There are various measures that are included in advanced containment design for the control of pressure and temperature. The objective is to limit the pressure to below an acceptable limit and to reduce the pressure down to atmospheric pressure as quickly as possible to limit fission product release to the environment. The systems should be passive so that they can still function reliably under severe accident conditions. The heat loads can arise from decay heat and in the event of a severe core meltdown from the Zircaloy/steam exothermic reaction and also possibly from MCCIs.

11.5.1.1 Passive Containment Cooling. This method is used with a steel containment with good heat transfer characteristics. Pressure reduction using this system will be relatively slow and will also depend on the partial pressure of non-condensable gases in the containment. External cooling is enhanced by external sprays.

The AP1000/600 containments comprise an inner steel containment shell surrounded by an exterior concrete shield building, Figure 11.1. The inner steel containment not only acts as a barrier to radioactive release but also serves as an integral part of the heat release system. It is prevented from over heating by a PCCS that provides a natural circulation draught of air cooling between the steel containment shell and the shield building (Scobel and Conway, 1990). This serves to enhance the heat removal from the PCCS.

A similar approach is adopted by the simplified PWR (SPWR) of Westinghouse — Mitsubishi design. This design has been scaled up to 900 and 1200 MW units (Lillington and Kimber, 1997; Naitoh et al., 1992).

Other conceptual designs (Lillington and Kimber, 1997; Kuczera, 1992) for example have been put forward by KfK, which consist of an inner steel containment surrounded by a strong re-enforced concrete wall. Both the inner steel and the concrete walls share the loads. There is an annulus between the two shells through which air flows by natural circulation.

11.5.1.2 Condenser Systems. In this system, heat is transferred to the external atmosphere via an intermediate circuit, which carries single — or two-phase water under natural or active system circulation. As for the PCCS described above, the effectiveness of heat transfer will depend on the partial pressure of non-condensables inside the containment. Pressure reduction is also relatively slow.

In the EP1000, a finned condenser is located at the top of the concrete primary containment. This transfers heat via a thermosyphon loop through the concrete containment walls to an external heat exchanger located in a tank. This is initially immersed in water but later in the accident is air-cooled (Yadigaroglu et al., 1998).

The cooling of the containment atmosphere by a condenser is also proposed for the SWR 1000 design. This transfers the heat to a secondary system connected to an external pool.

In the CANDU 6 system, a containment condenser transfers heat to a secondary side connected to the Passive Emergency Water System Tank (Hopwood, 1999).

In the ESBWR (Orsini and Pino, 1992), a PCCS is incorporated into the design of the containment to remove decay heat from the drywell (Figure 11.2). In this system, containment steam is condensed in an external pool. Non-condensables are discharged to the suppression pool.

11.5.1.3 Internal Containment Sprays. Internal spray systems may have both significant advantages but also disadvantages. The present designs tend to have active components and are, therefore, susceptible to not functioning in a hostile environment. Passive systems have been considered but have reduced capacity and may not function
correctly in the presence of aerosols. Sprays will also not reduce the pressure if there is a significant partial pressure of non-condensables, e. g. hydrogen from metal water reactions or other gases from core concrete interactions.

11.5.1.4 Sump Water Cooling. Systems to reduce the containment pressure by cooling the sump water are another possible method. However, there needs to be good natural circulation cooling of the sump water which is necessary for effective heat removal.