Containment building
It is designed, in any emergency, to contain the escape of radioactive steam or gas to a maximum pressure in the range of 275 to 550 kPa (40 to 80 psi).
In the United States, the design and thickness of the containment and the missile shield are governed by federal regulations (10 CFR 50.55a), and must be strong enough to withstand the impact of a fully loaded passenger airliner without rupture.
[5] While the Fukushima Daiichi plant had operated safely since 1971, an earthquake and tsunami well beyond the design basis resulted in failure of AC power, backup generators and batteries which defeated all safety systems.
Hydrogen leaking from the containment mixed with air, resulted in explosions in units 1, 3 and 4, complicating attempts to stabilize the reactors.
Early designs including Siemens, Westinghouse, and Combustion Engineering had a mostly can-like shape built with reinforced concrete.
During the theoretical leakage design basis accident, the reactor coolant flashes to steam in the drywell, pressurizing it rapidly.
The Mark I is the oldest, distinguished by a drywell which resembles an inverted lightbulb above the wetwell which is a steel torus containing water.
Both use a lightweight steel or concrete "secondary containment" over the top floor which is kept at a slight negative pressure so that air can be filtered.
A refueling platform has a specialized telescoping mast for lifting and lowering fuel rod assemblies with precision through the "cattle chute" to the reactor core area.
[6] The Mark III uses a concrete dome, somewhat like PWRs, and has a separate building for storing used fuel rods on a different floor level.
All three types also use the large body of water in the suppression pools to quench steam released from the reactor system during transients.
Unit 3 suffered a particularly spectacular explosion which created a plume of debris over 300 m high which resulted in a collapse of the north end of the top floor, and buckled concrete columns on its west side as can be seen by aerial photographs.
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.
All individual CANDU units on site are connected to this vacuum building by a large pressure relief duct which is also part of containment.
In the event of a leak in the high-pressure piping that carries the reactor coolant, these valves rapidly close to prevent radioactivity from escaping the containment.
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.
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.
A nuclear plant is required by its operating license to prove containment integrity prior to restarting the reactor after each shutdown.
[14] In 1988, Sandia National Laboratories conducted a test of slamming a jet fighter into a large concrete block at 775 km/h (482 mph).
A subsequent study by EPRI, the Electric Power Research Institute, concluded that commercial airliners did not pose a danger.
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.
