Chernobyl: 26 April, 1986

CHERNOBYL, Ukraine -- The accident in reactor no. 4 at the Chernobyl nuclear power station took place in the night of 25 to 26 April 1986, during a test.

On 26 April 1986, at 1:23:44, reactor no. 4 of the Chernobyl nuclear power station exploded. One hundred times more radiation was released than by the atom bombs dropped over Hiroshima and Nagasaki.

The operating crew planned to test whether the turbines could produce sufficient energy to keep the coolant pumps running in the event of a loss of power until the emergency diesel generator was activated.

In order to prevent the test run of the reactor being interrupted, the safety systems were deliberately switched off. For the test, the reactor had to be powered down to 25 per cent of its capacity. This procedure did not go according to plan: for unknown reasons, the reactor power level fell to less than 1 per cent.

The power therefore had to be slowly increased. But 30 seconds after the start of the test, there was a sudden and unexpected power surge. The reactor's emergency shutdown (which should have halted the chain reaction) failed.

Within fractions of a second, the power level and temperature rose many times over. The reactor went out of control. There was a violent explosion. The 1000-tonne sealing cap on the reactor building was blown off. At temperatures of over 2000°C, the fuel rods melted.

The graphite covering of the reactor then ignited. In the ensuing inferno, the radioactive fission products released during the core meltdown were sucked up into the atmosphere.

Determining the causes of the accident was not easy, because there was no experience of comparable events to refer to. Eyewitness reports, measurements carried out after the accident, and experimental reconstructions were necessary. The causes of the accident are still described as a fateful combination of human error and imperfect technology.

The test during which the accident happened was conducted under time pressure. Shortly after it started, on Friday 25 April 1986, the test run was interrupted for nine hours. Electricity still had to be supplied to the capital, Kiev.

The test then took place at night. Today, several flaws in the technical design of the reactor type are thought to have been decisive.

These include the handling of the control rods. In a reactor, the power level is controlled by raising and lowering the control rods: the fewer control rods are positioned between the fuel elements, the greater the reactor power. In this type of reactor, however, the management of the "braking" process has a fatal flaw.

If the control rods are raised and then, to "put on the brakes", lowered between the fuel elements, the initial effect is the exact opposite: reactor power is increased.

If, as was the case in the test at Chernobyl, too many control rods are raised at once and then reinserted simultaneously during an emergency shutdown, the power level rises so dramatically that the reactor is destroyed.

A similar error, but with much less severe consequences, had already occurred in a reactor of the same type in Lithuania in 1983. This experience, however, was not passed on to the operating crew in Chernobyl.

What technical measures were taken to extinguish the fire in the reactor? To put out the fire and thus stop the release of radioactive materials, firefighters pumped cooling water into the core of the reactor during the first ten hours after the accident.

This unsuccessful attempt to put out the fire was then abandoned. From 27 April to 5 May, more than 30 military helicopters flew over the burning reactor. They dropped 2400 tonnes of lead and 1800 tonnes of sand to try to smother the fire and absorb the radiation.

These efforts were however unsuccessful. In fact they made the situation worse: heat accumulated beneath the dumped materials. The temperature in the reactor rose again, and thus also the quantity of radiation emerging from it.

In the final phase of firefighting, the core of the reactor was cooled with nitrogen. Not until 6 May were the fire and the radioactive emissions under control.

The 600 men of the plant's fire service and the operating crew, who were employed in firefighting, were the most severely irradiated group. 134 of them received doses of radiation between 0.7 and 13 sieverts (Sv).

This means that within a few hours they received a quantity of radiation up to 13,000 times higher than 1 millisievert: in the European Union, 1 millisievert per year is the maximum effective dose of radiation to which individuals in the population near a nuclear power station should be exposed.

31 workers died shortly afterwards. A total of around 800 000 men were involved in the clean-up operations in Chernobyl up until 1989. Today, they are still suffering from the damage to their health. 300 000 of them are believed to have received doses of radiation of more than 0.5 Sv.

How many of them have died to date from the effects is a controversial question. According to government agencies in the three former Soviet States affected, about 25,000 "liquidators" have so far died. Estimates provided by the liquidator associations in the three countries are well in excess of the official figures. The Chernobyl Forum's 2005 Report on the other hand attributes a far lower number of liquidator deaths to the reactor disaster.

These discrepancies in numbers are due to different methods of assessment. The Chernobyl Forum bases its assessment on the assumption that a dose of under 500 mSv cannot result in death. Applied to the Hiroshima-Nagasaki data, however, this assumption would lead to an entirely new appraisal of the internationally recognised consequences of the two atom bomb explosions.

Besides, the liquidator statistics (number of casualties and amount of radiation received) were deliberately and accidentally distorted by the Soviet authorities, something which is nigh to impossible to rectify this stage.

Source: Chernobyl Info