Containment building

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A containment building, in its most common usage, is a steel or concrete structure enclosing a nuclear reactor. It is designed to, in any emergency, contain the escape of radiation despite pressures in the range of 60 to 200 psi ( 410 to 1400 kPa ). The containment is the final barrier to radioactive release, the first being the fuel ceramic itself, the second being the metal fuel cladding tubes, the third being the reactor vessel and coolant system.

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) [1].

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. There are several common designs, but for safety-analysis purposes containments are categorized as either "large-dry," "sub-atmospheric," or "ice-condenser."

For a boiling water reactor, the containment and missile shield fit close to the reactor vessel. The reactor building wall forms a secondary containment during 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.

The CANDU system is detailed in the "Nuclear Tourist" Reference below. Multiunit CANDU stations utilize a water spray equipped Vacuum Building to rapidly condense any steam from a postulated break and return containment to subatmospheric conditions. This minimizes any possible fission product release to the environment.

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 [2]. 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 [3] pages 15A-37 and -38, for CANDU see [4] 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. [5]

In the Soviet Union it was normal practice not to build containment buildings. This, along with the unstable nature of the RBMK reactors, led to the catastrophe of the Chernobyl accident [6]. In the case of these types of reactors it would be more proper to refer to the building housing the reactor as a reactor building rather than as a containment building.

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) [7] [8]. 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. [9]

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 [10] [11].