Equipment is provided to contribute directly to re­ducing the amount of radioactivity which reaches the external environment, once it has been released into the atmosphere inside the reactor building or primary containment. This equipment comprises the primary containment itself; its isolation system; the contain­ment spray svstem; and the secondary containment.

The primary containment is a massive prestressed reinforced concrete structure designed to withstand the high pressures reached following an energetic re­lease of mass and energy into its internal atmosphere, for example due to a LOCA, The basic shape is a right circular cylinder, about 46 m diameter and 42 m high, topped by a hemispherical dome. The general wall thickness is 1.3 m and the design pressure is 4.46 bar. The whole of the containment is lined inter­nally with a 6 mm thick welded carbon steel liner, which is contiguous with pre-engineered steel pene­tration assemblies that accommodate pipes and cables passing through the containment wall.

The containment isolation system ensures that all those penetrations not needed to remain open to pro­vide emergency services are closed automatically, should a rise in pressure or activity inside the containment be detected. This arrangement ensures that leakage through the containment boundary іь reduced to a very low level (less than 0.1го of the containment atmosphere during a 24 hour period) even at full de­sign pressure. Suitable leak rates tests are performed periodically to check that building integrity is being maintained and leakage is within limits.

The containment spray system is provided to assist in removing radioactivity from the primary contain­ment atmosphere so that it is no longer available for release via leakage from the containment building. This is achieved by spraying the maximum practicable proportion of the containment atmosphere with water droplets from sets of spray nozzles attached to a pair of spray headers affixed to the underside of the containment dome. Each header is supplied with re­latively cool borated water at about 800 m3/h from an external storage tank via a centrifugal pump located in the; auxiliary building basement; these pumps are started automatically when the atmospheric pressure rises above 2.4 bar.

The droplets tend to capture both particulate and volatile radioactive fission products as they fall through the atmosphere. Eventually the spray water reaches the containment floor and mixes with any water spilt from the primary or secondary circuits. It is then re­circulated as required by both the ECCS and the spray pumps drawing from the recirculation sumps. The pH of the recirculated water is adjusted by allowing it to dissolve quantities of tri-sodium phosphate ОІазРОд), normally stored as dry chemical in racks at the bot­tom of the containment; this offsets the boric acid dissolved in the water for reactivity control purposes and promotes slightly alkaline conditions, which are conducive to the continuing removal of fission pro­ducts such as elemental iodine.

The secondary containment, as its name implies, deals with any radioactive material that escapes from the primary containment. It consists of a steel framed and metalclad enclosure building, which completely covers the dome and other exposed faces of the pri­mary containment; the auxiliary building and the steam and feed cell, which are structural steel and reinforced concrete structures that adjoin the primary contain­ment; and part of the fuel building, comprising a buffer zone adjacent to the primary containment in the area of the fuel transfer tube, and connected to the enclosure building itself.

The enclosure building is partially supported by, but structurally separate from, the auxiliary building, to which it is sealed. It encloses all parts of the primary containment not surrounded by other buildings, and incorporates a housing which covers the platform outside the equipment hatch.

The auxiliary building, containing the fluid au- jliarv systems, encloses areas of the containment wall containing the majority of mechanical and electrical penetrations. Ducts and pipes penetrating the aux­iliary building wall are provided with redundant val­ues or dampers arranged to do>e automatical!;, on detection of high activity in the auxiliary building, or a safety injection signal indicative of a LOCA or SLB/FLB.

Activity leaking into the secondary containment is collected and processed by filtration equipment of at least 99% efficiency before it reaches the external environment. An emergency exhaust system is pro­vided to draw potentially-contaminated air from the auxiliary building, the steam and feed cell and the enclosure building. The action of the exhaust system creates a slight atmospheric depression inside the sec­ondary containment boundary to prevent untreated out-leakage. Redundant trains of high and moderate efficiency particulate air filters, charcoal absorber units and dehumidifiers are incorporated in the outlet duct­work, and the multiple redundant exhaust fans are powered from diesel-backed supplies. The secondary containment provisions described above are claimed to reduce the radioactive dose to members of the public from LOCA faults by at least a factor of 10.