page_title: Containment building
section_title: Boiling water reactors
hierarchical_section_title: Containment building / Types / Boiling water reactors
context_page_description: A containment building, in its most common usage, is a reinforced steel or lead structure enclosing a nuclear reactor. 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. The containment is the fourth and 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.
  Each nuclear plant in the US is designed to withstand certain conditions which are spelled out as "Design Basis Accidents" in the Final Safety Analysis Report. The FSAR is available for public viewing, usually at a public library near the nuclear plant.
  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, and must be strong enough to withstand the impact of a fully loaded passenger airliner without rupture.
context_section_description: In a BWR, the containment strategy is a bit different.  A BWR's containment consists of a drywell, where the reactor and associated cooling equipment is located, and a wetwell.  The drywell is much smaller than a PWR containment and plays a larger role. During the theoretical leakage design basis accident the reactor coolant flashes to steam in the drywell, pressurizing it rapidly.  Vent pipes or tubes from the drywell direct the steam below the water level maintained in the wetwell (also known as a torus or suppression pool), condensing the steam, limiting the pressure ultimately reached.  Both the drywell and the wetwell are enclosed by a secondary containment building, maintained at a slight sub-atmospheric or negative pressure during normal operation and refueling operations.
  Common containment designs are referred to by the names Mark I, Mark II, and Mark III. 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. The Mark II was used with late BWR-4 and BWR-5 reactors. It is called an "over-under" configuration with the drywell forming a truncated cone on a concrete slab. Below is a cylindrical suppression chamber made of concrete rather than just sheet metal. 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. The top level is a large open space with an overhead crane suspended between the two long walls for moving heavy fuel caskets from the ground floor, and removing / replacing hardware from the reactor and reactor well. The reactor well can be flooded and is straddled by pools separated by gates on either side for storing reactor hardware normally placed above the fuel rods, and for fuel storage. 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. 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.
  The Mark I containment was used in those reactors at the Fukushima I Nuclear Power Plant which were involved in the Fukushima I nuclear accidents. The site suffered from a combination of two beyond design-basis events, a powerful earthquake, which may have damaged reactor plumbing and structures, and 15 meter tsunami, which destroyed fuel tanks, generators and wiring, causing back up generators to fail, and battery-powered pumps also eventually failed. Insufficient cooling and failure of pumps needed to restore water lost to boiling off led to partial or possible complete meltdowns of fuel rods which were completely uncovered by water. This led to releases of significant amounts of radioactive material to the air and sea, and hydrogen explosions. The thin secondary containments were not designed to withstand hydrogen explosions, and suffered blown out or destroyed roofs and walls, and destruction of all equipment on the refueling floor including cranes and refueling platform. 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. Although they were fitted with modified hardened vent systems to vent hydrogen into exhaust stacks, they may have not been effective without power. Even before the Fukushima incident, Mark I containment had been criticized as being more likely to fail during a blackout.
  From a distance, the BWR design looks very different from PWR designs because usually a square building is used for the secondary containment. Also, because there is only one loop through the turbines and reactor, and the steam going through the turbines is also radioactive, the turbine building has 
caption: Cross-section sketch of a typical BWR Mark I containment