I—19. The weir wall pressure suppression containment system (Fig. I-6) in boiling water reactors consists of three different structures: the dry well, the containment envelope and the reactor building.

FIG. I-6. Schematic diagram of a weir wall pressure suppression containment system (the reactor building with its confinement function is not shown) for a boiling water reactor: 1, containment; 2, dry well; 3, suppression pool (weir well type); 4, containment spray system; 5, suppression pool cooling system; 6, hydrogen control system; 7, filtered air discharge system; 8, liner.

I-20. The function of the dry well structure is to enclose the reactor pressure vessel completely, and to create a pressure boundary to separate the reactor pressure vessel and its recirculation system from the containment vessel and the main body of the suppression pool. The dry well structure vents the steam — air mixture to the suppression pool. It also provides radiation shielding from the reactor and the piping of the nuclear steam supply system. The weir wall portion of the dry well structure functions as the inner wall of the suppression pool and serves to channel the steam released by a postulated LOCA through horizontal submerged vents into the suppression pool for condensation.

I—21. One of the functions of the reactor building is to provide protection against external missiles for the containment envelope, personnel and equipment. It also provides shielding from the fission products in the secondary confinement envelope, functions as a secondary containment barrier and provides a means for the collection and filtration of leaks of fission products from the steel containment vessel following a LOCA.

I—22. In postulated LOCA conditions, the pressure rise in the dry well reduces the water level between the weir wall and the wall of the dry well structure, uncovering the vents in the wall of the dry well structure, and forces the steam— air mixture in the dry well structure through the vents and into the suppression pool. The steam is condensed in the suppression pool water. Fission product noble gases and other non-condensables from the dry well structure escape from the surface of the pressure suppression pool into the containment envelope.

I—23. In the long term, an active spray system is used to reduce pressure and to reduce the concentration of airborne radionuclides within the containment envelope. This system takes water from the suppression pool by suction through a heat exchanger, following which the water is pumped to spray headers located in the dome of the containment envelope.