Metal and concrete protection wall assembled around Unit 4 of the Chernobyl nuclear plant is shown in a photo from September 1986.
At 1:23 AM on April 26, 1986, two explosions occurred in the Unit 4 reactor of the Chernobyl Nuclear Power Plant in Ukraine. The explosions wrecked the reactor block and part of the building, hurling burning graphite onto nearby buildings and starting fires. A radioactive plume released by the reactor swept north into the adjacent republic of Belarus and the western part of Russia before swirling west across Europe. Radioactive particles eventually were deposited across the whole Northern Hemisphere.
Metal and concrete protection wall assembled around Unit 4 of the Chernobyl nuclear plant is shown in a photo from September 1986.
The accident followed a safety experiment carried out immediately before a planned shutdown of the unit for maintenance. The experiment involved running the plant outside its design parameters at very low power and unfavorable cooling conditions. The two explosions destroyed the reactor core and exposed it to the atmosphere.
Although the conventional fires were rapidly extinguished by firefighters, a graphite fire in the reactor core was not put out until several days later. Large quantities of boron carbide, dolomite, lead, sand, and clay were dumped from helicopters onto the reactor.
Very little, if any, of the thousands of tons of materials dropped actually made it into the reactor shaft to cover the reactor core, according to nuclear engineer Alexander R. Sich [Nuc. Eng. Int., 46, 22 (1996)]. He suggests that "the accident stopped itself." The core melted and flowed into the lower part of the building. The "lava" then solidified and stopped releasing radioactivity.
Sich, who is with the Nuclear Safety Account of the European Bank for Reconstruction & Development in London, was the first Western scientist to live and work with members of the Chernobyl Complex Expedition, the small group of Russian and Ukrainian scientists studying the remains of Unit 4.
"About 71% of the fuel in the core (roughly 135 metric tons) remained uncovered in the reactor shaft after the explosion," writes Sich in the May/June 1996 issue of the Bulletin of the Atomic Scientists. Instead of being smothered by the materials dumped from the helicopters, "the core remained exposed to the environment, releasing radioactivity into the atmosphere for nearly 10 days, at which point the remnants cooled down on their own."
In the weeks after the accident, about 135,000 people were evacuated from a 30-km-radius exclusion zone set up around the plant. This included the complete evacuation of Pripet, a nearby town with a population of 45,000, on the day after the accident. The cleanup operation at Chernobyl involved about 800,000 people, of whom 200,000 were frontline workers known as "liquidators."
The total radioactivity of the material released from the reactor was estimated to be 200 times that of the combined releases from the atomic bombs dropped on Hiroshima and Nagasaki, according to a 1995 World Health Organization (WHO) report on the health consequences of the Chernobyl accident.
The accident exposed millions of people, notably in Belarus, Russia, and Ukraine, to varying doses of radiation. Worst affected were the Chernobyl plant staff and firemen from Pripet who were exposed to high radiation levels during the emergency operations.
"Chernobyl was an unprecedented disaster in the sense that it has affected, directly or indirectly, the lives of hundreds of thousands of people in the three affected republics [Belarus, Russia, and Ukraine] and indeed further afield," says David R. Kyd, director of the Division of Public Information at the International Atomic Energy Agency (IAEA), Vienna, Austria.
RED boxes indicate areas with significant levels of radionuclide contamination
The accident was a turning point for the nuclear power industry worldwide. "It demonstrated clearly that nuclear power in some parts of the world was not safe enough," notes the World Association of Nuclear Operators (WANO) in its report "Industry Progress Since the Chernobyl Accident, April 1986 - April 1996." The association points out that the accident "caused such a negative opinion of nuclear energy that, should such an accident occur again, the existence and future of nuclear energy all over the world would be compromised."
And 10 years later, some of the effects only now are emerging. "On the 10th anniversary of the accident of Chernobyl, the extent of its impact on affected populations has only begun to be fully realized," stated United Nations Secretary General Boutros Boutros-Ghali in a message to the international conference, "One Decade After Chernobyl: Summing Up the Consequences of the Accident," held earlier this month in Vienna. He added: "Even today, its health, social, economic, and environmental dimensions, both immediate and long term, remain to be defined."
The future of the Chernobyl plant is also uncertain. Today, the wrecked Unit 4 reactor is enclosed by a "shelter" or "sarcophagus." It "represents a standing potential risk," notes the Organization for Economic Cooperation & Development Nuclear Energy Agency (OECD/NEA) Committee on Radiation Protection & Public Health, in its report "Chernobyl Ten Years On: Radiological and Health Impact." The roof of the structure, for example, has numerous cracks. "It is clear that some work will need to be carried out to improve the integrity of the sarcophagus," says Vincent J. Madden, director of the WANO Coordinating Center in London.
The three other units at the plant are also cause for concern. The reactor in Unit 2 was shut down in October 1991 following a fire in the turbine hall. The reactors in Units 1 and 3 continue to operate, producing a total output of 1,800 MW of electrical power. Almost 6,000 people work at the plant every day.
The Chernobyl reactors are Soviet-designed and built RBMK-1000 reactors. Currently, 13 RBMK-1000 and two RBMK-1500 reactors are operating in Lithuania, Russia, and Ukraine. They are all boiling-water reactors with graphite moderators. Those at Chernobyl are first-generation RBMKs that became operational between 1977 and 1983.
Source: "Chernobyl Ten Years On," Organization for Economic Cooperation & Development's Nuclear Energy Agency
RBMKs have major design shortcomings that contributed to the Chernobyl accident. Immediately after the accident, modifications were made to improve the safety of all RBMKs in operation. The upgrading process continues today, but "there is scope to do more safety upgrading," says Madden.
The costs of dealing with Chernobyl continue to mount. On Dec. 20, 1995, in Ottawa, representatives of the Ukrainian government, governments of the Group of Seven (G-7) countries (Britain, Canada, France, Germany, Italy, Japan, and the U.S.), and the Commission of the European Communities signed a memorandum of understanding on the closure of the Chernobyl Nuclear Power Plant by about 2000.
According to this memorandum, the international community will provide grants and loans totaling at least $2.3 billion to support a comprehensive program that tackles restructuring of the power sector in Ukraine, nuclear safety and decommissioning of the Chernobyl plant, and the social impact of closure.
But the Ukrainian government has been asking the West to provide up to $4 billion to fund the Chernobyl closure program. Vladimir Kholosha, minister for the Protection of the Population from the Consequences of the Accident at the Chernobyl Nuclear Power Plant, pointed out at the Vienna conference that just making the sarcophagus safe would cost between $900 million and $1.5 billion. The figures are based on a feasibility study carried out under the aegis of the European Commission by a consortium of European companies called Alliance. "Such a large expenditure is beyond the means of Ukraine," says Kholosha.
Kholosha: health effects
insufficiently studiedEarlier this month in Moscow, G-7 leaders obtained a firm pledge from Ukrainian President Leonid Kuchma to close reactor Units 1 and 3 of the Chernobyl plant by 2000 in exchange for more than $3 billion in aid. The funds do not cover the cost of building another sarcophagus around the wrecked Unit 4 reactor.
And there are other costs, for example, for the disposal of radioactive waste from the Chernobyl site and for the creation of a safe radioactive waste management system. "Account must also be taken of the fact that the long-term effects of the Chernobyl accident on human health have still been insufficiently studied and forecast by scientists," notes Kholosha. "This could cause additional expenditures for medical care and social protection." Countermeasures are also needed in agricultural production and forestry use to reduce the radionuclide dose burdens on the Ukrainian population.
Belarus suffered particularly badly from the Chernobyl accident. Igor V. Rolevich, Belarus first deputy minister for emergencies, reported at the Vienna conference that "23% of the entire area of Belarus was contaminated with radionuclides as a result of the Chernobyl accident," although the contamination is extremely uneven and has a "spotlike" character.
Rolevich: damage to Belarus
tremendous"Scientists have calculated that the economic damage to Belarus from the accident is equal to 32 preaccident annual budgets, or $235 billion calculated over a 30-year recovery period," says Rolevich. This amount covers damage to agriculture, forestry, the construction industry, and the social sector, including housing.
Soviet Union government and medical commissions were set up soon after the accident to coordinate the medical follow-up activities in the contaminated Soviet republics. Compulsory health monitoring was required for all accident recovery workers, persons evacuated from the contaminated areas, those residents in areas with relatively high levels of contamination, and children of people in these groups. Regular or special health monitoring was provided for populations, totaling more than 4.5 million, who were exposed to lower levels of radioactive contamination.
Incoming and outgoing vehicles are checked for radiation at monitoring posts at the entrance to the 30-km exclusion zone around the Chernobyl plant.
According to the OECD/NEA report, about 270,000 people continue to live in contaminated areas of the former Soviet Union where protection measures are still required. Radiation doses received by populations outside the former Soviet Union have been relatively low. The report states: "After several years of accumulation of dosimetric data from all available sources and dose reconstruction calculations based on environmental contamination data and mathematical models, it is now possible to arrive at a reasonable, although not highly accurate, assessment of the ranges of doses received by the various groups of population affected by the accident."
The Chernobyl explosions resulted in the release of radioactive gases, aerosols, and finely fragmented fuel known as "hot particles" for several days. The OECD/NEA report lists 20 radionuclides that were released into the atmosphere. These included iodine-131 with a half-life of eight days; cesium-134 and cesium-137 with half-lives of two and 30 years, respectively; and several plutonium isotopes with half-lives ranging from 13 to 24,400 years. About 85% of the total release consisted of radionuclides with half-lives less than one month, 13% with half-lives of several months, 1% with half-lives of about 30 years, and 0.001% with half-lives more than 50 years.
According to the WHO report on the health consequences of the Chernobyl accident, 444 people were at the reactor site when the accident occurred and were therefore exposed to large amounts of radiation. Of the 300 who were admitted to hospitals, 134 were diagnosed as having acute radiation syndrome (ARS). Kyd points out that the current death toll directly attributable to the accident is 45, including 28 deaths from ARS during the first three months, three from other causes such as heart attack, and 14 further deaths among ARS sufferers over the past 10 years.
"Amazingly, more than 200 people hospitalized as ARS victims have survived, though they are in poor physical and psychological conditions," notes Kyd. The workers who recovered from ARS continue to suffer with emotional and sleep disturbances and about 30% of these workers suffer from gastrointestinal, cardiovascular, and immune function disorders.
No clinical symptoms of ARS were observed in the population evacuated from the 30-km exclusion zone or in the residents of other contaminated areas.
In Belarus, about 2.2 million people, including 600,000 children and juveniles, have been subjected to the prolonged impact of long-lived radionuclides, according to Rolevich. Since the accident, more than 130,000 people either have been resettled in clean areas in the republic or have left of their own volition. A total of 415 settlements in Belarus have been evacuated.
An overall assessment of the health impact of Chernobyl in the three republics has not been possible. To link radiation to health consequences, it is necessary to know the doses received by patients but, according to WHO, dose data are still incomplete. This is particularly the case for long-term health effects, such as some types of cancer that take decades to develop.
But some believe that the health effects of the accident have been huge. For instance, Yuri M. Shcherbak, Ukrainian ambassador to the U.S., wrote: "It is hard to know, even approximately, how many people have already died as a result of the accident" [Sci. Am., 273, 44 (1996)]. Even so, he added, "by comparing mortality rates before and after the accident, the environmental organization Greenpeace Ukraine has estimated a total of 32,000 deaths. There are other estimates that are higher, and some that are lower, but I believe a figure in this range is defensible."
Not everyone agrees with this gloomy assessment, however, and thus far the data don't support it. "It is very difficult to make any conclusions," says Elisabeth Cardis, head of the program on radiation and cancer at WHO's International Agency for Research on Cancer, in Lyons, France. "It is very difficult to make any comparisons between the health of exposed and nonexposed populations," she says.
Cardis: more thyroid cancer in
childrenShe points out that most of the information on the general population comes from statistical reporting made at the local level. "The problem is that there is no quality control, no check on completeness or on duplicates."
No centralized cancer registration system was in place in Russia and Ukraine at the time of the accident. However, a computerized national cancer registry has been functioning in Belarus since the 1970s. There are also other registries in the country. But they all need to be improved with adoption of international standards for coding, classification, and quality control, Cardis points out.
"There has been a dramatic increase in the incidence of thyroid cancer in children, mainly in Belarus and Ukraine, and also to a lesser extent in Russia," says Cardis. But she adds that, unfortunately, 10 years after the accident, it is not known whether increases in the incidence of other types of disease, including other types of cancer, can be linked to the Chernobyl accident.
"A number of the groups of diseases reported may be due to economic or psychological factors," she says. "The reported increase in mortality and decrease in the average life span in all the countries of the former Soviet Union does not appear to correlate at all with contamination from the Chernobyl accident."
To date, thyroid cancer in children up to the age of 15 is the only long-term health effect directly linked to the accident. "The normal incidence of thyroid cancer is 0.5 cases per million children per year," notes Kyd. "In northern Ukraine and southern Belarus, the numbers are far higher. The current rate at Gomel Oblast [Belarus] is 100 children per year, [which is] 200 times the normal level. In Belarus as a whole, the figure is 14.5 per million children per year."
"It was not until about March 1994 that we saw the first real recognition that this was not just a fabricated increase, but a real increase," says WHO radiation scientist Keith F. Baverstock. "The increase is certainly associated with the exposure to radioactive isotopes of iodine."
Baverstock: exposure to
radioactive iodineBaverstock points out that nearly all the iodine absorbed by the body through inhalation or food ingestion is concentrated in the thyroid gland. The diets of many people in the three affected republics were iodine-deficient. Their bodies therefore readily absorbed radioactive iodine.
Inhalation of radioactive iodine is likely to have been important near the Chernobyl plant at the time of the accident, according to University of Cambridge histopathology professor Sir Dillwyn Williams in a report to the Vienna conference. He adds: "Ingestion of radioiodine is largely dependent on the iodine content in milk and in vegetables. Iodine in milk is particularly important because deposited radioactive materials on grass eaten by cows is absorbed and concentrated in the cows' milk."
"At least 90 to 95% of these cases [of thyroid cancer] are curable," notes Kyd. "The total number of cases reported by the three republics is now 890. According to the latest projections, children under three years of age at the time of the accident will remain prone to developing thyroid cancer through adolescence and adulthood. The anticipated numbers are 4,000 to 8,000 cases in all, of which perhaps 400 to 800 will be fatal. The number of confirmed cases, where tissue has been available for independent analysis in the West, is 550."
On the basis of previous studies of survivors of the atomic bombs in Japan, there was a general expectation that the major radiological impact of the Chernobyl accident would be an increased number of leukemia cases in the populations exposed to radioactive contamination.
"It is very interesting that essentially very few of the republics are reporting any increase in leukemia," says Fred A. Mettler, professor of radiology at the University of New Mexico and head of the IAEA Health Effects Team.
"There's a possible reason for that, in that the most likely population to show this was the population moved out of the 30-km zone and dispersed to other areas," says Baverstock. "So it is entirely possible that the leukemia is now averaged out over a much larger population and therefore is not seen."
According to Cardis, increases in the incidence of cancers other than leukemia are usually not visible until at least 10 years after exposure. "In the past years, there were certainly a lot of reports in the media about increases of all kinds of cancer being [caused by] the accident," says Mettler. At the Vienna conference, he notes, there were many papers and posters noting that there has not been an increase in nonthyroid cancers.
The frequency of congenital malformations in newborns and human embryos has increased in heavily contaminated areas of Belarus following the accident. For example, a paper in Nature [380, 683 (1996)] shows that the rate of genetic mutation, known as "human minisatellite mutation," in children born in heavily polluted areas of the Moligev district of Belarus after the accident is "unusually high."
"Given the poor living conditions and health standards in the three republics, the main [health] problem is the deteriorating economies and collapsing health services in the three republics," says Paul Seaman, official spokesman for the British Nuclear Industry Forum (BNIF), who spent the first three months of this year working in Chernobyl. "People aren't getting paid. They are malnourished and can't look after themselves properly. This means that diseases like tuberculosis, which were previously eradicated, are now coming back. And hospitals aren't getting the money they need to survive." A real deterioration in general living and health standards is becoming apparent, he notes.
In view of the health problems in the three republics, the thyroid cancer cases, very few of which are fatal, cannot really be regarded as a major impact, says Terence R. Lee, professor of environmental psychology at the University of St. Andrews, Scotland. "Looking at the total situation for the three republics, there is not the slightest doubt that millions of people are affected by the secondary effects of the disaster, that is, the effect on their mental condition, which is in turn responsible for disturbances of a physical kind.
Lee: psychological effects of
accident"These secondary effects are very considerable and they are well established," continues Lee. "Particularly sensitive are the effects on the digestive system, the effects on blood pressure, the possible effects therefore on heart conditions, but more generally on sleeplessness, alcoholism, and so on. It has also been suggested that diabetes and similar disease conditions may be triggered by stress if there is a genetic or other predisposition to them."
Environmental groups take a different view on the health consequences of the Chernobyl accident. "The problem is that the nuclear industry doesn't really want the full impacts to be known," says Karen Richardson, a Greenpeace nuclear campaigner based in London. "The West hasn't really put a lot of money into researching the health effects, particularly in Belarus, which was the worst affected country. While that research isn't undertaken, they can just keep denying it."
Cardis agrees that more research on health effects is needed. "We are proposing two things. One is that there should be continued monitoring of health through population-based cancer and mortality registries. We are also proposing focused studies of, for example, leukemia among liquidators."
According to the OECD/NEA report, radioactive releases from the Chernobyl accident contaminated about 125,000 sq km of land in Belarus, Russia, and Ukraine. Just over 40% of this was used for agriculture. The rest was forest, water bodies, and urban centers. Plants and animals in parts of the 30-km exclusion zone received the highest doses, notes radioecologist Mona Dreicer, an independent consultant with Environmental Assessments, Washington, D.C.
Dreicer: countermeasures are
effectiveThe rate of migration of radionuclides such as cesium-137, strontium-90, and plutonium-239 in the soil is generally slow, although it varies with factors such as pH, rainfall, and agricultural tilling. Soil contamination with these radionuclides therefore remains high. For example, in Belarus, "18,000 sq km of agricultural land belonging to 729 farms have been contaminated and 2,640 sq km of agricultural land and 1,900 sq km of forest have, to all intents and purposes, been taken out of use forever," according to Rolevich. He refers to this land as a "plutonium reservation located in the center of Europe."
The Chernobyl accident occurred during the growing season. Within two weeks, the 30-km zone around Chernobyl received sufficiently high doses of radiation to result in significant damage to coniferous forests, which are particularly sensitive to radiation. The forests were used commercially for wood and provided an important source of berries, mushrooms, game, and other foods for the local population. Many pine trees died or suffered damage to their reproductive tissues. However, within three years, most of the ecological communities in the zone had recovered their reproductive function and dead trees now account for only 0.5% of the zone's forested area.
"The biodiversity there in mammals is as great in the most radioactive areas as it is in what we call the control regions," Baker says. "If you go to the most radioactive spots, the number of species and the number of animals is greater than it is outside the exclusion zone."
Baker and coworkers report in Nature [380, 707 (1996)] that the frequency of mutations in the mitochondrial DNA of voles living near the wrecked reactor in Unit 4 is hundreds of times greater than one would expect. However, despite these mutations, the vole populations are thriving.
Baker explains that the human population poses much more of a problem than the world's worst nuclear accident. "There's no grazing, no fires, no destruction of the habitat inside the zone, so the grass is very deep and the habitat is doing well," he says. "Outside the exclusion zone, the cattle have overgrazed the land so there's not much grass. And they've cut the trees for firewood."
"Although the environment, at least to the naked eye, seems to have recovered, right now we don't have a really good overview of the long-term effects on populations, species, or ecosystems," explains Dreicer.
Contamination of the soil by radionuclides with long half-lives, such as cesium-137, is a particular problem for people in rural areas. The radiocesium stays in the soil, gets taken up by crops, and is transferred to animal products such as milk and meat. "The cesium attaches itself fairly strongly to the soil," says Dreicer. "It remains concentrated in the upper 5 cm."
Human exposure to radionuclide contamination can be reduced by imposing agricultural countermeasures; for example, by stopping people from working in the fields, stopping the consumption of fresh vegetables, and stopping the pasturing of animals and poultry. But these measures need to be taken immediately after a nuclear accident, even before contamination levels are determined.
According to the OECD/NEA report, such measures were not introduced immediately in Ukraine and radiation doses to humans were therefore enhanced.
Changing the types of crops that are grown or adding chemicals such as lime or potassium fertilizers also decreases the uptake of radiocesium, says Dreicer. "Countermeasures have to be very specific for the situation, the type of radionuclide, and the type of environment. But in general, you can have really effective agricultural countermeasures and countermeasures in the environment that can protect the population."
Although most of the radioactive material released from the Chernobyl reactor was deposited over land, some did enter the aquatic environment. "The water is not nearly as radioactive," comments Baker. "The radionuclides settle out in the sediment."
Aquatic ecosystems have been shown to be tolerant of radioactive contamination, points out Dreicer. For example, no long-term direct effects of radiation on the populations of most organisms in the Chernobyl plant's cooling pond have been observed. The pond was the most heavily contaminated water body in the exclusion zone.
"The groundwater has not been poisoned as yet, but given the fact that the Unit 4 sarcophagus is not leak-tight, and there are many sites where irradiated equipment like trucks, bulldozers, and helicopters were dumped, the risk of contamination at some future date cannot be excluded," says Kyd.
The Chernobyl nuclear accident rates the highest classification - seven - on the International Nuclear Event Scale (INES), according to WANO. Other relatively minor accidents have also occurred at Chernobyl. For example, on Nov. 17, 1995, a small amount of nuclear fuel leaked from the Unit 1 reactor and exposed a worker to about one year's permitted radiation dose. The worst nuclear accident in the West occurred in the pressurized water reactor at Three Mile Island, Pa., in 1979. It rated a level five on INES.
Past experience indicates that there will be another Chernobyl-scale accident, Baverstock suggests. The increasing number of older reactors is especially a cause for concern.
"There are more than 430 nuclear reactors in the world, with more being built," says Kyd. "We will probably get up to about 480 by the end of the century." More significantly, the number of reactors that is aging is inevitably increasing. The first went on-line about 40 years ago.
The 300,000-metric-ton concrete and steel sarcophagus that was built at Chernobyl to entomb the destroyed reactor still contains uranium fuel. It is thought to include pellets and hot particles of enriched uranium dioxide, and three streams of solidified lava of fuel mixed with sand and concrete. According to the OECD/NEA report, several tons of fuel are dispersed as dust inside the sarcophagus.
The long-term stability of the sarcophagus is causing concern. Cracks have appeared in the roof allowing rainwater to penetrate the tomb. The resulting high humidity inside has led to corrosion of internal metallic structures. The upper biological shield, a concrete cover for the reactor core, is wedged between the reactor walls and could fall. And the condition of the lower floor slab, damaged during the accident, is also uncertain.
"The sarcophagus is known to be crumbling," says Richardson. "If it does crumble, there's a danger it's going to hit reactor 3 as well, the reactor which is next to it."
The roof is the most serious problem, according to Valentin Kupny, deputy general director of the "shelter object" at Chernobyl. "It is still not completely closed to rainwater," he says. "We cannot say what will fall and when." He points out that an ongoing program to repair the roof includes strengthening the roof beams.
Boris Shinkarenko, deputy chief of the workshop for the repair of radiation instruments of the Chernobyl nuclear plant, receives a preventative examination in 1987.
"The problem of the festering reactor 4 within its sarcophagus needs to be addressed before groundwater becomes contaminated by the melted fuel still inside, and before the risk becomes a real one that radioactive dust inside the structure could be resuspended in the atmosphere by any collapse of masonry," says Kyd.
The safety of the RBMK reactors that are still operating continues to be the focus of much attention. The Chernobyl reactor design was never built outside the former Soviet Union. It had certain features that would have prevented it from receiving a license in the West, states BNIF in one of its fact sheets on Chernobyl.
It is now generally accepted that the Chernobyl accident occurred, as WANO puts it, "because of a combination of the physics characteristics of the reactor, the design of the control rods, human error, and management shortcomings in the design and implementation of the [safety] experiment."
"The operational experiment took them somewhat into uncharted territories," says Madden.
The RBMK reactors have several design flaws. For a start, they lack high-pressure containment, used in Western-designed reactors to prevent the release of radioactive debris from an accident. RBMK reactors do have some systems to contain releases and more recent models have been upgraded to include pressure-suppression systems.
Another inherent design flaw of the reactor was the magnitude of its positive void coefficient. Western-designed reactors have negative void coefficients, which means that reactivity decreases if there is loss of cooling. Loss of cooling in a reactor with a large positive void coefficient results in increased reactivity and the reactor can become unstable. According to WANO, the positive values of the void coefficients in the RBMKs have now been brought down to well below danger levels.
The design of the RBMK reactor control rods also contributed to the accident. These rods reduce the fission rate when lowered into the reactor. At the time of the accident, the control rods lacked the capacity to halt the increased reactivity. "Now, changes to the design, together with the implementation of administrative measures to restrict operation at low power, prevent this sharp increase in reactivity," says WANO.
There have been other safety improvements since the accident, including the installation of emergency core-cooling systems and remote-control rooms in the first-generation Chernobyl-type RBMKs, and better operating rules and procedures.
"One of the contributory factors of the Chernobyl accident was isolation from worldwide operating experience, due in large part to the prevailing political system," states the WANO report. "There was little or no information exchange with other parts of the world. The RBMKs were isolated from the rest of the world, as were reactors of other designs in the former U.S.S.R. and Eastern Europe."
The nuclear industry has learned a number of things from Chernobyl, says Seaman. "First, it opened up the Soviet Union to the scrutiny of the international nuclear community through the International Atomic Energy Agency. Second, it led to the formation of the World Association of Nuclear Operators, an organization which brings together operators to look at safety and coordinate safety on a world scale."
Member of the environmental team from the International Atomic Energy Agency International Chernobyl Assessment Project collects potato samples in Trakavichi, Ukraine.
"Certain technical improvements in the RBMKs and developments in the training of operators have made a repeat of the accident less likely," according to BNIF. The forum recommends that international funds be made available to provide alternative means of generating electricity, either by completing safer nuclear power stations, or by building other types of capacity, such as gas turbines or coal-fired stations.
It continues: "The West's and the Ukraine's top priority is to ensure that there is no recurrence of the accident. The Ukraine's other priority is to generate as much electricity as possible for its energy-starved economy."
Greenpeace believes that these reactors should be closed immediately, according to Richardson. She refers to a leaked report from the Office of Energy Intelligence of the U.S. Department of Energy. "Many Soviet-design reactors operating in the successor states to the Soviet Union pose significant safety risks because of inherent design deficiencies, deteriorating economies, political turmoil, and weak regulatory oversight," states the report.
The report concludes that the Chernobyl plant in Ukraine has the highest accident likelihood of any plant examined to date: "Today, conditions at the Chernobyl nuclear plant are in many ways worse than those that existed prior to the disastrous 1986 accident. Serious problems abound in nearly every facet of the operation, raising the specter of another accident."
The target of all reactors is to get the risk of a major accident to less than one in a million per year, according to Kyd. But a speaker from the floor at the Vienna conference suggested that the risk for an RBMK reactor could be as high as one in a thousand, depending on whether future safety upgrades are carried out. If this is the case, then with 15 RBMK reactors still in operation, there is about a one in seven chance of another Chernobyl-scale accident in the next 10 years.
If societies want to continue using nuclear power, the benefits must be balanced against the risks, says Williams. He suggests that future generations will not forgive the present generation if the Chernobyl accident is not studied to predict the future consequences of such a disaster. Precautions must be taken to prevent it from happening again, and humanitarian assistance must be provided to those who suffered from it.
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