Pressurized water reactor (PWR) and boiling water reactor (BWR) are two light water reactor designs used in electricity generation. Both use water as a coolant and moderator; The main difference is where and how the steam that turns the turbine is produced.
In the PWR design, the primary circuit water passing through the reactor is kept under high pressure and does not boil. It transfers its heat to a separate secondary circuit in the steam generator. In the BWR design, water boils in the reactor vessel and the resulting steam goes directly to the turbine.
How does PWR work?
In PWR, the water heated in the reactor core remains in a liquid state thanks to high pressure. As this hot water passes through the pipes of the steam generator, it boils the water in the secondary circuit. The steam formed in the secondary circuit rotates the turbine.
In this arrangement, the reactor coolant and the steam going to the turbine are in different circuits. The pressure of the primary circuit is controlled by a system called a pressurizer.
How does BWR work?
In BWR, some of the water passing through the reactor core turns into steam directly in the reactor vessel. After the steam passes through the moisture separation stages, it is sent to the turbine via the main steam line. The steam coming out of the turbine is converted into water in the condenser and pumped back into the reactor vessel.
BWR does not have a separate steam generator. In contrast, the turbine side is the part of the system through which the steam produced in the reactor passes directly, and the radiological protection and maintenance approach is designed accordingly.
Main differences
| feature | PWR | BWR |
|---|---|---|
| Boiling in reactor vessel | Prevented in normal operation | It is part of the design |
| steam generator | is found | Not available |
| Main heat transport structure | Primary and secondary circuit | Direct steam cycle |
| pressurizer | Typically found | There are no separate pressurizers with the same function |
| Steam going to the turbine | It is produced in the secondary circuit | Produced in reactor vessel |
The table shows the general design logic. A particular power plant's systems, safety features, and operating procedures may vary by design generation.
Which is safer?
It is not correct to make a general security ranking based only on the name PWR or BWR. Security; Detailed design, physical barriers, backup and diversified systems, regulatory requirements, operational performance and security culture are evaluated and examined together.
Both reactor families have multiple safety systems to detect different abnormal conditions, shut down the reactor, cool the core and keep radioactive materials within limits.
Resources
- U.S. NRC, Boiling Water Reactors: https://www.nrc.gov/reactors/power/bwrs
- U.S. NRC, How Does a Nuclear Power Plant Make Electricity?: https://www.nrc.gov/education-regulatory-research/the-student-corner/science-101/how-does-a-nuclear-power-plant-make-electricity
- U.S. NRC, Reactor Concepts Training Course: https://www.nrc.gov/sites/default/files/doc_library/cdn/legacy/reading-rm/training/reactor-concepts-training-course.pdf