in its design an emergency passive air filtration system to mitigate releases into the lower containment penetration area. The air is filtered before being mixed with the cooling air of a PCCS system (similar to that in AP1000/600).

Ventilation systems may also be useful for designs with a secondary confinement if it became contaminated as a result of leakage from the primary containment. In some cases, primary containments are surrounded by additional containment buildings maintained at a slightly sub-atmospheric pressure. This is to ensure that residual fission products released from the primary containment do not escape. Controlled release from filters or stacks may then be considered.