| EXECUTIVE SUMMARY
Introduction
On 26 April, 1986, the Chernobyl nuclear power station, in Ukraine,
suffered a major accident which was followed by a prolonged release
to the atmosphere of large quantities of radioactive substances.
The specific features of the release favoured a widespread distribution
of radioactivity throughout the northern hemisphere, mainly across
Europe. A contributing factor was the variation of meteorological
conditions and wind regimes during the period of release. Activity
transported by the multiple plumes from Chernobyl was measured
not only in Northern and in Southern Europe, but also in Canada,
Japan and the United States. Only the Southern hemisphere remained
free of contamination.
This had serious radiological, health and socio-economic consequences
for the populations of Belarus, Ukraine and Russia, and to some
extent they are still suffering from these consequences. Although
the radiological impact of the accident in other countries was
generally very low, and even insignificant outside Europe, this
event had, however, the effect of enhancing public apprehension
all over the world on the risks associated with the use of nuclear
energy.
This is one of the reasons explaining the renewed attention and
effort devoted during the last decade to the reactor safety studies
and to emergency preparedness by public authorities and the nuclear
industry. This also underlies the continuing attention of the
public opinion to the situation at Chernobyl.
The forthcoming tenth anniversary of the accident appears, therefore,
the right moment to review the status of our knowledge of the
serious aspects of the accident impact, to take stock of the information
accumulated and the scientific studies underway, as well as to
assess the degree to which national authorities and experts have
implemented the numerous lessons that the Chernobyl accident taught
us.
This brief report, prepared under the aegis of the Committee on
Radiation Protection and Public Health (CRPPH) of the OECD Nuclear
Energy Agency, presents a collective view by OECD radiation protection
experts on this matter.
The accident
The Unit 4 of the Chernobyl nuclear power plant was to be shutdown
for routine maintenance on 25 April 1986. On that occasion, it
was decided to carry out a test of the capability of the plant
equipment to provide enough electrical power to operate the reactor
core cooling system and emergency equipment during the transition
period between a loss of main station electrical power supply
and the start up of the emergency power supply provided by diesel
engines.
Unfortunately, this test, which was considered essentially to
concern the non-nuclear part of the power plant, was carried out
without a proper exchange of information and co-ordination between
the team in charge of the test and the personnel in charge of
the operation and safety of the nuclear reactor. Therefore, inadequate
safety precautions were included in the test programme and the
operating personnel were not alerted to the nuclear safety implications
and potential danger of the electrical test.
This lack of co-ordination and awareness, resulting from an insufficient
level of "safety culture" within the plant staff, led
the operators to take a number of actions which deviated from
established safety procedures and led to a potentially dangerous
situation. This course of actions was compounded by the existence
of significant drawbacks in the reactor design which made the
plant potentially unstable and easily susceptible to loss of control
in case of operational errors.
The combination of these factors provoked a sudden and uncontrollable
power surge which resulted in violent explosions and almost total
destruction of the reactor. The consequences of this catastrophic
event were further worsened by the graphite moderator and other
material fires that broke out in the building and contributed
to a widespread and prolonged release of radioactive materials
to the environment.
Dispersion and deposition of radionuclides
The release of radioactive materials to the atmosphere consisted
of gases, aerosols and finely fragmented nuclear fuel particles.
This release was extremely high in quantity, involving a large
fraction of the radioactive product inventory existing in the
reactor, and its duration was unexpectedly long, lasting for more
than a week. This duration and the high altitude (about 1 km)
reached by the release were largely due to the graphite fire which
was very difficult to extinguish.
For these reasons and the concomitant frequent changes of wind
direction during the release period, the area affected by the
radioactive plume and the consequent deposition of radioactive
substances on the ground was extremely large, encompassing the
whole Northern hemisphere, although significant contamination
outside the former Soviet Union was only experienced in part of
Europe.
The pattern of contamination on the ground and in foodchains was,
however, very uneven in some areas due to the influence of rainfall
during the passage of the plume. This irregularity in the pattern
of deposition was particularly pronounced at larger distances
from the reactor site.
Reactions of national authorities
The scale and severity of the Chernobyl accident had not been
foreseen and took most national authorities responsible for public
health and emergency preparedness by surprise. The intervention
criteria and procedures existing in most countries were not adequate
for dealing with an accident of such scale and provided little
help in decision-making concerning the choice and adoption of
protective measures. In addition, early in the course of the accident
there was little information available and considerable political
pressure, partially based on the public perception of the radiation
danger, was being exerted on the decision-makers.
In these circumstances, cautious immediate actions were felt necessary
and in many cases measures were introduced that tended to err,
sometimes excessively so, on the side of prudence rather than
being driven by informed scientific and expert judgement.
Within the territory of the former Soviet Union, short-term countermeasures
were massive and, in general, reasonably timely and effective.
However, difficulties emerged when the authorities tried to establish
criteria for the management of the contaminated areas on the long
term and the associated relocation of large groups of population.
Various approaches were proposed and criteria were applied over
the years. Eventually, criteria for population resettlement or
relocation from contaminated areas were adopted in which radiation
protection requirements and economic compensation considerations
were intermingled. This was and continues to be a source of confusion
and possible abuse.
The progressive spread of contamination at large distances from
the accident site caused considerable concern in many countries
outside the former Soviet Union and the reactions of the national
authorities to this situation were extremely varied, ranging from
a simple intensification of the normal environmental monitoring
programmes, without adoption of specific countermeasures, to compulsory
restrictions concerning the marketing and consumption of foodstuffs.
Apart from the objective differences of contamination levels and
regulatory and public health systems between countries, one of
the principal reasons for the variety of situations observed in
the different countries stems from the different criteria adopted
for the choice and application of intervention levels for the
implementation of protective actions. These discrepancies were
in some cases due to misinterpretation and misuse of international
radiation protection guidelines, especially in the case of food
contamination, and were further enhanced by the overwhelming role
played in many cases by non-radiological factors, such as socio-economic,
political and psychological, in determining the countermeasures.
This situation caused concern and confusion among the public,
perplexities among the experts and difficulties to national authorities,
including problems of public credibility, as well as a waste of
efforts and unnecessary economic losses. These problems were particularly
felt in areas close to international borders due to different
reactions of the authorities and media in bordering countries.
However, all these issues were soon identified as an area where
several lessons should be learned and international efforts were
undertaken to harmonise criteria and approaches to emergency management.
Radiation dose estimates
Most of the population of the Northern hemisphere was exposed,
to various degrees, to radiation from the Chernobyl accident.
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 main doses of concern are those to the thyroid due to external
irradiation and inhalation and ingestion of radioactive iodine
isotopes, and those to the whole body due to external irradiation
from and ingestion of radioactive caesium isotopes. According
to current estimates, the situation for the different exposed
groups is the following:
- Evacuees - More than 100,000 persons were evacuated,
mostly from the 30-km radius area around the accident site, during
the first few weeks following the accident. These people received
significant doses both to the whole body and the thyroid, although
the distribution of those doses was very variable among them depending
on their positions around the accident site and the delays of
their evacuation.
Doses to the thyroid ranging from 70 millisieverts to adults up
to about 1,000 millisieverts (i.e., 1 sievert) to young
children and an average individual dose of 15 millisieverts [mSv]
to the whole body were estimated to have been absorbed by this
population prior to their evacuation. Many of these people continued
to be exposed, although to a lesser extent depending on the sites
of their relocation, after their evacuation from the 30-km zone.
- "Liquidators" - Hundreds of thousands
of workers, estimated to amount up to 800,000 and including a
large number of military personnel, were involved in the emergency
actions on the site during the accident and the subsequent clean-up
operations which lasted for a few years. These workers were called
"liquidators".
A restricted number, of the order of 400, including plant staff,
firemen and medical aid personnel, were on the site during the
accident and its immediate aftermath and received very high doses
from a variety of sources and exposure pathways. Among them were
all those who developed acute radiation syndrome and required
emergency medical treatment. The doses to these people ranged
from a few grays to well above 10 grays to the whole body from
external irradiation and comparable or even higher internal doses,
in particular to the thyroid, from incorporation of radionuclides.
A number of scientists, who periodically performed technical actions
inside the destroyed reactor area during several years, accumulated
over time doses of similar magnitude.
The largest group of liquidators participated in clean-up operations
for variable durations over a number of years after the accident.
Although they were not operating anymore in emergency conditions
and were submitted to controls and dose limitations, they received
significant doses ranging from tens to hundreds of millisieverts.
- People living in contaminated areas of the former Soviet
Union - About 270,000 people continue to live in contaminated
areas with radiocaesium deposition levels in excess of 555 kilobecquerels
per square metre [kBq/m2], where protection measures still continue
to be required. Thyroid doses, due mainly to the consumption of
cow's milk contaminated with radioiodine, were delivered during
the first few weeks after the accident; children in the Gomel
region of Belarus appear to have received the highest thyroid
doses with a range from negligible levels up to 40 sieverts and
an average of about 1 sievert for children aged 0 to 7. Because
of the control of foodstuffs in those areas, most of the radiation
exposure since the summer of 1986 is due to external irradiation
from the radiocaesium activity deposited on the ground; the whole-body
doses for the 1986-89 time period are estimated to range from
5 to 250 mSv with an average of 40 mSv.
- Populations outside the former Soviet Union - The
radioactive materials of a volatile nature (such as iodine and
caesium) that were released during the accident spread throughout
the entire Northern hemisphere. The doses received by populations
outside the former Soviet Union are relatively low, and show large
differences from one country to another depending mainly upon
whether rainfall occurred during the passage of the radioactive
cloud. These doses range from a lower extreme of a few microsieverts
or tens of microsieverts outside Europe, to an upper extreme of
1 or 2 mSv in some European countries. The latter value is of
the same order as the annual individual exposure from natural
background radiation.
Health impact
The health impact of the Chernobyl accident can be described in
terms of acute health effects (death, severe health impairment),
late health effects (cancers) and psychological effects liable
to affect health.
The acute health effects occurred among the plant personnel and
the persons who intervened in the emergency phase to fight fires,
provide medical aid and immediate clean-up operations. A total
of 31 persons died as a consequence of the accident, and about
140 persons suffered various degrees of radiation sickness and
health impairment. No members of the general public suffered these
kinds of effects.
As far as the late health effects are concerned, namely the possible
increase of cancer incidence, in the decade following the accident
there has been a real and significant increase of carcinomas of
the thyroid among the children living in the contaminated regions
of the former Soviet Union, which should be attributed to the
accident until proved otherwise. There might also be some increase
of thyroid cancers among the adults living in those regions. From
the observed trend of this increase of thyroid cancers it is expected
that the peak has not yet been reached and that this kind of cancer
will still continue for some time to show an excess above its
natural rate in the area.
On the other hand, the scientific and medical observation of the
population has not revealed any increase in other cancers, as
well as in leukaemia, congenital abnormalities, adverse pregnancy
outcomes or any other radiation induced disease that could be
attributed to the Chernobyl accident. This observation applies
to the whole general population, both within and outside the former
Soviet Union. Large scientific and epidemiological research programmes,
some of them sponsored by international organisations such as
the WHO and the EC, are being conducted to provide further insight
into possible future health effects. However, the population dose
estimates generally accepted tend to indicate that, with the exception
of thyroid disease, it is unlikely that the exposure would lead
to discernible radiation effects in the general population above
the background of natural incidence of the same diseases. In the
case of the liquidators this forecast should be taken with some
caution.
An important effect of the accident, which has a bearing on health,
is the appearance of a widespread status of psychological stress
in the populations affected. The severity of this phenomenon,
which is mostly observed in the contaminated regions of the former
Soviet Union, appears to reflect the public fears about the unknowns
of radiation and its effects, as well as its mistrust towards
public authorities and official experts, and is certainly made
worse by the disruption of the social networks and traditional
ways of life provoked by the accident and its long-term consequences.
Agricultural and environmental impacts
The impact of the accident on agricultural practices, food production
and use and other aspects of the environment has been and continues
to be much more widespread than the direct health impact on humans.
Several techniques of soil treatment and decontamination to reduce
the accumulation of radioactivity in agricultural produce and
cow's milk and meat have been experimented with positive results
in some cases. Nevertheless, within the former Soviet Union large
areas of agricultural land are still excluded from use and are
expected to continue to be so for a long time. In a much larger
area, although agricultural and dairy production activities are
carried out, the food produced is subjected to strict controls
and restrictions of distribution and use.
Similar problems of control and limitation of use, although of
a much lower severity, were experienced in some countries of Europe
outside the former Soviet Union, where agricultural and farm animal
production were subjected to restrictions for variable durations
after the accident. Most of these restrictions have been lifted
several years ago. However, there are still today some areas in
Europe where restrictions on slaughter and distribution of animals
are in force. This concerns, for example, several hundreds of
thousands of sheep in the United Kingdom and large numbers of
sheep and reindeer in some Nordic countries.
A kind of environment where special problems were and continue
to be experienced is the forest environment. Because of the high
filtering characteristics of trees, deposition was often higher
in forests than in other areas. An extreme case was the so-called
"red forest" near to the Chernobyl site where the irradiation
was so high as to kill the trees which had to be destroyed as
radioactive waste. In more general terms, forests, being a source
of timber, wild game, berries and mushrooms as well as a place
for work and recreation, continue to be of concern in some areas
and are expected to constitute a radiological problem for a long
time.
Water bodies, such as rivers, lakes and reservoirs can be, if
contaminated, an important source of human radiation exposure
because of their uses for recreation, drinking and fishing. In
the case of the Chernobyl accident this segment of the environment
did not contribute significantly to the total radiation exposure
of the population. It was estimated that the component of the
individual and collective doses that can be attributed to the
water bodies and their products did not exceed 1 or 2 percent
of the total exposure resulting from the accident. The contamination
of the water system has not posed a public health problem during
the last decade; nevertheless, in view of the large quantities
of radioactivity deposited in the catchment area of the system
of water bodies in the contaminated regions around Chernobyl,
there will continue to be for a long time a need for careful monitoring
to ensure that washout from the catchment area will not contaminate
drinking-water supplies.
Outside the former Soviet Union, no concerns were ever warranted
for the levels of radioactivity in drinking water. On the other
hand, there are lakes, particularly in Switzerland and the Nordic
countries, where restrictions were necessary for the consumption
of fish. These restrictions still exist in Sweden, for example,
where thousands of lakes contain fish with a radioactivity content
which is still higher than the limits established by the authorities
for sale on the market.
Potential residual risks
Within seven months of the accident, the destroyed reactor was
encased in a massive concrete structure, known as the "sarcophagus",
to provide some form of confinement of the damaged nuclear fuel
and destroyed equipment and reduce the likelihood of further releases
of radioactivity to the environment. This structure was, however,
not conceived as a permanent containment but rather as a provisional
barrier pending the definition of a more radical solution for
the elimination of the destroyed reactor and the safe disposal
of the highly radioactive materials.
Nine years after its erection, the sarcophagus structure, although
still generally sound, raises concerns for its long-term resistance
and represents a standing potential risk. In particular, the roof
of the structure presented for a long time numerous cracks with
consequent impairment of leaktightness and penetration of large
quantities of rain water which is now highly radioactive. This
also creates conditions of high humidity producing corrosion of
metallic structures which contribute to the support of the sarcophagus.
Moreover, some massive concrete structures, damaged or dislodged
by the reactor explosion, are unstable and their failure, due
to further degradation or to external events, could provoke a
collapse of the roof and part of the building.
According to various analyses, a number of potential accidental
scenarios could be envisaged. They include a criticality excursion
due to change of configuration of the melted nuclear fuel masses
in the presence of water leaked from the roof, a resuspension
of radioactive dusts provoked by the collapse of the enclosure
and the long-term migration of radionuclides from the enclosure
into the groundwater. The first two accident scenarios would result
in the release of radionuclides into the atmosphere which would
produce a new contamination of the surrounding area within a radius
of several tens of kilometres. It is not expected, however, that
such accidents could have serious radiological consequences at
longer distances.
As far as the leaching of radionuclides from the fuel masses by
the water in the enclosure and their migration into the groundwater
are concerned, this phenomenon is expected to be very slow and
it has been estimated that, for example, it will take 45 to 90
years for certain radionuclides such as strontium90 to migrate
underground up to the Pripyat River catchment area. The expected
radiological significance of this phenomenon is not known with
certainty and a careful monitoring of the evolving situation of
the groundwater will need to be carried out for a long time.
The accident recovery and clean-up operations have resulted in
the production of very large quantities of radioactive wastes
and contaminated equipment which are currently stored in about
800 sites within and outside the 30-km exclusion zone around the
reactor. These wastes and equipment are partly buried in trenches
and partly conserved in containers isolated from groundwater by
clay or concrete screens. A large number of contaminated equipment,
engines and vehicles are also stored in the open air.
All these wastes are a potential source of contamination of the
groundwater which will require close monitoring until a safe disposal
into an appropriate repository is implemented.
In general, it can be concluded that the sarcophagus and the proliferation
of waste storage sites in the area constitute a series of potential
sources of release of radioactivity that threatens the surrounding
area. However, any such releases are expected to be very small
in comparison with those from the Chernobyl accident in 1986 and
their consequences would be limited to a relatively small area
around the site. On the other hand, concerns have been expressed
by some experts that a much more important release might occur
if the collapse of the sarcophagus should induce damage in the
Unit 3 of the Chernobyl power plant, which currently is still
in operation.
In any event, initiatives have been taken internationally, and
are currently underway, to study a technical solution leading
to the elimination of these sources of residual risk on the site.
Lessons learned
The Chernobyl accident was very specific in nature and it should
not be seen as a reference accident for future emergency planning
purposes. However, it was very clear from the reactions of the
public authorities in the various countries that they were not
prepared to deal with an accident of this magnitude and that technical
and/or organisational deficiencies existed in emergency planning
and preparedness in almost all countries.
The lessons that could be learned from the Chernobyl accident
were, therefore, numerous and encompassed all areas, including
reactor safety and severe accident management, intervention criteria,
emergency procedures, communication, medical treatment of irradiated
persons, monitoring methods, radioecological processes, land and
agricultural management, public information, etc.
However, the most important lesson learned was probably the understanding
that a major nuclear accident has inevitable transboundary implications
and its consequences could affect, directly or indirectly, many
countries even at large distances from the accident site. This
led to an extraordinary effort to expand and reinforce international
co-operation in areas such as communication, harmonisation of
emergency management criteria and co-ordination of protective
actions. Major improvements were achieved in this decade and important
international mechanisms of co-operation and information were
established, such as the international conventions on early notification
and assistance in case of a radiological accident, by the IAEA
and the EC, the international nuclear emergency exercises (INEX)
programme, by the NEA, the international accident severity scale
(INES), by the IAEA and NEA and the international agreement on
food contamination, by the FAO and WHO.
At the national level, the Chernobyl accident also stimulated
authorities and experts to a radical review of their understanding
of and attitude to radiation protection and nuclear emergency
issues. This prompted many countries to establish nationwide emergency
plans in addition to the existing structure of local emergency
plans for individual nuclear facilities. In the scientific and
technical area, besides providing new impetus to nuclear safety
research, especially on the management of severe nuclear accidents,
this new climate led to renewed efforts to expand knowledge on
the harmful effects of radiation and their medical treatment and
to revitalise radioecological research and environmental monitoring
programmes. Substantial improvements were also achieved in the
definition of criteria and methods for the information of the
public, an aspect whose importance was particularly evident during
the accident and its aftermath.
Conclusion
The history of the modern industrial world has been affected on
many occasions by catastrophes comparable or even more severe
than the Chernobyl accident. Nevertheless, this accident, due
not only to its severity but especially to the presence of ionising
radiation, had a significant impact on human society.
Not only it produced severe health consequences and physical,
industrial and economic damage in the short term, but, also, its
long-term consequences in terms of socio-economic disruption,
psychological stress and damaged image of nuclear energy, are
expected to be long standing.
However, the international community has demonstrated a remarkable
ability to apprehend and treasure the lessons to be drawn from
this event, so that it will be better prepared to cope with a
challenge of this kind, if ever a severe nuclear accident should
happen again.
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