Core Chernobyl-4 reactor accident

Only two earlier reactor accidents caused significant releases or radionuclides the one at Windscale (United Kingdom) in October 1957 and the other at Three Mile Island (United States) in March 1979 (UNSCEAR-1982). While it is very difficult to estimate the fraction of the Windscale radionuclide core inventory that was released to the atmosphere, it has been estimated that the accident released twice the amount of noble gases that was released at Chernobyl, but 2,000 times less and Cs (DOE-1987). The Three Mile Island accident released approximately 2% as much noble gases and 0.00002% as much l as the Chernobyl accident. 

Even if terrorists succeeded in detonating an explosive at a reactor site, the health consequences would be limited. The reactor accident at the Three Mile Island, Pennsylvania nuclear power plant caused a small release of radiation, insufficient to cause any radiation injuries. Bypassing several safety systems caused the Chernobyl reactor incident, involving two explosions, fires and reactor core meltdown. This accident caused the following early phase health effects (1) ... 

The potential dangers of nuclear power were tragically demonstrated by the accidents at Three Mile Island, Pennsylvania (1979), and Chernobyl in the former U.S.S.R. (1986). Both accidents resulted from the loss of coolant to the reactor core. The reactors at Three Mile Island were covered by concrete containment buildings and therefore released a relatively small amount of radioactive material into the atmosphere. But because the Soviet Union did not require containment structures on nuclear power plants, the Chernobyl accident resulted in 31 deaths and the resettlement of 135,000 people. The release of large quantities of 1-131, Cs-134, and Cs-137 appears to be causing long-term health problems in that exposed population. 

The Chernobyl accident was about as bad a nuclear reactor accident as it is possible to imagine. The reactor core burned without any containment for ten days. It is difficult to postulate any accident that could lead to a greater release of radioactivity to the environment. A total of 31 people died either at the time of the accident or within a few weeks from radiation sickness. The nearby town of Pripyat, with a population of 49,(X)0, was evacuated within a few days and an exclusion zone of 30 km radius around the Chernobyl plant was declared. 

The past safety record of nuclear reactors, other than the Soviet Chernobyl-type RBMK reactors, is excellent Excluding RBMK reactors, there had been about 9000 reactor-years of operation in the world by the end of 1999, including about 2450 in the United States.1 In this time there was only one accident involving damage to the reactor core, the 1979 Three Mile Island accident, and even at TMI there was very little release of radionuclides to the outside environment. 

The Chernobyl accident was in many ways the worse possible scenario having an exposed reactor core and roofless building. Two plant workers died from the blast and fire, 22 other plant workers and 6 firefighters received huge radiation doses and died within months. 

Two accidents of vastly differing severity have occurred at nuclear power plants. On 28 March 1979, an accident occurred in the nuclear power plant at Three Mile Island, Pennsylvania, USA. The radiation was contained and the small amount released had negligible effects on the health of individuals at the plant. On 26 April 1986 an accident occurred in the nuclear power plant 10 miles from the city of Chernobyl, then part of the Soviet Union. The chain reaction in the radioactive core of one of the four reactors became uncontrolled. Steam pressure rose to dangerous levels there were several explosions and a subsequent fire took several hours to extinguish. Large amounts of radioactive material were scattered over a wide area and into the atmosphere (later descending in a dilute form in rain all over the world). 

Despite the safety regulations, accidents have occurred with nuclear reactors and reprocessing plants, primarily due to mistakes of the operators. By these accidents parts of the radioactive inventory have entered the environment. Mainly gaseous fission products and aerosols have been emitted, but solutions have also been given off. In the Chernobyl accident, gaseous fission products and aerosols were transported through the air over large distances. Even molten particles from the reactor core were carried with the air over distances of several hundred kilometres. 

This is the most serious accident in the history of the development of nuclear energy. It was caused by illegal operations. The reactor core was completely destroyed and about 50 MCi of noble gas was released in the first day, April 26th. Furthermore, about 50 MCi of other fission products were released into the atmospheric environment until May 6th. The radioactivity from Chernobyl was detected at many places in the northern hemisphere. A large area of Europe received significant surface deposition of radioactive materials such as l and Cs. 

On 26 April 1986 at 0123 hours local time an accident occurred at the fourth unit of the Chernobyl nuclear power station. The accident destroyed the reactor core and part of the building in which the core was housed. The radioactive materials released were carried away in the form of gases and dust particles by air currents. In this manner, they were widely dispersed over the territory of the Soviet Union, over many other (mostly European) countries and, in trace amounts, throughout the northern hemisphere. 

The Chernobyl accident involved the largest short-term release from a single source of radioactive materials to the atmosphere ever recorded. Of the materials released from the reactor core, four elements have dominated the short-term and long-term radiological situation in the affected areas of the USSR iodine (primarily caesium ( Cs, Cs), strontium (primarily Sr) and plutonium ( Pu, " Pu). In addition, highly radioactive fuel fragments (hot particles) were released. 

The design of the Chernobyl plant was flawed in other ways as well. Western reactors are designed when operating to maintain negative power coefficients of reactivity that prevent such runaway accidents. The Chernobyl plant would not have been issued a license to operate in the U.S. or other Western countries. The Chernobyl accident was in many ways the worse possible scenario having an exposed reactor core and roofless building. Thirty-one plant workers and firemen died directly from the radiation exposure and it is projected that over 3,400 local residents will eventually acquire and die of cancer due to radioactive exposure. 

At 01.23 hours on 26 April 1986, Unit No. 4 of the Chernobyl Nuclear Power Station was operating at low power prior to a scheduled shutdown when a sudden, very rapid and uncontrolled power increase occurred. This resulted in the destruction of the reactor core and severe damage to the reactor building. There was a release of radioactive material from the core over the subsequent 9 days. The accident is now known to have occurred as a result of a test which was being carried out by the operators. The manner in which the test was conducted, including the disabling of all the safety systems provided to protect the plant, was the immediate cause of the accident. However, aspects of the reactor design resulted in the adverse response of the reactor which caused its destruction. 

This defect is met in commercial BWR designs by additional circulation of the coolant around the reactor core, over and above the coolant removed as steam through the turbines. Controllers link the flow rate to the power demand as felt on the turbine, so that the increased coolant input sweeps out the steam bubbles and compensates for the void effect. The RBMK has a similar additional coolant circulation which is necessary, perhaps, for its satisfactory operation with a negative water void coefficient but would exacerbate the disadvantage of a positive coefficient. This again was a feature of the Chernobyl accident. 

Certainly the most catastrophic nuclear accident occurred on April 26, 1986, at the Chernobyl unit 4 reactor near Kiev, Ukraine. The accident resulted in a core meltdown, explosion, and fire. 

Figures 3-5 and 3-6 show the significant differences between the dynamics of the Chernobyl and the TMI accidents. Figure 3-5 illustrates the crucial phase of the Chernobyl accident and shows how it essentially comprised an uncontained explosion of the reactor. Figure 3-6 shows the damaged state of the TMI-2 reactor core and vessel after the accident, and results from many years of research (OECD, 1993).

As a consequence of the Chernobyl accident (see Section 7.4.3.), the RBMK-type reactors have been improved to reduce their sensitivity to the core steam void fraction and to increase the effectiveness of safety provisions. Besides improvements in the shutdown systems, increase in the enrichment of the fuel and installation of a greater number of fixed absorber rods in the reactor core are the most important measures for increasing reactor safety. 

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