What distinguishes a pressurized water reactor (PWR) from a boiling water reactor (BWR)?

The distinguishing feature of a pressurized water reactor (PWR) compared to a boiling water reactor (BWR) lies in the pressure at which water is maintained within the reactor core. In a PWR, water is kept under high pressure, typically around 155 bar (2250 psi), which prevents it from boiling within the reactor core, despite reaching high temperatures. The reactor uses this high-pressure water as a coolant and also as a moderator to facilitate the nuclear fission process.

In contrast, in a BWR, water is allowed to boil in the reactor core, producing steam directly at atmospheric pressure. This steam is then channeled to turn a turbine to generate electricity. The operational differences in pressure and temperature between PWRs and BWRs significantly influence the design and safety systems, as well as the overall thermal efficiency of each reactor type.

Other aspects, such as fuel type and the presence of a moderator, do not provide a clear differentiation between the two reactor types, as both types commonly use uranium fuel and a moderator, such as water. The key differentiator is fundamentally linked to how water is utilized in the reactor's cooling and steam generation processes.