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Vol. 9 No. 1 - January 2003

Radioecology and the Chernobyl Disaster

By: J.N.B. Bell

Radioecology is the study of the pathways of radionuclides in the environment and their impact on biota. It is a relatively young science, having been born following the development of nuclear weapons. Initially concern was related to the fate of radionuclides discharged into the stratosphere following the testing of nuclear bombs aboveground, with subsequent long-term deposition to the earth's surface on a global scale. Subsequently, interest switched to the consequences of releases from nuclear installations, such as power stations and reprocessing plants, arising routinely or as a result of accidents. In terms of accidents, the Chernobyl Disaster of 1986 had the most widespread and serious environmental consequences, which led to a major boost in research into a wide range of radioecological topics.

It is now over 16 years since the accident at the Chernobyl nuclear power station in the northern Ukraine, then part of the USSR, yet the ensuing environmental problems remain to this day and will continue into the future. The accident occurred on 26 April 1986 (a date which I remember well, because it is my birthday!), as a result of an unauthorized experiment coupled with a design fault, which rendered the reactor inherently unsafe. The accident resulted in a massive surge of power, which could not be controlled, due to removal or switching off of control devices, causing a steam explosion, which blew the top off the reactor. This was followed by a major fire, which burned for 10 days before becoming extinguished. The fires resulted in discharge of vast quantities of a cocktail of radionuclides to the atmosphere. A large amount of the radioactivity was deposited in the surrounding regions of both Ukraine and Byelorussia, this being often associated with the larger particles arising from the fire, but the plume containing smaller particles and gases was dispersed a very long distance, ultimately circling the northern hemisphere.

The human consequences of the accident were appalling. Some 150,000 people had to be evacuated from the nearfield contaminated area and a large amount of farmland taken out of production. To this day much of the surrounding area is a permanent exclusion zone and the city of Prypiat, which formerly contained c.55,000 people is the largest ghost town in the world, with a population of zero. An immediate ecological effect of the accident was the radiation-induced death of trees close to the reactor, resulting in the so-called "Red Forest". Subsequently abnormalities were observed in the form of very large pine needles and oak leaves. These heavily contaminated trees were disposed of by burial on the site of the Red Forest. Around the reactor the topsoil was scraped off for disposal and the reactor itself was entombed in concrete to form a sarcophagus in order that the other reactors in the complex could continue to operate.

As was normal in the Soviet Union the authorities failed to publicise bad news and it was not until the plume had been detected abroad that the first announcement was made. Thus on 28 April at 21.00 Radio Moscow reported "An accident has occurred at the Chernobyl nuclear power plant - one of the atomic reactors has been damaged. Measures are being taken to liquidate the consequences of the accident. Those affected are being given aid, and a government commission has been created". This was essentially a standard USSR statement and could be described as the understatement of the century!

The first detection of the radioactive plume outside the USSR occurred at the Forsmark nuclear power station on the east coast of Sweden, when the alarms sounded. When it was determined that no accident had taken place on site it was realised that a massive nuclear disaster had occurred in some place east of Sweden. This initial plume passed over central Europe, depositing radioactivity largely as a result of interception by rain. Then in early May the plume moved northwards, passing over the United Kingdom, with deposition of radioactivity primarily being associated with rainstorms. Immediately the countries concerned started to take measurements of the radionuclides of concern in environmental samples, such as air, water and soil and in foods, such as vegetables, milk and meat, as the isotopes concerned - iodine - 131, caesium - 134 and caesium - 137 entered food chains. The half-life of I131 is only 8 days, but values for Cs134 and Cs137 are 2 and 30 years, respectively, so that a longer term problem should have been anticipated.

Different countries reacted very differently in terms of countermeasures taken to reduce radioactive dose to humans. In Germany different states also gave varying advice, including not allowing children to play in sand and avoiding the consumption of certain vegetables, while in The Netherlands the sale of sheepsmilk cheese was prohibited for some time. Particular problems arose in Arctic Scandinavia, where the Lapps are heavily reliant on their reindeer herds as a source of food. Reindeer feed on reindeer moss, which is in fact a lichen and, as such is adapted to take up nutrients from the atmosphere. The latter fact and the absence of the protective cuticle found in higher plants resulted in the reindeer moss accumulating an extremely high level of radiocaesium, which is then passed through the food chain to reindeer. In view of highly elevated levels of caesium in reindeer the authorities took action by slaughtering large numbers of the animals, thereby resulting in considerable social hardship to the people concerned. In the UK it was decided that any immediate countermeasures, such as a ban on sale of milk from heavily contaminated areas, would cause more social disruption and stress than was warranted by any hypothetical life saved by reduction in collective dose. So the only advice given in the immediate aftermath of the accident (other than "do not panic"!) was not to drink rainwater in certain parts of Scotland.

Thus it was apparent that radioactivity was entering food-chains. In fact the behaviour of radiocaesium in this respect had been studied back to the days of bomb-testing, when regular measurements were made of levels in air and milk across the UK. Another radionuclide, strontium-90 had similarly been studied in milk and air, and as an undergraduate I remember newspaper cartoons of housewifes testing milk bottles with Geiger counters and people making jokes about drinking their daily dose of radioactivity! In the UK the government was extremely reassuring and it is worth noting what was said at the time. On 6th May 1986 the Secretary of State for the Environment stated in the House of Commons that "The effects of the cloud have already been assessed and none presents a risk to health in the UK". On 13th May he reiterated "As long as there are no further discharges from the Chernobyl accident, the incident may be regarded as over for this country by the end of the week, although its traces will remain". Further reassurance was given by the Minister of Agriculture on the 19th May when he told the House of Commons "We have always been a long way from the stage when we might need to contemplate imposing any sort of restriction".

One month later from this last statement the Ministry of Agriculture, Fisheries and Food suddenly imposed a wide scale ban on the movement and slaughter of sheep over a large area of North West England, surrounding the mountainous Lake District, and in North Wales, including the mountainous Snowdonia area. Then 4 days later the Secretary of State for Scotland announced a similar ban in 3 regions of that country.

The disruption caused by these actions was severe in the extreme, in view of the paramount importance of sheep farming in the local economies. The ban was imposed, because radiocaesium levels in sheep meat were being found to exceed the regulatory threshold of 1000 Bq kg-1. The only way of testing if this threshold was being exceeded was by killing the animal concerned and a sample of meat being sent off to a laboratory for analysis. 4,200,000 sheep out of a national flock of 24,600,000 were affected in this manner and, clearly, it was not possible to determine the level of radioactivity in each of these. Thus during the summer of 1986 there was a race to develop a "live monitor", which could be used to measure the level of radioactivity in sheep without the need for slaughter. By the autumn this monitor had been used to assess precisely where the sheep exceeded the threshold level and it was possible to lift the ban on large areas of land. The latter were essentially in lowlands, with the remaining banned areas being in the uplands, with rough grazing of natural vegetation.

Why did the government fail to predict that in many upland areas sheep would show rising radiocaesium levels, rather than falling ones, as had been predicted? The problem lay essentially in the fact that advice by Ministry of Agriculture, Fisheries and Food Scientists was based on experiments involving measurement of uptake by crops of caesium from agricultural lowland soils. Such soils contain a significant proportion of clay particles onto which caesium rapidly becomes more or less irreversibly fixed and thus is unavailable for uptake into plants. An example of such an experiment was carried out at Imperial College in the mid 1980s. Winter wheat was grown over two successive growing seasons in two agricultural soils - a sandy loam and a silty clay - which had been contaminated with Cs137 before the start of the experiment. The uptake was measured at maturity in each year: in 1984 and 1985, the wheat growing in the sandy loam contained 105 and 38 Bq kg-1, respectively, while the corresponding figures for the silty clay were 180 and 29 Bq kg-'. This fall was not due to depletion of radiocaesium in the soil, but rather by fixation on clay particles. In lowland agricultural areas of the UK this is precisely what happened, with a sharp fall in caesium levels in agricultural grasses, from the time of initial contamination. Indeed the areas derestricted after the first live monitoring exercise were all places with normal agricultural soils.

The upland areas where restrictions remained were very different from lowland agricultural systems. They contain semi-natural habitats, with peaty soils with very low clay contents. In addition they are invariably highly acidic and often waterlogged, both characteristics tending to render metals more available for uptake through the roots. The vegetation in these places is very different from lowland grasses. It consists of coarse grasses, sedges and Ericaceous species, as well as other plants adapted to these harsh conditions. The plants concerned are adapted to maximise nutrient uptake from these very poor quality soils. It should be remembered that caesium is an analogue of potassium, which is a macronutrient, and thus it is hardly surprising that the radionuclide was accumulated to extremely high levels in the upland vegetation. Sheep graze this wild vegetation and thus radiocaesium enters the food chain. The effect of growing plants on simulated upland and agricultural soils was demonstrated in a further experiment at Imperial College. In this case artificial soils were constructed containing either 18% clay and 10% peat or 2% clay and 90% peat and Cs137 mixed into these. Heather (Calluna vulgaris), one of the species grazed in uplands, was then grown in these soils for 25 days and the uptake of the radionuclide measured at the end of this period. The results of this study were dramatic, with the high peat/low clay soil plants containing 11395 Bq g-1 compared with 177 Bq g-1 in the high clay/low peat soil. Current thinking is that the high organic matter content, rather than low clay levels, is the predominant factor leading to the high mobility of radiocaesium in upland ecosystems.

The problems of contamination of ecosystems in Europe after the Chernobyl accident led to extensive research programmes into environmental pathways of radiocaesium in the countries concerned. These concentrated on a whole range of natural/semi-natural ecosystems, which had largely been ignored in earlier research. Besides soil characteristics, a range of vegetation related factors were examined, such as absorption through the foliage, rooting depth and seasonal growth patterns, as well as the influence of grazing by sheep and soil ingestion. A study at Imperial College showed that heather could absorb large quantities of caesium through the foliage, with subsequent translocation to elsewhere in the shoot, whereas two other ericaceous species - bilberry (Vaccinium myrtilus) and cross-leaved heath (Erica tetralix)- showed lower uptake. Research in Scotland with artificial contamination of a grassland showed a high level of variability in uptake of caesium between species, with an insignificant species which represented a tiny fraction of cover in the sward, the mouse-eared chickweed (Cerastium fontanum), showing a particularly high level of accumulation. Chernobyl caused a major rethink in the world of radioecology, with the realisation that there were many potential food-chain pathways to humans other than via agriculture. Thus interest developed into uptake of radiocaesium into wild fruits, game birds and animals and edible fungi. The latter are highly effective in taking up caesium from the soil. This caused particular problems in contaminated areas, where hunting of deer takes place. These animals have a strong liking for the macrocarps of fungi and in the autumn these can form a substantial part of their diet, thus resulting in high levels of radiocaesium in their flesh. In Sweden active consideration was given to altering the hunting season to avoid the major times of fungal fruiting body production, and Prussian blue was introduced into saltlicks in the forest to act as an antagonist to uptake of caesium.

It is now nearly 17 years since the Chernobyl disaster. What is the situation in contaminated areas at the present time and what is the prognosis for the future? In the UK there continue to be restrictions on sheep in parts of the contaminated uplands, although the number of farms affected continues to fall, year by year. Thus in Cumbria 867,000 sheep on 1670 farms were restricted in June 1986, with this failing by January 1998 to 14,000 sheep on 10 holdings. The corresponding figures in Wales fell from 2,000,000 sheep on 5,100 farms to 180,000 sheep on 359 farms. Extrapolating into the future it has been estimated that the last restrictions are likely to be lifted as long as 30 years into the future, demonstrating the long-term influence of the Chernobyl accident at locations very remote from the scene of the accident.

Around Chernobyl itself large areas remain as an effectively permanent exclusion zone, representing a vast radioactive nature reserve, where human influence has been removed. This now teams with wildlife, including rare species, enjoying the absence of interference by humans. The Red Forest has been replaced by new growth of pine and birch trees, which contain large quantities of "Sr. At this location there are concerns about downward migration of radioactivity into the groundwater, with appropriate monitoring programmes having been established. The exclusion zone represents a large open-air laboratory, with enormous potential for research into radioecology, biogeochemical cycling and ecosystem recovery after removal of human influence. A particularly important mechanism for research in the Chernobyl area is the establishment of the Chernobyl International Radioecology Laboratory. This is based in the city of Slavutych, located outside the heavily contaminated area, which was constructed after the accident to house the workers who operated the remaining reactors. This laboratory is supported primarily by funds from the USA, but with contributions also from the UK, France, Germany and Japan. It aims to provide facilities for the international community of radioecologists to carry out research in the exclusion zone. It has a second location, for more field-based work, in the city of Chernobyl, which is still inhabited, but with the population living there for 2 weeks on and 2 weeks off to reduce their dose. Much of the research at the Laboratory is by radioecologists from the USA. This includes studies on small mammals, which are particularly vulnerable, in view of their bodies being in very close proximity to contaminated soil, so that they receive a high dose of radioactivity. The results of this research are extremely interesting. These animals are surviving at locations where they are exposed to potentially lethal doses. At first it was thought that they might be wandering into the most contaminated areas or else were not breeding. However, research indicated that neither of these situations applied and that the animals appeared to be adapting to these very high levels of radioactivity. Such adaptation flies in the face of conventional wisdom and possibly should cause us to re-examine the effects of radiation on biota, including humans.

The story of Chernobyl is a tragic one and has represented a major set-back for the nuclear power industry. The last operational reactor at Chernobyl closed around two years ago, with serious consequences for employment in this remote region, although there are jobs concerned with the massive decommissioning activities that will continue into the future. There are salutary lessons to be learned from the UK government's misguided response to contamination of the food chain. Any upland ecologist could have told the agricultural scientists that radiocaesium could be expected to remain highly mobile in the upland organic soils of the UK. This is a classic example of the need to understand how the natural environment works in order to provide good environmental protection and management.

Nigel Bell is Professor of Environmental Pollution at Imperial College, London based in the Department of Biological Sciences and the Department of Environmental Science & Technology

This article has been reproduced from the archives of EnviroNews - Newsletter of ISEB India.

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