Risk Assessment: Tools, Techniques, and Their Applications

12.3 Case Study: Chernobyl

On 25 April, 1986, prior to a routine shutdown, the reactor crew at Chernobyl 4 began preparing for an experiment to determine how long turbines would spin and supply power to the main circulating pumps following a loss of main electrical power supply. This test had been carried out at Chernobyl the previous year, but the power from the turbine ran down too rapidly, so new voltage regulator designs were to be tested (6).

Multiple operator actions, including the disabling of automatic shutdown mechanisms, preceded the attempted experiment on the morning of April 26. By the time that the operator began to shut down the reactor, it was in an extremely unstable condition. The design of the control rods caused a dramatic power surge as they were inserted into the reactor.

The interaction of extremely hot fuel with the cooling water led to fuel disintegration, along with rapid steam production and an increase in reactor pressure. The design characteristics of the reactor were such that substantial damage to even three or four fuel assemblies could—and did—result in the failure of the reactor vessel. Extreme pressure in the reactor vessel caused the 1000 ton cover plate of the reactor to become partially detached. The fuel channels were damaged and the control rods jammed, which by that time were only halfway down. Intense steam generation then spread throughout the entire core. The steam resulted from water being dumped into the core because of the rupture ...