GEOL 115: The Earth’s Energy Resources

The operators at the Chernobyl plant were performing a series of tests that required the reactor be operated at low power. The sequence of events is summarized in Mevedev (1991) and at http://www.stacken.kth.se/~foo/texts/chernobyl.html. Because of the high positive reactivity void coefficient and some other issues the crew was unable to control the reactor power as precisely as desired. During this whole process they pulled a large number of control rods out of the reactor. As reactor power began to increase they attempted to stop the increase in power by reinserting a large number of control rods simultaneously. The resulting initial increase in reactivity caused the reactor to heat up so much that the control rods warped and got stuck. As the reactor continued to heat zirconium cladding on the fuel assembly began to react with the water and steam to produce hydrogen following the reaction
Zr + 2H₂O = ZrO₂ + 2H₂
The accumulation of steam in the reactor system caused a steam explosion followed by a hydrogen explosion as the hydrogen was exposed to the atmosphere. These explosions destroyed the reactor and started a series of fires that took weeks to extinguish.