Light-water reactors

Light-water reactors (LWRs) are power reactors that are cooled and moderated with ordinary water. There are two basic types: the pressurized-water reactor (PWR) and the boiling-water reactor (BWR).

In the PWR, water at high pressure and temperature removes heat from the core and is transported to a steam generator. There the heat from the primary loop is transferred to a lower-pressure secondary loop also containing water. The water in the secondary loop enters the steam generator at a pressure and temperature slightly below that required to initiate boiling. Upon absorbing heat from the primary loop, however, it becomes saturated and ultimately slightly superheated. The steam thus generated ultimately serves as the working fluid in a steam-turbine cycle.


  • Strong negative void coefficient — reactor cools down if water starts bubbling because the coolant is the moderator, which is required to sustain the chain reaction
  • Secondary loop keeps radioactive stuff away from turbines, making maintenance easy.
  • Very much operating experience has been accumulated and the designs and procedures have been largely optimized.


  • Pressurized coolant escapes rapidly if a pipe breaks, necessitating lots of back-up cooling systems.
  • Can’t breed new fuel — susceptible to “uranium shortage”