The Chernobyl accident

Chernobyl is 100 km north of Kiev in the Ukraine, close to borders with Russia and Belarus. Ukraine was part of the Soviet Union up to 1989 and at Chernobyl there was a major nuclear power station with four graphite-moderated boiling water (Soviet RMBK-type) reactors. In the early hours of Saturday 26 April, 1986, an accident occurred in reactor 4 at the power station. It remains the most serious accident to occur at a nuclear power station anywhere in the world.

The accident happened during a turbine test prior to the reactor being shutdown for re-fuelling. The test was started early on Friday 25 April 1986 but was interrupted for operational reasons during the afternoon. Because of this delay, staff had to operate the reactor outside its design parameters for an extended time, which included switching off basic safety systems and withdrawing nearly all the control rods from the reactor.

When the turbine test was actually carried out in the very early hours of the following morning, the operators feared the reactor had become unstable and tried to insert all the control rods at once. Unfortunately, a design flaw in the control rods meant the reactor became even more unstable and it suddenly surged to full power and beyond. This caused the fuel to melt and a pressure explosion followed by a hydrogen explosion blew the 1000 tonne lid off the reactor.

There was substantial damage to the reactor building and large amounts of radioactive fission products were released into the environment. A subsequent fire in the open reactor continued for more than a week and dispersed radioactive material until an improvised seal was constructed using sand, clay, lead and boron dropped from military helicopters.

Two workers at the power station died from physical injuries on 26 April and over 200 workers and firemen who tackled the blaze after the explosion were hospitalised with radiation burns. Twenty-eight people from this group died from acute radiation exposure syndrome during the following days and weeks.

Measures were taken by the local and regional authorities to reduce radiation exposures during the first day or so, and a large scale evacuation of surrounding towns and villages was undertaken during the following days and weeks. However there was no initial widespread use of stable iodine tablets or restrictions on the consumption of milk or other foods to reduce uptake of iodine-131.

The Soviet authorities drafted in a large number of people to make the reactor site and local area safe after the accident. They were monitored in the same way as radiation workers would have been and the size of the radiation dose they received was recorded. They became known as 'liquidators' and were allowed to build up a radiation dose of 250 millisieverts (about the same amount they would have accumulated naturally in a lifetime) before being relocated. As many as 200,000 liquidators were employed in the region during the first few months and nearly 600,000 in total over the years afterwards. The latter group of liquidators received far lower doses on average.

Figure 1. Chernobyl Reactor 4 shortly after the accident in April 1986

Chernobyl Reactor 4 shortly after the accident in April 1986.

The environmental impact

The explosive breaching of the reactor containment led to the release of significant quantities of radioactive materials into the environment. Estimates of the size of the release vary, but it is generally agreed that about five per cent of the reactor's radioactive fission products were widely dispersed.

Gaseous and volatile fission products travelled the farthest. Iodine-131 and caesium-134 and -137 were detected in many parts of western and northern Europe, and traces were detected further afield, in Japan and the USA for example. Other less mobile radionuclides, such as uranium-235 and plutonium-239, tended to stay in the vicinity of Chernobyl.

The Soviet Union permanently evacuated certain areas of the country due to contamination from the accident. This action was taken to avoid the steadily increasing radiation dose that people would receive from eating food grown and harvested in the locality. This is still the basis for continuing restrictions in various parts of the Ukraine, Belarus and Russia. The external radiation dose in these areas is not significantly higher than average.

There are contradictory reports about how wildlife is surviving in the restricted zones. Some scientists report a thriving scenario for many species of fauna and flora, probably due to the absence of humans and farming practices. Others report health problems for some species and argue this is due to radioactive contamination of the food chain.

The health impact

In addition to the immediate health impact caused by acute radiation exposures to workers and fire fighters, there has been a significantly increased incidence of thyroid cancer in people who lived nearby. It is particularly marked in those who were very young at the time, including children born a few months after the accident. Iodine concentrates in the thyroid and it is widely accepted that iodine-131 exposures in this area in 1986 are the cause of this increase in thyroid cancer.

As time progresses there are reports of some health effects in recovery workers and fire fighters, including leukaemia and cataracts. The reports argue the need for a properly funded long term study of these groups to see if there is anything unusual occurring.

There are other health effects that are claimed to be due to the accident, for example an increase in heart disease in the local population. However establishing the direct cause of these effects is much less clear-cut. The stress and disruption caused by the accident itself could have had various effects on health, particularly the evacuation and subsequent blight of the local area. These health effects could therefore have been caused by the accident, but there is no direct link to significant radiation exposure.

The impact in the UK

Chernobyl fallout was first detected in northern Poland on Monday, 28 April, 1986 and soon afterwards across the Baltic Sea in Sweden. The plume then travelled southwards over Germany and up through France to the UK, arriving in southeast England on the morning of Friday, 2 May.

The plume had travelled over 1000 miles before reaching the UK and so was considerably dispersed. It was nevertheless very detectable with suitable instruments and there was some widespread 'dry' deposition (i.e., not caused by rainfall). Rainfall in North Wales, NW England and Scotland deposited more fission products on the ground via 'wet' deposition. Iodine-131 was measured in air at about 1or 2 becquerels per cubic metre and could be detected in milk products in these and other regions, but not at levels that would have triggered a ban on consumption.

Caesium-137 could also be detected in soil samples and this still has an impact in parts of the UK. In most areas the caesium-137 was washed into the ground where it binds strongly with clay and other soils and is therefore removed from the food chain. However in certain parts of the UK the soil and grasses combine to recycle the caesium-137 and it can then enter the food chain. This occurs in some hill farm areas and has required restrictions on produce from sheep farms in these areas.

Measurements were carried out to see how much radiation people had absorbed during, and after, the passage of the plume. In southern UK, people had levels of about 300 becquerels of caesium-137 in their bodies in 1986 which dropped to 50 becquerels in 1989 and then were back to pre-incident levels in 1990. For comparison, adults have about 4,000 becquerels of naturally occurring potassium-40 in their bodies. Potassium is essential for life and potassium-40 emits electrons and a gamma ray with a very similar energy to the gamma ray from caesium-137.

Figure 2. The concrete sarcophagus for Chernobyl Reactor 4, completed in 1986

Chernobyl sarcophagus

The Chernobyl accident

The environmental impact

The health impact

The impact in the UK

Further reading