Chernobyl - Appendices
March 2001
The health effeccts of the Chernobyl accident have been the subject of unprecedented study by health professionals and unprecedented speculation and exaggeration by parts of the media.
|
Chernobyl Accident: Simplified sequence of events |
|
The sequence of events which follows has been compiled following a review of a large number of reports and it represents what is considered the most likely sequence of events, but there remain some uncertainties. |
|
April 25: Prelude | |
|
01:06 |
The scheduled shutdown of the reactor started. Gradual lowering of the power level began |
|
03:47 |
Lowering of reactor power halted at 1600 MW(thermal). |
|
14:00 |
The emergency core cooling system (ECCS) was isolated (part of the test procedure) to prevent it from interrupting the test later. |
|
The fact that the ECCS was isolated did not contribute to the accident; however, had it been available it might have reduced the impact slightly. | |
|
14:00 |
The power was due to be lowered further; however, the controller of the electricity grid in Kiev requested the reactor operator to keep supplying electricity to enable demand to be met. Consequently, the reactor power level was maintained at 1600 MW(t) and the experiment was delayed. |
|
Without this delay, the test would have been conducted during `day shift'. | |
|
23:10 |
Power reduction recommenced. |
|
24:00 |
Shift change. |
|
April 26: Preparation for the test | |
|
00:05 |
Power level had been decreased to 720 MW(t) and continued to be reduced. |
|
It is now recognised that the safe operating level for a pre-accident configuration RBMK was about 700 Mwt because of the positive void coefficient. | |
|
00:28 |
Power level was now 500 MW(t). |
|
Control was transferred from the local to the automatic regulating system. Either the operator failed to give the `hold power at required level' signal or the regulating system failed to respond to this signal. This led to an unexpected fall in power, which rapidly dropped to 30 MW(t). | |
|
00:32 |
(approximate time). In response, the operator retracted a number of control rods in an attempt to restore the power level. |
|
Station safety procedures required that approval of the chief engineer be obtained to operate the reactor with fewer than the effective equivalent of 26 control rods. It is estimated that there were less than this number remaining in the reactor at this time. | |
|
01:00 |
The reactor power had risen to 200 MW(t). |
|
01:03 |
An additional pump was switched into the left hand cooling circuit in order to increase the water flow to the core (part of the test procedure). |
|
01:07 |
An additional pump was switched into the right hand cooling circuit (part of the test procedure). |
|
Operation of additional pumps removed heat from the core more quickly. This reduced the water level in the steam separator. | |
|
01:15 |
Automatic trip systems to the steam separator were deactivated by the operator to permit continued operation of the reactor. |
|
01:18 |
Operator increased feed water flow in an attempt to address the problems in the cooling system. |
|
01:19 |
Some manual control rods withdrawn to increase power and raise the temperature and pressure in the steam separator. |
|
Operating policy required that a minimum effective equivalent of 15 manual control rods be inserted in the reactor at all times. At this point it is likely that the number of manual rods was reduced to less than this (probably eight). However, automatic control rods were in place, thereby increasing the total number. | |
|
01:21:40 |
Feed water flow rate reduced to below normal by the operator to stabilise steam separator water level, decreasing heat removal from the core. |
|
01:22:10 |
Spontaneous generation of steam in the core began. |
|
01:22:45 |
Indications received by the operator, although abnormal, gave the appearance that the reactor was stable. |
|
The test | |
|
01:23:04 |
Turbine feed valves closed to start turbine coasting. This was the beginning of the actual test. |
|
01:23:10 |
Automatic control rods withdrawn from the core. An approximately 10 second withdrawal was the normal response to compensate for a decrease in the reactivity following the closing of the turbine feed valves. |
|
Usually this decrease is caused by an increase in pressure in the cooling system and a consequent decrease in the quantity of steam in the core. The expected decrease in steam quantity did not occurdue to reduced feedwater to the core. | |
|
01:23:21 |
Steam generation increased to a point where, owing to the reactor's positive void coefficient, a further increase of steam generation would lead to a rapid increase in power. |
|
01:23:35 |
Steam in the core begins to increase uncontrollably. |
|
01:23:40 |
The emergency button (AZ-5) was pressed by the operator. Control rods started to enter the core. |
|
The insertion of the rods from the top concentrated all of the reactivity in the bottom of the core. | |
|
01:23:44 |
Reactor power rose to a peak of about 100 times the design value. |
|
01:23:45 |
Fuel pellets started to shatter, reacting with the cooling water to produce a pulse of high pressure in the fuel channels. |
|
01:23:49 |
Fuel channels ruptured. |
|
01:24 |
Two explosions occurred. One was a steam explosion; the other resulted from the expansion of fuel vapour. |
|
The explosions lifted the pile cap, allowing the entry of air. The air reacted with the graphite moderator blocks to form carbon monoxide. This flammable gas ignited and a reactor fire resulted. | |
|
Thereafter, over nine days: | |
|
Some 8 of the 140 tonnes of fuel, which contained plutonium and other highly radioactive materials (fission products), were ejected from the reactor along with a portion of the graphite moderator, which was also radioactive. These materials were scattered around the site. In addition, caesium and iodine vapours were released both by the explosion and during the subsequent fire. | |
Abstract
Apart from the dramatic increase in thyroid cancer in those exposed as children, there is no evidence to date (1996) of a major public health impact as a result of radiation exposure due to the Chernobyl accident in the three most affected countries (Belarus, Russia and Ukraine).
Although some increases in the frequency of cancer in exposed populations have been reported, these results are difficult to interpret, mainly because of differences in the intensity and method of follow-up between exposed populations and the general population with which they are compared. If the experience of the survivors of the atomic bombing of Japan and of other exposed populations is applicable, the major radiological impact of the accident will be cases of cancer. The total lifetime numbers of excess cancers will be greatest among the 'liquidators' (emergency and recovery workers) and among the residents of 'contaminated' territories, of the order of 2000 to 4600 among each group (the size of the exposed populations is 200,000 liquidators and 6,800,000 residents of 'contaminated' areas). These increases would be difficult to detect epidemiologically against an expected background number of 41,500 and 800,000 cases of cancer respectively among the two groups.
However, the exposures for populations due to the Chernobyl accident are different (in type and pattern) from those of the survivors of the atomic bombing of Japan (and doses received early after the accident are not well known). Predictions derived from studies of these populations are therefore uncertain. Indeed, although an increase in the incidence of thyroid cancer in persons exposed as children as a result of the Chernobyl accident was envisaged, the extent of the increase was not foreseen.
Only ten years have passed since the accident. It is essential, therefore, that monitoring of the health of the population be continued in order to assess the public health impact of the accident, even if any increase in the incidence of cancers as a result of radiation exposure due to the Chernobyl accident, except for leukaemia among liquidators and thyroid cancer, is expected to be difficult to detect. Studies of selected populations and diseases are also needed in order to study observed or predicted effects; careful studies may in particular provide important information on the effect of exposure rate and exposure type in the low to medium dose range and on factors which may modify radiation effects. As such, they may have important consequences for the radiation protection of patients and the general population in the event of any future accidental exposure.
Background
paper # 3 from:
One Decade After Chernobyl - Summing up
the consequences of the accident, Proceedings of
international conference, Vienna, April 1996, sponsored by EU,
IAEA & WHO. The summary
of the results of this joint conference is available on
the web.
The World Health Organisation (WHO) issued a summary of the main findings of the November 1995 international conference in Geneva on the health consequences of the Chernobyl disaster, and called for more research in the future.
Speaking after the conference ended, Dr Wilfried Kreisel, the WHO's Executive Director in charge of Health and Environment, said: "The legacy will stay with us for a long time in the shape of radiation-induced diseases and psycho-somatic disorders. We shall be doing a disservice if we fail to extract benefits for mankind out of this monumental human tragedy. If history is not to repeat itself, we should learn very well the lessons of Chernobyl."
Dr Kreisel said one of the conference's main achievements had been to establish scientific consensus on the known health consequences of the accident. He dismissed as "fiction" claims by Ukrainian officials earlier in 1995 that more than 100,000 people had died as a result of the accident, saying the proven death toll so far was about 40. He said 30 of those deaths were from direct exposure at the time, and there had been some 10 fatal cases to date of radiation-induced thyroid cancer.
As regards
the conference's main findings, the WHO statement
reads:
"Scientists attending the conference have identified
three main areas of concern: the large increase in
psychological disorders, especially among accident recovery
workers and people living in the highly contaminated areas;
the health impact of the thyroid cancer incidence among
children; and future cancers which could occur in people,
particularly leukaemia, breast cancer, bladder cancer and
kidney diseases.
"There is a definite increase in thyroid cancer, mostly in children but also in adolescents, following the Chernobyl accident, with nearly 400 cases in Belarus, 220 cases in the Ukraine and 62 cases in the Russian Federation. These cancers are extremely aggressive and locally invasive. There is strong circumstantial evidence that an increase in thyroid cancers in these three countries is due to radioactive fall-out which followed the Chernobyl accident. This is demonstrated by:
"Although, so far, there has been no statistically measurable increase in leukaemia and other similar blood disorders, scientists are warning that the peak may occur within the next few years, accompanied by greater incidence in breast cancer, cancer of the bladder and kidney diseases. Preliminary reports on the health status of accident recovery workers indicate that the increase is beginning to show.
"The participants at the conference concluded that psycho-social effects is a priority area, which should be addressed in a much more serious way both by the three affected states and by the international community. Similarities in behaviour patterns on the part of the victims and unaffected populations were observed among the Japanese bomb survivors and those affected by the Chernobyl fallout. In Japan, bomb survivors were discriminated against by prospective employers because they might contract cancer at a later time. Similarly, people evacuated from contaminated areas to clear areas after the Chernobyl accident were shunned by local residents because the evacuees were provided with new houses and pensions.
"Evacuation and relocation of large groups of people after the Chernobyl accident, as well as constant concern and fear of radiation exposure have resulted in a rising number of health disorders being reported to local outpatient clinics. Studies show that headaches, chest pains, intestinal disorders, sleep disturbance, loss of concentration and alcohol abuse are common. This is clearly a priority area not only for the governments and public health services of the three affected states, but for the international community as a whole. Similar psycho-social impact can be observed in the wake of earthquakes, fires, floods and other natural and man-made disasters."
See also WHO booklet Health Consequences of the Chernobyl Accident: Summary Report, issued to coincide with the conference.
Source: NucNet Background #18/95.
Uranium Information Centre Ltd GPO Box 1649N, Melbourne 3001, Australia