Containment - Bird's Eye View
A key principle in the design of the nuclear plant is defense in depth. There are intended to be 3 barriers between the radioactive fission products and the public in order to reduce the likelihood of radioactive releases. These 3 barriers are - the fuel cladding, the reactor coolant piping, and the containment. An EdF figure best illustrates this concept.
The containment is a large steel building, usually with a hemispherical dome. Containments are often not visible since they are usually surrounded by a steel or concrete building that provides additional boundaries and aesthetically pleasing designs.
The containment is capable of sustaining high internal pressures. Design is in the range of 45 to 60 psig, however, much higher pressures, even exceeding 100 psig may be sustained. Ice condenser containments are usually designed for about 12 psig. In either case, the containment is designed for a double ended rupture of the largest diameter pipe in the structure. These pipes are usually 3 feet in diameter.
The containment houses the reactor, reactor cooling or recirculation system and pumps. For PWRs, the pressurizer is also in the containment.
Containments have fans or chiller units for cooling during normal conditions and in the unlikely event of an accident. To provide additional cooling and pressure suppression, several ring headers are mounted in the uppermost part of the containment. These ring headers are used to spray water (often borated) to reduce pressure.
Below the containment basement, there are sumps that can be used during accident conditions. The sumps can be routed to cooling systems so that the fluid may be recirculated and cooled. A grating system is installed over the containment sump (click for photo of sump) to prevent materials from being sucked into the cooling system, usually called the Residual Heat Removal system.
|Using the center of the photo as a reference:
The refueling pool is directly below. During a refueling the head is removed from the reactor and is stored within the containment. The upper internals are removed and stored underwater in the refueling pool.
The crane (orange-color) is used to move new and old fuel. Directly below is the reactor.
Steam generators can be seen in the lower right and just to the upper right from the center.
Ventilation ducts can be seen along the wall of the containment.
The reactor coolant pumps, pressurizer, and emergency core cooling accumulators are also found in the containment.
|Photo (of photo at a plant) by J.A. Gonyeau|
The following equipment is located within the containment. Numbers below refer to the containment diagram.
|101||Unit Vent||114||Equipment Hatch Lifting Gear|
|102||Auxiliary Boiler Stack||115||Reactor Head Decontamination & Storage Area|
|103||Containment Spray header||116||Refueling Machine|
|104||Steel Liner||117||Internals Lifting Rig|
|107||Main Steam Lines||120||Reactor Cooling Pumps|
|108||Main Feedwater Pipes||121||Accumulators|
|109||Steam Generators||122||Reactor Coolant Pump Motor Oil Collection Tank|
|110||Hydrogen Mixing Fans||123||Incore Instrumentation|
|111||Control Rod Drive Mechanism Fans & Plenums||124||Upper Internals Storage Stand|
|112||Containment Coolers||125||Reactor Coolant Drain Tank|
|113||Equipment Hatch Lifting Gear||126||Pressurizer Relief Tank|
There are 3 types of BWR containments. These are discussed and illustrated on pages 3-15 through 3-18 of the NRC's Boiling Water Reactor Systems Manual. The designs are also nicely illustrated in the Union of Concerned Scientists' page on BWR containment overpressure, as illustrated below.
Courtesy Union of Concerned Scientists
Mark I is the traditional torus and inverted light bulb design. This was the first generation of BWR containment, as used in the BWR 1 through 4 designs. Twenty two units have this style of containment. The design includes:
The drywell, which surrounds the reactor vessel and recirculation loops,
A suppression chamber (also known as the torus) which stores a large body of water (suppression pool),
An interconnecting vent network between the drywell and the suppression chamber, and
The secondary containment, which surrounds the primary containment (drywell and suppression pool) and houses the spent fuel pool and emergency core cooling systems.
Mark II containment was the second generation containment as used with late BWR-4 and BWR-5 reactors. The Mark II design is an over-under configuration. The design includes:
Drywell, in the form truncated cone, located directly above the suppression pool.
Suppression chamber is cylindrical and separated from the drywell by a reinforced concrete slab.
Drywell head is an elliptical steel dome on top of the drywell.
The inerted drywell atmosphere is vented into the suppression chamber through as series of downcomer pipes penetrating and supported by the drywell floor.
The Mark III containment is used with the newest BWR reactor designs. The design includes:
Drywell, a cylindrical, reinforced concrete structure with a removable head, designed to withstand and confine steam generated during a pipe rupture inside the containment and to channel the released steam into the suppression pool via a weir wall and horizontal vents.
The suppression pool contains a large volume of water for rapidly condensing steam directed to it.
A leak tight, cylindrical, steel containment vessel surrounds the drywell and the suppression pool to prevent gaseous and particulate fission products from escaping to the environment following a pipe break inside containment.
The Refueling operations page links to photos and graphics showing containment activities during a refueling.
Copyright © 1996-2006. The Virtual Nuclear Tourist. All rights reserved. Revised: January 5, 2006.