How does a BWR nuclear reactor work?

There are different types of nuclear reactors, but there are two designs that account for more than 80% of all those in operation in the world: the Pressurised Water Reactor (PWR) and the Boiling Water Reactor (BWR).

In both cases, water acts as both moderator and coolant and circulates through the core, increasing its temperature. The basic difference is that for PWRs because they operate at very high pressure, the water does not boil and remains liquid throughout the reactor coolant circuit, whereas in BWRs, the water leaves the core as steam.

In the case of a PWR reactor, there are three isolated coolant circuits:

• Primary circuit. This is the one in contact with the fuel.
• Secondary circuit. Water-steam.
• External cooling circuit. This is the one in contact with the environment.

In a BWR reactor, there are two cooling circuits isolated from each other. The first extracts heat from the fuel, generating steam, and the second is the external cooling circuit that is in contact with the environment. However, there is no secondary water-steam circuit, but the coolant itself circulates through the reactor vessel and core, evaporating as it passes through the core.

The saturated steam leaving the reactor vessel is directed to the turbine. The operation of the turbine, alternator, condenser, and feedwater system is entirely similar to that of a PWR plant, although in this case, the system drives the water to the reactor vessel itself.

External recirculation loops are provided in the reactor vessel which, through the combined use of centrifugal and jet pumps, allow rapid regulation of the coolant/moderator flow rate and hence control of reactor power.

The control rods are located in the lower part of the vessel and are inserted inside the vessel by means of a hydraulic system that uses the coolant itself as a fluid, by means of a passive system of high-pressure accumulators.