Chernobyl - Appendices: Positive Void Coefficient

 March 2001

Positive void coefficient is a term often associated with the RBMK reactors, the type involved in the Chernobyl disaster. Reactors that have a positive void coefficient can be unstable at low power and may experience a rapid, uncontrollable power increase. While reactors other than the RBMK type have positive void coefficients, they incorporate design features to prevent instability from occuring.

 Details

In a water cooled reactor steam may accumulate to form pockets, known as voids. If excess steam is produced, creating more voids than normal, the operation of the reactor is disturbed, because

• the water is a more efficient coolant than steam
• the water acts as a moderator and neutron absorber while steam does not.

A reactor is said to have a positive void coefficent if excess steam voids lead to increased power generation, and a negative void coefficient if excess steam voids leads to a decrease in power. The coefficient is simply a measure of the speed of change of state of the reactor.

When the void coefficient is positive, the power can increase very rapidly because any power increase that occurs leads to increased steam generation, which in turn leads to a further increase in power. Such increases are, therefore, very difficult to control.

When the void coefficient is negative, excess steam generation will tend to shut down the reactor. This is, of course, not a safety problem.

 Most of the world's operating power reactors have negative void coefficients. In those reactors where same water circuit acts as both moderator and coolant, excess steam generation reduces the slowing of neutrons necessary to sustain the nuclear chain reaction. This leads to a reduction in power.

In some reactor designs however, the moderator and coolant are in separate circuits, or are of different materials. In these reactors, excess steam reduces the cooling of the reactor, but as the moderator remains intact the nuclear chain reaction continues.

In some of these reactors, most notably the RBMK, the neutron absorbing properties of the cooling water are a significant factor in the operating characteristics. In such cases, the reduction in neutron absorbtion as a result of steam production, and the consequent presence of extra free neutrons, enhances the chain reaction. This leads to excess power production.

This excess power production causes additional heating. The additional heat raises the temperature in the cooling circuit and more steam is produced. More steam means less cooling and less neutron absorbtion, and the problem gets worse.

All of this can happen very rapidly. If it is not stopped, and it is very difficult to stop because it feeds itself, there will be the sort of event that happened at Chernobyl unit 4.

In order to avoid problems with positive void coefficient there are two approaches. Either the reactor characteristics can be altered to reduce the positive void coefficient or systems can be provided that will shut the reactor down very quickly if an increase in power is detected.

Since the Chernobyl disaster, the RBMK reactor design has been altered and units have been backfitted to protect them against the effects of the positive void coefficient. 

Email : wna@world-nuclear.org