What type of reactor uses the primary water boiled directly to create steam?

A boiling water reactor (BWR) is designed such that the primary water, which acts as both the coolant and the moderator, is allowed to boil directly within the reactor core. This boiling process generates steam that is then utilized to drive turbines for electricity generation. The direct conversion of heat to steam minimizes the need for a secondary steam generator, simplifying the system and increasing efficiency.

In contrast, a pressurized water reactor (PWR) keeps the primary water under high pressure to prevent it from boiling, and instead, it transfers heat to a secondary loop where the steam is generated. Advanced gas-cooled reactors and liquid metal fast breeder reactors utilize different cooling and moderation mechanisms, such as gas or liquid metals, which do not involve directly boiling water in the primary system.

Understanding these fundamental differences helps clarify why the boiling water reactor is uniquely identified by its direct steam generation characteristic.