Boiling water reactors (BWRs) are a type of light water nuclear reactor, on a par with pressurized water reactors(PWRs). This article presents the history and characteristics of BWR technology, aimed at an audience that is knowledgeable but unfamiliar with this particular subject. It also explores the differences between BWRs and PWRs.
History
The development of boiling water reactors began in the USA in the 1950s, under the aegis of the General Electric Company (1). The first commercial reactor of this type, the Dresden-1, was commissioned in 1960. Although BWRs are less widespread than PWRs, they still account for around 20% of the world’s operating nuclear reactors in 2021.
Characteristics of boiling water reactors
Boiling water reactors use light water (H2O) as coolant and neutron moderator. The nuclear fuel, generally enriched uranium, is contained in fuel rod assemblies. Unlike PWRs, BWRs do not use a pressure vessel to keep the water in a liquid state at high temperatures. Instead, water is allowed to boil directly in the reactor core, producing steam which is used to drive a turbine and generate electricity.
Boiling water reactors are designed with several safety systems, such as containment barriers, emergency cooling systems and automatic shutdown systems in the event of abnormal conditions. New-generation reactors, such as the Advanced Boiling Water Reactor (ABWR) and the Economic Simplified Boiling Water Reactor (ESBWR), incorporate significant improvements in safety and efficiency over previous designs.
Differences between BWRs and PWRs
One of the main differences between boiling water reactors (BWRs) and pressurized water reactors (PWRs) lies in their design and mode of operation. In a BWR, boiling water is produced directly in the reactor core, whereas in a PWR, the water is kept under pressure and heated in a primary circuit before transferring its heat to a secondary circuit via a steam generator (9).
This difference has several implications. Firstly, BWRs use a single cooling circuit, unlike PWRs, which use two separate circuits. This makes BWRs slightly less complex and expensive to build and operate. However, it also means that radioactivity is more likely to spread through the cooling circuit in the event of a leak or accident.
Secondly, the pressure in the reactor core is generally lower in a BWR than in a PWR. This can reduce the risk of vessel rupture in the event of overpressure, but it can also lead to challenges in terms of transient management and cooling efficiency.
Thirdly, BWRs generally have lower power levels than PWRs, ranging from 600 to 1,300 megawatts electrical (MWe), whereas PWRs have power ratings of 1,000 to 1,700 megawatts electrical (MWe). This means that BWRs may be more suitable for smaller installations or less-developed power grids.
Finally, the thermal-hydraulic characteristics of BWRs and PWRs are different, which can affect reactor design and safety management. For example, the formation of steam bubbles in the reactor core can lead to power fluctuations in a BWR, whereas this is generally not a problem in a PWR.
Statistics
In 2021, there were around 90 boiling water nuclear reactors in operation worldwide, mainly in the USA, Japan and Sweden. These reactors have a total capacity of around 85,000 electrical megawatts (MWe). Among the new-generation reactors under development, ABWRs have a capacity of 1,300 to 1,400 MWe, while ESBWRs have a capacity of 1,520 MWe.
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- https://www.voix-du-nucleaire.org/reacteurs-nucleaires-a-eau-bouillante/