Schematic diagram of a boiling water reactor. Source: US Nuclear Regulatory CommissionThe boiling water reactor (BWR) is a type of nuclear powered fission reactor used to generate electricity. In a BWR, water is used as a coolant and moderator. Steam is produced in the reactor under pressure, and is used to drive a turbine. These reactors were originally designed by Allis-Chalmers and General Electric (GE). The BWR is one of the major types of nuclear power plants used for the generation of electrical power using heat generated by nuclear reactions in nuclear fuel. Along with its light-water cousins, the pressurized water reactor (PWR), and the pressurized heavy water reactor (CANDU PHWR), these reactors constitute the vast majority of electricity generation reactors currently in use. BWRs are the second most common type of nuclear reactor the PWR, accounting for about 20% world’s 441 commercial nuclear power plants operating in 2007.

Basic operation

In a typical commercial boiling water reactor (1) the reactor core creates heat, (2) a steam-water mixture is produced when very pure water (reactor coolant) moves upward through the core absorbing heat, (3) the steam-water mixture leaves the top of the core and enters the two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line, (4) the steam line directs the steam to the main turbine causing it to turn the turbine generator, which produces electricity. The unused steam is exhausted to the condenser where it is condensed into water. The resulting water is pumped out of the condenser with a series of pumps, reheated, and pumped back to the reactor vessel. The reactor's core contains fuel assemblies which are cooled by water, which is force-circulated by electrically powered pumps. Emergency cooling water is supplied by other pumps which can be powered by onsite diesel generators. Other safety systems, such as the containment cooling system, also need electric power.

The BWR design has many similarities to the pressurized water reactor (PWR), 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.

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, so the turbine hall can be entered soon after the reactor is shut down.

A BWR fuel assembly comprises 90-100 fuel rods, and there are up to 750 assemblies in a reactor core, holding up to 140 metric tons of uranium.

Reactor control

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 a BWR. 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. Some early BWRs and the proposed ESBWR (Economic Simplified BWR) designs use only natural circulation with control rod positioning to control power from zero to 100% because they do not have reactor recirculation systems.

Changing (increasing or decreasing) the flow of water through the core is the normal and convenient method for controlling power. When operating on the so-called "100% rod line," power may be varied from approximately 30% to 100% of rated power by changing the reactor recirculation system flow by varying the speed of the recirculation pumps. 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.

Comparison with other reactors

Light water is ordinary water. In comparison, some other water-cooled reactor types use heavy water, such as the Canadian made CANDU reactor series. (See heavy water reactor.) In heavy water, the deuterium isotope of hydrogen replaces the common hydrogen atoms in the water molecules (D₂O instead of H₂O, molecular weight 20 instead of 18).

The pressurized water reactor (PWR) was the first type of light-water reactor developed because of its application to submarine propulsion. The civilian motivation for the BWR is reducing costs for commercial applications through design simplification and lower pressure components. There are no naval BWR type reactors. The description of BWRs below describes civilian reactor plants in which the same water used for reactor cooling is also used in the Rankine cycle turbine generators.

In contrast to the pressurized water reactors that utilize a primary and secondary loop, in civilian BWRs the steam going to the turbine that powers the electrical generator is produced in the reactor core rather than in steam generators or heat exchangers. There is just a single circuit in a civilian BWR in which the water is at lower pressure (about 75 times atmospheric pressure) compared to a PWR so that it boils in the core at about 285°C. The reactor is designed to operate with steam comprising 12–15% of the mass of the two-phase coolant flow (exit quality) in the top part of the core, resulting in less moderation, lower neutron efficiency and lower power density than in the bottom part of the core. In comparison, there is no significant boiling allowed in a PWR because of the high pressure maintained in its primary loop (about 158 times atmospheric pressure).

Sources

• International Atomic Energy Agency, Nuclear energy knowledge resources, Accessed 31 October 2008.
• United States Nuclear Regulatory Commission, Power reactors, Accessed October 31, 2008.
• Wikipedia contributors, Boiling water reactor, Wikipedia The Free Encyclopedia, Accessed 31 October 2008.
• Hore-Lacy, Ian (Lead Author); World Nuclear Association (Content Partner); Cutler J. Cleveland (Topic Editor). 2008. Nuclear power reactor. In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth August 22, 2006; Last revised March 3, 2008; Retrieved October 30, 2008].