Word from the President
Chernobyl, the accident scenario and its global
As the world recalls the Chernobyl accident twenty
years ago, this report aims to provide a brief description of
the facts surrounding the Chernobyl accident (known and assumed),
to examine its possible causes and to provide answers to commonly
asked questions on issues like health, social and socio-political
impacts, environmental considerations etc. It also draws some
conclusions on the current state of affairs twenty years after
an event that troubled man’s collective conscience.
The accident: what happened ?
Chernobyl’s N°4 reactor was a graphite
moderated light water reactor (RBMK) with an output of 1000 MWe.
It was a pressure tubes boiling water reactor with direct steam
feed to the turbines.
A standard maintenance stop for reactor N°4
was planned on April 25. To run the RBMK type plant requires the
generation of electrical power, mainly for cooling. In the event
of a power failure, emergency generators start up a few seconds
later. Due to problems with the new emergency generators, it was
decided to carry out a test on the cooling pumps, which required
the bypassing of safety systems. The aim of the test was to check
if the inertia of the turbines provided enough power to keep the
cooling pumps operational during the time required to start the
Here is chronological run-down of the chain of events that took
place in the days and hours that led up to the accident:
Friday April 25 1986:
01.00 a.m.: the operators decrease the power of the reactor
02.00 p.m.: the reactor runs at half power
11.00 p.m.: decision to start the test.
Due to an error in the regulation, the power is much lower
than normal. Rather than stopping the reactor (and the test),
the operators try to increase the power again by lifting many
more control bars than allowed (6-8 rather than 30). The problem
is that at low power, the reactor has a positive void coefficient
Saturday April 26 1986:
01.22 a.m.: the test begins while the reactor
continues operating under non-authorised conditions. The operators
switch off the safety mechanism that should stop the reactor
in case of loss of steam supply to the turbine.
01.23.04 a.m.: the turbines shut down and
the cooling pumps stop. This increases the steam content in
the tubes and the reactor power increases rather than decreases
due to the positive void coefficient.
01.23.40 a.m.: an attempt is made to manually
stop the reactor by releasing the control bars (211). The
control bars take about 20 seconds to reach the core, and
their design is such that reactivity increases during the
initial seconds. Fuel elements start breaking up. A few seconds
later, shocks are felt and explosions are heard. Steam explosions
destroy the reactor core and blow the roof off the reactor
building. Fires start all over the place. The worst civil
nuclear accident in history has just occurred.
01.28 a.m.: the first fire-fighters arrive
on the scene
02.30 a.m.: the largest fires are under
05.00 a.m.: the graphite fire starts
Today, the causes and the consequences of the
accident have been thoroughly studied and many lessons have been
The main causes of the accident, as identified by Western experts
Unsafe and unstable reactor design:
In addition to generating electricity, the RBMK reactors at
Chernobyl were also designed and adapted for the production
of plutonium for military purposes, as fuel can be loaded
and unloaded during operation. This double function restricted
the reactor’s built-in safety mechanisms. Consequently,
the accident cannot be disassociated from the politico-military
context of the former Soviet Union at that time, even if there
are no indications that at any time plutonium was produced
there for military purposes
The operators’ lack of theoretical
training and knowledge: During the cold war, safety
was clearly not a priority. There was a critical lack of safety
culture at Chernobyl, which was amplified by an global lack
of understanding and training
The culture of strict confidentiality
that reigned in the former Soviet Union due to the strong
interdependency of civil and military nuclear applications:
Within the context of the 1980’s, operators were not
supposed to think critically or take initiatives in case of
emergency situations, which were never even officially considered.
The question of exactly how many casualties resulted
from the Chernobyl catastrophe remains on everyone's mind today,
twenty years later - even though quantifying human suffering in
terms of fatalities is much too restrictive. The following data
mainly come from a report published by the Chernobyl Forum (Chernobyl's
Legacy: Health, Environmental and Socio-Economic Impacts and Recommendations
to the Governments of Belarus, the Russian Federation and Ukraine1)
and another one written by SCK-CEN report entitled Chernobyl,
20 years later2. Distinction has to be made between
the different categories of casualties, as follows:
Fatalities that occurred among people who
received high radiation doses during the 4 months that followed
the explosions (in total, 134 people suffered from radiation
sickness). It is highly probable that fatalities also occurred,
a few years after the accident, among people who had initially
suffering from radiation sickness but had seemed to have recovered
Fatalities estimated among rescue workers
and the so called 'liquidators' who did not suffer from radiation
Fatalities estimated among the general population
Two employees died from injuries
caused by the explosions that were not connected to radiation.
One other employee probably died from an acute cardiac arrest
brought on by the explosions.
28 employees or rescuers died within 4 months and there is no
doubt that their death is as a result of the accident. A further
19 workers died between 1987 and 2004. As they were among those
suffering from radiation sickness, it seems more than probable
that the majority of them died from the consequences of the accident,
although some certainly died from other causes. Some authors limit
radiation-related deaths to 11.
Remark: Out of 134 people,
28 died from extremely high radiation doses. That leaves 106 people.
Of those 106, between 11 and 19 died over a period of more than
15 years. Those are “normal death rates?” Radiation-related
models indicate much higher rates. Why the discrepancy?
Among the rescue workers and
the liquidators (initially about 350.000, later
up to 600.000), about 1000 received radiation doses ranging from
2 – 20 Gy. The average effective dose among all 600.000
liquidators is estimated to be around 100 mSv. Therefore, the
doses range between 25 and 250 times the natural radiation dose.
21 cases of leukaemia have been detected among workers who received
more that 150 mSv. That is about twice the normal rate of occurrence
(in other population groups, no increase has been seen). An increase
in solid or thyroid cancers has also been noted, but this is certainly
due to vastly improved screening methods. A radiation-induced
increase in incidence of these cancers cannot, however, be excluded.
But statistically this is not significant due to the very low
Models, mainly based on observations made with
regard to survivors of Hiroshima and Nagasaki, led to about 2000
radiation-induced cancers being made attributable to rescue workers
or liquidators, during their lifetime. Furthermore, the extrapolated
numbers depend upon the life expectancy model used. The current
life expectancy in the Ukraine or Belarus is now as low as 55
- 65 years for adult males. Many solid cancers may not have the
time to develop by this age group.
All numbers derived from such models are subject
to great uncertainty, but the ongoing discussion about the linear
threshold model, or a simple threshold, or even hormesis, is irrelevant
here because the doses are much too high for those potential effects
to have played a role.
Among the general population,
there is very little doubt that the increase of thyroid cancer
in children (about 5000 detected cases) is due to contamination,
probably by iodine and caesium isotopes trapped by an iodine deficient
thyroid. Unfortunately, about 15 children have died. It has been
suggested that screening explains the increase in observed cases
of thyroid cancer. The observed correlation with soil contamination
points to radiation effects. It should be realised, however, that
in Western countries one half of all elderly people have thyroid
cancer that goes totally unnoticed (autopsy data). No other increase
in cancer incidence has been observed. However, it may yet ocur,
or it may be too small to be detected.
Based probably on a linear non-threshold model,
the report by the Chernobyl Forum (September 2005 version) predicts
some 2000 extra cancer deaths among the general population, taking
into account average radiation doses above background levels.
It is stated that this is an increase of 3% on normal cancer incidence
levels. This means that since the normal incidence of cancer death
is about 25%, a total exposed population of about 250.000 people
was considered. Obviously, nobody will be able to prove or disprove
2000 extra cases among what is a normal rate of occurrence - unless
the cancers are of a very specific nature.
The linear non-threshold model assumes that there
is no threshold level below which no detrimental radiation effect
is observed. A model with even a small threshold level would greatly
decrease the number of 2000 cancer cases. Also, it does not seem
reasonable to speak about “extra cancer deaths” as
if those people would not have died without radiation. Would it
not be better to speak about “early cancer deaths?”
Furthermore, an increase in cancer deaths does not necessarily
mean decreased life expectancy in general. It may be that survivors
live longer (the “healthy survivor” effect) and, therefore,
that cancer may not be the only indicator for radiation effects
to be taken into account.
It is probably fair to conclude that, apart from
thyroid cancer among children, no statistically significant increase
in cancer incidence has been observed today, and if it were to
occur, it will not have a major impact on the average health status
of the population in the Chernobyl area. Indeed, even though statistically
significant, and certainly dramatic from a personal point of view,
10 or even 100 extra deaths due to a particular or rare cancer
have no impact on public health in areas where chronic factors
such as alcohol abuse, malnutrition, smoking etc. have a compound
effect on health.
As far as incidence of malformations is concerned,
about which numerous false information has been communicated and
misleading photographs published, no relevant radiation-induced
increase has been identified. The only probable non-cancer health
effect is an increase in the incidence of cataracts among liquidators
On April 27 at 11.00 a.m., the population of
the town of Pripyat was told that it was going to be evacuated.
Two and a half hours later, all inhabitants had left their homes
forever, along with friends, people living in the neighbourhood,
cats and dogs. The evacuation was progressively extended to include
people living within a 30 km radius of the stricken reactor. This
brought the total number of evacuees to about 116.000. In the
years following the accident, the number of people that were relocated
grew to more than 330.000. It is not hard to imagine the psychological
damage cause by this forced evacuation and relocation - not only
among the resettled people, but also among the residents of the
areas of resettlement who feared and disapproved of the mass arrival
of busloads of 'contaminated foreigners'.
This forced relocation gave rise to mental health
problems, alcohol and tobacco abuse etc..., in what the Chernobyl
forum reports as "the largest public health problem unleashed
by the accident today".
The permanent relocation of such a large number
of people, irrespective of age and social background, can certainly
be questioned. Many public health arguments used to justify the
relocation policy were either irrelevant or temporary by nature.
Some formerly evacuated areas have now been resettled. This is
a positive development, but probably happened much too late.
If one accepts that the first evacuations had
to be decided upon in an emergency situation, it is not clear
what other reasons could have led to the evacuation of an extra
200,000 residents months or years after the accident.
Some 100.000 people are considered as permanently
disabled as a result of the accident and 7 million people receive
compensation because of it. Today, between 5 and 7% of government
spending in Ukraine and Belarus is allocated to various Chernobyl-related
How many people are objectively entitled to
specific support and how many have obtained support from “less
acceptable channels” - simply in order to escape unbearable
poverty - remains an open question.
The effects on the environment are well-documented
and less subject to fuzzy interpretations as they are often measurable.
However, the economical or political decisions taken on the basis
of the measured data, such as the restrictions on the sale of
milk products and vegetables have taken account of many other
factors than public health alone. These were taken not only the
accident region, but also worldwide. The decision-making process
also showed how difficult it is for experts to communicate their
findings to the authorities, and for the authorities to know which
experts to listen to.
About 4300 km2 are in the no-go zone.
Another area of about 7000 km2 is considerably contaminated
by 137Cs. In Belarus, Ukraine and Russia, a further
130.000 km2 were less severely contaminated. Similar
levels also occurred in 60.000 km2 in other parts of
In the inhabited but contaminated areas, the
radiation burden due to remaining radioactivity in soil and food
is now down to less than 1 mSv/y/person. This is due, among other
reasons, to natural decay, but also to countermeasures affecting
soil contamination levels and farming methods.
In Europe, different countermeasures were taken.
Not all of them were justifiable. The fact that some radioactivity
could be measured, certainly when expressed in Bq, was interpreted
by some that danger was inevitable.
It seems that among wildlife in the most contaminated
regions, malformations occurred in the first generation of offspring,
but no obvious hereditary effects have been observed. What has
been observed, however, is flourishing biodiversity. This is to
be expected when the main predator - man - is no longer present.
One remaining problem that has potential consequences
that go far beyond strongly-contaminated areas is contamination
of groundwater and downstream water-ecosystems by 137Cs
and 90Sr. It adds to existing problems due to industrial
The regulatory impact of the accident has been
profound, both at national and international levels. The major
international actors involved since the beginning are the IAEA,
EURATOM, ICRP, NEA, WHO, WANO and others.
The Chernobyl accident gave rise to a fundamental
worldwide change in approach when it comes to safety. The world
certainly is much safer now that it was before, not only with
respect to safety of nuclear power, but also with regard to other
industrial areas - where the pioneering role of regulation in
the nuclear industry gave rise to similar initiatives in other
One perverse effect of stricter regulations is
to induce increased fear among the population. The general view
is that if something requires strong regulations it has to be
very dangerous to begin with. It is certainly correct top say
that nuclear power, air travel, even driving a car are all very
dangerous if regulations are not respected. This is what regulations
In 1972, the Club of Rome predicted that, in
the US alone, nuclear energy would supply 900.000 MW by year 2000.
The Three Mile Island and Chernobyl accidents drastically changed
all previous projections. Whereas in Western Europe and the US
the installation of new power plants came to a full stop, this
was not the case in the East.
The accidents occurred at a time of increasing
environmental awareness and changing views worldwide. The green
political movement demonised nuclear power as an evil technology.
This situation has slowly changed, due mainly to the more objective
analysis of pro's and con's, strong economic arguments put forward
by the power industry, awareness of the potentially harmful effects
of CO2 emissions and the problem of security of supply.
In 1985, President Gorbachov decided to impose
a certain degree of transparency to Russian politics. This was
called 'Glasnost' and was part of his 'perestroika' policy. Then
the Chernobyl accident occurred and the old culture of secrecy
once again took the upper hand. Under heavy pressure from the
West to provide open information on the accident, Gorbachov imposed
full glasnost, thereby annihilating one of the strongest pillars
of the Soviet regime. That regime fell apart soon after. Chernobyl
was a major catalyst in triggering the chain reaction of events
that would soon lead to the disintegration of the Soviet Union.
Twenty years have passed since Chernobyl. Twenty
years without a significant nuclear accident in a power plant.
Twenty years later, the public seems to have gradually changed
it's perception of nuclear energy, against the backdrop of what
is often referred to as 'the nuclear renaissance'. This could
be seen as evidence of the maturity of nuclear technology, of
the adequacy of the safety culture, of effective regulations etc...
. But it may also be proof of loss of memory.
We know that coal mining alone kills thousands
of people every year. We know that car accidents kill more people
during a single weekend than Chernobyl ever will. Let’s
not forget that public perception is not about cold figures, but
instead about feelings. One single major accident in a power plant
could - in a matter of minutes - ruin twenty years of considerable
1) The Report by the Chernobyl
Forum 2003-2005, second revised version, Chernobyl's Legacy: Health,
Environmental and Socio-Economoic Impacts and Recommendations
to the Governments of Belarus, the Russian Federation and Ukraine,
IAEA/PI/A.87 Rev.2/06-09181, April 2006.
The Chernobyl Forum is composed
of the IAEA, WHO, UNDP, FAO, UNEP, UN-OCHA, UNSCEAR, the World
Bank Group, Belarus, the Russian Federation and Ukraine.
2) Chernobyl, 20 years later, a report published
by SCK•CEN (available in April 2006):