Boiling water reactor

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A boiling water reactor (BWR) is a light water reactor design used in some nuclear power stations. It has many similarities to the pressurized water reactor, except that there is only a single circuit in which the water is at lower pressure (about 75 times atmospheric pressure) so that it boils in the core at about 285°C. The reactor is designed to operate with 12-15% of the water in the top part of the core as steam, and hence with less moderating effect and thus efficiency there.

Reactor power is controlled via two methods: by inserting or withdrawing control rods and by changing the water flow through the reactor core.

The steam passes through drier plates (steam separators) above the core and then directly to the turbines, which are thus part of the reactor circuit. Since the water around the core of a reactor is always contaminated with traces of radionuclides, it means that the turbine must be shielded and radiological protection provided during maintenance. The cost of this tends to balance the savings due to the simpler design. Most of the radioactivity in the water is very short-lived (mostly N-16, with a 7 second half life), so the turbine hall can be entered soon after the reactor is shut down.

Like the pressurized water reactor, the reactor core continues to produce heat from radioactive decay after the fission reactions have stopped, making nuclear meltdown possible in the event that all safety systems have failed and the core does not receive coolant. Also like the pressurized water reactor, a boiling-water reactor has a negative void coefficient, that is, the thermal output decreases as more steam is formed inside the reactor. However, unlike a pressurized water reactor, a sudden increase in steam pressure can result in a sudden decrease in the amount of steam in the reactor, which will produce an increase in the power output of the reactor. Thus the size of this negative void coefficient can not be too large, for safety reasons.

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 tonnes of uranium. The secondary control system involves restricting water flow through the core so that steam in the top part means moderation is reduced.

Advantages

• Simple configuration, no steam generator heat-exchangers.
• Greater thermal efficiency than a PWR operating at the same core temperature.
• Pressure vessel is subject to little irradiation, and so does not become brittle with age.

Disadvantages

• Complex design and operational calculations.
• Much larger pressure vessel than for a PWR of similar power, with correspondingly higher costs.
• Contamination of the turbine by fission products.

de:Siedewasserreaktor ja:沸騰水型原子炉