Boiling water reactors (BWR) are a type of nuclear reactor that use light water (ordinary water, as opposed to heavy water) as their coolant and neutron moderator. They are the second most used reactor for nuclear power generation in the world, next to the pressurized water reactor (PWR), with 80 in operation as of 2015.
With maximum operating temperatures of around 285oC, the Carnot efficiency (maximum efficiency) of boiling water reactors in the production of power is 46%. The realistic efficiency of these reactors in power production however is around 33-34%.
See the section below to view a basic operating cycle of a boiling water power plant.
Boiling water reactors must use enriched uranium as their nuclear fuel, due to their use of light water. This is because light water absorbs too many neutrons to be used with natural uranium, so the fuel content of fissile Uranium-235 must be increased. This is done through a uranium enrichment processes, in which the concentration of Uranium-235 is increased from 0.7% to around 4%.
The enriched uranium is packed into fuel rods, which are assembled into a fuel bundle, as seen in Figure 2. There are about 90-100 rods in each bundle, with up to 750 bundles in a reactor. This corresponds to nearly 140 tonnes of uranium!
As mentioned before, light water is used as the coolant and moderator for a boiling water reactor. Light water is much more abundant than heavy water, as it makes up 99.99% of natural water.
The use of light water as a coolant is beneficial as it can cool down the fuel at the same time as it heats up to steam. The steam then directly flows to turbines which spin a generator to generate electricity. This simplifies the design of boiling water reactors, as there is only one primary loop needed which results in a lower cost of development.
Light water does not make as good of a moderator as heavy water or graphite as a result of its relatively high absorption of neutrons. However its use as a moderator makes for an interesting and important safety feature; if there is a loss of coolant accident (LOCA), there will also be a loss of moderator causing the nuclear chain reaction to stop. Also if the moderating water overheats and turns to steam inside the bottom reactor core, there will be less moderator and therefore the chain reaction will stop.
Boiling water reactors must operate at fairly high pressures in order for the water to perform adequately; at high pressures the water can remain in liquid form at temperatures much higher than its normal 100oC boiling point. Therefore boiling water reactors operate at around 7 MPa, around 70 times higher than atmospheric pressure.
Although very high, these pressures are still less than half of the pressures needed for pressurized water reactors. Since the pressures in boiling water reactors are less than those of pressurized ones, the specific power is less. This means that the boiling water reactor needs to be larger in order to provide the same power as a pressurized reactor, which results in higher costs. However the walls do not need to be as thick as in PWRs, so the costs are essentially the same.
Since the BWR has only one primary loop, as mentioned earlier, the very same water that flows through the reactor core is what is used for the spinning of the turbines. This is in contrast to pressurized water reactors, where the hot water in the core is not converted to steam, and therefore two loops are necessary. This is the main difference between BWRs and PWRs.
Less water needs to flow through a BWR than through a PWR, however there are complications because the water becomes radioactive in its circulation. This means that there must be extra shielding in the non-reactor parts of the power plant.
Control rods are placed in the bottom of the reactor, rather than the top as in PWRs. This is because in a boiling water reactor there is more steam at the top (visible in Figure 3 below), and placement of control rods in the steam portion would have a lesser effect than their placement in the water portion of the reactor. There is higher reactivity of the fuel in the water portion due to its moderation, so the control rods are needed to change the reactivity as deemed fit by the operators.
Seen below is the flow of water through a boiling water reactor power plant, and its production of electricity.