The Night Shift Test Shortly after midnight, 26 April 1986

You are advising the shift supervisors in the control room of Unit 4 at the Chernobyl nuclear power plant in the Soviet Union.

A test is planned to check whether a slowing turbine could briefly power the reactor’s cooling pumps during a power cut. It was due during the day but was postponed for hours at the request of the Kyiv electricity grid controller. Shortly after midnight, during a switch in control mode, the reactor’s power fell unexpectedly to about 30 megawatts (thermal), far below the planned test level.

Known information

The test programme called for reactor power of roughly 700–1,000 MW (thermal).
Operators have managed to raise power back only to about 200 MW.
Some safety systems have been disconnected as part of the test programme.
The reactor is due to be shut down for maintenance, and a missed test might not be repeated for a long time.
Operating instructions set a minimum number of control rods that must remain effectively inserted in the core.

Reveal what happened (spoilers: decide first)

Fact

What actually happened

The operators decided to run the test at about 200 MW. (Carry on with the test)

The test began at about 1:23 a.m. Within seconds, a sudden power surge caused explosions that destroyed the reactor and started fires. Two plant workers died that night, and 28 more died of acute radiation sickness in the following months. Large areas of Ukraine, Belarus and Russia were contaminated, and more than 100,000 people were evacuated in 1986.

Important

Possible consequences · Analysis and speculation, not fact

Shutting down would very probably have avoided the accident that night, although the design weaknesses identified afterwards would have remained in RBMK reactors until they were discovered and modified.

History does not reveal what would definitely have happened under the other choice. Many factors could have changed the outcome.

Context · Why it was decided this way

Early Soviet accounts placed most of the blame on operator errors. The International Atomic Energy Agency’s 1992 INSAG-7 report gave more weight to flaws in the RBMK reactor design, including how it behaved at low power and the design of its control rods, which the operators had not been properly informed about.

Investigators also pointed to a weak safety culture: the test programme was poorly reviewed, and at the time of the test fewer control rods were effectively in the core than operating rules required.