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. 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 reactor accident
April 26, 1986, 01:23:04
|- to start of plant tests (designed to see how Nuclear Power Plant would respond to a turbine/generator trip|
|concurrent with a loss of offsite power)|
01:23:40 - to press of the reactor emergency close down button AZ-5.
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.
April 26 till May 5,1986 - radionuclide release from destroyed reactor Chernobyl Unit 4
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.
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. Also urgent measures for protecting of the population had been taken.
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.
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 of 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. 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 the monitoring data about 116,000 people from 188 settlements in Ukraine and Belarus were evacuated by the end of August 1986.
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 absence of deterministic effects of exposure to population 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: 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.
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);
Countermeasures for reducing internal exposure doses: replacement of local milk and other food products with "clean" products secured a significant decrease of the internal exposure doses.