Topical Session 1: Clinically observed effects

The Clinically Observed Effects of the Chernobyl Accident in 1986 are as follows:

• Total number of patients treated for Acute Radiation Syndrome following the accident - 237

• The number of patients who survived the acute phase of the syndrome - 209

• Number of persons who died in the acute phase of radiation syndrome (in 1986) - 28

Following the acute phase, 14 additional patients have died in the ten years following the accident. Their deaths, however, do not correlate with the original severity of the acute radiation syndrome and may not be directly attributable to the radiation exposure at the time of the accident.

There is evidence that the quality of life of the surviving patients may need improvement. The more severely affected patients suffer from multiple ailments and are still in need of treatment. Their mental health is less than optimal. Conclusions of the session are:

1. Medical care and follow-up should be provided to the survivors for many years (two to three decades) from qualified medical centres.

2. Research efforts, in the event of possible radiation emergencies, should focus on the emergency care needed for victims within the first two days of any radiation accident.

Topical Session 2: Thyroid effects

Thyroid carcinoma is an extremely rare disease. In the United Kingdom, the incidence is 0.5 per million children per year; in other countries it may reach one per million, three per million, and very occasionally 6 per million.

In the case of southern Belarus and northern Ukraine since 1990 there have been substantial increases in the number of cases which prior to the Chernobyl accident are believed to have been comparable to the UK figure. In Gomel Oblast, the rate has reached 200 times the British figure.

Gomel is thus currently showing over one hundred cases per million children per year whilst the rate for Belarus as a whole is 14.6 per million.

Thyroid cancer of the papillary type known to be radiation-related and which attacks the muscles of the thyroid represents 94 to 98 per cent of the cases seen in Belarus and Ukraine, compared to 30 percent in the United Kingdom. (Other types of thyroid cancer which are relatively rare are follicular and madullary, the latter not induced by an uptake of radioiodine.)

Uptake of radioiodine from Chernobyl appears to have occurred in children starting as of three months into their mothers' pregnancy to approximately three years of age at the time of the accident. Plotting the incidence of cases (some 500 to 600 in total so far) indicates that vulnerability to contracting cancers after the age of three at the time of the accident falls off rapidly. Geographic plotting correlates closely with the known distribution of fallout. It can therefore be reasonably concluded, given the geographical spread and the age correlation, that the great majority of the thyroid cancer cases in children showing this steep increase are due with virtual certainty to the uptake of radioiodine from the Chernobyl fallout. It can also be assumed, although more study is necessary, that children in the relevant age group will continue to carry the risk of developing thyroid cancers for several decades.

The only mitigating circumstance is that thyroid cancer has a low mortality rate following appropriate treatment (normally surgery followed by medication such as thyroxine).

Topical Session 3: Estimated long term health effects of the Chernobyl accident

Besides the dramatic thyroid cancer in persons exposed when they were children, no evidence of a major public health impact is in evidence as a result of the radiation exposure in the Chernobyl accident in the three affected countries (Belarus, Russia or the Ukraine). It is believed that the major radiological impact of the accident will be greatest in the group known as the "liquidators" (those who worked at the Chernobyl plant and helped with the clean up activities following the accident).

The conclusions reached in the session are that continued studies are needed in the future and they include:

1. Passive monitoring of the radiation exposed populations (in the form of a population registry) from the Chernobyl accident to prove data about disease patterns.

2. Focused studies of selected populations with exposures in the low to medium radiation dose range.

3. Studies of radiation-induced cancer risks through studies of the thyroid cancers occurring in the young people. Although the studies may not benefit today's patients from the Chernobyl accident, they will be valuable for protection of future populations with possible radiation exposure.

Topical Session 4: Other health-related effects: psychological consequences, stress, anxiety

Widespread public anxiety and pessimism about the Chernobyl accident among hundreds of thousands of people in the affected areas appears to be out of all proportion to the verifiable radiation induced health effects. This stress is nonetheless very real and has caused widespread damage to the general health and wellbeing of the population.

A large proportion of the relevant inhabitants - whether evacuated or not - complain of ailments they believe to be due to radiation exposure. The level of general health is in any case low, and radiation fears are compounded by poor public understanding of radiation; initial secrecy; subsequent lack of effective communication; and the collapse of the former centralized political and economic systems. Distrust of 'authorities' is widespread. It may be in this comparatively low-cost field of better communication that more needs to be done to help offset what is probably the most pervasive after-effect of the 1986 accident.

Topical Session 5: Consequences for the Environment

Studies of the direct impacts of radiation on the natural and human environments surrounding the Chernobyl accident indicate that: 1. At high radiation levels, surrounding the site of the accident, the natural environment showed short term impacts but any long term impacts remain to be seen and require further study; 2. Countermeasures are effective when taken to reduce the transfer of contamination from the environment to humans. These measures produce some waste, reduce some of the radiation dose and are very site specific; their practicality must be considered; 3. The majority of lifetime radiation doses have been already received by the human population in the years since the accident.

Future research is recommended to improve the ability to assess the present and future doses to the affected populations and to apply effective countermeasures. The work should include: monitoring of migration of groundwater around the small radioactive waste burial sites to protect drinking water supplies for future; research on radiation dose estimates for internal human doses based on diet measurements which differ from whole body count doses; research on whether food processing of wild foods (mushrooms, etc.) can control contamination levels enough to allow restrictions to be removed for human consumption; research on whether natural and seminatural (forrests and meadows) can be completely rehabilitated during the next generation; research on whether work on cost effectiveness of countermeasures and long-term intervention is needed for the use of decision makers in the future.

Topical Session 7: Nuclear safety remedial measures

Remedial measures taken immediately after the Chernobyl accident included a ban on experiments, disabling reactor protection systems or installation in the control room of any extra controls for reactor equipment; instructing staff to strictly adhere to regulations on operating reactivity margin and coolant/feedwater flow rate ration; and setting stricter requirements on procedures for restart of the reactor after shutdown.

Since then, the following safety concerns have been addressed: design features of the reactor and shutdown systems relevant for the control of reactivity; core cooling of first generation of RBMKs; consequences of a possible multiple tube rupture; and protection against fires and flooding.

Void reactivity has been reduced by installing 80-90 additional absorbers; an increase in operative reactivity margin up to 43- 45 manual control rods; and adjustment of fuel enrichment to 2.4%. In addition to increasing the speed of control rod insertion, a new fast acting shutdown system is being added.

As regards remaining problems for RBMKs, it must be recognized that sound implementation of defence in depth is vital. This means that RBMK safety concepts need to ensure appropriate redundancy, diversity, functional segregation and quality of systems assuring the basic safety function under accident conditions. Quality assurance, in particular of the material properties of pressure tubes, needs due attention.

As regards the sarcophagus, the most important issues are the risk of the release of radioactive dust caused by the possible collapse of structural elements; contamination of the environment by water from the shelter; and possible recriticality of masses containing fuel material.

In the short term the stability of the sarcophagus needs to be reinforced while measures are pursued on how to construct a second sarcophagus in the medium and long term.