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Sarcophagus

Most of the radioactivity of the destroyed reactor is contained within the Sarcophagus built around the destroyed unit - at present about 200 tonnes of irradiated and fresh nuclear fuel, mixed with other materials in various forms. The total activity of this material is estimated to be 700 x 1015 Bq of long lived radionuclides. The Sarcophagus has fulfilled its protective function for over the past 16 years. In the long term, however, its stability and the quality of its confinement are in doubt .

Research carried out in the Sarcophagus over a lengthy period has shown that part of the nuclear fuel fragments from the destroyed reactor is in the form of lava-like fuel containing mass (FCM) which breaks down gradually. As a result, large quantities (about 30 tonnes) of radioactive dust are produced.

Analysis of the Sarcophagus shows that it is a potentially hazardous object. The main hazards of the Sarcophagus object are linked to:

- the possibility of radioactive dust blow-out beyond the Sarcophagus that could be triggered by the breakdown of the Sarcophagus structure, earthquakes, hurricanes, initiation of self-sustaining fission reaction etc.;

- discharge of radioactivity in liquid phase into the soil under the construction travelling beyond the Sarcophagus object and penetrating ground water and the water drains of the Pripyat River;

- emergency personnel exposure in the event of emergency situations resulting from both initial events and engineering work on the Sarcophagus object;

- injuries to personnel not linked to radiation agents (falling structural elements, fires, electric shocks, etc.).

Modeling of structural collapse shows that it could lead to a release of radioactive dust and the irradiation of the personnel employed at the site and, partially, of the plant. However, even in the worst case, widespread effects (beyond the 30-km zone) are not expected.

It has been established that the Sarcophagus is currently safe from the point of view of a self-sustaining chain fission reaction (SCFR) in the nuclear fuel remnants. But given the accumulation of general negative trends in the fuel containing mass which weaken its nuclear stability, it cannot be excluded that individual configurations of fuel masses inside the Sarcophagus could reach the critical point for SCFR in the event of changes in their geometry or contact with water. However, even if such guaranty condition were to lead to elevated radiation levels inside the Sarcophagus, large off-site releases would not be expected.

Moreover it is probable that water containing radionuclides discharged from the Sarcophagus can lead to major contamination of ground water in the surrounding area.

It has to be admitted that knowledge of the state of the Sarcophagus and the processes taking place inside it remains inadequate, owing to high radiation fields, difficulty of access of certain premises and the complex implementation of the necessary research. All this makes it difficult to reliably assess the risks linked to possible accidents involving the Sarcophagus or the processes inside it.

Risks linked to the significant amount of radioactive materials, including nuclear matter, in the Sarcophagus and the ongoing processes undermining its environmental protection capabilities made it a matter of urgent necessity to devise plans and take action to convert the Sarcophagus into an ecologically safe system.

The main goal of the strategy of transforming the Sarcophagus into an ecologically safe system, defined by Ukraine in 1996, was to reduce risks and to ensure safe operation of the Sarcophagus, as well as to extract remainders of nuclear fuel and to isolate and bury them as soon as possible in accordance with current national and international standards.

In this connection, the Ukrainian Government signed an agreement with the countries of the G-7 and the European Community Commission on co-operation in making the Sarcophagus ecologically safe object, and the "Shelter Implementation Plan" (SIP) was developed, with the following goals:

1. to reduce the probability of the Sarcophagus collapsing by stabilising its construction;

2. to reduce the consequences of an accident in the event of the Sarcophagus collapsing;

3. to increase the nuclear safety of the Sarcophagus;

4. to increase the safety of workers and the surrounding natural environment.

5. to develop a long-term strategy and basis for transforming the "Shelter" into an ecologically safe system.

In 1997 it was decided to set up an International Chernobyl Fund under the administrative management of the European Bank for Reconstruction and Development with a view to carrying out the Plan.

Work on the SIP will make the ChNPPs 4th unit safer and will obviate concern over a possible collapse of the Sarcophagus. Implementation of immediate projects will provide the necessary basis for work on making the Sarcophagus safe, creating a new cover and extracting fuel containing mass, although uncertainties requiring operative solutions will remain at that stage.

See more information on Chernobyl NPP site http://www.chnpp.gov.ua/index.php?option=com_content&view=article&id=228&Itemid=&lang=en

 

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Environmental contamination

Radionuclide in the Exclusion Zone

The Exclusion Zone around the Chernobyl NPP (hereinafter Exclusion Zone) is the area most contaminated by the accident. Its population was evacuated outside the area in 1986, and all economic activity not related to the Chernobyl NPP (ChNPP) was stopped. The Exclusion Zone covers a surface area of 4300 km2.

At present, the main radionuclides causing Exclusion Zone contamination are 137s, 90Sr and a -emitters of transuranium elements (TUE) 238, 239, 240Pu, 241Am. The amounts deposited in the Exclusion Zone are set out in Table 1.

Table 1 - Estimates of the amounts of radionuclides in the Exclusion Zone (not including radioactivity in the Sarcophagus) as of 2002 (Bq).

 

Amount in radioactive waste disposals

Amount on the ground surface

 

Radionuclides

 

RWTD*

 

RWTLS*

 

Overall

In bottom sediments of the cooling pond

Total

137Cs

3.4 1015

1.1 1015

5.6 1015

0.16 1015

10.1 1015

90Sr

2.8 1015

0.7 1015

2.6 1015

0.1 1015

6.1 1015

241Pu

2.7 1015

0.7 1015

2.5 1015

0.1 1015

5.9 1015

a - emitters TUE (238Pu +    + 239,240Pu + 241Am)

1.4 1014

 

0.4 1014

 

1.3 1014

 

5.0 1012

 

3.1 1014

___________________________

NOTE: RWTD - radioactive waste disposal sites, RWTLS - radioactive waste temporary localisation sites.

At present 137Cs accounts for more than 90% of external exposure of personnel working in Exclusion Zone and the population of contamination areas.

The main difference in Exclusion Zone 137Cs contamination between today and May 1986 is that activity lies in the depth of soil in most of the zone and also activity reduction due to radioactive decay.

The field of  90Sr contamination readily correlates on the whole with that of 137Cs. The greatest intensity of contamination is observed on the "narrow" Westward Plume. The maximal densities of contamination range as far as 40 MBqm-2 at 2-5 km from the ChNPP and fall to 0.5 MBqm-2 at 30 km from the NPP. The contamination in the vicinity of Chernobyl is about 0.5 MBqm-2, with 40180 kBqm-2 for most of the periphery of the 30-km zone and 180370 kBqm-2 along the north border-line.

The Pu and 241Am (TUE) contamination densities almost fully correlate with the 90Sr field chart in this area.

The concentrations of TUEs in general are traced in the same locations as those of 90Sr. They reach ~ 700 kBq m-2 for two isotopes of plutonium and ~ 500 kBq m-2 for 241Am. They fall to 2 kBq m-2 and lower on the zone periphery. Besides the Westward Plume, there is a wide Northward Plume traced on all maps and a similar though certainly less active Southward Plume .

General characteristics of radioactive waste storage sites

Experts estimate the total activity of long-lived radionuclides (137Cs, 90Sr, Pu isotopes, 241Am) stored in RWTDs and RWTLSs at roughly 12 1015 Bq (see Table 1) . RWTLSs and RWTDs were created in 1986-1987 in the course of urgent post-accident clean-up of the 10-km zone around the ChNPP contaminated as a result of the accident.

The clean-up activity was undertaken with the aim of quickly isolating a considerable amount of contaminated soils, timber, building constructions and demolished building parts, heavy equipment etc., by burying them in trenches to prevent air transport of radioactive dust. The burial procedures themselves were performed mainly without taking into account requirements to create engineering barriers to prevent radionuclide migration. As a result, the ground water in the vicinity of these sites is gradually being contaminated.

The Exclusion Zone as a protection barrier against radionuclide

migration to populated areas

The territory of the Exclusion Zone is the most heavily contaminated by 137Cs, 90Sr and TUE dose-created radionuclides. It is the main depot of "Chernobyl" radionuclides and as such a dangerous source of radionuclide transfer to neighbouring areas. Even so, the results of research in recent years show that the Exclusion Zone is also a very effective complex barrier against the migration of deposited radionuclides to populated areas of Ukraine and Belarus.

The importance of the Exclusion Zone as a barrier is confirmed by estimates of the proportion of total radionuclides released outside the zone each year (not including the Sarcophagus). Release does not exceed 0.002%, even in the most pessimistic estimates, but if the radioactivity of the Sarcophagus is taken into account this value is reduced to 2 10-5 % per annum. Taking into account the steady trend to reduce radionuclide release outside the Exclusion Zone in time, it may be concluded that the Zone's natural landscape complex, strengthened by technological protection systems (hydrological dikes, forest fire protection stripes, et. al.), forms an effective barrier to radionuclide migration.

Since the regularity of radionuclide migration and fixing in complex natural systems is not studied extensively enough, complex scientific monitoring in the Exclusion Zone is one of the main prerequisites for minimising risks linked to radionuclide migration beyond the Zone.

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Contamination outside the Exclusion Zone

The territories of Belarus, Russia and Ukraine have undergone the heaviest contamination on account of the Chernobyl disaster. Since air masses containing airborne radioactive substances transited over the north of the globe for several weeks, the contamination spread to almost all of the countries in Europe with the greatest impact in the Scandinavian and Alpine regions. The formation of radioactive fields of contamination outside the borders of the former Soviet Union began during the night of 27 April 1986 and was actually completed in the first ten days. Rainfall at that time caused the formation of zones with high levels of radionuclide fallout on the territories of Sweden, Finland, Germany, Austria, Switzerland, Greece, Bulgaria, Romania and Georgia

Table 2 and Figs. 2 and 3 give an idea of 137Cs distribution in relation to the distance from ChNPP. Rin and Rout are the inner and outer radii of the radioactive zones with q as an average density of contamination. Q is the percentage contribution of a given zone's activity to the total activity released, and S is the percentage ratio of the zone as compared to the total area of Europe. Almost 55% of the total 137Cs amount was deposited within an 800 km distance on the territory of former Soviet Union countries.

Table 2 - Distribution of 137Cs over the territory of Europe

Rin

Rout

S territory

Q 137Cs

q 137Cs

[km]

[km]

%

%

[kBq m-2]

0

10

0.0034

1.70

5,030

10

30

0.0275

4.69

1,730

30

100

0.3129

7.19

235

100

400

5.1587

24.11

48

400

800

15.275

16.49

11

800

1,400

30.176

25.46

8.6

1,400

2,000

32.695

15.47

5.7

2,000

3,000

16.355

4.89

3.1

 

 

 

 

 

0

10

0.003

1.70

5,030

0

30

0.03

6.39

2,100

0

100

0.34

13.58

400

0

400

5.50

37.69

70

0

800

20.78

54.18

27

0

1,400

50.95

79.64

16

0

2,000

83.65

95.11

12

0

3,000

~ 100

~ 100

10

 

 

 

 

 

800

3,000

79.226

45.82

6.0

1,400

3,000

49.05

20.36

4.2

 

 

Fig. 2 indicates the maximum 137Cs distribution to be 1,000-1,400 km away from the ChNPP, which corresponds to the higher contamination levels in the Alps, the Balkans and Finland. Another local maximum of contamination is 1,500-1,900 km away from the NPP, caused by anomalous contamination of the territories of Sweden and Norway.

 

Figure 2 - Correlation between the area of Europe and amount of 137Cs in relation to distance from Chernobyl NPP (% of total quantities)

a - % of the area of Europe

a - % of the amount of 137Cs

Distance from ChNPP (km)

 

Figure 3 - Average density of 137Cs contamination against distance from ChNPP

 

 

a - overall amount of 137Cs (%)

a - density of 137Cs contamination (kBq km-2)

Distance from ChNPP (km)

The main factors in the distribution of radioactive contamination across Europe were massifs and plateaus, with the zones of heavier fallout configured more than 500 km away from ChNPP. These were the Carpathian mountains, the Donetsk ridge, Apennines, Scottish Uplands, as already mentioned the Alps and Balkans, etc. It was there that small 20 100 km2 areas with a contamination density of over 100 kBq m-2 were formed.

Almost two thirds of the caesium released was concentrated on the territories of Belarus, Russia and Ukraine. Around 10% of the overall release was deposited in the Scandinavian peninsula, and half as much in the countries of the Alpine region.

In general terms, the situation in Europe is as follows: the Chernobyl accident has left its mark on two-thirds of the continent, where radiocaesium contamination exceeds pre-accident quantities.

Radioactive contamination of water bodies occurred as a result of both direct fallout of radioactive aerosols on the surface of water basins and of secondary effects such as radioactivity washout from the surface of water catchment areas, inflow of contaminated water from the more contaminated water bodies and areas into the less contaminated ones, mass exchange between bottom sediment and aquatic masses, discharge of contaminated subsurface water into surface water bodies etc.

The highest levels of contamination were registered during the period of maximal fallout in the first ten days of May 1986, after which water contamination levels started to decline. The overall radioactivity in the river Pripyat fell from about 106 Bq l-1 during the first days of the accident to 104 - 103 Bq l-1 by the beginning of June. Nowadays the maximal levels of 90Sr in the Pripyat standing at 20 Bq l-1, with up to 100 Bq l-1 and more detected in the water bodies of the neighbouring zone.

Chernobyl radionuclides have been detected in various rivers of the European part of the USSR and Western Europe. Recovery of pre-accident levels of background radioactive contamination has been rather slow in many rivers, whereas it has not occurred in some rivers even 16 years after the fallout in 1986. Notwithstanding the very low levels of contamination currently found in these rivers, the content of 137s and 90Sr is found to increase during periods of spring run-offs and rainfall each year.

In the years since the accident and in periods of water protection operations the dynamics of radioactive contamination of the Pripyat and Dnepr rivers have been entirely shaped by the hydrological pattern of the run-off formation over the watershed drainage area, river water levels, and the transformation of the physico-chemical forms of 90Sr and 137Cs radioactive contamination in the soils of the watershed and the Pripyat floodplain. In 1986 a substantial contribution to radioactive run-off of the Dnepr upstream from its influx into the Kyiv reservoir came from the river Sozh, which collects its water from the contaminated areas of the Gomel region in Belarus and partially within the so-called Bryansko-Tulskiy caesium spot of the fallout. From 1993 onwards the levels of water contamination in the Dnepr mouth stabilised and in 1995 they fell closer to the pre-accident levels for the best part of a year.

As far as contamination of ground water is concerned in the years since the ChNPP accident, the initial contamination of underground water both of the first surface aquifer (in Quaternary deposits) and of the deeper aquiferous horizons has occurred also at a substantial distance from the plant, but within the Exclusion Zone.

In the most cases the radioactive contamination of underground water caused by the accident is comparable with natural radiation levels and significantly lower than the permitted levels for portable water (for 137Cs and 90Sr).

Air contamination was substantial only during the first two months after the accident and then fell to acceptable levels. It is now insubstantial even inside the Chernobyl NPP site.

Several items of the normal diet were contaminated by radioactive matter. In the initial period following the accident, basic foodstuffs such as milk and green vegetables had contamination levels in excess of what is considered acceptable by the WHO/FAO "Codex Alimentarius" Commission in terms of maximum permitted contamination levels for foodstuffs sold on the international market.

As a result of the natural radioactive decay, migration and soil fixing of radionuclides and also of the countermeasures implemented, the main mass of foodstuffs have present radionuclide contamination levels which are lower than the permitted national standards (similar to international one). However contamination of some of the milk received from private farming exceeds the permitted levels. The contamination of some forest produce (mushrooms, berries) also exceeds the permitted levels. As a result, part of the population in the contaminated areas receives significantly higher doses than the population on average.

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Consequences of the accident for public health

Exposure doses received by "liquidators"

About 200,000 persons who participated in 1986-1987 in the "liquidation" of the accident's consequences received average doses of around 100 mSv. Around 10% of them received doses of around 250 mSv; a smaller percentage received doses exceeding 500 mSv. Several dozen of those providing immediate response to the accident received potentially lethal doses of a few thousands of millisieverts which, unfortunately, was not documented.

Exposure doses received by the population

The 116,000 people evacuated from the Exclusion Zone in 1986 had already been exposed to radiation. Fewer than 10% had received doses of more than 50 mSv and fewer than 5% had received doses of more than 100 mSv.

The radioiodines released delivered radiation doses to the thyroid gland. Iodine was absorbed into the bloodstream, generally by ingestion in foodstuffs, mainly contaminated milk, and also by inhalation of the initial radioactive cloud, and accumulated in the thyroid gland. Doses to the thyroid were anticipated to be particularly high compared with those to other body organs, especially for children. Equivalent doses to the thyroid were estimated at up to several Gy (made primarily on the basis of measurements reported for 150,000 people in Ukraine and also in Belarus and the Russian Federation) .

Clinical effects

A total of 237 individuals exposed to radiation in the course of their work were stated to be suffering from clinical syndromes attributable to radiation exposure and were admitted to hospital. Acute radiation syndrome (ARS) was diagnosed in 134 cases. Of these 134 patients, 28 died as a consequence of radiation injuries, all within the first three months.

It should be noted that the data concerning ARS cases did not cover heavily exposed individuals who, for various reasons, were not examined by qualified medical personnel.

There is no doubt that hospitalised patients received the best possible treatment in line with the state of knowledge at the time, in the most experienced centre available. However, the treatment entailing bone marrow transplantation recommended at the time was of little benefit.

At present, the more severely affected patients suffer from multiple ailments, including effects of mental stress, and are in need of up to date treatment and preventive measures against secondary effects. Health care should be provided for these patients, and their state of health should be monitored over the forthcoming two to three decades.

A highly significant increase in the incidence of thyroid cancer among those persons in the affected areas who were children in 1986 is the clear evidence to date of public health impact of radiation exposure as a result of the Chernobyl accident. In 1991, the report on the International Chernobyl Project had stated that "it is expected that there will be a radiogenic excess of thyroid cancer cases in the decades to come. This risk relates to thyroid doses received in the first months after the accident..." . This increase in incidence has been observed in Belarus, Ukraine and the Russian Federation. In most cases the diagnoses have been confirmed by international experts.

The number of people with thyroid cancer began to increase around 5 years after the accident. This number continues to rise. In some areas the incidence is over a hundred times higher than before the accident. Over 10 000 cases have already been reported.

The increase has been observed in children who were born before or within six months of the accident; the incidence of thyroid cancer in children born more than six months after the accident drops significantly to the low levels expected in unexposed population groups. Moreover, most of the cases of thyroid cancer are concentrated in areas thought to have been contaminated by radioiodines as a result of the accident. Thus both temporal and geographical distributions clearly indicate a correlation between the increase in incidence and radiation exposure due to the Chernobyl accident .

The data presented indicate that the majority of thyroid 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 in cases could only to a minor degree be attributed to more accurate and widespread detection due to screening.

The pathology of virtually all the thyroid cancer cases shows papillary carcinomas, many with an unusual solid/follicular pattern of growth. The type of molecular alterations so far studied has not shown any significant differences from tumours of the same type in thyroids not exposed to radiation. However, these alterations are more frequent in tumours of thyroids exposed to radiation.

Analysis by age at exposure confirmed the hypothesis that very young children were most at risk. It is now considered that the increased incidence of thyroid cancer in those exposed as young children may persist. This could increase the prevalence of thyroid cancer in the affected group in the future, and adequate resources will be required to deal with it.

The extent of the future incidence of thyroid cancers as a result of the Chernobyl accident is very difficult to predict. There remain uncertainties in dose estimates and, although it is not certain that the present increase in incidence will be sustained in the future, it will most probably persist for several decades. If the current high relative risk is sustained, there will be a substantial increase over the coming decades in the incidence of thyroid carcinoma in adults who received high radiation doses as children.

Besides the confirmed increase in the incidence of thyroid cancer, there have been some reports of increases in the incidence of specific malignancies in some populations living in contaminated territories and in the "liquidators". These reports, however, may require further investigation.

Leukaemia, a rare disease, has become a major concern after radiation exposure. Few fatalities due to radiation-induced leukaemia would theoretically be expected according to predictive models (based on data from the survivors of the atom bomb drops on Japan and others). The total expected excess fatalities due to leukaemia would be of the order of 470 among the 7 million residents of "contaminated" areas and "strict radiation monitoring zones", which would be impossible to distinguish from the spontaneous incidence of about 25,000 fatalities.

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