Ishiguro, W. Shimazu, and H. Kafeel and A. Turan, Axi-symmetric simulation of a two phase vertical thermosyphon using Eulerian two-fluid methodology, Heat and Mass Transfer, vol. Tung, R. Johnson, C. Chieng, and Y. Alizadehdakhel, M. Rahimi, and A. Ye, M. Zheng, M. Wang, R. Zhang, and Z. Xiong, The design and simulation of a new spent fuel pool passive cooling system, Annals of Nuclear Energy, vol. All Rights Reserved. Log In. Paper Titles. Article Preview.
Abstract: Heat pipe is considered being used as a passive system to remove residual heat that generated from reactor core when incident occur or from spent fuel pool. Add to Cart. Applied Mechanics and Materials Volume Main Theme:. Edited by:. Online since:. January Cited by. Related Articles. Paper Title Pages. Authors: S. Jeong, K. Abstract: In order to utilize low enthalpy geothermal heat sources, a thermosyphon is a good device which can extract heat without using electric power.
The heat transfer in the thermosyphon occurs through the circulation of a working fluid through a sequence of evaporation, vapor transfer, condensation, and liquid return. A two-phase thermosyphon system using carbon dioxide CO2 as a working fluid has been investigated both experimentally as well as theoretically. Carbon dioxide is the only non-flammable and non-toxic fluid that has the potential to offer environmental safety in a system.
A copper tube thermosyphon of total length of 1, mm with inside and outside diameters of 9. The temperature distribution along the thermosyphon was monitored and theninput heat to evaporator section and output heat from condenser were measured as well. The effects of temperature difference between evaporator and condenser section and coolant mass flow rates on the performance of the thermosyphon were determined.
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The results indicate that the heat flux transferred increased with increasing coolant mass flow rate and temperature difference between evaporation and condenser section. The experimental analysis of the thermosyphon system confirms that the proposed system must be a reliable and highly efficient as well as environmentally friendly alternative to common ground-coupled systems. Abstract: Considering the problem of the concentrating solar cell efficiency restricted by the temperature.
The closed two-phase thermosyphon was used to dissipation heat in concentrating solar cell at high heat flux, which adopted water as the working fluid. The temperature distribution of evaporator had significant effect on solar cell performance and heat pipe efficiency. During the computing process, the heat flux, filling ratio of liquid and saturation temperature were taken into account.
The smaller saturation temperature would bring the better heat transfer characters.
Abstract: Two-phase mechanically pumped cooling loop MPCL has emerged as a highly effective means for dissipating large amounts of heat from a small heat transfer area and provided a robust solution for significant design with flexibility, precise temperature control, and is othermalization. Results indicate this new structure loop heat pipe can meet the design requirements and secure to work well. This miniature loop heat pipe realized visualization research of phase change phenomenon to some extent.
Abstract: Heat pipes have been widely used as one of the alternative methods to absorb more heat in the cooling systems of electronic devices. One of the ways to improve the thermal performance of heat pipes is to change the fluid transport properties and flow features of working fluids using nanofluids. The purpose of this research was to investigate the effect of Al 2 O 3 -water nanofluids concentration and fluid loading to the thermal resistance between evaporator and adiabatic section of copper straight sintered copper powder wick heat pipe.
In this research, sintered powder wick heat pipes were manufactured and tested to determine the thermal resistance of the sintered powder wick heat pipes which charged with water and Al 2 O 3 -water nanofluids. Nine years after its erection, the sarcophagus structure, although still generally sound, raises concerns for its long-term resistance and represents a standing potential risk.
In particular, the roof of the structure presented for a long time numerous cracks with consequent impairment of leaktightness and penetration of large quantities of rain water which is now highly radioactive. This also creates conditions of high humidity producing corrosion of metallic structures which contribute to the support of the sarcophagus.
Moreover, some massive concrete structures, damaged or dislodged by the reactor explosion, are unstable and their failure, due to further degradation or to external events, could provoke a collapse of the roof and part of the building. According to various analyses, a number of potential accidental scenarios could be envisaged.
They include a criticality excursion due to change of configuration of the melted nuclear fuel masses in the presence of water leaked from the roof, a resuspension of radioactive dusts provoked by the collapse of the enclosure and the long-term migration of radionuclides from the enclosure into the groundwater.
The first two accident scenarios would result in the release of radionuclides into the atmosphere which would produce a new contamination of the surrounding area within a radius of several tens of kilometres. It is not expected, however, that such accidents could have serious radiological consequences at longer distances. As far as the leaching of radionuclides from the fuel masses by the water in the enclosure and their migration into the groundwater are concerned, this phenomenon is expected to be very slow and it has been estimated that, for example, it will take 45 to 90 years for certain radionuclides such as strontium90 to migrate underground up to the Pripyat River catchment area.
The expected radiological significance of this phenomenon is not known with certainty and a careful monitoring of the evolving situation of the groundwater will need to be carried out for a long time. The accident recovery and clean-up operations have resulted in the production of very large quantities of radioactive wastes and contaminated equipment which are currently stored in about sites within and outside the km exclusion zone around the reactor. These wastes and equipment are partly buried in trenches and partly conserved in containers isolated from groundwater by clay or concrete screens.
A large number of contaminated equipment, engines and vehicles are also stored in the open air. All these wastes are a potential source of contamination of the groundwater which will require close monitoring until a safe disposal into an appropriate repository is implemented. In general, it can be concluded that the sarcophagus and the proliferation of waste storage sites in the area constitute a series of potential sources of release of radioactivity that threatens the surrounding area.
However, any such releases are expected to be very small in comparison with those from the Chernobyl accident in and their consequences would be limited to a relatively small area around the site. On the other hand, concerns have been expressed by some experts that a much more important release might occur if the collapse of the sarcophagus should induce damage in the Unit 3 of the Chernobyl power plant, which currently is still in operation.
In any event, initiatives have been taken internationally, and are currently underway, to study a technical solution leading to the elimination of these sources of residual risk on the site. The Chernobyl accident was very specific in nature and it should not be seen as a reference accident for future emergency planning purposes. The lessons that could be learned from the Chernobyl accident were, therefore, numerous and encompassed all areas, including reactor safety and severe accident management, intervention criteria, emergency procedures, communication, medical treatment of irradiated persons, monitoring methods, radioecological processes, land and agricultural management, public information, etc.
However, the most important lesson learned was probably the understanding that a major nuclear accident has inevitable transboundary implications and its consequences could affect, directly or indirectly, many countries even at large distances from the accident site. This led to an extraordinary effort to expand and reinforce international co-operation in areas such as communication, harmonisation of emergency management criteria and co-ordination of protective actions.
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At the national level, the Chernobyl accident also stimulated authorities and experts to a radical review of their understanding of and attitude to radiation protection and nuclear emergency issues. This prompted many countries to establish nationwide emergency plans in addition to the existing structure of local emergency plans for individual nuclear facilities.
In the scientific and technical area, besides providing new impetus to nuclear safety research, especially on the management of severe nuclear accidents, this new climate led to renewed efforts to expand knowledge on the harmful effects of radiation and their medical treatment and to revitalise radioecological research and environmental monitoring programmes. Substantial improvements were also achieved in the definition of criteria and methods for the information of the public, an aspect whose importance was particularly evident during the accident and its aftermath.
The history of the modern industrial world has been affected on many occasions by catastrophes comparable or even more severe than the Chernobyl accident. Nevertheless, this accident, due not only to its severity but especially to the presence of ionising radiation, had a significant impact on human society. Not only it produced severe health consequences and physical, industrial and economic damage in the short term, but, also, its long-term consequences in terms of socio-economic disruption, psychological stress and damaged image of nuclear energy, are expected to be long standing.
However, the international community has demonstrated a remarkable ability to apprehend and treasure the lessons to be drawn from this event, so that it will be better prepared to cope with a challenge of this kind, if ever a severe nuclear accident should happen again. The Chernobyl Power Complex, lying about km north of Kiev, Ukraine Figure 1 , consisted of four nuclear reactors of the RBMK design, Units 1 and 2 being constructed between and , while Units 3 and 4 of the same design were completed in IA Two more RBMK reactors were under construction at the site at the time of the accident.
Figure 1. The site of the Chernobyl nuclear power complex modif. To the South-east of the plant, an artificial lake of some 22 km2, situated beside the river Pripyat, a tributary of the Dniepr, was constructed to provide cooling water for the reactors. This area of Ukraine is described as Belarussian-type woodland with a low population density. About 3 km away from the reactor, in Pripyat, there were 49, inhabitants. The town of Chernobyl, which had a population of 12,, is about 15 km to the South-east of the complex.
Within a km radius of the power plant, the total population was between , and , The RBMK Figure 2 is a Soviet designed and built graphite moderated pressure tube type reactor, using slightly enriched 2 per cent uranium uranium dioxide fuel. It is a boiling light water reactor, with. Water pumped to the bottom of the fuel channels boils as it progresses up the pressure tubes, producing steam which feeds two MW e [megawatt electrical] turbines.
The water acts as a coolant and also provides the steam used to drive the turbines. The vertical pressure tubes contain the zirconium-alloy clad uranium-dioxide fuel around which the cooling water flows. A specially designed refuelling machine allows fuel bundles to be changed without shutting down the reactor. The moderator, whose function is to slow down neutrons to make them more efficient in producing fission in the fuel, is constructed of graphite.
A mixture of nitrogen and helium is circulated between the graphite blocks largely to prevent oxidation of the graphite and to improve the transmission of the heat produced by neutron interactions in the graphite, from the moderator to the fuel channel. The core itself is about 7 m high and about 12 m in diameter. There are four main coolant circulating pumps, one of which is always on standby. The reactivity or power of the reactor is controlled by raising or lowering control rods, which, when lowered, absorb neutrons and reduce the fission rate. The power output of this reactor is 3, MW t [megawatt thermal] or 1, MW e , although there is a larger version producing 1, MW e.
Various safety systems, such as an emergency core cooling system and the requirement for an absolute minimal insertion of 30 control rods, were incorporated into the reactor design and operation. The most important characteristic of the RBMK reactor is that it possesses a "positive void coefficient". This means that if the power increases or the flow of water decreases, there is increased steam production in the fuel channels, so that the neutrons that would have been absorbed by the denser water will now produce increased fission in the fuel.
However, as the power increases, so does the temperature of the fuel, and this has the effect of reducing the neutron flux negative fuel coefficient. The net effect of these two opposing characteristics varies with the power level. At the high power level of normal operation, the temperature effect predominates, so that power excursions leading to excessive overheating of the fuel do not occur. However, at a lower power output of less than 20 per cent of the maximum, the positive void coefficient effect is dominant and the reactor becomes unstable and prone to sudden power surges.
This was a major factor in the development of the accident. The Unit 4 reactor was to be shutdown for routine maintenance on 25 April It was decided to take advantage of this shutdown to determine whether, in the event of a loss of station power, the slowing turbine could provide enough electrical power to operate the emergency equipment and the core cooling water circulating pumps, until the diesel emergency power supply became operative.
The aim of this test was to determine whether cooling of the core could continue to be ensured in the event of a loss of power. This type of test had been run during a previous shut-down period, but the results had been inconclusive, so it was decided to repeat it. Therefore, inadequate safety precautions were included in the test programme and the operating personnel were not alerted to the nuclear safety implications of the electrical test and its potential danger. The planned programme called for shutting off the reactor's emergency core cooling system ECCS , which provides water for cooling the core in an emergency.
Although subsequent events were not greatly affected by this, the exclusion of this system for the whole duration of the test reflected a lax attitude towards the implementation of safety procedures. As the shutdown proceeded, the reactor was operating at about half power when the electrical load dispatcher refused to allow further shutdown, as the power was needed for the grid. In accordance with the planned test programme, about an hour later the ECCS was switched off while the reactor continued to operate at half power.
It was not until about hr on 25 April that the grid controller agreed to a further reduction in power. For this test, the reactor should have been stabilised at about 1, MW t prior to shut down, but due to operational error the power fell to about 30 MW t , where the positive void coefficient became dominant. The operators then tried to raise the power to , MW t by switching off the automatic regulators and freeing all the control rods manually.
It was only at about hr on 26 April that the reactor was stabilised at about MW t. Although there was a standard operating order that a minimum of 30 control rods was necessary to retain reactor control, in the test only control rods were actually used. Many of the control rods were withdrawn to compensate for the build up of xenon which acted as an absorber of neutrons and reduced power. This meant that if there were a power surge, about 20 seconds would be required to lower the control rods and shut the reactor down.
In spite of this, it was decided to continue the test programme. There was an increase in coolant flow and a resulting drop in steam pressure. The automatic trip which would have shut down the reactor when the steam pressure was low, had been circumvented. In order to maintain power the operators had to withdraw nearly all the remaining control rods. The reactor became very unstable and the operators had to make adjustments every few seconds trying to maintain constant power. At about this time, the operators reduced the flow of feedwater, presumably to maintain the steam pressure.
Simultaneously, the pumps that were powered by the slowing turbine were providing less cooling water to the reactor. The loss of cooling water exaggerated the unstable condition of the reactor by increasing steam production in the cooling channels positive void coefficient , and the operators could not prevent an overwhelming power surge, estimated to be times the nominal power output. The sudden increase in heat production ruptured part of the fuel and small hot fuel particles, reacting with water, caused a steam explosion, which destroyed the reactor core.
A second explosion added to the destruction two to three seconds later. The accident occurred at hr on Saturday, 26 April , when the two explosions destroyed the core of Unit 4 and the roof of the reactor building. In the IAEA Post-Accident Assessment Meeting in August IA86 , much was made of the operators' responsibility for the accident, and not much emphasis was placed on the design faults of the reactor. Later assessments IA86a suggest that the event was due to a combination of the two, with a little more emphasis on the design deficiencies and a little less on the operator actions.
The two explosions sent a shower of hot and highly radioactive debris and graphite into the air and exposed the destroyed core to the atmosphere. The plume of smoke, radioactive fission products and debris from the core and the building rose up to about 1 km into the air. The heavier debris in the plume was deposited close to the site, but lighter components, including fission products and virtually all of the noble gas inventory were blown by the prevailing wind to the North-west of the plant.
Fires started in what remained of the Unit 4 building, giving rise to clouds of steam and dust, and fires also broke out on the adjacent turbine hall roof and in various stores of diesel fuel and inflammable materials. Over fire-fighters from the site and called in from Pripyat were needed, and it was this group that received the highest radiation exposures and suffered the greatest losses in personnel. These fires were put out by hr of the same day, but by then the graphite fire had started. Many firemen added to their considerable doses by staying on call on site.
The intense graphite fire was responsible for the dispersion of radionuclides and fission fragments high into the atmosphere. The emissions continued for about twenty days , but were much lower after the tenth day when the graphite fire was finally extinguished. While the conventional fires at the site posed no special firefighting problems, very high radiation doses were incurred by the firemen.
However, the graphite moderator fire was a special problem. Very little national or international expertise on fighting graphite fires existed, and there was a very real fear that any attempt to put it out might well result in further dispersion of radionuclides, perhaps by steam production, or it might even provoke a criticality excursion in the nuclear fuel.
A decision was made to layer the graphite fire with large amounts of different materials, each one designed to combat a different feature of the fire and the radioactive release. Boron carbide was dumped in large quantities from helicopters to act as a neutron absorber and prevent any renewed chain reaction. Dolomite was also added to act as heat sink and a source of carbon dioxide to smother the fire. Lead was included as a radiation absorber, as well as sand and clay which it was hoped would prevent the release of particulates.
While it was later discovered that many of these compounds were not actually dropped on the target, they may have acted as thermal insulators and precipitated an increase in the temperature of the damaged core leading to a further release of radionuclides a week later. By May 9, the graphite fire had been extinguished, and work began on a massive reinforced concrete slab with a built-in cooling system beneath the reactor.
This involved digging a tunnel from underneath Unit 3. About four hundred people worked on this tunnel which was completed in 15 days, allowing the installation of the concrete slab. This slab would not only be of use to cool the core if necessary, it would also act as a barrier to prevent penetration of melted radioactive material into the groundwater. In summary, the Chernobyl accident was the product of a lack of "safety culture". The reactor design was poor from the point of view of safety and unforgiving for the operators, both of which provoked a dangerous operating state.
The operators were not informed of this and were not aware that the test performed could have brought the reactor into explosive conditions. In addition, they did not comply with established operational procedures. The combination of these factors provoked a nuclear accident of maximum severity in which the reactor was totally destroyed within a few seconds. The "source term" is a technical expression used to describe the accidental release of radioactive material from a nuclear facility to the environment.
Not only are the levels of radioactivity released important, but also their distribution in time as well as their chemical and physical forms. The initial estimation of the Source Term was based on air sampling and the integration of the assessed ground deposition within the then Soviet Union. This was clear at the IAEA Post-Accident Review Meeting in August IA86 , when the Soviet scientists made their presentation, but during the discussions it was suggested that the total release estimate would be significantly higher if the deposition outside the Soviet Union territory were included.
Subsequent assessments support this view, certainly for the caesium radionuclides Wa87, Ca87, Gu The initial estimates were presented as a fraction of the core inventory for the important radionuclides and also as total activity released. In the initial assessment of releases made by the Soviet scientists and presented at the IAEA Post-Accident Assessment Meeting in Vienna IA86 , it was estimated that per cent of the core inventory of the noble gases xenon and krypton was released, and between 10 and 20 per cent of the more volatile elements of iodine, tellurium and caesium. This estimate was later revised to 3.
This corresponds to the emission of 6 t of fragmented fuel. At that time, it was estimated that 1 to 2 exabecquerels EBq were released. These estimates of the source term were based solely on the estimated deposition of radionuclides on the territory of the Soviet Union, and could not take into account deposition in Europe and elsewhere, as the data were not then available.
The total caesium release was estimated to be 70 petabecquerels PBq of which 31 PBq were deposited in the Soviet Union. Later analyses carried out on the core debris and the deposited material within the reactor building have provided an independent assessment of the environmental release. After an extensive review of the many reports IA86 , Bu93 , this was confirmed.
For iodine, the most accurate estimate was felt to be 50 to 60 per cent of the core inventory of 3, PBq. The current estimate of the source term De95 is summarised in Table 1. The release pattern over time is well illustrated in Figure 3 Bu The initial large release was principally due to the mechanical fragmentation of the fuel during the explosion.
It contained mainly the more volatile radionuclides such as noble gases, iodines and some caesium. The second large release between day 7 and day 10 was associated with the high temperatures reached in the core melt. The sharp drop in releases after ten days may have been due to a rapid cooling of the fuel as the core debris melted through the lower shield and interacted with other material in the reactor. Although further releases probably occurred after 6 May, these are not thought to have been large. Figure 3. Daily release rate of radioactive substances into the atmosphere modif.
The release of radioactive material to the atmosphere consisted of gases, aerosols and finely fragmented fuel. Gaseous elements, such as krypton and xenon escaped more or less completely from the fuel material. In addition to its gaseous and particulate form, organically bound iodine was also detected. The ratios between the various iodine compounds varied with time. As mentioned. Table 1. Current estimate of radionuclide releases during the Chernobyl accident modif.
Other volatile elements and compounds, such as those of caesium and tellurium, attached to aerosols, were transported in the air separate from fuel particles. Air sampling revealed particle sizes for these elements to be 0. Unexpected features of the source term, due largely to the graphite fire, were the extensive releases of fuel material and the long duration of the release.
Elements of low volatility, such as cerium, zirconium, the actinides and to a large extent barium, lanthanium and strontium also, were embedded in fuel particles. Larger fuel particles were deposited close to the accident site, whereas smaller particles were more widely dispersed. Other condensates from the vaporised fuel, such as radioactive ruthenium, formed metallic particles.
These, as well as the small fuel particles, were often referred to as "hot particles", and were found at large distances from the accident site De During the first 10 days of the accident when important releases of radioactivity occurred, meteorological conditions changed frequently, causing significant variations in release direction and dispersion parameters. Deposition patterns of radioactive particles depended highly on the dispersion parameters, the particle sizes, and the occurrence of rainfall.
The largest particles, which were primarily fuel particles, were deposited essentially by sedimentation within km of the reactor. Small particles were carried by the wind to large distances and were deposited primarily with rainfall. The radionuclide composition of the release and of the subsequent deposition on the ground also varied considerably during the accident due to variations in temperature and other parameters during the release. Caesium was selected to characterise the magnitude of the ground deposition because 1 it is easily measurable, and 2 it was the main contributor to the radiation doses received by the population once the short-lived iodine had decayed.
The three main spots of contamination resulting from the Chernobyl accident have been called the Central, Bryansk-Belarus, and Kaluga-Tula-Orel spots Figure 4. The Central spot was formed during the initial, active stage of the release predominantly to the West and North-west Figure 5. The Bryansk-Belarus spot, centered km to the North-northeast of the reactor, was formed on April as a result of rainfall on the interface of the Bryansk region of Russia and the Gomel and Mogilev regions of Belarus.
Figure 4. Main spots of caesium contamination See also Figure 4 at end of file. Figure 5. Central spot of caesium contamination See also Figure 5 at end of file. The Kaluga-Tula-Orel spot in Russia, centered approximately km North-east of the reactor, was formed from the same radioactive cloud that produced the Bryansk-Belarus spot, as a result of rainfall on April. Radioactivity was first detected outside the Soviet Union at a Nuclear Power station in Sweden, where monitored workers were noted to be contaminated. It was at first believed that the contamination was from a Swedish reactor.
When it became apparent that the Chernobyl reactor was the source, monitoring stations all over the world began intensive sampling programmes. The radioactive plume was tracked as it moved over the European part of the Soviet Union and Europe Figure 6. Initially the wind was blowing in a Northwesterly direction and was responsible for much of the deposition in Scandinavia, the Netherlands and Belgium and Great Britain. Later the plume shifted to the South and much of Central Europe, as well as the Northern Mediterranean and the Balkans, received some deposition, the actual severity of which depended on the height of the plume, wind speed and direction, terrain features and the amount of rainfall that occurred during the passage of the plume.
The radioactive cloud initially contained a large number of different fission products and actinides, but only trace quantities of actinides were detected in most European countries, and a very small number were found in quantities that were considered radiologically significant. This was largely due to the fact that these radionuclides were contained in the larger and heavier particulates, which tended to be deposited closer to the accident site rather than further away.
Figure 6. Areas covered by the main body of the radioactive cloud on various days during the release.
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Most countries in Europe experienced some deposition of radionuclides, mainly caesium and caesium, as the plume passed over the country. In Austria, Eastern and Southern Switzerland, parts of Southern Germany and Scandinavia, where the passage of the plume coincided with rainfall, the total deposition from the Chernobyl release was greater than that experienced by most other countries, whereas Spain, France and Portugal experienced the least deposition. It was reported that considerable secondary contamination occurred due to resuspension of material from contaminated forest.
This was not confirmed by later studies. While the plume was detectable in the Northern hemisphere as far away as Japan and North America, countries outside Europe received very little deposition of radionuclides from the accident. No deposition was detected in the Southern hemisphere Un In summary it can be stated that there is now a fairly accurate estimate of the total release. The duration of the release was unexpectedly long, lasting more than a week with two periods of intense release. Another peculiar feature was the significant emission about 4 per cent of fuel material which also contained embedded radionuclides of low volatility such as cerium, zirconium and the actinides.
The composition and characteristics of the radioactive material in the plume changed during its passage due to wet and dry deposition, decay, chemical transformations and alterations in particle size. The area affected was particularly large due to the high altitude and long duration of the release as well as the change of wind direction. However, the pattern of deposition was very irregular, and significant deposition of radionuclides occurred where the passage of the plume coincided with rainfall.
Although all the Northern hemisphere was affected, only territories of the former Soviet Union and part of Europe experienced contamination to a significant degree. The scale and severity of the Chernobyl accident with its widespread radioactive contamination had not been foreseen and took by surprise most national authorities responsible for emergency preparedness.
No provisions had been made for an accident of such scale and, though some radiation protection authorities had made criteria available for intervention in an accident, these were often incomplete and provided little practical help in the circumstances, so that very few workable national guidelines or principles were actually in place. Those responsible for making national decisions were suddenly faced with an accident for which there were no precedents upon which to base their decisions.
In addition, early in the course of the accident there was little information available, and considerable political pressure, partially based on the public perception of the radiation danger, was being exerted on the decision-makers. In these circumstances, cautious immediate action was felt necessary, and measures were introduced that tended to err, sometimes excessively so, on the side of prudence rather than being driven by informed scientific and expert judgement. The town of Pripyat was not severely contaminated by the initial release of radionuclides, but, once the graphite fire started, it soon became obvious that contamination would make the town uninhabitable.
Late on 26 April it was decided to evacuate the town, and arrangements for transport and accommodation of the evacuees were made. The announcement of evacuation was made at hr the following day. Evacuation began at hr, and Pripyat was evacuated in about two and one half hours.
As measurements disclosed the extensive pattern of deposition of radionuclides, and it was possible to make dose assessments, the remainder of the people in a km zone around the reactor complex were gradually evacuated, bringing the total evacuees to about , Other countermeasures to reduce dose were widely adopted Ko Decontamination procedures performed by military personnel included the washing of buildings, cleaning residential areas, removing contaminated soil, cleaning roads and decontaminating water supplies. Special attention was paid to schools, hospitals and other buildings used by large numbers of people.
Streets were watered in towns to suppress dust. However, the effectiveness of these countermeasures outside the km zone was small. An attempt to reduce thyroid doses by the administration of stable iodine to block radioactive uptake by the thyroid was made Me92 , but its success was doubtful. This value was lower by a factor of 2 to 3 than that recommended by the International Commission on Radiological Protection ICRP for the same countermeasure.
Nevertheless, this value proposed by the NCRP was strongly criticised as being a very high level.
The situation was further complicated by the political and social tension in the Soviet Union at that time. Later, a special Commission was established which developed new recommendations for intervention levels. These recommendations were based on the levels of ground contamination by the radionuclides caesium, strontium and plutonium People who continued to live in the heavily contaminated areas were given compensation and offered annual medical examinations by the government.
Residents of less contaminated areas are provided with medical monitoring. Current decisions on medical actions are based on annual doses. Compensation is provided for residents whose annual dose is greater than 1mSv. The use of locally produced milk and mushrooms is restricted in some of these areas. Relocation is considered in Russia for annual doses above 5 mSv. As is mentioned in the section on psychological effects, in Chapter V, the Soviet authorities did not foresee that their attempts to compensate those affected by the accident would be misinterpreted by the recipients and increase their stress, and that the label of "radiophobia" attributed to these phenomena was not only incorrect, but was one that alienated the public even more.
Some of these initial approaches have been recognised as being inappropriate and the authorities are endeavouring to rectify their attitude to the exposed population. The progressive spread of contamination at large distances from the accident site has caused considerable concern in Member countries, and the reactions of national authorities to this situation have been extremely varied, ranging from a simple intensification of the normal environmental monitoring programmes, without adoption of any specific countermeasures, to compulsory restrictions concerning the marketing and consumption of foodstuffs.
This variety of responses has been accompanied by significant differences in the timing and duration of the countermeasures. In general, the most widespread countermeasures were those which were not expected to impose, in the short time for which they were in effect, a significant burden on lifestyles or the economy. These included advice to wash fresh vegetables and fruit before consumption, advice not to use rainwater for drinking or cooking, and programmes of monitoring citizens returning from potentially contaminated areas. In reality, experience has shown that even these types of measures had, in some cases, a negative impact which was not insignificant.
Protective actions having a more significant impact on dietary habits and imposing a relatively important economic and regulatory burden included restrictions or prohibitions on the marketing and consumption of milk, dairy products, fresh leafy vegetables and some types of meat, as well as the control of the outdoor grazing of dairy cattle. In some areas, prohibitions were placed on travel to areas affected by the accident and on the import of foodstuffs from the Soviet Union and Eastern European countries.
In most Member countries, restrictions were imposed on the import of foodstuffs from Member as well as non-Member countries. The range of these reactions can be explained primarily by the diversity of local situations both in terms of uneven levels of contamination and in terms of national differences in administrative, regulatory and public health systems. However, one of the principal reasons for the variety of situations observed in Member countries stems from the criteria adopted for the choice and application of intervention levels for the implementation of protective actions.
In this respect, while the general radiation protection principles underlying the actions taken in most Member countries following the accident have been very similar, discrepancies arose in the assessment of the situation and the adoption and application of operational protection criteria. These discrepancies were further enhanced by the overwhelming role played in many cases by non-radiological factors, such as socio-economic, political and psychological, in determining the countermeasures.
This situation caused concern and confusion among the public, perplexities among the experts and difficulties to national authorities, especially in maintaining their public credibility. This was, therefore, identified as an area where several lessons should be learned from the accident and efforts directed towards better international harmonisation of the scientific bases and co-ordination of concepts and measures for the protection of the public in case of emergency.
Nowhere was this problem better illustrated than by the way that contaminated food was handled. In some countries outside the Soviet Union the main source of exposure to the general population was the consumption of contaminated food. Mechanisms to handle locally produced as well as imported contaminated food had to be put in place within a few weeks of the accident. National authorities were in an unenviable position. They had to act quickly and cautiously to introduce measures to protect the "purity" of the public food supply and, what is more, they had to be seen to be effective in so doing.
This inevitably led to some decisions which even at the time appeared to be over-reactions and not scientifically justified. In addition, dissenting opinions among experts added to the difficulties of the decision-makers. Some countries without nuclear power programmes and whose own food was not contaminated, argued that they did not need to import any "tainted" food and refused any food containing any radionuclides whatsoever.
This extreme and impracticable measure might well have been regarded as an example of how well the authorities of those countries were protecting the health of their population. Sometimes this attitude appeared to promote a neighbourly rivalry between countries to see which could set the more stringent standards for food contamination, as though, by so doing, their own citizens were more protected. The result was that often slightly contaminated food was destroyed or refused importation to avoid only trivial doses. Thus, food items with a trivial consumption and dose , such as spices, were treated the same as items of high consumption such as vegetables.
However, these values were later relaxed for some food items in order to remove inconsistent treatment of food groups. In some special circumstances, decisions had to be made based on the local situation.
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For example, in some Northern European communities, reindeer meat is a major component of the diet; due to the ecological circumstances, these animals tend to concentrate radiocaesium, which will then expose the populations which depend on them. Special countermeasures, such as pasturing reindeer in areas of lower contamination, were introduced in some countries to avoid this exposure. Table 2. Codex Alimentarius Guideline values for food moving in international trade FA Levels above these do not necessarily constitute a health hazard, and if found, the competent national authority should review what action should be taken.
Often the national authorities were not able accurately to predict the public response to some of their advice and pronouncements. For example, in some European countries, soon after the accident the public were advised to wash leafy vegetables. The national authority felt that this was innocuous advice as most people washed their vegetables anyway, and they were unprepared for the public response which was to stop buying these vegetables.
This resulted in significant economic loss to local producers which far outweighed any potential benefit in terms of radiological health. In some countries, the public was told that the risks were very small but, at the same time, were given advice on how to reduce these low risks. It was very difficult to explain this apparently contradictory advice, and the national authority came under criticism from the media Sj Outside the Soviet Union, the initial confusion led to inconsistent and precipitate actions which, although understandable, were sometimes ill-advised and unjustified.
However, it should be emphasised that great progress has been made since this early confusion. As a result of the actions of the international organisations to harmonise intervention criteria and the willingness of countries to cooperate in this endeavour, a firm groundwork for uniform criteria based on accepted radiation protection principles has been established, so that relative consistency can hopefully be expected in their implementation in the event of a possible future nuclear accident.
In summary, the Chernobyl accident took authorities by surprise as regards extent, duration and contamination at long distance. As no guidelines were available for such an accident, little information was available and great political and public pressure to do something were experienced, overprecautious decisions were often taken in and outside the Soviet Union.
The psychological impact of some official decisions on the public were not predicted and variable interpretations or even misinterpretations of ICRP recommendations, especially for intervention levels for food, led to inconsistent decisions and advice. These added to public confusion and provoked mistrust and unnecessary economic losses. However, there were exceptions and very soon international efforts started to harmonise criteria and approaches to emergency management.
The exposure of the population as a result of the accident resulted in two main pathways of exposure. The first is the radiation dose to the thyroid as a result of the concentration of radioiodine and similar radionuclides in the gland. The second is the whole-body dose caused largely by external irradiation mainly from radiocesium.
The absorbed dose to the whole body is thought to be about 20 times more deleterious, in terms of late health effects incidence, than the same dose to the thyroid IC The population exposed to radiation following the Chernobyl accident can be divided into four categories: 1 the staff of the nuclear power plant and workers who participated in clean-up operations referred to as "liquidators" ; 2 the nearby residents who were evacuated from the km zone during the first two weeks after the accident; 3 the population of the former Soviet Union, including especially the residents of contaminated areas; and 4 the population in countries outside the former Soviet Union.
A number of liquidators estimated to amount up to , took part in mitigation activities at the reactor and within the km zone surrounding the reactor. The most exposed workers were the firemen and the power plant personnel during the first days of the accident. Most of the dose received by the workers resulted from external irradiation from the fuel fragments and radioactive particles deposited on various surfaces. About , people were evacuated during the first days following the accident, mainly from the km zone surrounding the reactor.
Prior to evacuation, those individuals were exposed to external irradiation from radioactive materials transported by the cloud and deposited on the ground, as well as to internal irradiation essentially due to the inhalation of radioactive materials in the cloud. The relative contributions to the external whole-body dose from the main radionuclides of concern for that pathway of exposure and during the first few months after the accident are shown in Figure 7.
It is clear that tellurium played a major role in the first week after the accident, and that, after one month, the radiocaesiums caesium and caesium became predominant.
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Subsequently, however, caesium decayed to levels much lower than those of caesium, which became after a few years the only radionuclide of importance for practical purposes. It is usual to refer to caesium only, even when the mix of caesium and caesium is meant, because the values for the constituents can be easily derived from those for caesium Figure 7. Relative contribution of gamma radiation from individual radionuclides to the absorbed dose rate in air during the first several months after the Chernobyl accident Go With regard to internal doses from inhalation and ingestion of radionuclides, the situation is similar: radioiodine was important during the first few weeks after the accident and gave rise to thyroid doses via inhalation of contaminated air, and, more importantly, via consumption of contaminated foodstuffs, mainly cow's milk.
After about one month, the radiocaesiums caesium and caesium again became predominant, and, after a few years, caesium became the only radionuclide of importance for practical purposes, even though strontium may in the future play a significant role at short distances from the reactor. About 4 million people live in those areas. In those areas, called "strict control zones", protection measures are applied, especially as far as control of consumption of contaminated food is concerned.
Early after the accident, the All-Union Dose Registry AUDR was set up by the Soviet Government in to record medical and dosimetric data on the population groups expected to be the most exposed: 1 the liquidators, 2 the evacuees from the km zone, 3 the inhabitants of the contaminated areas, and 4 the children of those people. In , the AUDR contained data on , persons. Outside the former Soviet Union, the radionuclides of importance are, again, the radioiodines and radiocaesiums, which, once deposited on the ground, give rise to doses from ingestion through the consumption of foodstuffs.
Deposited radiocaesium is also a source of long-term exposure from external irradiation from the contaminated ground and other surfaces. Most of the population of the Northern hemisphere was exposed, in varying degrees, to radiation from the Chernobyl accident. Most of the liquidators can be divided into two groups: 1 the people who were working at the Chernobyl power station at the time of the accident viz. They number a few hundred persons; and 2 the workers, estimated to amount up to ,, who were active in at the power station or in the zone surrounding it for the decontamination, sarcophagus construction and other recovery operations.
On the night of 26 April , about workers were on the site of the Chernobyl power plant. All of the dosimeters worn by the workers were over-exposed and did not allow an estimate of the doses received. However, information is available on the doses received by the persons who were placed in hospitals and diagnosed as suffering from acute radiation syndrome. Using biological dosimetry, it was estimated that of these patients received whole-body doses from external irradiation in the range Gy, that 55 received doses between 2 and 4 Gy , that 21 received between 4 and 6 Gy, and that the remaining 21 received doses between 6 and 16 Gy.
In addition, it was estimated from thyroid measurements that the thyroid dose from inhalation would range up to about 20 Sv, with individuals in the The second category of liquidators consists of the large number of adults who were recruited to assist in the clean-up operations.
They worked at the site, in towns, forests and agricultural areas to make them fit to work and live in. Several hundreds of thousands of individuals participated in this work. Initially, 50 per cent of those workers came from the Soviet armed forces, the other half including personnel of civil organisations, the security service, the Ministry of Internal Affairs, and other organisations. The total number of liquidators has yet to be determined accurately, since only some of the data from some of those organisations have been collected so far in the national registries of Belarus, Russia, Ukraine and other republics of the former Soviet Union So Also, it has been suggested that, because of the social and economic advantages associated with being designated a liquidator, many persons have contrived latterly to have their names added to the list.
There are only fragmented data on the doses received by the liquidators. Attempts to establish a dosimetric service were inadequate until the middle of June of ; until then, doses were estimated from area radiation measurements. The liquidators were initially subjected to a radiation dose limit for one year of mSv. In this limit was reduced to mSv and in to 50 mSv Ba The registry data show that the average recorded doses decreased from year to year, being about mSv in , mSv in , 30 mSv in and 15 mSv in Se95a.
It is, however, difficult to assess the validity of the results as they have been reported. It is interesting to note that a small special group of 15 scientists who have worked periodically inside the sarcophagus for a number of years have estimated accumulated whole-body doses in the range 0. While no deterministic effects have been noted to date, this group may well show radiation health effects in the future.
Immediately after the accident monitoring of the environment was started by gamma dose rate measurements. About 20 hours after the accident the wind turned in the direction of Pripyat, gamma dose rates increased significantly in the town, and it was decided to evacuate the inhabitants. About 20 hours later the 49, inhabitants of Pripyat had left the town in nearly 1, buses. About a further 80, people were evacuated in the following days and weeks from the contaminated areas. Information relevant for the assessment of the doses received by these people have been obtained by responses of the evacuees to questionnaires about the location where they stayed, the types of houses in which they lived, the consumption of stable iodine, and other activities Li The iodine activity in thyroid glands of evacuees was measured.
More than 2, measurements of former inhabitants of Pripyat had sufficient quality to be useful for dose reconstruction Go95a.