Reactor Types: PWR and BWR

Two reactor designs generate over 85% of the world's nuclear electricity. Both use ordinary water as coolant and moderator. They differ in one key design choice: whether the water that touches the fuel also touches the turbine.

A pressurized water reactor (PWR) keeps reactor water under high pressure (about 155 atmospheres) so it never boils, reaching 320 degrees C as a hot liquid. This primary loop transfers heat through a steam generator to a separate secondary loop, where lower-pressure water boils into steam that drives the turbine. The turbine never contacts radioactive water. A boiling water reactor (BWR) lets the reactor water boil directly, sending steam from the reactor vessel straight to the turbine. Simpler design (one loop instead of two), but the turbine and associated piping become mildly radioactive.

Compare the tradeoffs. PWR: two-loop design adds cost and complexity but isolates the turbine from radioactive contamination. BWR: single-loop design is simpler and operates at lower pressure (about 75 atmospheres), but maintenance workers must manage radioactive steam systems.

Why do PWRs outnumber BWRs roughly 2:1 in the global fleet?

The Navy connection. Admiral Rickover chose PWR for submarines because the compact, high-pressure design fit in a hull. The US nuclear Navy trained the engineers who built civilian plants. Commercial PWR designs descended directly from naval reactors, giving PWR a head start that compounded through standardization.

Both designs are proven and safe. The PWR's dominance is historical, not inherently technical.

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