Box 18E The Chernobyl nuclear accident

Chernobyl site after the accident

Source: Frank Spooner Pictures


Description of the accident

At the end of 1985 the USSR was operating 14 RBMK pressure tube reactors, each generating 1000 MW(e) of electricity. Four reactors were at Chernobyl on the Pripyat river, about 100 km north of Kiev (Ukraine) and 140 km south of Gomel (Belarus). Unit 4 came into operation in December 1983 and was destroyed accidentally on 26 April 1986. A combination of factors and circumstances contributed to this catastrophe. First, there were design drawbacks with reactors of this type and an imperfect control and protection system. Secondly, some erroneous actions taken by the operators guided by poor technical specifications and provisions for safe operation resulted in an unstable operation mode of the reactor.

The combination of these circumstances culminated in an instantaneous increase in the thermal power at 01.23 Moscow time on 26 April 1986 (Lakey, 1993). Two non-nuclear (steam/hydrogen) explosions in quick succession penetrated the reactor pressure vessel and blew the roof off the Unit 4 reactor building. Concrete, graphite and other debris heated to very high temperatures then escaped to the atmosphere. Flames and fragments spread 30 secondary fires around the Unit 3 and the turbogenerator building (IAEA, 1991).

The radionuclides, normally accumulated inside the reactor core, immediately began to be released into the environment, and this continued for ten days. Widespread distribution of airborne radioactivity thereby resulted across Europe, and eventually, at appreciably lower levels, throughout the northern hemisphere. The deposition of airborne radioactivity resulted in the contamination of soil, plants and animals, leading to foodstuff contamination.

Consequences and countermeasures

On the site

The first response to the fire alarm was the power station fire team of 15 men followed by the Pripyat Fire Brigade. Eventually 15 men were fighting the spot fires close to the damaged reactor, under very difficult circumstances. Later on, reinforcements from Kiev arrived. All the first firemen received serious radiation doses. To achieve thyroid blocking, potassium iodide tablets were issued to plant personnel around 03.00. By 5.00 the fire brigade had extinguished the main fire sites, except for the fire in the central reactor hall, where graphite continued burning. In order to extinguish the burning graphite and to suppress the radioactive release, more than 5000 tonnes of material (mostly lead, sand and clay) was dumped by helicopters onto the reactor during the subsequent days. This process continued until June 1986.

There were 31 deaths and, of the 301 people who were initially diagnosed, 203 people were confirmed to show clinical effects due to radiation exposure or burns. In order to isolate the destroyed reactor, it was decided to build a containment, better known as the 'sarcophagus', around the destroyed reactor building. This work was finished by mid-November 1986. The gaps in the structure, due to the difficult conditions under which it was built, as well as the need to ventilate it, are continuously monitored for radioactive emissions (Lakey, 1993).

The near field

The town of Pripyat, 3 km from the reactor, was closed in the first few hours and people were instructed to seek shelter indoors. Later, iodine tablets were distributed to households by volunteers. Despite the social impact and disadvantages associated with relocation and resettlement, on 27 April the authorities decided to evacuate the inhabitants of the town using some 1100 buses and 200 lorries. On 2 May the evacuation zone was expanded to an area with a radius of 30 km from the plant ­ the exclusion zone (Map 18.2). A real exodus then started, nd thousands of inhabitants and thousands of farm animals were transported. This was completed on 6 May. New buildings were constructed in Kiev, Zhitomir and Chernigov to shelter the evacuees.

After the people in the 30 km zone were moved, decontamination work started. Over a period of several months efforts were made to reduce external exposure due to radioactive materials that were deposited on surfaces: removal of soil to a depth of 15 cm, asphalting and covering of soil and cleaning of buildings. Later on, these efforts were found to be only moderately effective and natural decontamination, ie, natural decay of radionuclides and migration through the soil, seemed to be more efficient.

To prevent the highly contaminated ground from being washed away, or the radionuclides from migrating through the soil and thus contaminating the rivers and the Kiev reservoir, 140 dams and dikes were built. Initially a deep concrete wall was constructed around the plant to prevent the passage of contamination into the groundwater. Subsequently a long concrete dike was constructed along the bank of the Dnepr to separate it from ground liable to flooding, which would in turn allow activity from the heavily contaminated soil to be transferred downstream and thus appear in drinking water.

The far field

The amount of caesium-137 deposited in Europe, outside the former Soviet Union (Map 18.3), varied from less than 1 kBq/m2 (in the southern part of the UK, western France, the Iberian peninsula, and some parts of Italy and southern Greece) to 100 kBq/m2 (in some 'hot spots' in Austria, Greece and Scandinavia, this value was even exceeded) (De Cort et al, 1990). This patchy deposition pattern led to important variations in foodstuff contamination, which induced differences in individual doses. The estimated doses range from 200 µSv (UK) to 960 µmSv (Greece) (UNSCEAR, 1988).

The European Commission and the Council of Ministers had to act urgently to set common intervention levels for food control in order to avoid large trade conflicts within the EC since, according to the Treaty of Rome, Member States are allowed to override all normal requirements for the free access of imports if no common rules exist concerning health on a particular issue. On 30 May 1986 a Regulation was adopted with the following permitted caesium levels to the import of food into the EC: 370 Bq/kg for milk and foodstuffs for infants during the first four to six months of life; and 600 Bq/kg for all other products. In the meantime some 20 countries outside the EC have also adopted these levels (Luykx, 1993).

Health effects

Although precise knowledge of the full consequences of the accident is unlikely, some unexpected effects have emerged. Five years after the accident, a sharp increase in the reported incidence of thyroid cancer among children has been observed in Belarus. To date there is no proof of an increase in the incidence of any other type of radiation inducible cancer. However, the indirect adverse effects on health may be more important than the direct effects due to radiation (WHO, in press).

Follow-up

In 1992 the CEC/CIS joint programme on the consequences of the Chernobyl accident was initiated. The programme was initially conceived for implementation within the framework of CHECIR (Chernobyl Centre for International Research) which was established by an agreement between the former USSR and the IAEA in 1990. However, because of the political changes that occurred in the former USSR in 1991, a different procedure had to be followed, which culminated in an agreement being signed in June 1992 between the Commission and the three new states of Belarus, the Russian Federation and Ukraine to establish the joint programme of work. The main purpose of the CEC/CIS collaborative programme is to complement and to assist those in the three republics responsible for evaluating and mitigating the consequences of the accident, and to gain an improved understanding and knowledge of the health and environmental impact of radioactive contamination and of how it can be reduced. (Kelly and Cecille, 1994).


Map 18.2 - Caesium contamination around Chernobyl after the accident

Source: Gagarmski, 1990


Map 18.3 - Cumulative deposition in Europe of Cs-137 from the Chernobyl accident

Source: De Cort et al, 1990