CANDU plants

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

Дата добавления: 2015-05-31