The Chernobyl Accident: RBMK Reactors
The Chernobyl reactor was an RMBK reactor (Once again the link again is from the Paks Reactor web site in Hungary). RBMK reactors are graphite moderated reactors that also have water and steam. Water can moderate neutrons and absorb some neutrons whereas steam (because of its low density relative to liquid water) is an ineffective moderator or neutron absorber.
If a reactor is moderated by water, conversion of water to steam within the reactor will cause the reactor to lose power (The moderator is lost, thus the fuel is less able to capture the free neutrons). In contrast, conversion of water to steam in an RBMK reactor will cause the reactor to increase power (The graphite moderator is still in place. The density of water molecules in steam is much less than it is in liquid water, thus the volume of the reactor occupied by the steam will capture far fewer neutrons than if it were filled with liquid water. ere but the neutron absorption capacity of the water has been eliminated). This phenomenon is called positive reactivity void coefficient. This characteristic and another that we will discuss tomorrow were major contributing factors to the disaster.
For a good reference on the Chernobyl disaster see
Medvedev G. 1991. The truth about Chernobyl. Basic Books, 274 pp.
(Edited 4/28/08)
Posted: April 27th, 2008 under Chernobyl, Nuclear Power.