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BWR Mk I Containments.

BWR Mk I containment consists of a light-bulb shape drywell, a torus (wetwell) containing water required for suppression of the fluid pressure and temperature discharged during the postulated design basis accident. Large diameter steel vent pipes connect the drywell and the torus (see Figs. 1.7 and 1.8). The steel drywell encloses the reactor pressure vessel with removable head, a steel-lined concrete sacrificial shield wall (in Fig. 1.8, it is termed as biological shield), and associated

FIG. 1.7

BWR Mk I STEEL CONTAINMENT [7] pumps and recirculation pipes. The reactor pressure vessel is supported by a pedestal supported on a reinforced concrete floor. The drywell shell at the bottom of the light bulb is embedded between the RC floor and the concrete fill, which is bearing on the reactor building basemat. Ashar and Tan [7] provide the performance experience of MK I containments. The steel drywell encloses the reactor pressure vessel with removable head, a steel-lined sacrificial shield wall (in Fig. 1.8, it is termed as biological shield wall) and associated pumps and recirculation pipes.

The reactor pressure vessel is supported by a pedestal which in turn is supported on a reinforced concrete floor. The drywell shell at the bottom of the light bulb is embedded between the RC floor and the concrete fill, which is bearing on the reactor building basemat. Figure 1.8 shows the components of a typical MK I containment. As Fig. 1.8 is taken from an operating reactor, it shows an area of corrosion found during a routine inspection. Readers can find additional description of this and other events in NUREG-1522 (1995) [8].

If a pipe rupture occurs inside the drywell, the drywell becomes pressurized by steam blowing down from the reactor coolant system. The fluid discharged during an accident has to go through the vent pipes and is collected in the vent header, which in turn distributes it to the downcomers immersed below the water surface. The torus is supported by steel columns, which are bearing on the reactor building basemat. In Fig. 1.8, a sand cushion is shown, just below the vent pipe. The function of the sand cushion is to allow the drywell structure to flex under high temperatures and pressures. Above the operating floor (see Fig. 1.7), the refueling bay area is typically surrounded by a steel structure with blow-out panels. During high winds, the panels are blown out to relieve the pressure on the structure. Figure 1.9 shows the basic layout of an Mk I concrete containment.

The basic function of drywell, suppression chamber, and the vent pipes is the same as that noted for Mk I steel containment. In the case of Mk I concrete containments, the drywell and the torus are steel-lined concrete containments. Most of the other details related to the inside of the drywell and torus are similar to that described for the Mk I steel containment.

In Fig. 1.9, more details are shown related to the structures surrounding the drywell and the torus. The Mk I containment (RC or steel) is enclosed by a low leakage reinforced concrete Reactor Building, which serves as a barrier against external missile, as well as a low leakage secondary containment. Refueling cavity, where the fuel assemblies are transferred from the reactor to the adjoining spent fuel pool is located on the top of the drywell. Out of 22 Mk I containments, 20 are steel and 2 are steel-lined concrete containments. Table 1A.5 of Appendix 1A of this chapter shows the plant-specific parameters of Mk I containments.

FIG. 1.8

BWR Mk I CONTAINMENT COMPONENTS [7]

FIG. 1.9

BWR Mk I CONCRETE CONTAINMENT [5]

 
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