I—24. The term ‘negative pressure containment’ is used to describe a containment system that typically consists of the following subsystems (Fig. I—7):

(a) A containment envelope that comprises the reactor buildings, the connecting pressure relief duct, vacuum ducts, the vacuum building and all the containment extensions.

(b) A pressure relief system which comprises the pressure relief blowout panels that isolate the reactor buildings from the connecting pressure relief duct and the pressure relief valves that isolate this relief duct from the vacuum building.

(c)

A vacuum system that maintains a subatmospheric pressure inside the vacuum building, so that when this building is connected to the

FIG. I-7. Schematic diagram of a negative pressure containment system for a pressurized heavy water reactor: 1, reactor buildings; 2, vacuum building; 3, pressure relief duct; 4, blow-out and blow-in panels; 5, pressure relief valve; 6a, upper chamber; 6b, evacuation system; 7, vacuum building evacuation system; 8, vacuum building spray system; 9, dousing tank; 10, filtered air discharge system.

containment the atmosphere from the containment passes into the vacuum building.

(d) An energy suppression system, comprising a dousing tank, upper chamber vacuum system and spray header, which is housed inside the vacuum building and which can absorb all the energy released to the vacuum building.

(e) An atmospheric control system that controls the atmosphere within the reactor buildings.

(f) A filtered air discharge system to help to maintain subatmospheric pressure within the containment envelope in the long term after an accident. The reactor buildings are maintained at slightly negative gauge pressures in both operational states and post-accident conditions.

I—25. Energy management is achieved by relieving the peak pressure in the reactor building to the vacuum building via the pressure relief system, which is actuated by a small increase in pressure in the reactor building. Additional energy suppression takes place when the steam drawn into the vacuum building is condensed by the spray system, which is automatically actuated by a change in pressure in the vacuum building. Long term heat removal from the containment is achieved by the atmospheric control system that cools the building air and by the heat exchangers in the recirculation system of the emergency core cooling system. Radionuclide management is accomplished by plate-out on the internal surfaces of the containment envelope, by washout afforded by the spray and by the leaktightness of the containment envelope.