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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 |
