I-14. The pressure suppression containment system (Fig. I-5) in boiling water reactors is divided into two main compartments: a dry well housing the reactor coolant system and a wet well partly filled with water, whose function is to condense steam in the event of a LOCA. The two compartments are connected by pipes that are submerged in the water of the wet well. Spray systems are usually installed in both the dry well and the wet well. The reactor building surrounding the containment forms a secondary confinement which captures leaks from the containment. The containment envelope usually consists of either a concrete structure with a steel liner for leaktightness or a steel shell.

I-15. The purpose of the pressure suppression system is to reduce the pressure if a pipe in the reactor coolant system ruptures. The steam from a leak in these pipes enters the dry well and is passed through pipes into the water of the suppression pool (wet well), where it condenses, and the pressure in the dry well is reduced. The pressure suppression system helps in reducing the concen­trations of airborne radioiodines by scrubbing radionuclides from the steam.

I-16. The wet well is also used as a heat sink for the automatic pressure relief system. This serves to limit the pressure rise in the reactor coolant system when the reactor cannot discharge steam to the turbine condenser system. The steam still produced by residual heat after shutdown of the reactor is passed into the water in the wet well via safety relief valves connected to the steam pipes within the dry well.

I-17. The concrete or steel structure of the reactor building surrounding the containment serves as protection against external events.

I-18. The reactor building is held at a slightly negative gauge pressure in both operational states and accident conditions. In the event of an accident, leaks from the dry well into the reactor building are extracted and filtered by an air removal system to permit the use of controlled emission from the plant stack.