Boiling water reactors (BWRs) use heat released by nuclear fission to directly boil water that produces steam to drive a generator. These reactors are considered light water reactors, because they use ordinary water. Thirty-five of all nuclear reactors in the U.S., or approximately one-third, are boiling water reactors (BWRs).
Boiling water reactors (BWRs) are the second most common type of commercial nuclear reactors in the U.S. The most common is the pressurized water reactor.
The boiling water reactor is the same type of reactor as those severely damaged in Japan, when an earthquake and tsunami struck the Fukushima Daiichi Nuclear Power Plant in March 2011. Overheating and damage inside the reactors triggered ongoing radiation leaks that require evacuation of a 12 mile radius.
How a Boiling Water Reactor Works
In BWRs, the reactor pressure vessel is a large, 7-inch thick forged steel cylinder with domed ends. This vessel is kept inside a sealed containment building to prevent any radiation from escaping into the atmosphere if there is a failure or accident. These building are dome-shaped and are made of steel-reinforced concrete.
Unlike pressurized water reactors, BWRs actually boil the water. The core inside the containment structure of the typical BWR produces heat as 370-800 fuel assemblies sustain the nuclear chain reaction. The core is cooled by ordinary – or light – water, which also serves as the moderator, circulated with electric pumps.
- The water moves upward through the core, absorbing heat and producing a steam-water mixture.
- The steam-water mixture is forced out the top of the core into two stages of moisture separation where water droplets are removed before the steam is allowed to enter the steam line.
- The steam is piped to the turbine, causing it to turn the plant’s generator, producing electricity which is distributed to the grid.
- The unused steam is exhausted into the condenser where it is condensed into water, which is pumped out of the condenser, reheated and pumped back to the reactor vessel.
The hot core at the center of the reactor is kept from overheating by a flow of cooling water circulated by pumps. This cooling function is so vital that reactors have backup generators designed to continue the water flow if the reactor goes out or malfunctions.
Refueling a Boiling Water Reactor
A boiling water reactor can run for 18 months to two years before it requires refueling. Refueling involves replacement of roughly 250 out of 764 (approximately one-third) of the bundled uranium dioxide pellets in a reactor pressure vessel. When the reactor is refueled, the spent fuel from the reactor is moved to onsite spent fuel pools or if the pools are at capacity, it’s stored in dry casks. These casks may also be used to transport spent fuel. Both storage methods are designed to keep radiation shielded and prevent it from escaping into the atmosphere.