RBMK: Encyclopedia II - RBMK - Positive void coefficient

RBMK - Positive void coefficient

Ordinary (light) water absorbs neutrons fairly readily, and so removing water from the core (such as happens when it boils and is replaced by steam) tends to increase the rate at which the nuclear reaction proceeds. In a water-moderated reactor, this effect is countered by the reduction in moderation, but in the RBMK the moderating effect of the water is small compared to that of the graphite, so the overall effect is positive. This is called a "positive void coefficient". The RBMK as designed also had a "positive power coefficient", meaning that an increase in reactor power tends to further increase the rate of reaction. Large positive void and power coefficients can produce runaway conditions and have not been permitted in other reactor designs, but it was not possible to eliminate them from the RBMK if natural uranium fuel was to be used.

The RBMK was also intended to use recycled uranium from reprocessed PWR fuel, which has a low remaining enrichment. In this configuration it was also unstable. These characteristics brought the RBMK to the world's notice in 1986, when one of the four RBMK reactors cooling systems at Chernobyl exploded in the worst civilian nuclear accident to date.

Adapted from the Wikipedia article "Positive void coefficient", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki