Boiling Water Reactor

In the boiling water reactor the same water loop serves as moderator, coolant for the core, and steam source for the turbine.

In the boiling water reactor (BWR), the water which passes over the reactor core to act as moderator and coolant is also the steam source for the turbine. The disadvantage of this is that any fuel leak might make the water radioactive and that radioactivity would reach the turbine and the rest of the loop.

A typical operating pressure for such reactors is about 70 atmospheres at which pressure the water boils at about 285¡C. This operating temperature gives a Carnot efficiency of only 42% with a practical operating efficiency of around 32%, somewhat less than the PWR.

Pressurized Water Reactor

In the pressurized water reactor, the water which flows through the reactor core is isolated from the turbine.

In the pressurized water reactor (PWR), the water which passes over the reactor core to act as moderator and coolant does not flow to the turbine, but is contained in a pressurized primary loop. The primary loop water produces steam in the secondary loop which drives the turbine. The obvious advantage to this is that a fuel leak in the core would not pass any radioactive contaminants to the turbine and condenser.

Another advantage is that the PWR can operate at higher pressure and temperature, about 160 atmospheres and about 315 C. This provides a higher Carnot efficiency than the BWR, but the reactor is more complicated and more costly to construct. Most of the U.S. reactors are pressurized water reactors.

Liquid-Metal Fast-Breeder Reactor

In the LMFBR, the fission reaction produces heat to run the turbine while at the same time breeding plutonium fuel for the reactor.