These draft conclusions and recommendations were formulated in the weeks preceding the EC/IAEA/WHO International Conference: One Decade after Chernobyl - summing up the consequences of the accident (held in the Austria Center Vienna, 8 - 12 April 1996) on the basis of the Background Papers prepared in advance by expert panels. They were modified during the conference itself to reflect the discussions and Session Chairmen's recapitulations. They have not been edited and do not necessarily reflect the views of governments of Member States of the Sponsoring Organisations. Nevertheless after finalisation the Joint Secretariat of the Conference recommend that they be used as the factual basis for making decisions concerning future work and collaboration having the aim of alleviating the consequences of the accident.

Background

1. On 26 April 1986 the most serious accident in the history of the nuclear industry occurred at the Chernobyl nuclear power plant in the former Soviet Union, near the present borders of Ukraine, Belarus and Russia. As a result of the accident, the reactor was destroyed and, over the ensuing 10 days, large quantities of radionuclides were ejected into the environment. During these ten days, severe actions had to be taken to bring the release of radioactive material under control, to deal with the reactor debris, and subsequently to construct by November 1986 the so-called "sarcophagus" to cover the damaged reactor core.

2. The overall response to the accident was conducted by a large number of ad hoc workers, including operators of the plants, emergency volunteers such as firefighters, and military personnel, as well as many non-professional people. These people became known by the term translated from Russian - liquidators. Approximately 200,000 liquidators worked in the region of Chernobyl during the period 1986-87, when the exposures were most significant. In total some 600,000 to 800,000 persons took part in the cleanup activities to 'liquidate' the consequences of the Chernobyl accident. This figure also includes persons who participated in the clean-up after the accident (cleaning up around the reactor, construction of the sarcophagus, decontamination, building of roads, destruction and burial of contaminated buildings, forests and equipment), as well as many others, including physicians, teachers, cooks, and interpreters who worked in the 'contaminated' territories and received on average low doses.

3. Over the period 27 April to mid-August 1986, about 116,000 members of the public were evacuated to protect them against high levels of radiation. A so-called "exclusion zone" was established on the most contaminated territories, to which access was prohibited to the general public. This zone was continued into the succeeding three independent countries of Belarus, Russia and Ukraine and covers in total 4300 km².

   Releases and deposition

4. The total activity of radioactive material released is estimated today to be around 12 x 10¹⁸ Bq, including some 6 to 7 x 10¹⁸ Bq noble gases. This estimate is higher than that reported in 1986 by the authorities of the former USSR which was based on summing up the activity within the former USSR. This reassessment does not alter the estimations of individual doses. The amount released was about 3 to 4 % of the used fuel present in the reactor at the time, as well as up to 100% of noble gases and 20 - 60 % volatile radionuclides.

5. The release had a complex composition of radionuclides. The radioactive isotopes of iodine and caesium were of special radiological significance. The iodines, with their short lifetimes, had the greater radiological impact in the short term; the caesiums, with lifetimes of the order of tens of years, will have the greater radiological impact in the long term. The current estimates of the activity of the releases of these elements alone are as follows: ¹³¹I ~ 2 x 10¹⁸ Bq, ¹³⁴Cs ~0.06 x 10¹⁸ Bq and ¹³⁷Cs ~0.09 x 10¹⁸ Bq. These values represent about 50 - 60 % of the radioiodine in the reactor core at the time of the accident, and about 20 - 40 % of the radiocaesium.

6. Material released into the atmosphere dispersed and eventually deposited back on the surface of the earth, where it was measurable over the whole northern hemisphere. Most material was deposited in the regions around the plant site, with great variations in deposition density. The areas of land in the surrounding territories of Belarus, Ukraine and Russia with activity levels of ¹³⁷Cs in excess of 185 kBq/m² are estimated as 16,500 km², 4,600 km² and 8,100 km² respectively. These territories were designated as contaminated or affected areas, or "areas of strict control".

   Radiation doses

7. The 200,000 persons who participated in 1986-1987 in 'liquidation' of the accident received average doses in the order of a hundred mSv*. Around 10% of these received doses of the order of 250 mSv, a few percent received doses greater than 500 mSv, while perhaps several tens of the people who responded initially to the accident received potentially lethal doses.

8. Among the public, the 116,000 people evacuated from the exclusion zone in 1986 were among those who received the highest doses (10% received more than 50 mSv and 5% more than 100 mSv).

9. The radioiodines delivered radiation doses to the thyroid gland. Iodine, either inhaled from the initial radioactive cloud or ingested in foodstuffs, mainly contaminated milk, was absorbed into the bloodstream and accumulated in the thyroid gland. According to the measurements carried out on 150,000 people in the Ukraine and also in Belarus and Russia, the 1986 INSAG report on the accident, and according to all other international evaluations to date, doses to the thyroid were suspected to be particularly high compared with other body organs, especially those of children. Equivalent doses to the thyroid were reported to be up to several sievert or more**. It is difficult to reconstruct individual thyroid doses for the entire population.

10. Doses to the populations in various countries of the northern hemisphere caused by the accident have been assessed by UNSCEAR, including average doses to various countries. This committee estimated that the individual doses outside the former Soviet Union due to the accident have been as follows:

    - the highest national average first year dose was 0.8 mSv;

    - the highest regional average committed dose over the 70 years to 2056 was 1.2mSv

11. The International Chernobyl Project, which was carried out in 1990 in order to determine the safety of continued living on contaminated territories, estimated that a committed dose (for the seventy years from 1986 to 2056) for people living in the most contaminated territories is around 160 mSv. Recent more detailed studies have produced similar results. For the time period from 1996 to 2056, the committed doses to the population living in the areas with a contamination density of 185 - 555 kBq/m² will be, with some exceptions, of the order of 5 to 20 mSv; for the population living in the areas with a contamination density of 555 - 1480 kBq/m² the doses in this period will be of the order of 20 to 50 mSv.

    Clinically observed effects

12. The Chernobyl accident resulted in a total number of 237 individuals who were suspected of suffering from acute radiation sickness (ARS). Of these, 28 died due to radiation exposure. Two more individuals died due to non-radiation causes at the accident site, bringing the total to 30 deaths. One additional death was thought to have been due to a coronary thrombosis.

13. The diagnosis of ARS was confirmed in 134 cases of the 237 people hospitalized. Of these 134, gastrointestinal damage was a severe problem in 11 patients who received doses greater than 10 Gy, and resulted in early and lethal changes in intestinal function. Deaths in 26 of the 28 patients who died in the first 3 months after exposure, were associated with skin lesions involving over 50% of the total body surface area.

14. After this acute phase, 14 additional patients have died over the last ten years. Their deaths do not correlate with the original severity of ARS and may therefore not be directly attributable to the radiation exposure.

15. There is little doubt that the ARS patients, also those with severe skin injury, have received the best possible treatment in line with the state of knowledge at the time in the most experienced centre available. The therapy of bone marrow transplantation recommended at the time was of little benefit. From today's knowledge this is very well understandable, in view of the intrinsic immunological risks of the procedure, the heterogeneous exposure characteristic for the accident situation and the complicating other injury, such as unmanageable intestinal radiation damage or skin lesions. The bone marrow damage can in future cases best be managed by rapid administration of haemopoietic growth factors, of which the most optimal combination and dose scheduling, however, still needs to be worked out. Also for other damage, new diagnostic tools have become available which may contribute to a more accurate prognosis and more tailored treatment.

16. There is good evidence that the quality of life of the surviving patients may be amenable to improvement. At least the more severely affected patients suffer presently from multiple ailments and are in need of up to date treatment and secondary prevention; also their mental health might be affected. More has to be done in the future to distinguish among the disease patterns encountered between those attributable to the radiation exposure and those due to confounding factors intrinsic to the population. The follow-up of these patients needs to be assured for the forthcoming two to three decades.

    Thyroid effects

17. Ten years after the Chernobyl accident, the highly significant increase in thyroid cancer in those exposed as children in the three most affected countries is the only evidence to date of a public health impact of radiation exposure as a result of the accident.

18. This great increase in childhood thyroid cancer has been observed in Belarus and Ukraine and to a lesser extent in the Russian Federation following the Chernobyl accident. The number of reported cases up to the end of 1995 are about 800 in children between 0-15 years old at the time of diagnosis. More than 400 of these cases are observed in Belarus. The diagnosis has been confirmed in most cases by international experts.

19. The increase has been observed only in children who were born or had already been conceived before the time of the accident. The incidence of thyroid cancer in children born more than 6 months after the accident drops dramatically to the low levels expected in unexposed populations. Most of the thyroid cancer cases are concentrated in areas that have been heavily affected by radioactive contamination released by the damaged reactor. Thus both temporal and geographical distribution clearly indicate the relationship to radiation exposure due to the Chernobyl accident.

20. Analyses by cohort at age of exposure confirmed the hypothesis that very young children were at the greatest risk. It is now believed that there may be a continuing increase in the incidence of thyroid cancer particularly in those exposed as young children. This could make thyroid cancer relatively common in the three republics in the future, requiring major resources to cope with the increase.

21. Radioiodine was one of the major radioactive components released by the reactor. The fact that the thyroid gland concentrates iodine supports the concept that one or more radioactive isotopes of iodine are the causative agents.

22. The latency period between the accident and the diagnosis at present ranges between 4 and 10 years, with a mean of about 6 years. This latency period is a little earlier than expected on the basis of previous experience related to acute exposure to external radiation.

23. At presentation the majority of the tumours were in an advanced stage showing extension to tissues outside the thyroid gland and/or lymph node metastases and less frequently distant metastases. This finding is strong evidence that the observed increase cannot be attributed solely to increased ascertainment due to screening.

24. The pathology of virtually all the cases shows papillary carcinomas, many with an unusual solid/follicular pattern of growth. The molecular biology of the cases so far studied has not shown any major differences from tumours of the same type in thyroids not exposed to radiation.

25. So far a very small number of children (three) have died of this disease. Although only short term follow-up data are available at present, these post-Chernobyl papillary thyroid cancers in children, in spite of their aggressiveness, appear to respond favorably to standard therapeutic procedures if appropriately applied. This emphasizes the need of an accurate and continuing follow-up of the affected children in order to establish adequate therapy. Life-long administration of L-thyroxine to these children is mandatory after thyroidectomy.

26. Iodine prophylaxis through the distribution of pharmacological doses of iodine is a recognized measure for reducing the thyroid exposure to radioiodine and should be applied under strictly defined conditions to populations at risk in the unfortunate case of a future accident. In any case, correction of iodine deficiency prevalent in the affected areas is recommended through the consumption of iodized salt in food.

    Longer term health effects

27. Apart from thyroid cancer in young people, there have been some reports of increases in the incidence of specific malignancies and possibly of acceleration of appearance of cancer among liquidators and some populations living in contaminated territories. These reports, however, are not consistent and could reflect differences in follow-up of exposed populations and increased ascertainment following the Chernobyl accident. These should be investigated further. In particular, to date no consistent attributable increase has been detected in the rate of leukaemia, one of the major concerns after radiation exposure. Leukaemia is a rare disease, and the estimated numbers of radiation induced leukemia deaths according to predictive models (based on data from the Japanese atomic bomb survivors and others) are small: of the order of 200 among the 3.7 million residents of the contaminated territories and 200 among the 200,000 liquidators (who worked in 1986-87). According to current models, 150 of these 200 excess leukemia cases among the liquidators, would have been expected to have been seen in the first ten years.

28. Increases in the frequency of a number of non-specific detrimental health effects other than cancer among exposed populations, particularly among liquidators, have been reported. It is difficult to interpret these findings because exposed populations undergo a much more intensive and active health follow-up than the general population.

29. Existing population based cancer and mortality registries should be strengthened, or, where appropriate, setup. In addition, specific studies to investigate the reported and predicted increases, particularly in leukemia and thyroid cancer among liquidators, should be carried out. This should be done using carefully designed protocols applied uniformly to analyse and possibly separate confounding factors.

    Other health related effects: psychological consequences, stress, anxiety etc.

30. Several important studies and programmes have been conducted during the past ten years in the area of social and psychological effects and reactions to the Chernobyl accident.

31. There are significant non-radiation-related health disorders and symptoms, such as anxiety, depression and various psychosomatic disorders attributable to mental stress among the population in the region. Psychosocial effects, unrelated to radiation exposure, resulted from the lack of information immediately after the accident, the stress and trauma of compulsory relocation, the breaking of social ties, and the fear that radiation exposure is damaging and could damage their and their children's health in the future. It is understandable that a population who had not been told the complete truth for several years after the accident continue to mistrust official statements and instead feel that all kinds of illnesses that are now increased must be due to radiation. This misperception of radiation risks and the resulting distress are extremely damaging to the people.

32. The highly politicized handling of the accident s consequences has led to psycho-social effects among the population that are extensive, serious and long-lasting. Severe effects include a feeling of helplessness and despair leading to social withdrawal, and little hope for a positive future. The effects are being prolonged by the protracted debate over radiation risks, necessary countermeasures and general social policy, and also by the appearance of the thyroid cancers attributed to the early exposures. It is extremely difficult to distinguish these effects from effects associated with the collapse of the USSR and economic hardship.

33. There is an urgent need to develop trust the personal ability to influence one's life situation and to induce positive change; to encourage small-scale and communal projects aiming at positive development of the local situation, and support organisations promoting rehabilitation of the population concerned; to increase public knowledge of radiation and protective behaviour; and to develop, integrate and sustain existing networks involving local authorities, specialists and researchers in the social and psychological field of activities.

    Environment

34. In the first few weeks after the accident, lethal doses were reached for some radiosensitive local ecosystems, notably for coniferous trees and for some small mammals within 10 km of the reactor site. By the autumn of 1986 dose rates had fallen by a factor of 100. By 1989 the natural environment in these localities had begun to recover. No dramatically obvious long term impacts on populations or ecosystems have been observed. However, possible long term genetic impacts remain to be studied.

35. The main pathways to man resulting from the environmental releases are from external irradiation from the activities deposited on the ground and internal irradiation from the contamination of foodstuffs. In the first few weeks after the accident, radioiodines were the most important radionuclides to consider. Since 1987, most of the dose is due to Cs-137, with a minor contribution from Sr-90, while Pu-239 plays a very small role. Countermeasures are relatively inefficient to reduce external exposures but can be very efficient to reduce internal exposures.

36. Several items of the normal diet were contaminated by radioactive materials. Early after the accident, key foodstuffs such as milk and green vegetables had contamination levels in excess of what is today considered acceptable by the WHO/FAO Codex Alimentarius Commission. These levels are now globally established through the International Basic Safety Standards for Protection against Ionizing Radiation and for the Safety of Radiation Sources. There are some questions as to the effectiveness of controls in the early stage of the accident.

37. In the long term, effective application of agricultural countermeasures can result in significant reduction in the uptake of caesium into food. These countermeasures strongly depend on local conditions, such as soil type. For example, in some localities where the caesium deposition on the ground is relatively small, the transfer to milk can be very high. In general, no food produced by the collective farm system exceeds the WHO/FAO Codex Alimentarius levels today, although some foods produced by private farmers do exceed these levels.

38. The seminatural environment, i.e. with characteristics intermediate between agricultural land that is managed and wholly natural environment, may have a dominant influence on the future doses to the human population.

39. Depending on the type of soil, the transfer from soil to milk of cows grazing on meadows varies by several hundred. Food products from animals that graze in seminatural pastures, forests and mountain areas and wild foods (game, berries, mushrooms) gathered by the population will continue to show high Cs-137 levels over the next decades and are likely to be an important source of future internal doses.

40. Concerning buried radioactive materials in the exclusion zone, local dose rates can be considerable. Furthermore, without orderly management of the provisional depositories, in the long term there is a considerable risk to local groundwater.

    Social, economic, institutional and political impact

41. Between 1990 and the end of 1995, owing to ever-increasing societal pressure and the political situation following clarification of the radiological situation, there was further resettling of people in Ukraine (about 53 000 persons), Belarus (about 107 000 persons) and Russia (about 50 000 persons). Evacuation and resettling has created a series of serious social problems, linked to the hardships of adjusting to the new living conditions.

42. The demographic situation in the contaminated regions has worsened: the birth rate has decreased; the work force is migrating from 'contaminated' areas to the 'clean' ones, creating labour and professional shortages. The social and economic status of people living and working in the contaminated territories is heavily dependent on state subsidies.

43. The controls imposed to limit radiation exposure in the contaminated territories impede normal industry and agriculture. The attitude of the general population towards products produced in these areas make it difficult for produce to be sold or exported, leading to reductions in the local population's income. Restrictions on people's lifestyle counter to normal traditions make everyday life extremely difficult and distressing.

44. Despite the major rehabilitation actions undertaken over the last years, the majority of the population considers that the information concerning the current radiation situation and of the radionuclide concentrations measured in foodstuffs is insufficient.

45. It is expected from the results of radioecological monitoring that the compensation system in force could be reconsidered; then some of the funds could be reoriented towards new industrial and agricultural projects.

46. The Chernobyl accident's consequences, aggravated by the political, economic and social changes of the past years, have led to a worsening in people's quality of life and health, and to negative impacts on social activity. The situation has been further complicated by the spreading of incomplete and distorted information on the accident consequences and measures for their alleviation (during the first years).

    Nuclear Safety

47. In accordance with a dynamic approach to safety all nuclear power plants that do not meet an internationally acceptable level of safety need appropriate upgrading or should be shut down. In 1991, the IAEA conference The Safety of Nuclear Power: Strategy for the Future formulated a consensus that "safety standards of older operating plants should be reasonably compliant with current safety objectives". Active commitment to this objective remains of prime importance for ensuring an acceptable level of safety for nuclear installations and for increasing public confidence in nuclear energy.

48. The main causes of the Chernobyl accident was the coincidence of severe deficiencies in the reactor physical design and in the design of the shut-down system and the violation of procedures. The lack of 'safety culture' in the responsible organisations of the Soviet Union led to the inability to remedy such design weaknesses, even though they had been known before the accident.

49. In addition to those features of direct relevance for the causes of the accident, the original design of RBMK plants was affected by further deficiencies. In particular the original design of the first RBMK generation falls short of current safety objectives. Remaining deficiencies such as the partial containment concept require further attention.

50. A significant number of nuclear safety remedial measures were undertaken during the past decade at the existing RBMK plants: technical and organisational measures taken immediately after the accident, as well as safety upgrading performed between 1987 and 1991 essentially removing the design deficiencies contributing to the accident. Progress has also been achieved in areas such as plant management, training of personnel, non-destructive testing, and safety analysis. As a result, a repetition of the same accident scenario seems practically no longer possible today. However other accidents with substantial releases can not be excluded.

51. Some of the concerns regarding safety might also apply to other reactors designed to earlier standards if no sufficient improvements have been performed in the meantime. The importance of recurrent safety reviews is widely recognized in this regard.

52. For all RBMK plants there exist plans for further safety upgrading regarding those RBMK design deficiencies that are not directly related to the Chernobyl accident. The implementation of these plans is essentially behind the needs because the new independent states lack the necessary resources due to their tremendous economic problems.

53. Accelerated implementation of what is agreed necessary and has already been planned has been identified as a top priority for the national nuclear programmes as well as for international co-operation:

    - necessary safety improvements must be carried out independently from consideration of early decommissioning of the plants;

    - more resources must be made available for the safety of the RBMK plants currently operated;

    - strengthening the status of national regulatory authorities and their support organisations.

54. Similar backfits as for other RBMK units were also performed at the Chernobyl NPP. However, safety concerns with these units are not only related to the remaining generic design deficiencies, e.g. to the quality of equipment.

55. The Ukrainian decision to shut down the remaining units should not result in neglecting safety needs and backfits during the remaining time of operation.

    Sarcophagus

56. The sarcophagus around the destroyed reactor has met its protective objectives for ten years now. In the long term however its stability and the quality of its confinement are questionable. A collapse could lead to a release of radioactive dust with significant exposure of the personnel employed at the site. About 200 tonnes of irradiated and fresh nuclear fuel are in the "Sarcophagus" now, mixed with building materials etc. mainly in the form of dust. The total activity is estimated to be 700 x 1015 Bq of long-lived radionuclides. Even in the worst case, widespread effects (outside the 30 km zone) are not expected.

57. It has been found that the sarcophagus is currently safe from a criticality point of view. However, it cannot be completely excluded that here exists configurations of fuel masses inside the sarcophagus that could reach a critical state when in contact with water. However, even if this could lead to significant radiation levels inside the sarcophagus, large off-site releases are not expected. The impact of such a state on site personnel needs clarifying.

58. There are very different opinions about the significance of the risk of an accident in unit 3 caused by a collapse of the sarcophagus. More detailed investigations on this issue are required.

59. The safety of the remaining units and the stability of the sarcophagus are not the only important problems to be solved at the Chernobyl site. Further concerns relate to the contamination, in particular to the radioactive material buried at the site. All these issues are closely interrelated and an integrated concept is therefore required to solve them. The proposed construction of a second shelter should be part of that approach. The actions financed by the European Commission in this area contribute to achieving this objective. However, the approach needs to be generalised and should integrate more effectively the know-how of the competent organisations of the former USSR.

60. A cost-effective procedure requires suitable steps according to the progress of investigations and the financial conditions. The first step should be the stabilisation of the existing sarcophagus. This step would significantly reduce the risk of a collapse of the shelter and thus provide the time required for careful planning of further measures (e.g. a second shelter).

    Perspective and future prognosis

61. Complete rehabilitation of the 30km zone is currently not possible owing to the following: the existence of 'hot spots' of contamination very near residential areas; possible local radioactive contamination of ground water; hazard associated with the possible collapse of the Sarcophagus; severe restrictions imposed on diet and lifestyle.

62. Any estimates of the total number of fatal and non-fatal cancers (in the case of thyroid) attributable to the accident should be interpreted with caution recognizing the uncertainties involved with the assumptions made. Such projection, however, provide the perspectives on the magnitude of the problem (long-term health effects) and help to identify areas of special attention at the present time (e.g. leukaemia in the liquidators and thyroid cancer among people who were children at the time of the accident) as well as in the future.

63. There is a major discrepancy between the number of thyroid cancers appearing in those who were children at the time of the accident and the number of cancers that are predicted based on standard thyroid dosimetry and current risk projection models. This difference may be the result of several factors unique to the accident which are not typically incorporated into standard models. It is important to clarify these issues as well as to continue the thyroid tumour detection programmes.

64. There will most probably be an increase in the incidence of thyroid cancer that will persist for several decades. While it is not possible to predict with certainty on the basis of current data, the estimated number of thyroid cancers among those who were children in 1986 is in the range of a few thousands. The number of fatalities should be much lower than this number if cancer is diagnosed in the early stage and appropriate treatment is given. This group should therefore continue to be closely monitored throughout their lives.

65. Many factors, such as economic hardship, are having a marked effect on the health of the population in general, including the various groups exposed following the accident. Examination of the statistics for the exposed population in neglect of the clear general increase in morbidity and mortality in the former Soviet Union can lead to the misinterpretation that the trends seen were due to the accident.

66. The public perception of the present and future impact of the accident may have been exaggerated by the difficult socioeconomic circumstances in the USSR at the time, by the countermeasures that the authorities took to minimise the accident's impact, and by the public's impression of the risks from the continuing levels of radioactive contamination.

67. Past experience of accidents unrelated to radiation has shown that the psychological impact may persist for a long period. In fact, ten years after the Chernobyl accident, the evolution of symptoms has not ended. It can be expected that the importance of this effect will decrease with time. However, the continuing debate over radiation risks and countermeasures, combined with the fact that effects of the early exposures are now being seen (i.e. the significant rise in thyroid cancers among children exposed in 1986), may prolong the symptoms. The psychological impact cannot be completely dissociated from that of the breakup of the USSR, and any forecast should therefore take into account the economic, political and sociological circumstances of the three countries. The symptoms such as anxiety associated with mental stress may be among the most important legacies of the accident.

68. In view of the low risk associated with the present radiation levels in many of the contaminated areas, the benefits of future efforts to reduce doses still further to the public would be outweighed by the negative psychological and economic impacts. All potential impacts of future measures to be taken in response to the accident should be taken into account. In addition, measures to mitigate the psychological impact should be considered.

69. It is important to develop strategies that take into account both the real radiological risk and the economic, social and psychological impacts in order to yield the greatest benefit in human terms.