Issue No.12 Spring
(April 2006)


ENS News

Chernobyl, the accident scenario and its global impact

Critical thinking

ENS Events

PIME 2006 - Chairman's opening speech

Pime 2006 - Summary

TopNux 2006

ENA 2006

RRFM 2006

TopSeal 2006

TopFuel 2006

Member Societies & Corporate Members

Three Baltic States saying “YES” to Nuclear Energy

Positive current and future outlook in Nuclear Field in Slovenia

YGN Report

International Youth Nuclear Congress 2006

PIME 2006 - ENS YGN Report

European Institutions

Nuclear Industry's response to EU Energy Green Paper

WENRA presents its Harmonisation Reports

ENS World News

A look at the Promise and Problems of Nuclear Energy

NucNet News

ENS Members

Links to ENS Member Societies

Links to ENS Corporate Members

Editorial staff

RRFM 2006RRFM 2006

RRFM 2006
30 April - 3 May 2006 in Sofia, Bulgaria

TopSeal 2006

TopSeal 2006

17.9. - 20.9.2006 Olkiluoto, Finland
TopFuel 2006

TopFuel 2006

22 - 26 October 2006 in Salamanca

























































































































































Word from the President

Chernobyl, the accident scenario and its global impact

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 emergency generators.

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 control

  • 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 learnt.

The main causes of the accident, as identified by Western experts are:

  • 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 from it

  • Fatalities estimated among rescue workers and the so called 'liquidators' who did not suffer from radiation sickness

  • 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 numbers involved.

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 and children.

Social consequences

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 compensation packages.

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 Europe.

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 pollution.

Chernobyl, the accident scenario and its global impact


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 industries.

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 are for.

Nuclear power

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 effort.


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):

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