Chernobyl: The Night Reactor Four Exploded

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Just after one in the morning on 26 April 1986, a routine electrical test inside reactor No. 4 at the Chernobyl Nuclear Power Plant went catastrophically wrong. Within seconds, steam explosions tore the reactor building apart. By the time the sun rose over the nearby city of Pripyat, the Soviet Union had its worst nuclear accident on record — and the world had the most expensive disaster in history, with a final cost estimated at $700 billion.

The strange thing is that nobody set out to blow up a reactor. They were trying to answer a question about safety: what happens if the power fails?

A Question About Safety

Reactor No. 4 was an RBMK design, cooled by more than 45 million litres of water an hour, pushed through 1,661 fuel channels by electrically driven pumps. If the plant ever lost outside power, backup diesel generators would kick in — but not instantly. They needed 60 to 75 seconds to reach full load and produce the 5.5 megawatts required to run just one main pump. During that gap, the reactor’s own coolant pumps would have nothing driving them, a dangerous scenario if a coolant pipe ruptured at the same time.

Engineers had a theoretical fix. As a reactor’s steam turbine spins down after losing power, it still carries rotational momentum — energy that might be harvested to run emergency cooling pumps just long enough to bridge the gap before the generators caught up. The idea had been tested in 1982, 1984, and 1985. Each attempt failed, for different reasons: insufficient voltage, then another electrical problem, then faulty recording equipment. The test scheduled for 25–26 April 1986 was the fourth attempt to finally get an answer.

Whoever wrote the test procedure treated it as a straightforward electrical experiment on the generator, not as something that touched the reactor’s safety systems. Under existing regulations, that classification meant the test needed no sign-off from the reactor’s chief designer or the nuclear safety regulator — only approval from the plant’s chief engineer, which it got. Crucially, the procedure called for disabling the emergency core cooling system for the duration of the test.

A Delay, a Shift Change, and a Vanishing Window

The plan was simple on paper. Reduce reactor power gradually. Run the turbine generator at normal speed, disconnect it from the grid, and measure how long its coasting momentum could power the main coolant pumps before the emergency generators took over.

It started on schedule. At 01:06 on 25 April, operators began lowering power, and by the start of the day shift it stood at 1,600 MW, half the reactor’s normal output. The day shift had been briefed and was ready to run the test at 14:15, and the emergency core cooling system was switched off in preparation. Then, at 14:00, a different regional power station went offline unexpectedly, and the Kiev grid controller asked Chernobyl to delay any further power reduction — the capital needed that electricity for the evening peak.

So the test waited. The day shift went home. The evening shift took over, and the emergency cooling system stayed disabled the entire time — not because anyone thought it was still needed, but because switching it back on and off again was a manual job involving two or three people turning valve wheels the size of sailboat helms. Nothing went wrong during those eleven unprotected hours, but the fact that the plant ran that long with a key safety system offline, outside the test itself, pointed to a loose safety culture at the plant.

By the time Kiev finally cleared the reactor to resume its shutdown at 23:04, the evening shift was packing up too. The test that was supposed to be finished by the day shift now fell to the night crew, who had barely any time to prepare. Anatoly Dyatlov, the plant’s deputy chief engineer and one of the test’s own authors, was on-site to direct it — the senior-most person in the room. Unit supervisor Aleksandr Akimov ran the night shift, and 25-year-old Leonid Toptunov, just three months into working independently as senior reactor control engineer, controlled the reactor itself.

The Power Collapses — and Then Surges

Getting the reactor down to the test’s target range of 700–1,000 MW went smoothly enough; operators hit 720 MW just after midnight. But reactors left at reduced power accumulate xenon-135, a byproduct that absorbs the neutrons needed to sustain the chain reaction. At full power, xenon-135 burns off as fast as it forms. At low power, it builds up — a known, predictable complication, but one that makes the reactor sluggish and harder to control.

Then came a moment of confusion in the control room. Switching between automatic regulator systems, one activated properly and pulled its control rods out; the other failed because of an imbalance in its sensors. Toptunov adjusted the controls to stabilize things — and the reactor’s power instead collapsed, crashing to 30 megawatts or less, a near-shutdown. Exactly why remains unclear: most accounts blame operator error, though Dyatlov himself maintained the fault lay in the automatic regulator system, not the young engineer at the controls.

Whatever the cause, the reactor was now stuck at roughly 5% of the minimum power the test required — and the built-up xenon made it stubbornly resistant to recovery. To force power back up, the crew pulled numerous control rods from the core. It took several minutes to coax the reactor back to 160 MW, with most of the control rods withdrawn near their upper limits — technically still within permitted bounds, but leaving little room for error. Over the following twenty minutes, power crept up to 200 MW, far below the test’s intended range, as the night shift pressed on toward an experiment now running in conditions nobody had designed it for.

It was from this fragile, rod-starved, xenon-poisoned state — power far too low, safety margins stripped away, the emergency cooling system still switched off — that the test continued toward its final, fatal steps.

What we still don’t know

The precise cause of the sudden power collapse around 00:28 on 26 April is still not settled. Most accounts place the blame on an operator error by Toptunov, but Dyatlov attributed it instead to a fault in the automatic regulator system, and the exact circumstances remain unknown. The long-term human toll is similarly contested: estimates of eventual deaths from the disaster range from around 4,000, as assessed by the United Nations for Ukraine, Belarus, and Russia, to as many as 16,000 across Europe when wider projections are included. Even the scale of thyroid cancer caused directly by the accident is described only as “a large fraction” of the roughly 6,000 childhood cases recorded by 2005 — not a precise, settled figure.

Sources

– This article is a synthesis of the Wikipedia article on Chernobyl disaster (en.wikipedia.org/wiki/Chernobyl_disaster), which itself cites the underlying scholarship. Readers wanting primary sources should follow that article’s reference list.