Chernobyl, Ukraine

Cause of the Disaster

On the night that the accident occurred, the operating crew was ordered to perform tests to ascertain whether, in the event of a loss of external power, the spinning turbines could produce sufficient power to keep the coolant pumps running until the emergency diesel generator started up.

The safety systems of the reactor were switched off deliberately to prevent any interruption to the tests. The reactors’ design was such that in the event of a power failure, the reactor would SCRAM (control rods would be inserted into the reactor, which in turn would shut down). However, even when the control rods were fully inserted, the reactor still required cooling, as the fission products from reaction would continue to produce heat as they decayed—and this heat could in turn lead to damage of the reactor's core.

Aside from “human error” and “defective technology,” another area where the RBMK reactor came under heavy criticism is in the design of the interface between “people” and “machines.”

The design of the RBMK reactor has also come under heavy criticism. One notable feature of the reactor is that it has an unusually large “void reactivity coefficient;” this is a parameter that defines how the reactor will respond to the formation of steam bubbles in the water coolant. In many designs, as steam bubbles form, the reactor will produce less energy—this is because water is used to slow down fast neutrons, which are less likely to split atoms of uranium than slower neutrons. As water turns to steam, fewer neutrons are slowed, splitting fewer uranium atoms and releasing less energy. In the RBMK reactor design, graphite was used as a moderator to slow down neutrons, and light water, which absorbs neutrons, to cool the core. The bubbles of steam will not absorb neutrons as readily as water; therefore, as bubbles of steam form, the energy production and reactor temperature increases.

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