The boiling water reactor (BWR) design is used in about one third of the commercial power generation reactors in use in the world today. It is the second most popular reactor design behind pressurized water reactors. The BWR design was the result of collaboration between the Idaho National Laboratory and General Electric in the mid-1950s. Today, GE Hitachi Nuclear Energy is the main manufacturer of the BWRs.
In the BWR design, ordinary water that has been processed to remove minerals is used as the coolant and the neutron moderator. The heat from the fission reaction in the core of the reactor is used to turn the coolant water to steam. The water is kept under pressure so that the temperature at which it turns to steam rises from two hundred and twelve degrees Fahrenheit to five hundred and fifty degrees Fahrenheit.
The steam from the reactor core is then used to turn a turbine to generated electric power. The steam then enters a condenser where cooling water flowing through a second system is used to turn the steam back into liquid water. This water is drawn from a river, lake or ocean and is returned at a higher temperature.
Water returning to the core is draws heat from the steam in the system and fed into the bottom of the core where it rises and turns back to steam. Because the steam is part of the primary water system that includes the core, the water and the steam contain radionuclides leaked from the fuel rods. The part of the reactor that contains the steam turbine must be well sealed and shielded to prevent escape of radiation.
The water in BWRs also acts as a moderator for the fission reaction in the core. As the fast neutrons generated by fission collide with water molecules, they are slowed down which increases their ability to trigger more fission events in the core. As the water heats, the molecules move apart and reduce the ability of the water to slow down the fast neutrons. This in turn reduces the activation of fission events which cools the core. This creates a negative feedback control system which keeps the core within a particular temperature range. The position of the control rods determines the center or set point of the temperature range. Light water is an excellent moderator and allows the construction of compact reactor cores.
Controlling flow of water with recirculation pumps is a useful secondary control system that is present in some BWR designs. There are steam bubbles in the water around the core. If the water flow is increased, the steam bubbles are removed more quickly and the water is denser. This increases neutron moderation which increases fission and power generation. Slowing the flow of water decreases neutron moderation and decreases the fission reaction, lowering heat production and power generation.
The fuel used in BWRs is uranium dioxide (UO2) ceramic pellets in Zircaloy fuel rods. The BWR has evolved through 6 different designs which are divided into Generation I, Generation II, Generation III. The number of fuel rods in a bundle and the number of bundles in a core have changed during this evolutionary process.
Boron control rods are used in BWRs to start the reactor, shut down the reactor and to control short terms changes in power demand. The control rods can also compensate for changes in the fuel due to depletion or poisoning by isotopes generated during fission.
The boiling water reactors operate at about half the pressure of the pressurized water reactors. The pressure vessel in the BWR gets less radiation than the pressure vessel in the PWR and does not become brittle as quickly. There are fewer components in a BWR compared to a PWR which reduces the possibility of a rupture and leaking of coolant. With natural convection circulation of water, the possibility of failure of cooling pumps is eliminated. Boric acid is not used in the coolant water eliminating corrosion. A single major manufacturer supports standardized design. BWRs are not used for propulsion systems and are not as useful for developing nuclear weapons. This makes BWRs desirable for export from the United States.
|
1. Reactor pressure vessel (RPV) |
10. Generator |