Chernobyl Disaster Explained: The Night Reactor No. 4 Exploded

Reactor No. 4 Exploded at 1:23:44 a.m. on April 26, 1986

The explosion at Reactor No. 4 of the Chernobyl Nuclear Power Plant in Soviet Ukraine at 1:23:44 a.m. on April 26, 1986, released approximately 400 times more radiation than the atomic bomb dropped on Hiroshima. The disaster was the direct result of a flawed reactor design — the RBMK-1000 — combined with a safety test conducted by undertrained operators who violated established protocols and disabled multiple safety systems. Thirty-one people died in direct acute radiation exposure and fire within months of the accident; the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) attributed approximately 6,000 thyroid cancer cases in children to radioactive iodine exposure, and the long-term cancer death toll from the disaster remains contested, with estimates ranging from 4,000 (WHO) to tens of thousands (other scientific analyses).

The RBMK Reactor and Its Fatal Design Flaw

The RBMK-1000 (Reaktor Bolshoy Moshchnosti Kanalnyy — High Power Channel-type Reactor) was a Soviet-designed graphite-moderated, water-cooled reactor with several characteristics that distinguished it — and ultimately destabilized it — compared to Western light-water reactor designs.

The positive void coefficient was the RBMK's most dangerous characteristic. As water in the reactor boiled into steam (voids), the reactor became more reactive rather than less — a runaway feedback loop. This property was known to Soviet designers but was classified and not disclosed to plant operators. The graphite-tipped control rods created a secondary hazard: when operators attempted an emergency SCRAM (full rod insertion) by pressing the AZ-5 button, the graphite tips momentarily increased reactivity in the bottom of the core before the absorbing portion of the rod reached those positions — a brief power surge in an already destabilized reactor.

The Safety Test and What Went Wrong

The night of April 25–26, 1986, operators were conducting a safety test designed to determine whether the reactor's turbines, spinning down after a power shutdown, could generate enough electricity to run emergency cooling pumps for 60–75 seconds until diesel backup generators came online. This test had been attempted three times previously — in 1982, 1984, and 1985 — and failed each time due to insufficient turbine output.

• The test was delayed by 9 hours for grid demand reasons, shifting it from a day-shift crew to the less experienced night shift
• At 11:10 p.m. on April 25, operators inadvertently allowed reactor power to drop to near zero (xenon poisoning); it took 30 minutes to stabilize power at just 200 megawatts thermal — far below the planned 700–1,000 MW test level
• To conduct the test at low power, operators withdrew most control rods far below minimum safety specifications (regulations required at least 15 rods inserted; operators had approximately 6–8 inserted)
• When the test began at 1:23 a.m., operators also disabled the emergency core cooling system (ECCS) to prevent it from interrupting the test
• At 1:23:40, operator Leonid Toptunov pressed the AZ-5 emergency shutdown button; the graphite tips of all rods triggered a massive power surge — power reached an estimated 30,000 MW (ten times operating capacity) in approximately 3 seconds

The Explosions and Initial Response

Two explosions — probably one steam explosion followed by a second explosion (the exact nature of which remains debated, with some researchers arguing it was a prompt criticality nuclear excursion) — destroyed the reactor and blew the 1,000-ton reactor lid off the building. A fire ignited in the graphite moderator and surrounding structures. First responders — local firefighters from Pripyat and the plant's own fire brigade — arrived within minutes without knowledge that they were walking into a radiation field that delivered lethal doses within hours. Lieutenant Volodymyr Pravyk and Lieutenant Viktor Kibenok, both 23 years old, led the initial firefighting response; both died within weeks.

The Liquidators and the Exclusion Zone

Between 1986 and 1990, approximately 600,000 people — termed liquidators — were deployed to clean up the site and build the concrete sarcophagus that entombed the destroyed reactor. Workers included military reservists, miners, helicopter crews, and construction workers, many of whom received significant radiation doses.

Pripyat, a city of 49,000 people built for the plant's workers, was not evacuated until 36 hours after the explosion — a delay driven by Soviet authorities' reluctance to acknowledge the scale of the disaster. The 30-kilometer exclusion zone established around the plant remains largely unpopulated today, though portions of it have been reopened for limited tourism. The new containment structure — the New Safe Confinement arch, completed in 2016 at a cost of €1.5 billion — was slid over the aging original sarcophagus to contain the approximately 200 tonnes of melted nuclear fuel (corium) still inside the remains of Reactor No. 4.