After reading this essay you will learn about the risk of nuclear accidents.
Although accidents involving the release of radioactive material occur at any stage of the nuclear fuel cycle, most attention has been focused on reactor accidents. This is not because reactor accidents are more likely, but because the potential consequences for general public of some reactor accidents are much greater than from accidents at other stages in the fuel cycle.
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Several studies have been made in which the probability reactor accidents of various degrees of severity were estimated, and in which the environmental impacts of the radiation release associated with these hypothetical accidents were calculated. The type of accident considered involves the reactor safety features that control the cooling and running of the reactor.
If they fail, the temperature of the reactor core rises and the fuel melts and may breach the main containment above the reactor. Such a ‘meltdown‘ results in the release of large quantities of radioactive material. It needs to be stressed that in a thermal reactor the fuel is near to the most active state and there is no way in which the nuclear assemble can go supercritical and explode like a nuclear weapon.
In contrast, the plutonium fuel used in fast-breeder reactors could possibly go supercritical during a core collapse meltdown. In general, studies have suggested that a large accidental release could cause large numbers of early fatalities and large numbers of latent cancers, but the probability of such a release is low.
However, it is now generally agreed that there is a large range of uncertainty in the numerical results quantifying the risks of an accident, as reactor accidents have highlighted.
One example of a hypothetical accident at a nuclear power station is provided by the UK Royal Commission on Environmental Pollution (1976). It assumes that 10 per cent of the gaseous and volatile fission products are released from a 1000 MW(e) nuclear reactor as a cloud of radioactive material.
The reactor is sited at a semi-urban UK site. The main health hazard will be from iodine131 (half-life of 8 days), which will irradiate the thyroid and caesium137 (half-life of 30 years), which will cause prolonged contamination of the countryside and buildings.
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Since the weather—especially wind direction and wind speed—will markedly affect the behaviour of the radioactive cloud, estimates of the number of people exposed and the hazards can only be expressed in terms of probabilities.
The inhalation of iodine131 could cause thyroid cancer in people as far away as 24 km and there is a 20 per cent probability that, over a period of 10-20 years, between 1,000 and 10,000 people could develop thyroid cancer (less than 10 per cent of these would be fatal cancers).
The most probable outcome is 100-150 deaths from thyroid cancer over the same period. These figures could be ten times higher or lower—depending on the circumstances.
The iodine131 deposited on the ground vegetation in a few weeks. However, the longer-lived caesium137 would be present in the soil and building? for many decades. The radiation levels could necessitate evacuation for weeks, months or longer, even for people up to 50 km from the reactor site.
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Similar theoretical studies have been made in other countries such as that by Professor Norman Rasmussen in the United States, who concluded that the worst possible reactor accident would cause 3,300 deaths, 45,000 illnesses and 1,500 fatal cancers which would appear decades later.
The Chernobyl Accident, 1986:
If the public’s confidence in the safety record of the nuclear power industry faltered following the Three Mile Island (USA) accident, the accident at Chernobyl in the former Soviet Union caused nuclear reactor safety to be completely re-examined.
After only 4,000 reactor years worldwide, Chernobyl approached the worst conceivable nuclear reactor accident, which was previously estimated to be of a probability somewhere near 1 in 10,000 or even 1 in a million per reactor year.
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The Chernobyl accident released 50-100 million curies of radioactivity, according to the official Soviet report, or as much as 287 million curies of radioisotopes with half-lives greater than one day according to Hohenemser et al. This makes it 100 times worse than the Wind scale accident and 1,000 times worse than the Three Mile Island accident.
The accident at the graphite moderated, light-water-cooled pressure tube reactor at Chernobyl occurred on 26 April 1986 when an explosion produced an uncontrollable fire which lasted several days and led to vast quantities of radionuclides being lifted high into the atmosphere.
The plume of radionuclides was swept across Europe during the following 7-10 days, exposing up to 400 million people in 15 nations to high levels of ionising radiation (Fig. 24.5). In areas where heavy rainfall scavenged the radionuclides from the atmosphere, radiation levels peaked up to several hundred times higher than background levels.
The accident resulted in around 30 Soviet citizens immediately dying from radiation sickness (the majority of fatalities—most reactor workers or firemen—had received whole-body doses ranging from 600 rem. to 1,600 rem.). Another 237 people were treated for radiation sickness, some of whom needed bone-marrow transplants to restore their white blood-cell count.
The number of casualties near the reactor was less than might be expected because the radiation plume was made especially buoyant by the intense fire and so, like the effect of a tall industrial stack, local deposition of radionuclides was reduced whereas the long-range transport of radionuclides was enhanced.
Within a 30 km radius of Chernobyl, 135,000 people were evacuated. Evacuation was not prompt because the Soviet authorities were initially unwilling to publicize the seriousness of the accident and because few Soviet citizens had access to private transport.
These people now face annual medical inspection for the rest of their lives to assess what the radiation may have bequeath to them in terms of a legacy of cancer or birth defects.
Of the evacuated, 24,000 are estimated to have received an average 0.43 Sv (43 rem.). Some of the consequences for this group include over 120 spontaneous leukemia’s during the next 50 years and increased risk of genetic disorders in future generations.
Estimates of the number of expected cancers resulting from exposure to the radionuclides vary quite markedly, according to the assumptions made in the model employed. Estimates range from several thousands to tens of thousands of thyroid cancers arising from the exposure to iodine131 and a similar number of cancers elsewhere in the body from caesium137.
Only a few per cent of the thyroid tumours would be fatal, whereas perhaps half of the cancers from caesium137 would be fatal. Hawkes et al, (1986) estimate the likely number of deaths in the Soviet Union at between 5,000 and 10,000.
The official Soviet estimate of the projected cancer deaths in western Russia after Chernobyl is put at 4,000-5,000 due to exposure to iodine131 and as many as 40,000 from exposure to caesium137.
Hohenemser and Renn (1988) suggest 28,000 additional cancer deaths worldwide will be the result of Chernobyl contamination, including 10,000-15,000 in the Soviet Union, 1,500-3,000 in Poland, 4,000 in Western Europe and only 10-20 in the United States.
Anspaugh et al, estimate 17,000 additional fatal cancers in Europe in the next 50 years but highlight that the Chernobyl legacy adds an increment of only 0.01 per cent to European lifetime fatal cancer risk. Some researchers point out that the radiation dose is so small that the body can perfectly repair all resulting damage to DNA and chromosomes.
There are many problems involved in calculating the number of damages and deaths resulting from the Chernobyl accident. The ground-level deposition of radionuclides is not known accurately into the existence of local hot spots of radiation, where heavy outfall scavenged the radionuclides from the atmosphere, may not be readily incorporated into some models.
In many places in Europe, fallout from Chernobyl exceeded the peak of the weapons-testing fallout in 1963 (Fig. 24.6). Their problems associated with estimating the health effects of the Chernobyl radiation release include the difficulty of taking into account all of the radionuclides involved and all of the protective measures adopted by governments and individuals to reduce the radiation dose people received.
Protective measures included staying indoors during the passage of the radiation cloud; the removal of the outer leaves and washing of vegetables and the washing and peeling of fresh fruit; the ban by Western European countries of the importation of fresh food from areas of Eastern Europe within 1,000 km of Chernobyl; the restriction in consumption of fresh milk and free-range eggs; the removal of cattle and sheep from open pasture or a ban on their sale (for example, 2.5 million sheep in parts of north Wales were prohibited temporarily from being moved or slaughtered); the taking of prophylactic potassium iodide tablets to saturate the thyroid with iodine so as for minimise the amount of radioactive iodine being taken in (this measure was adopted most extensively in Poland); the advice not to drink fresh rainwater; and the warning to the public about possible contamination during outdoor activity.
As with most other air pollution disasters, the Chernobyl accident highlighted many inadequacies in the way in which the authorities reacted to the accident. The then Soviet government failed to provide a warning of the accident to European governments and when information was offered it was incomplete.
Many European governments—such as France and the United Kingdom—failed either to provide adequate answers to the public’s urgent questions or introduce precautionary measures quickly enough.
Radiation monitoring networks were inadequate in many European countries for the purpose of rapidly pinpointing radiation hot spots, though, in hindsight, a rainfall map of Europe for the period provided useful approximation.
In spite of nuclear plant accidents dependence on nuclear reactors for energy harvesting increased day by day.
An estimate of nuclear plant’s establishment in various countries is depicted in Table 24.1:
On the whole, the CO2 emission from various fossil fuel combustion and subsequent land use changes in different continents of the world is fairly significant (Fig. 24.7). In fact, two types of emissions are recognised here: those resulting from the burning of fossil fuels and those resulting from land use changes (e.g. forest clearance).
However, the advantages and disadvantages to nuclear power plant is given:
