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Some facilities have an advanced containment/protection system
The ultimate safety system inside and outside of every BWR are the numerous levels of physical shielding that both protect the reactor from the outside world and protect the outside world from the reactor.
There are five levels of shielding:
1. The fuel rods inside the reactor pressure vessel are coated in thick Zircalloy shielding;
2. The reactor pressure vessel itself is manufactured out of 6 inch thick steel, with extremely temperature, vibration, and corrosion resistant surgical stainless steel grade 316L plate on both the inside and outside;
3. The primary containment structure is made of steel 1 inch thick;
4. The secondary containment structure is made of steel-reinforced, pre-stressed concrete 1.2–2.4 meters (4–8 ft) thick.
5. The reactor building (the shield wall/missile shield) is also made of steel-reinforced, pre-stressed concrete 0.3 m to 1 m (1–3 feet) thick.
If every possible measure standing between safe operation and core damage fails, the containment can be sealed indefinitely, and it will prevent any substantial release of radiation to the environment from occurring in nearly any circumstance.
Varieties of containment/protection measures
As illustrated by the descriptions of the systems above, BWRs are quite divergent in design from PWRs. Unlike the PWR, which has generally followed a very predictable external containment design (the stereotypical dome atop a cylinder), BWR containments are varied in external form but their internal distinctiveness is extremely striking in comparison to the PWR. There are five major varieties of BWR containments:
· The "pre-modern" containment (Generation I); spherical in shape, and featuring a steam drum separator, or an out-of-RPV steam separator, and a heat exchanger for low pressure steam, this containment is now obsolete, and is not used by any operative reactor.
· the Mark I containment, consisting of a rectangular steel-reinforced concrete building, along with an additional layer of steel-reinforced concrete surrounding the steel-lined cylindrical drywell and the steel-lined pressure suppression torus below. The Mark I was the earliest type of containment in wide use, and many reactors with Mark Is are still in service today. There have been numerous safety upgrades made over the years to this type of containment, especially to provide for orderly reduction of containment load caused by pressure in a compounded limiting fault. The reactor building of the Mark I generally is in the form of a large rectangular cube of reinforced concrete.
· the Mark II containment, similar to the Mark I, but omitting a distinct pressure suppression torus in favour of a cylindrical wetwell below the non-reactor cavity section of the drywell. Both the wetwell and the drywell have a primary containment structure of steel as in the Mark I, as well as the Mark I's layers of steel-reinforced concrete composing the secondary containment between the outer primary containment structure and the outer wall of the reactor building proper. The reactor building of the Mark II generally is in the form of a flat-topped cylinder.
· the Mark III containment, generally similar in external shape to the stereotypical PWR, and with some similarities on the inside, at least on a superficial level. For example, rather than having a slab of concrete that staff could walk upon while the reactor was not being refuelled covering the top of the primary containment and the RPV directly underneath, the Mark III takes the BWR in a more PWRish direction by placing a water pool over this slab. Additional changes include abstracting the wetwell into a pressure-suppression pool with a weir wall separating it from the drywell.
· Advanced containments; the present models of BWR containments for the ABWR and the ESBWR are harkbacks to the classical Mark I/II style of being quite distinct from the PWR on the outside as well as the inside, though both reactors incorporate the Mark III-ish style of having non-safety-related buildings surrounding or attached to the reactor building, rather than being overtly distinct from it. These containments are also designed to take far more than previous containments were, providing advanced safety. In particular, GE regards these containments as being able to withstand a direct hit by a tornado of Old Fujitsa Scale 6 with winds of 330+ miles per hour. Such a tornado has never occurred. They are also designed to withstand seismic accelerations of .2 G, or nearly 2 meters per second in any direction.
Pressurised water reactors
As the cold war ended in 1991, newer nuclear power plants often have spherical design while pre-1991 reactors are often “can shaped” with a much more robust and massive missile shield.
For a pressurised water reactor, the containment also encloses the steam generators and the pressuriser, and is the entire reactor building. The missile shield around it is typically a tall cylindrical or domed building designed to withstand a moderate missile attack.
A large, 4000-7000 kg barrack buster (WMD), should have no problem destroying the structure and the reactor inside.
PWR containments are typically large (up to 10 times larger than a BWR) because the containment strategy during the leakage design basis accident entails providing adequate volume for the steam/air mixture that results from a loss-of-coolant-accident to expand into, limiting the ultimate pressure (driving force for leakage) reached in the containment 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).
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. In other words;
“can” shaped containment buildings are much more effectively protected from explosive blasts than spherical designs which is often designed to prevent leakage accidents.
Typical examples are:
- Three Mile Island was an early PWR design by Babcock and Wilcox, and has 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
- The twin PWR reactor containments at the Cook Nuclear Plant in Michigan
- German plants exhibits 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 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.
Boiling water reactors
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 refuelling 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 also use the large body of water in the suppression pools 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 steam 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.
Typical examples are:
- A representative one – Kernkraftwerk Krummel, unit German BWR has containment around both the turbine and reactor buildings
- A typical two-unit BWR at the Brunswick Nuclear Generating Station
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.
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.
Typical examples are:
- 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
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