Having set the limits on any radioactive release which may be permitted from the plant and having designed the plant to be accident free and so to give rise to no emissions, nevertheless it is prudent to design the contain­ment to provide protection in the event of an accident. Thus a containment evaluation accident is chosen as the worst accident to which the plant could ever be subjected, and this accident is used to assess the adequacy of the containment design. In many cases there will be an iteration between the calculation of accident conditions and the design of the containment.

Figure 5.5 shows that for the particular containment concept chosen, a radioactive dose in excess of the limits set by 10 CFR 100 could be ex­perienced following a core disruptive accident or a sodium fire with high radioactive contamination in the sodium only if a large number of safety features had failed or were not provided. Such a fault tree demonstrates that, for radioactive limits to be exceeded, a large number of conditions must arise at the same time. This can be seen by the large number of INHIBIT and AND gates present.

The sodium fire analysis is treated in Chapter 4 and the core disruptive accident will be treated in full in Sections 5.4 and 5.5 of this chapter. It suffices here to say that the two accidents either alone or together could give pressures in the range of 5-35 psia within the containment volume and temperatures of 200-700°F on the inner surfaces of the containment walls. Under these conditions the containment building is designed to comply with the leakage rates required to meet the dose limitations at the site boundaries.