Chernobyl accident

Reactor shutdown and power excursion

The rods moved at 0.4 m/s, taking 18 to 20 seconds to travel the full height of the core, about 7 m (23 ft). As the control rods moved downward into the reactor, their extensions initially displaced neutron-absorbing water with neutron-moderating graphite. It was known that in the RBMK design a scram could therefore initially increase the reaction rate within the core. This behaviour had been discovered in 1983, when the insertion of control rods in a similar reactor at Ignalina Nuclear Power Plant had induced a power spike. Countermeasures were, however, not implemented. The IAEA investigative report INSAG-7 later stated, "Apparently, there was a widespread view that the conditions under which the positive scram effect would be important would never occur. However, they did appear in almost every detail in the course of the actions leading to the Chernobyl accident."