Void Coefficient - Reactor Designs

Reactor Designs

• Boiling water reactors generally have negative void coefficients, and in normal operation the negative void coefficient allows reactor power to be adjusted by changing the rate of water flow through the core. However, the negative void coefficient can cause an unplanned reactor power increase in events (such as sudden closure of a steamline valve) where the reactor pressure is suddenly increased. In addition, the negative void coefficient can result in power oscillations in the event of a sudden reduction in core flow, such as might be caused by a recirculation pump failure. Boiling water reactors are designed to ensure that the rate of pressure rise from a sudden steamline valve closure is limited to acceptable values, and they include multiple safety systems designed to ensure that any sudden reactor power increases or unstable power oscillations are terminated before fuel or piping damage can occur.
• Pressurized water reactors operate with no voids at all, and the water serves as both moderator and coolant. Thus a large negative void coefficient ensures that if the water boils or is lost the power output will drop.
• CANDU reactors have positive void coefficients that are small enough that the control systems can easily respond to boiling coolant before the reactor reaches dangerous temperatures (see References).
• RBMK reactors, such as the reactors at Chernobyl, have a dangerously high positive void coefficient. This was necessary for the reactor to run on unenriched uranium and to require no heavy water. Before the Chernobyl accident these reactors had a positive void coefficient of 4.7 beta and after the accident that was lowered to 0.7 beta. This was done so all RBMK reactors could resume safe operating and produce much needed power for the then USSR and its satellites.
• Fast breeder reactors do not use moderators, since they run on fast neutrons, but the coolant (often lead or sodium) may serve as a neutron absorber and reflector.
• Magnox reactors, advanced gas-cooled reactors and pebble bed reactors are gas-cooled and so void coefficients are not an issue. In fact, some can be designed so that total loss of coolant does not cause core meltdown even in the absence of active control systems. As with any reactor design, loss of coolant is only one of many possible failures that could potentially lead to an accident. In case of accidental ingress of liquid water into the core of pebble bed reactors a positive void coefficient may occur.