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Quick answers
What happened at Chernobyl on 26 April 1986?
During a delayed safety test, reactor 4 of the RBMK-1000 plant underwent a rapid power excursion. Explosions destroyed the core and started a graphite fire. The event is classed as INES Level 7, the same top rank later assigned to Fukushima Daiichi in 2011 for a different accident sequence.
How many people died?
UNSCEAR records two immediate deaths and 28 ARS deaths among staff and emergency workers within four months — the 30 official early deaths often cited, sometimes listed as 31 with a later cardiac case. Later cancer totals are modelled. Thyroid cancer rose among those exposed as children; UNSCEAR does not treat a six-figure death toll as an observed count.
Was it only operator error?
INSAG-1 (1986) emphasised operators. INSAG-7 (1992) concluded that design features and a weak safety culture were the main causes, with the night’s rule-breaking as the trigger. Both reports remain part of the public file.
When was Pripyat evacuated?
On 27 April 1986, about 36 hours after the explosions. Residents were told the leave would be short. The city was not reoccupied. Slavutych was later built for plant staff.
What is the New Safe Confinement?
A steel arch, the largest movable land structure of its kind, built beside unit 4 and slid over the 1986 sarcophagus on 29 November 2016, with later commissioning. It is meant to last about a century and to allow dismantling work.
What international law followed?
The 1986 Early Notification and Assistance conventions, then the 1994 Convention on Nuclear Safety, plus IAEA safety-culture doctrine. Domestic Soviet criminal law produced the 1987 plant-management trial.
Investigation ran on two tracks: a Soviet criminal and party file, and an IAEA scientific file.
English readers meet the second track first; the first track produced the 1987 sentences.
Agencies
- Soviet government commission — Immediate technical and political inquiry, with Legasov among the scientific leads.
- IAEA / INSAG — International cause reports in 1986 and 1992 that became the English reference narrative.
- UNSCEAR and WHO — Dose reconstruction and health-effect annexes, notably the 2008 report.
- Ukrainian and Russian successor archives — Post-1991 releases of plant logs, maps and party correspondence that filled gaps in INSAG-1.
What followed
- Stabilise the ruin
Helicopter drops, tunnel work and the first sarcophagus were emergency engineering, not a forensic pause.
- Brief Vienna
August 1986 Post-Accident Review Meeting produced INSAG-1 from a still-partial Soviet account.
- Try the managers
The 1987 court located blame in plant leadership while the design remained a state secret in public.
- Open the design
After 1989–91, RBMK details allowed INSAG-7 to re-rank causes.
- Register doses
UN and national registries tracked liquidators, evacuees and thyroid cases rather than a single death list.
- Fund a second shelter
The Shelter Fund and Novarka contract turned the ruin into an international construction case.
Turning points
- Forsmark, 28 April 1986 — Foreign detection ended the chance of a purely domestic file.
- INSAG-7, 1992 — The cause statement English regulators still teach.
- NSC move, 2016 — A visible end to the 1986 sarcophagus as the only lid on unit 4.
The investigation succeeded as engineering history. It did not produce a single uncontested lifetime casualty number.
Public-record encyclopedia. Not legal advice. Not a how-to. Graphic detail is withheld. Wars belong in a separate atlas.