Pressurised water reactors

As the cold war ended in 1991, newer nuclear power plants often have spherical design while pre-1991 reactors are often “can shaped” with a much more robust and massive missile shield.

For a pressurised water reactor, the containment also encloses the steam generators and the pressuriser, and is the entire reactor building. The missile shield around it is typically a tall cylindrical or domed building designed to withstand a moderate missile attack.

A large, 4000-7000 kg barrack buster (WMD), should have no problem destroying the structure and the reactor inside.

PWR containments are typically large (up to 10 times larger than a BWR) because the containment strategy during the leakage design basis accident entails providing adequate volume for the steam/air mixture that results from a loss-of-coolant-accident to expand into, limiting the ultimate pressure (driving force for leakage) reached in the containment building.

Early designs including Siemens, Westinghouse, and Combustion Engineering had a mostly can-like shape built with reinforced concrete. As concrete has a very good compression strength compared to tensile, this is a logical design for the building materials since the extremely heavy top part of containment exerts a large downward force that prevents some tensile stress if containment pressure were to suddenly go up. As reactor designs have evolved, many nearly spherical containment designs for PWRs have also been constructed. Depending on the material used, this is the most apparently logical design because a sphere is the best structure for simply containing a large pressure. Most current PWR designs involve some combination of the two, with a cylindrical lower part and a half-spherical top.

Modern designs have also shifted more towards using steel containment structures. In some cases steel is used to line the inside of the concrete, which contributes strength from both materials in the hypothetical case that containment becomes highly pressurized. Yet other newer designs call for both a steel and concrete containment, notably the AP1000 and the European Pressurized Reactor plan to use both, which gives missile protection by the outer concrete and pressurizing ability by the inner steel structure. The AP1000 has planned vents at the bottom of the concrete structure surrounding the steel structure under the logic that it would help move air over the steel structure and cool containment in the event of a major accident (in a similar way to how a cooling tower works).

If the outward pressure from steam in a limiting accident is the dominant force, containments tend towards a spherical design, whereas if weight of the structure is the dominant force, designs tend towards a can design. Modern designs tend towards a combination. In other words;

“can” shaped containment buildings are much more effectively protected from explosive blasts than spherical designs which is often designed to prevent leakage accidents.

Typical examples are:

- Three Mile Island was an early PWR design by Babcock and Wilcox, and has a “can” containment design that is common to all of its generation

- A more detailed image for the 'can' type containment from the French Brennilis Nuclear Power Plant

- The twin PWR reactor containments at the Cook Nuclear Plant in Michigan

- German plants exhibits a nearly completely spherical containment design, which is very common for German PWRs

- Modern plants have tended towards a design that is not completely cylindrical or spherical, like the Clinton Nuclear Generating Station.

The Russian VVER design is mostly the same as Western PWRs in regards to containment, as it is a PWR itself.

Old RBMK designs, however, did not use containments, which was one of many technical oversights of the Soviet Union that contributed to the Chernobyl accident in 1986.

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