Chernobyl Disaster

Lore & Background

The test was meant to show whether a turbine running down after a power loss could keep the coolant pumps going until the backup diesel generators reached full load. Earlier attempts in 1982, 1984 and 1985 had failed or given no result. The procedure treated the exercise as a purely electrical test, and its authors were not aware of how the RBMK-1000 reactor would behave under the planned conditions.

The test was scheduled for the day shift of 25 April 1986, but at 14:00 the grid controller in Kiev asked for the power reduction to be postponed, and it was 23:04 before the shutdown was allowed to resume. The emergency core cooling system had been disabled for the test and was left that way throughout the delay. The operators went ahead after an accidental drop in reactor power. When the reactor was shut down in those conditions, design flaws produced a power surge; components ruptured, coolant was lost, and the explosions followed.

The blast killed two engineers and severely burned two others. Of the 237 workers hospitalized, 134 showed symptoms of acute radiation syndrome, and 28 of them died within three months.

Reader's Guide

The Soviet government set up a 10 km exclusion zone 36 hours after the accident and evacuated around 49,000 people; the zone was later widened to 30 km and approximately 68,000 more people were moved. The disaster was not acknowledged publicly until two days after the explosion, when raised radiation levels were detected in Sweden. The response involved more than 500,000 personnel.

Estimates of the long-term death toll vary widely, from up to 4,000 in Ukraine, Belarus and Russia to 16,000 across Europe. As of 2005, 6000 cases of childhood thyroid cancer had occurred in the affected populations, 15 of them fatal.

Pripyat was abandoned and replaced by the purpose-built city of Slavutych. A sarcophagus over the reactor was completed in December 1986, and the New Safe Confinement was built around it in 2016–2018. The clean-up is scheduled for completion by 2065. The only other accident given the same maximum rating is the one at Fukushima in 2011.

Did You Know?

• The reactor ran for 11 hours without its emergency core cooling system before the test.
• The same test had been tried without success in 1982, 1984 and 1985.

Reactor cooling after shutdown

In nuclear reactors, most heat comes from fission, but over 6% is from radioactive decay, which continues after shutdown. Continued coolant circulation is essential to prevent core overheating or meltdown. Chernobyl's RBMK reactors used water coolant circulated by electrically driven pumps; reactor no. 4 had 1,661 fuel channels requiring over 45 million liters of coolant per hour.

In a total power loss, three backup diesel generators took 60–75 seconds to reach full load. Special counterweights on pumps provided coolant via inertia to bridge the gap. A potential risk existed if a station blackout occurred with a coolant pipe rupture, requiring the emergency core cooling system (ECCS). It was theorized that the steam turbine's rotational momentum could generate power for the ECCS via feedwater pumps for about 45 seconds, not fully bridging the gap but alleviating the situation.

Safety test

The turbine run-down energy capability needed experimental confirmation; previous tests in 1982, 1984, and 1985 had failed due to insufficient excitation voltage, electrical system issues, or recording equipment problems. The test was scheduled for 1986 during a controlled power-down of reactor no.

4 for maintenance. The test procedure was written without awareness of unusual RBMK-1000 behavior under planned conditions; it was regarded as an electrical test and did not require approval from the chief design authority or nuclear safety regulator. The test called for disabling the emergency core cooling system, with approval from the site chief engineer. The procedure involved reducing thermal power to 700–1000 MW, running the turbine at normal speed, powering four pumps with off-site power and four with the turbine, then closing steam supply to trigger automatic shutdown, measuring voltage and RPMs, and letting the turbine free-wheel down after emergency generators supplied full power.

Unexpected drop of the reactor power

The test plan required gradual power reduction to 700–1000 MW thermal; 720 MW was reached at 00:05 on 26 April. However, due to xenon-135 buildup from decay of iodine-135, power continued to drop, a process called reactor poisoning. When power fell to about 500 MW, control was switched to automatic regulators.

AR-1 activated, removing all its control rods, but AR-2 failed due to an ionization chamber imbalance. An operator reduced power to stabilize sensors, causing a sudden drop to 30 MW or less. The exact cause is unknown; most reports attribute it to operator error, but another source cited a fault in AR-2. The reactor was producing only 5% of the minimum test power.

Low reactivity hindered xenon burn-off and power rise. Operators removed many control rods; after several minutes, power was restored to 160 MW at 00:39, with most rods at upper limits but within normal limits. Over the next 20 minutes, power increased to 200 MW.