What is Boiling Water Reactor?

A nuclear reactor in which ordinary water serves as both coolant and moderator, boiling directly in the reactor core to produce steam that drives the turbine-generator in a single-loop design.

What category does Boiling Water Reactor belong to?

Boiling Water Reactor is a reactor-types concept in nuclear energy and SMR technology.

Boiling Water Reactor — Nuclear & SMR Glossary

The Boiling Water Reactor (BWR) is the second most common commercial nuclear reactor type, with approximately 60 units operating worldwide. Unlike a PWR, where the primary coolant remains liquid under high pressure, a BWR allows water to boil directly within the reactor core, producing steam that passes through moisture separators and dryers before flowing directly to the turbine-generator. This single-loop design eliminates the need for steam generators and reduces the overall system complexity and capital cost. However, it means that slightly radioactive steam reaches the turbine building, requiring additional radiation shielding in the steam cycle compared to PWR designs.

The most significant BWR-based SMR in development is GE-Hitachi's BWRX-300, a simplified 300 MWe natural circulation boiling water reactor that represents the leading SMR construction project in North America. Ontario Power Generation (OPG) received a CNSC License to Construct for the BWRX-300 at the Darlington Nuclear Generating Station in April 2025, and construction commenced the following month, making it the first SMR physically under construction in North America. OPG plans up to four BWRX-300 units at Darlington and announced a CAD $3 billion investment deal to de-risk the project in October 2025. The BWRX-300 design simplifies the traditional BWR by using natural circulation in the reactor vessel (eliminating recirculation pumps), an isolation condenser system for passive decay heat removal, and a steel-plate composite containment that reduces construction time.

Beyond Darlington, GE-Hitachi is pursuing BWRX-300 deployments with the Tennessee Valley Authority (TVA) at the Clinch River site in Tennessee and in Poland. The design leverages GE-Hitachi's extensive BWR operating experience, including the ESBWR (Economic Simplified Boiling Water Reactor) licensing effort with the NRC, and draws on the global fleet of GE BWR/6 and ABWR designs. GE Vernova (NYSE: GEV), the parent company, brings significant manufacturing and balance-of-plant capabilities to the BWRX-300 program. The design's use of standard LEU fuel (below 5% enrichment) avoids the HALEU supply chain constraints facing advanced reactor designs, a meaningful advantage in the current fuel availability environment.