Process Safety · Case study

Bhopal, 1984: methyl isocyanate release

Water entered a storage tank of methyl isocyanate (MIC). The runaway reaction that followed released a toxic cloud over a sleeping city.

Date2–3 Dec 1984

WhereUnion Carbide India Ltd, Bhopal

MaterialMethyl isocyanate (MIC)

ReleasedAbout 40 tonnes

DeathsThousands: the Madhya Pradesh government confirmed 3,787; other estimates are far higher

ExposedOver 500,000 people

What happened

The plant made the pesticide carbaryl using MIC, a volatile, highly toxic and water-reactive intermediate stored in large underground tanks. Late on 2 December 1984, water got into tank E610. MIC reacts exothermically with water; the temperature and pressure rose, the relief valve lifted, and MIC vapour poured out of the vent stack for about two hours. The cloud drifted over densely populated neighbourhoods next to the plant.

The way the water got in has been disputed (a pipe-washing operation without a blind, or deliberate introduction), but the scale of the disaster came from what failed to stop it.

Sequence of events

Late eveningWater enters MIC tank E610, which holds about 40 t of MIC.

Before midnightWorkers notice eye irritation; the tank pressure is rising.

Around midnightPressure and temperature climb fast; the relief valve lifts.

Next 1–2 hoursMIC vapour pours out of the vent stack; the scrubber and flare cannot stop it.

By morningThe cloud has covered large parts of the city; thousands are dead or dying.

Why it happened

Large inventory of a highly hazardous intermediate: far more MIC was stored than the process needed (an inherent-safety failure).
Safeguards out of service: the refrigeration that should have kept MIC near 0 °C had been shut down; the vent gas scrubber was on standby and too small for a runaway; the flare was out of service for maintenance; water sprays could not reach the height of the release.
Maintenance and procedures: no slip-blinds when washing lines; alarms and gauges known to be unreliable.
Siting and emergency planning: dense housing up to the fence, no community warning or evacuation plan, no information on antidotes or sheltering.

Lessons for your plant

Minimise inventories of highly hazardous intermediates: “what you don’t have can’t leak”.
A safeguard that is out of service is not a safeguard. Track impairments and stop operation when critical layers are missing.
Size scrubbers, flares and relief for the worst credible runaway, not the normal vent load.
Plan for the community: land-use planning, sirens, shelter-in-place advice, coordination with local authorities.
Water-reactive materials need positive isolation (blinds) during line washing.

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Sources

Summary written for training. Figures are as published by the investigating bodies and widely cited sources; see the sources for full details.