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 in a BWR the steam going to the turbine is produced in the reactor core rather than in a steam generator or heat exchanger. In a BWR there is only a single circuit in which the water is at lower pressure than in a PWR (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, resulting in less moderation, lower neutron efficiency and lower power density than in the bottom part of the core.

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

Positioning (withdrawing or inserting) control rods is the normal method for controlling power when starting up the reactor and operating up to approximately 70% of rated power. As control rods are withdrawn, neutron absorption decreases in the control material and increases in the fuel, so reactor power increases. As control rods are inserted, neutron absorption increases in the control material and decreases in the fuel, so reactor power decreases.

Changing (increasing or decreasing) the flow of water through the core is the normal method for controlling power when operating between approximately 70% and 100% of rated power. As flow of water through the core is increased, steam bubbles ("voids") are more quickly removed from the core, the amount of liquid water in the core increases, neutron moderation increases, more neutrons are slowed down to be absorbed by the fuel, and reactor power increases. As flow of water through the core is decreased, steam voids remain longer in the core, the amount of liquid water in the core decreases, neutron moderation decreases, fewer neutrons are slowed down to be absorbed by the fuel, and reactor power decreases.

Steam produced in the reactor core passes through steam separators and dryer plates above the core and then directly to the turbines, which are thus part of the reactor circuit. Because the water around the core of a reactor is always contaminated with traces of radionuclides, the turbine must be shielded during normal operation, and radiological protection must be provided during maintenance. The increased cost related to operation and maintenance of a BWR tends to balance the savings due to the simpler design and greater thermal efficiency of a BWR when compared with a PWR. 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 BWR 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 the proportion of steam to liquid water increases inside the reactor. However, unlike a pressurized water reactor which contains no steam in the reactor core, a sudden increase in BWR steam pressure (caused, for example, by a blockage of steam flow from the reactor) will result in a sudden decrease in the proportion of steam to liquid water inside the reactor and will produce an increase in the power output of the reactor. Because of this negative void coefficient effect in BWRs, operating components and safety systems are designed to ensure that no credible, postulated failure can cause a pressure and power increase that exceeds the safety systems' capability to quickly shutdown the reactor before damage to the fuel or to components containing the reactor coolant can occur.

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to approximately 750 assemblies in a reactor core, holding up to approximately 140 tonnes of uranium. The number of fuel assemblies in a specific reactor is based on considerations of desired reactor power output, reactor core size and reactor power density.

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.

External Links

Boiling Water Reactors, US Nuclear Regulatory Commission [1] (http://www.nrc.gov/reactors/bwrs.html)

Advanced BWR General Description [2] (http://www.engr.sjsu.edu/jrhee/me210/Table%20of%20Contents.pdf) (Table of Contents, with active links to text)

Technical details and features of Advanced BWRs [3] (http://www.nuc.berkeley.edu/designs/abwr/abwr.html)

Text Book Chapter on "Nuclear Power Reactors" describing various reactor types [4] (http://book.nc.chalmers.se/KAPITEL/CH20NY3.PDF)