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.