Containment building

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A containment building, in its most common usage, is a steel or reinforced concrete structure enclosing a nuclear reactor. It is designed to, in any emergency, contain the escape of radiation to a maximum pressure in the range of 60 to 200 psi ( 410 to 1400 kPa). The containment is the final barrier to radioactive release (part of a nuclear reactor's Defence in depth strategy), the first being the fuel ceramic itself, the second being the metal fuel cladding tubes, the third being the reactor vessel and coolant system.^[1]

The containment building itself is typically an airtight steel structure enclosing the reactor normally sealed off from the outside atmosphere. The steel is either free-standing or attached to the concrete missile shield. In the United States, the design and thickness of the containment and the missile shield are governed by federal regulations (10 CFR 50.55a) ^[2].

While the containment plays a critical role in the most severe nuclear reactor accidents, it is only designed to contain or condense steam in the short term (for large break accidents) and long term heat removal still must be provided by other systems. In the Three Mile Island accident the containment pressure boundary was maintained, but due to insufficient cooling, some time after the accident, radioactive gas was intentionally let from containment by operators to prevent over pressurization. This, combined with further failures caused the release of radioactive gas to atmosphere during the accident.^[3]

1 Types
2 Requirements
3 References
4 See also

[edit] Types

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.

Containment systems for nuclear power reactors are distinguished by size, shape, materials used, and suppression systems. The kind of containment used is determined by the type of reactor, generation of the reactor, and the specific plant needs.

Suppression systems are critical to safety analysis and greatly affect the size of containment. Suppression refers to condensing the steam after a major break has released it from the cooling system. Because decay heat doesn't go away quickly, there must be some long term method of suppression, but this may simply be heat exchange with the ambient air on the surface of containment. There are several common designs, but for safety-analysis purposes containments are categorized as either "large-dry," "sub-atmospheric," or "ice-condenser."

[edit] Pressurized Water Reactors

For a pressurized water reactor, the containment also encloses the steam generators and the pressurizer, and is the entire reactor building. The missile shield around it is typically a tall cylindrical or domed 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).

Three Mile Island was an early PWR design by Babcock and Wilcox, and shows 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

A German plant exhibiting 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 this painted containment at 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.

[edit] Boiling Water Reactors

In a BWR, the drywell plays a larger role than for a PWR and the containment and missile shield fit close to the reactor vessel. The reactor building wall forms the secondary containment during normal operation and refueling operations. The containment designs are referred to by the names Mark I (oldest; drywell/torus), Mark II, and Mark III (newest). All three types house a large body of water used to quench steam released from the reactor system during transients.

From a distance, the BWR design looks very different from PWR designs because usually a square building is used for containment. Also, because there is only one loop through the turbines and reactor, and the water going through the turbines is also slightly radioactive, the turbine building has to be considerably shielded as well. This leads to two buildings of similar construction with the taller one housing the reactor and the short long one housing the turbine hall and supporting structures.

A representitive one-unit German BWR showing containment around both the turbine and reactor buildings

A typical two-unit BWR at the Brunswick Nuclear Generating Station

[edit] CANDU Plants

CANDU power stations make use of a wider variety of containment designs and suppression systems than other plant designs. Due to the nature of the core design, the size of containment for the same power rating is often larger than for a typical PWR, but many innovations have reduced this requirement.

Many multiunit CANDU stations utilize a water spray equipped Vacuum Building. All individual Candu units on site are connected to this Vacuum building by a very large pipe and as a result require a small containment themselves. The Vacuum building rapidly condenses any steam from a postulated break, allowing the unit's pressure to return to subatmospheric conditions. This minimizes any possible fission product release to the environment.^[4]

Additionally, there have been similar designs that use double containment, in which containment from two units are connected allowing a larger containment volume in the case of any major incident. This has been pioneered by one Indian HWR design where a double unit and suppression pool was implemented.

The most recent Candu designs, however, call for a single conventional dry containment for each unit.^[5]

The Bruce A Generating Station, showing a large vacuum building serving 4 separate units that have a BWR-like shielding around them individually

The Qinshan Nuclear Power Plant is two-unit site where the containment system is autonomous for each unit

A single unit of the Pickering Nuclear Generating Station, showing a slightly different shape from a typical PWR containment, which is mostly due to the larger footprint required by the Candu design

[edit] Requirements

Title 10 of the Code of Federal Regulations, Part 50, Appendix J provides the basic design criteria for lines penetrating the containment wall. Each large pipe penetrating the containment, such as the steam lines, has isolation valves on it, configured as allowed by Appendix J; generally two valves ^[6]. For smaller lines, one on the inside and one on the outside. For large, high-pressure lines, space for relief valves and maintenance considerations cause the designers to install the Appendix J valves near to where the lines exit containment. In the event of a leak in the high-pressure piping that carries steam and feedwater, these valves on high pressure systems rapidly close to prevent radioactivity from escaping the containment. Valves on lines for standby systems penetrating containment are normally closed.

During normal operation, the containment is air-tight and access is only through marine style airlocks. High air temperature and radiation from the core limit the time, measured in minutes, people can spend inside containment while the plant is operating at full power. In the event of a worst-case emergency, called a "design basis accident" in NRC regulations, the containment is designed to seal off and contain a meltdown. Redundant systems are installed to prevent a meltdown, but as a matter of policy, one is assumed to occur and thus the requirement for a containment building. For design purposes, the reactor vessel's piping is assumed to be breached, causing a "LOCA" (Loss Of Coolant Accident) where the water in the reactor vessel is released to the atmosphere inside the containment and flashes into steam. The resulting pressure increase inside the containment, which is designed to withstand the pressure, triggers containment sprays ("dousing sprays") to turn on to condense the steam and thus reduce the pressure. A SCRAM ("neutronic trip") initates very shortly after the break occurs. The safety systems close non-essential lines into the air-tight containment by shutting the isolation valves. Emergency Core Cooling Systems are quickly turned on to cool the fuel and prevent it from melting. The exact sequence of events depends on the reactor design, for ABWR see ^[7] pages 15A-37 and -38, for CANDU see ^[8] slides 21, 23 and 25.

Containment buildings in the U.S. are subjected to Containment Integrated Leakage Rate Tests (CILRTs) on a periodic basis, both to identify the possible leakage in an accident and to locate and fix leakage paths. ^[9]

In 1988, Sandia National Laboratories conducted a test of slamming a jet fighter into a large concrete block at 481 miles per hour (775 km/h) ^[10]^[11]. The airplane left only a 2.5-inch deep gouge in the concrete. Although the block was not constructed like a containment building missile shield, it was not anchored, etc., the results were considered indicative. A subsequent study by EPRI, the Electric Power Research Institute, concluded that commercial airliners did not pose a danger. ^[12]

The Turkey Point Nuclear Generating Station was hit directly by Hurricane Andrew in 1992. Turkey Point has two fossil fuel units and two nuclear units. Over $90 million of damage was done, largely to a water tank and to a smokestack of one of the fossil-fueled units on-site, but the containment buildings were undamaged ^[13]^[14].