Interesting review by the BBC of the Chernobyl nuclear disaster.
Reactions went awry, apparently, when some engineers tried to test a hypothesis on a production system. The system quickly heated out of control during the test, and was unable to recover.
The reactor began to overheat and its water coolant started to turn to steam.
At this point it is thought that all but six control rods had been removed from the reactor core – the minimum safe operating number was considered to be 30.
Because the reactor was not housed in a reinforced concrete shell, as is standard practice in most countries, the building sustained severe damage and large amounts of radioactive debris escaped into the atmosphere.
They are still working on building a containment system, twenty years later, and now need £600m to replace the present system that is failing. Wonder what the cost of the containment shell, and/or a proper development and test environment, would have been prior to the accident.