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 enhances power production which 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.
This positive feedback can occur very rapidly, potentially leading to a serious accident such as at Chernobyl unit 4 in 1986 where the power peaked at several hundred times the normal full rating. 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 equipped to protect them against the runaway effects of the positive void coefficient . There are currently 13 RBMKs in the world and one under construction due for commercial operation in 2003.