On 26 April 1986, the most serious accident in the history of the nuclear industry occurred at Unit 4 of the Chernobyl nuclear power plant in the former Ukrainian Republic of the Union of Soviet Socialist Republics, near the present borders of Belarus, the Russian Federation and Ukraine. The reactor was destroyed and, over the ensuing 9 days or so, large quantities of radioactive material were released into the environment.
The total activity of all the radioactive matter released in the accident is today estimated to have been around 1.2 x 1019 Bq, including some 7 x 1018 Bq due to noble gases. About 3% of the used fuel in the reactor at the time of the accident as well as up to 100% of noble gases and 20-60% of the volatile radionuclides were released. This current estimate of activity of the matter at the time of its release is higher than the estimate advanced in 1986 by the authorities of the former USSR, compiled by calculating the activity of the matter deposited within the countries of the former USSR on 6 May 1986.
The radionuclide composition of the matter released in the accident was complex. The radioactive isotopes of iodine and caesium are of the greatest radiological significance: the iodines, with their short radioactive half-lives, had the greater radiological impact in the short term; the caesiums and strontiums, with half-lives of the order of tens of years, have the greater radiological impact in the long term. The estimates for the activity of the amounts of the key radionuclides released are as follows: 131I: ~1.3 - 1.8 x 1018 Bq; 134Cs: ~0.05 x 1018 Bq; 137Cs: ~0.09 x 1018 Bq. These values correspond to about 50-60% of the 131I in the reactor core at the time of the accident and about 20-60% of the two radioactive isotopes of caesium .
The radioactive matter released into the atmosphere was widely dispersed and eventually deposited onto the ground surface. It was measurable over practically the entire northern hemisphere. Most of the material was deposited in the region around the plant site, with wide variations in deposition density. The areas of the surrounding territories of Belarus, Russia and Ukraine in which activity levels of 137Cs in excess of 37 kBq/m2 were measured were estimated at 46,500 km2, 57,000 km2 and 41,800 km2 respectively.
Chronology of the accident
The initial state of the unit just before the accident at 01:23 on 26.04.1986 was as follows: power: 200 MW (thermal output), operating reactivity capacity of 8 manually controlled rods. Neither the output power of the reactor nor any other parameters of the reactor installation demanded any intervention by personnel or automatic devices during the period from when tests began to the pressing of the reactor emergency close down button AZ-5 (at 01:23:40 am).
The movement of the rods associated with emergency protection and manual control, initiated by the AZ-5 command, caused significant deformations of the power production field. The local increase of power production after the AZ-5 button was pressed was such that, during about 5 seconds, total power production of the reactor increased by several tens of times compared to the initial level.
This caused the destruction of the reactor unit and the discharge of radionuclides from the core. The loss of cooling of remnants in the core caused the spent fuel to ignite on account of the residual energy production, priming a further release of radionuclides.
It was established that the core was completely destroyed, with parts of the wreckage located within the destroyed reactor installations and others thrown onto the roofs of neighbouring buildings while pieces of the ventilation pipe were scattered about the surrounding area. Owing to the residual energy production and burning of the graphite blocks, the temperature of the spent fuel exceeded 2,000oC. This resulted in the active fuel dispersion and blow-out of radionuclides with convection air flows reaching a height of 3,000 m.
The main cause of the accident
The main cause of the Chernobyl accident lay in the coincidence of severe deficiencies in the design of the reactor and of the shutdown system and the violation of procedures. The lack of “safety culture” in the responsible organizations of the Soviet Union resulted in an inability to remedy such design weaknesses, even though they had been known of before the accident.
In addition to those features of direct relevance to the causes of the accident, the original design of plants with RBMK reactors (Soviet light water cooled graphite moderated reactors) had further deficiencies. In particular, the original design of the first generation of RBMK reactors falls short of present safety objectives. Remaining deficiencies, such as the partial containment, require further attention.
Emergency measures had to be taken to bring the release of radioactive material under control, to deal with the debris from the reactor, and subsequently to construct a confinement structure, the so-called `sarcophagus', which was completed in November 1986, to contain the remains of the reactor core.
In order to reduce this blow-out and eliminate the possibility of the self-sustained chain fission reaction (SCFR) occurring, about 5,000 tonnes of different materials were dropped by helicopters into the reactor ruins:
2,400 tonnes of lead (to cool the fuel),
40 tonnes of boron carbide (to prevent SCFR occurrence),
800 tonnes of dolomite (to generate carbon dioxide),
1,800 tonnes of sand and clay (to filter radioactive blow-out).
Moreover, water was channelled through the remainder of the emergency cooling system to cool the core.
The other problem requiring an urgent response was preventing the reactor support plate from burning through and molten material falling into the premises under the rector, which were assumed to be full of radioactive water. For this purpose, a tunnel was built under the destroyed reactor, the water was pumped out and an emergency cooling system of the reactor support plate by liquid nitrogen was created.
After this, the surrounding area was partially cleaned up and by November 1986 the unique Sarcophagus structure was built to help restore partial control over the accident source.
The response to the accident was carried out by a large number of ad hoc workers, including operators of the plant, emergency volunteers such as fire-fighters, and military personnel, as well as many non-professional personnel. All these people became known by the Russian term likvidator. About 200,000 `liquidators' worked in the region of Chernobyl during the period 1986?1987, when radiation exposures were highest. They were among some 600,000 to 800,000 persons who were registered as involved in activities relating to alleviating the consequences of the accident. This includes persons who participated in the cleanup after the accident (including cleaning up around the reactor, construction of the sarcophagus, decontamination, road building, and destruction and burial of contaminated buildings, forests and equipment), as well as many other general personnel who worked in the territories designated as `contaminated’ and who generally received low doses.
The measures implemented to contain the accident helped to reduce releases from the destroyed reactor but none the less release activity was 1,2.1019 Bq (300 МCi), which resulted in the contamination of a huge territory inhabited by more than 7 million people.
Urgent Measures for protecting the population
The preliminary decision to evacuate the town of Pripyat, which is located less than 3 km from the ChNPP, was taken on the afternoon of 26 April 1986, when the dose rate in some parts of the town reached several mSv/hour. By 9 pm on 26 April 1986, 1,350 buses, 2 railway trains and 3 motor ships were brought into the Chernobyl district (12 km from the town of Pripyat). At 10 pm the USSR Ministry of Public Health decided that the emergency evacuation of the town was necessary. By standing idle for half a day 12-15 km from the powerful source of the radioactive blow-out, the city buses became contaminated and contaminated the city of Kyiv after their return there. The organised evacuation of the town of Pripyat and the Yanov railway station (49,360 and 254 people respectively), including about 17,000 children and 80 bed-bound patients, was carried out on 27 April 1986, between 2 pm and 5 pm.
On the basis of data received from an aerial survey, mobile radiological laboratories and radiometric prospecting towards the beginning of May, the radiation situation was estimated at a distance of 1,000 km around the NPP. Accordingly, evacuation from the neighbouring zones began on 2 May and finished on 7 May. The decision to evacuate was taken for the residential areas within the limits of the isopleth of 0.05 mSv/hour, where estimated exposure doses during the first year after the accident could exceed 0.1 Sv, the value established by the USSR Ministry of Public Health for evacuation. This zone included 76 inhabited areas in Ukraine and 50 in Belarus, evacuation from which was accomplished in several stages .
Altogether, about 116,000 people from 188 settlements in Ukraine and Belarus were evacuated by the end of August 1986. At the same time, 60,000 cattle and other farm animals were removed from the 30-km zone of the ChNPP.
A so-called `exclusion zone’ was established, which included territories with the highest dose rates, to which public access was prohibited. This exclusion was continued in the independent successor countries of Belarus and Ukraine after the dissolution of the Soviet Union. The exclusion zone covers in total 4300 km2.
On the whole, despite a number of problems linked to the absence of prior planning for the evacuation of a large number of people, the emergency evacuation was carried out within a tight deadline and in a fairly organised manner.
The populations of the towns of Pripyat and Chernobyl were housed in other Ukrainian towns, mostly in Kyiv and Chernigov, while 1,040 families were evacuated outside Ukraine: to Moldova, Russia and the Baltic republics.
Iodine prophylactics in the town of Pripyat began on the morning of 26 April 1986. It was significantly delayed in the other residential areas but, in total, iodine prophylactics were dispensed to 5 million people, including 1.6 million children.
The almost complete failure to use such an effective protective measure as sheltering and a delay in widely applying iodine prophylactics were mainly due to the secrecy shrouding all the problems of the Chernobyl accident. This excluded wide general public information about the danger of exposure linked to the radionuclide blow-out and caused additional exposure of the population.
However it should be noted that the main purpose of the short-term phase of accident assessment and response was fulfilled: there were only isolated instances of deterministic effects of exposure among the population. From the criteria of external exposure, evacuation was implemented in time, while it was late in terms of thyroid exposure. This fact has served as a basis for correcting national accident response plans.
The virtual absence of deterministic effects of exposure is connected above all with the fact that the first release hit a sparsely populated area and not the town of Pripyat with 50,000 people. Besides this, the height of release was up to 3,000 m, resulting in the contamination of a large area but with a lower contamination density.
Long-term measures for protection of the population
The main task of the long-term measures for protection of the population is to minimise the population and personnel exposure doses for a period of 50-70 years.
Countermeasures for reducing external exposure. At the early post-accident period, when there were still many short-lived gamma-emitters, a great deal of work was carried out to reduce external doses in the most contaminated areas. This work included: resettlement, removal of the top layer of soil in the most contaminated areas in the most heavily frequented places, clean-up of buildings, replacement of contaminated roofs and constructions, asphalting of the roads etc.
One of the most effective measure according to the criterion of the exposure dose reduction was the clean-up of school areas (removal of the top layer of soil, hard-paving of playgrounds, clean-up of buildings using surface-active agents etc.), which led (according to the direct measurements data) to an average drop of 30% in the external exposure dose for children, who spend a great deal of time in these places.
A similar set of projects, carried out in rural settlements with the aim of reducing external exposure doses, did not give true values of exposure dose reduction according to the results of direct measurements. Theoretical calculation confirmed that the total effectiveness of work in terms of the external exposure dose reduction criterion is less than 10% (for 1989). In subsequent periods of time, when the main sources of external exposure became 137Сs, the clean-up of areas, in terms of the external exposure dose reduction criterion is still less effective.
In urban conditions, the most widely used measures to reduce external exposure doses in 1986 was daily washing of the roads, pavements, and yard areas with hard pavement (in the city of Kyiv these areas amounted to 25 km2);
Many of the countermeasures implemented (for example, removal of leaves, elimination of the spots with high contamination levels etc.) could be considered as highly effective in terms of reducing public anxiety. The experience of Chernobyl showed that assessment of the expediency of different responses should not always be based on the value of the prevented dose.
Countermeasures for reducing internal exposure doses. More than 50% of the internal exposure dose in the contaminated areas of Ukraine is determined by the radioactive caesium content of milk. That is why replacement of local milk and other food products with "clean" products secured a significant decrease of the internal exposure doses.
Resettlement. The first phase of resettlement began in summer 1986, when the radiation situation in the Chernobyl area was specified. At that time a further 3,000 people or so were resettled from 15 residential areas. The main phase of resettlement began after complex monitoring of the contaminated areas and estimation of the expected effective exposure doses through use of local food products, mainly meat and milk. The Government adopted a number of regulatory documents on population protection linked to the accident and resettlement to "clean" areas with full pecuniary compensation for the buildings abandoned and the provision of housing at the expense of the government.
Resettlement in other areas was planned according to effective dose per year
(Deff > 5 mSv). This dose was defined using a simplified method, which attached substantial significance to the conservative coefficient when calculating the yearly effective exposure dose received by the inhabitants of specific localities.
The dose-related effectiveness of moving people out of two regions - Rivno (44 localities whose inhabitants are scheduled for resettlement) and Zhitomir (37 localities) - was studied in Ukraine. Doses were calculated using the results of direct measurements.
The actual collective dose for these residential areas (excluding thyroid gland dose) amounted to 1970 man-Sv from 1986 to 1991. The avertable dose due to the relocation in 1991 will amount to 630 man-Sv for these residential areas. The ratio of the collective dose for 70 years to the collective dose for 1986-1991 (2500/1970 man-Sv) gives a value of dose effectiveness of relocation equal to 1.3.
In some cases of resettlement to the "clean" areas, the total dose did not decrease, but increased through additional exposure from sources of natural radioactivity, by several times as compared to the value of the avertible dose. The choice of resettlement destinations was not optimal in all cases.
Restrictions on the use of locally produced foodstuffs. The dose efficiency of this countermeasure was assessed using the example of the Dubrovitskiy region of the Rivne oblast (54 localities, with assessments calculated for the adult population totalling about 50,000 people).
Doses were calculated using information concerning areas' 137Cs contamination density, a model of internal exposure taking into account the radiocaesium content of locally produced foodstuffs (situation without countermeasures) and a model of real results of radiocaesium measurements in the bodies of the region's inhabitants.
Collective dose values for 1991 showed that the "expected" (without countermeasures) collective dose of internal exposure had to be 2.5 man-Sv and the "real" collective dose 0.21 man-Sv. Therefore restrictions on the use of locally produced foodstuffs and other countermeasures lead to an 11-fold reduction in internal exposure doses.