What happened in Chernobyl?

On April 26, 1986, a major accident occurred at Unit 4 of the nuclear power station at Chernobyl, Ukraine, in the former USSR.

The operating crew was planning 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.

To prevent any interruptions to the power of the reactor, the safety systems were deliberately switched off. To conduct the test, the reactor had to be powered down to 25 percent of its capacity. This procedure did not go according to plan and the reactor power level fell to less than 1 percent. The power therefore had to be slowly increased. But 30 seconds after the start of the test, there was an unexpected power surge. The reactor's emergency shutdown (which should have halted a chain reaction) failed.

The reactor's fuel elements ruptured and 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. The graphite burned for nine days, churning huge quantities of radiation into the environment. The accident released more radiation than the deliberate dropping of a nuclear bomb on Hiroshima, Japan in August 1945.

The clean up

Initial attempts to extinguish the burning reactor involved fire fighters pouring cooling water into the reactor, and were abandoned after 10 hours. 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 also unsuccessful. In fact they made the situation worse: heat accumulated beneath the dumped materials. The temperature in the reactor rose again, along with the quantity of radiation emerging from it. In the final phase of fire fighting, the core of the reactor was cooled with nitrogen. Not until 6 May were the fire and the radioactive emissions under control.

Despite the obvious dangers the response to the disaster needed people. Not just a few but thousands of people whose lives and health were sacrificed in vain attempts to contain the disaster. These people were termed 'liquidators'.

The 600 men of the plant's fire service and the operating crew were the most severely irradiated group. In this group 130 men were irradiated with doses equivalent to 650 years worth of a radiation worker's annual limit. Thousands of military personal and other workers were drafted in to move deadly radioactive material with little or no protection.

31 workers died shortly afterwards. A total of between 600,000 and 800,000 men were involved in the clean-up operations in Chernobyl up to 1989. Of these men, 300,000 received radiation doses 500 times the limit for the public over one year. Today, the ones who still survive are still suffering from the damage to their health.

How many of them have died to date from the disaster 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. Also the liquidator statistics (number of casualties and amount of radiation received) were distorted by the Soviet authorities so definitive numbers may never be known.

The end of the disaster?

On 22 December 1988, Soviet scientists announced that the sarcophagus now enclosing the reactor was designed for a lifetime of only 20 to 30 years.

Three years after the nuclear accident, the Soviet government halted construction of the fifth and sixth reactor units at the Chernobyl nuclear power complex. After prolonged international negotiations, the entire complex was closed on 12 December 2000, 14 years after the accident.

What is the sarcophagus?

Following the explosion, a massive concrete 'sarcophagus' (cover) was constructed around the damaged no. 4 Reactor. This sarcophagus encases the damaged nuclear reactor and was designed to halt the release of further radiation into the atmosphere. The first task in containing the destroyed reactor was to build a 'cooling slab' under the reactor to prevent the still-hot reactor fuel from burning a hole in the base of the reactor. Coal miners were drafted in to dig this tunnel under the reactor and by 24 June four hundred coal miners had built the 168 m long tunnel under the reactor.

By November 1986 the sarcophagus containing the reactor was completed using more than 7,000 tonnes of steel and 410,000m3 of concrete.

The sarcophagus was designed with a lifetime of only 20 to 30 years in mind. The greatest problem is a lack of stability: it was hastily constructed, and corrosion of supporting beams threaten the integrity of the entire structure. Water is leaking through the sarcophagus via holes in its roof,  becomes radioactively contaminated, then seeps through the floor of the reactor into the soil below.

Scientists predict that the next nuclear catastrophe in the scale of Chernobyl will be in Chernobyl itself, due to the fragile status of its protective shield.

There is no certainty as to how much fuel has been left inside the reactor but most estimates put it at more than 95 percent of its original contents. Also dumped inside the sarcophagus are thousand of cubic metres of nuclear waste created by fragments of the destroyed reactor building and contaminated soil that has also been dumped into the sarcophagus.