Pressurized Water Reactor

Pressurized Water Reactor
Name	Pressurized Water Reactor
Generation	II, III, III+
Concept	Thermal reactor, Light water reactor
Status	Operational
Manufacturer	Westinghouse Electric Company, Framatome, Mitsubishi Heavy Industries, Korea Electric Power Corporation

Pressurized Water Reactor. A Pressurized Water Reactor (PWR) is the most prevalent type of nuclear reactor used for electricity generation worldwide. It employs ordinary light water as both a coolant and a neutron moderator, kept under high pressure to prevent boiling within the reactor core. The heat generated by nuclear fission in the core is transferred via a primary circuit to a steam generator, where it produces steam in a separate secondary circuit to drive turbines connected to electrical generators.

Design and operation

The fundamental design principle centers on maintaining the water in the primary loop at a pressure typically around 15.5 megapascals, which raises its boiling point significantly above its normal saturation temperature. This high-pressure water circulates through the core, absorbing heat from the nuclear fuel assemblies without turning to steam. The heated water then flows to U-tubes within one or more steam generators, transferring its thermal energy to a lower-pressure secondary loop. In the secondary system, the feedwater boils, generating saturated or slightly superheated steam that is routed to the turbine hall. After passing through the turbine, the steam is condensed back into water by a condenser cooled by a tertiary system, often from a large water body like the Mississippi River or a cooling tower, and is pumped back to the steam generators.

Components

The primary system includes the reactor pressure vessel, typically fabricated by companies like Japan Steel Works, which houses the core and internal structures. Massive reactor coolant pumps, such as those historically made by Byron Jackson Pump Company, circulate the pressurized water. The system also incorporates a pressurizer, a separate vessel that maintains system pressure using electric heaters and spray nozzles. Key components of the secondary side are the steam generator, the main steam isolation valve, and the turbine generator set, often supplied by General Electric or Siemens. The entire reactor is contained within a robust containment building, frequently a pre-stressed concrete structure designed to withstand internal pressures and external events.

Safety systems

Multiple engineered safety features protect against the release of radioactive material. These include high-pressure safety injection systems, such as the accumulator tanks, which can rapidly flood the core with borated water in a loss-of-coolant accident. Large-capacity spent fuel pools provide cooling for used fuel assemblies. For severe accidents, many designs incorporate passive nuclear safety features like core catchers. Systems are designed to meet stringent standards set by the Nuclear Regulatory Commission in the United States and equivalent bodies like the Autorité de sûreté nucléaire in France.

Fuel cycle

PWRs primarily use low-enriched uranium dioxide fuel, typically enriched to between 3% and 5% uranium-235. Fuel is fabricated into sealed zirconium alloy cladding tubes assembled into fuel rod bundles. After an operational cycle of 12 to 24 months in reactors like the VVER or the System 80, the spent fuel is moved to on-site storage. While some countries, such as France and Japan, pursue nuclear reprocessing at facilities like La Hague site or Rokkasho Reprocessing Plant, most, including the United States, currently utilize direct disposal in geological repositories like the proposed Yucca Mountain nuclear waste repository.

Comparison with other reactor types

The principal alternative light water reactor design is the Boiling Water Reactor, where water boils directly in the core and the steam drives the turbine, simplifying the system but potentially increasing radioactivity in the turbine building. Compared to heavy water reactors like the CANDU reactor, which use deuterium oxide as a moderator and can utilize natural uranium, PWRs require enriched fuel but have a more compact core design. Gas-cooled reactors, such as the Advanced Gas-cooled Reactor, use carbon dioxide or helium and can operate at higher temperatures, while Generation IV reactor concepts like the sodium-cooled fast reactor aim for improved sustainability and safety.

History and development

The technology was pioneered in the United States for naval propulsion, notably in the USS Nautilus (SSN-571), under the leadership of Hyman G. Rickover. The first commercial PWR for power generation was the Shippingport Atomic Power Station in Pennsylvania, which began operation in 1957. Major design lineages evolved from the work of Westinghouse Electric Company, including the widely built Combustion Engineering and Babcock & Wilcox designs. International variants were developed, such as the French CP series from Framatome and the Russian VVER series. Ongoing development focuses on advanced Generation III+ reactor designs like the AP1000, EPR (nuclear reactor), and APR1400, which emphasize enhanced safety and economics.

Category:Nuclear reactors Category:Nuclear power reactor types