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BWR Mk II Containments.

Mk II design retained the basic pressure suppression function of the Mk I containment, but rearranged the drywell and suppression pool into an “over/under” configuration as shown in Fig. 1.10. The suppression pool is below the drywell with vertical downcomers, directing the LOCA-induced steam to below the surface of the suppression pool water. This simpler vent configuration results in lower head loss in the downcomers, resulting in a lower containment design pressure than that in the Mk I design. There are eight Mk II containments in the United States. One is constructed of steel, five are reinforced concrete containments, and two are prestressed concrete containments.

Figure 1.10 shows a typical concrete containment configuration. The conical upper portion of the containment is the drywell, and the cylindrical lower portion is the suppression chamber or wet-well. The downcomer pipes penetrate the operating floor of the drywell and are immersed below the water surface in the

FIG. 1.10

BWR Mk II CONTAINMENT [5] suppression pool. Metal liner (carbon steel or stainless steel) is attached to the inside of the drywell and the wetwell, as well as to the basemat floor. The reactor vessel is supported by a pedestal that bears down on the top of the reinforced concrete basemat. The reactor vessel is surrounded by a biological shield. The reactor vessel head, as well as the drywell head, is removable for maintenance and refueling activities. The layout of the Mk II steel containment is similar to that of the concrete containment, except that reinforced concrete shield building encloses the steel containment. A low leakage reactor building encloses the entire containment (drywell and the wetwell) and protects the safety-related equipment against external missiles (see Fig. 1.10). In Mk I and Mk II containments, the spent fuel pools are located adjacent to the refueling cavity and are located inside the reactor building.

Table 1A.6 of Appendix 1A of this chapter shows the information related to all Mk II containments.

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