Boiling water reactor

A boiling water reactor (BWR) is a type of light water reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR). In a BWR, the reactor core heats water, which turns to steam and then drives a steam turbine. The BWR was developed by the General Electric company in the mid-1950s.

Design and Operation

Diagram of a Boiling Water Reactor

The BWR is characterized by its direct cycle design, where the water used as a coolant and neutron moderator is allowed to boil directly in the reactor core. The steam produced is then used to drive a turbine connected to a generator, producing electricity. This design eliminates the need for a separate steam generator, which is a component of the PWR design.

The reactor core of a BWR is composed of fuel assemblies, which contain uranium dioxide fuel pellets. These pellets are encased in zirconium alloy tubes, known as fuel rods. The core is submerged in water, which acts as both a coolant and a neutron moderator. Control rods, made of materials that absorb neutrons, are inserted into the core to control the rate of the nuclear reaction.

Components

Cross-section of a UK Advanced Boiling Water Reactor

The main components of a BWR include:

• Reactor Pressure Vessel (RPV): Contains the reactor core and the water that is boiled to produce steam.
• Steam Separator and Dryer: Located at the top of the RPV, these components remove moisture from the steam before it is sent to the turbine.
• Turbine and Generator: The steam drives the turbine, which is connected to a generator that produces electricity.
• Condenser: After passing through the turbine, the steam is condensed back into water and returned to the reactor.
• Feedwater Pump: Pumps the condensed water back into the reactor to be reheated.

Safety Features

BWRs are equipped with several safety systems to ensure safe operation. These include:

• Emergency Core Cooling Systems (ECCS): Designed to provide cooling to the reactor core in the event of a loss of coolant accident.
• Containment Structure: A robust structure that encloses the reactor vessel and associated components to prevent the release of radioactive materials.
• Control Rods: Can be rapidly inserted into the core to shut down the nuclear reaction in an emergency.

Advantages and Disadvantages

The BWR design offers several advantages, including a simpler design with fewer components compared to PWRs, which can lead to lower construction and maintenance costs. However, the direct cycle design means that the turbine and other components are exposed to radioactive steam, requiring additional shielding and maintenance.

Applications

Leibstadt Nuclear Power Plant

BWRs are used in many countries around the world for electricity generation. Notable examples include the Leibstadt Nuclear Power Plant in Switzerland and the Browns Ferry Nuclear Plant in the United States.

Advanced Boiling Water Reactors

The Advanced Boiling Water Reactor (ABWR) is a more recent development that incorporates several design improvements for enhanced safety and efficiency. The ABWR design includes features such as improved control rod drive mechanisms and advanced safety systems.

Related Pages

• Nuclear reactor
• Pressurized water reactor
• Nuclear power
• Light water reactor