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  • The site
  • The RBMK-1000 reactor
  • Events leading to the accident
  • The accident
  • The graphite fire


  • The source term
  • Atmospheric releases
  • Chemical and physical forms
  • Dispersion and deposition
  • Within the former Soviet Union
  • Outside the former Soviet Union


  • Within the former Soviet Union
  • Outside the former Soviet Union


  • The liquidators
  • The evacuees from the 30-km zone
  • Doses to the thyroid gland
  • Whole-body doses
  • People living in the contaminated areas
  • Doses to the thyroid gland
  • Whole-body doses
  • Populations outside the former Soviet Union


  • Acute health effects
  • Late health effects
  • Thyroid cancer
  • Other late health effects
  • Other studies
  • Psychological effects
  • Within the former Soviet Union
  • Outside the former Soviet Union


  • Agricultural impact
  • Within the former Soviet Union
  • Within Europe
  • Environmental impact
  • Forests
  • Water bodies


  • The Sarcophagus
  • Radioactive waste storage sites


  • Operational aspects
  • Scientific and technical aspects



Chapter II


The source term

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 1986 (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, Gu89). The initial estimates were presented as a fraction of the core inventory for the important radionuclides and also as total activity released.

Atmospheric releases

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 100 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. The early estimate for fuel material released to the environment was 3 ± 1.5 per cent (IA86). This estimate was later revised to 3.5 ± 0.5 per cent (Be91). This corresponds to the emission of 6 t of fragmented fuel.

The IAEA International Nuclear Safety Advisory Group (INSAG) issued in 1986 its summary report (IA86a) based on the information presented by the Soviet scientists to the Post-Accident Review Meeting. At that time, it was estimated that 1 to 2 exabecquerels (EBq) were released. This did not include the noble gases, and had an estimated error of ±50 per cent. 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.

However, more deposition data (Be90) were available when, in their 1988 Report (UN88), the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) gave release figures based not only on the Soviet data, but also on worldwide deposition. The total caesium-137 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. These studies estimate that the release fraction of caesium-137 was 20 to 40 per cent (85 ± 26 PBq) based on an average release fraction from fuel of 47 per cent with subsequent retention of the remainder within the reactor building (Be91). After an extensive review of the many reports (IA86, Bu93), this was confirmed. For iodine-131, the most accurate estimate was felt to be 50 to 60 per cent of the core inventory of 3,200 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 (Bu93). 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. from IA86a)

Chemical and physical forms

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. from De95)

       Core inventory                        Total release during
      on 26 April 1986                           the accident

 Nuclide    Half-life    Activity         Percent of     Activity   
                           (PBq)           inventory       (PBq)    

33Xe 5.3 d 6 500 100 6500 131I 8.0 d 3 200 50 - 60 ~1760 134Cs 2.0 y 180 20 - 40 ~54 137Cs 30.0 y 280 20 - 40 ~85 132Te 78.0 h 2 700 25 - 60 ~1150 89Sr 52.0 d 2 300 4 - 6 ~115 90Sr 28.0 y 200 4 - 6 ~10 140Ba 12.8 d 4 800 4 - 6 ~240 95Zr 1.4 h 5 600 3.5 196 99Mo 67.0 h 4 800 >3.5 >168 103Ru 39.6 d 4 800 >3.5 >168 106Ru 1.0 y 2 100 >3.5 >73 141Ce 33.0 d 5 600 3.5 196 144Ce 285.0 d 3 300 3.5 ~116 239Np 2.4 d 27 000 3.5 ~95 238Pu 86.0 y 1 3.5 0.035 239Pu 24 400.0 y 0.85 3.5 0.03 240Pu 6 580.0 y 1.2 3.5 0.042 241Pu 13.2 y 170 3.5 ~6 242Cm 163.0 d 26 3.5 ~0.9

above, 50 to 60 per cent of the core inventory of iodine was thought to have been released in one form or another. 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 0.5 to 1 mm.

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 (De95).

Dispersion and deposition

Within the former Soviet Union

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 100 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-137 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-131 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). Ground depositions of caesium-137 of over 40 kilobecquerels per square metre [kBq/m2] covered large areas of the Northern part of Ukraine and of the Southern part of Belarus. The most highly contaminated area was the 30-km zone surrounding the reactor, where caesium-137 ground depositions generally exceeded 1,500 kBq/m2 (Ba93).

The Bryansk-Belarus spot, centered 200 km to the North-northeast of the reactor, was formed on 28-29 April as a result of rainfall on the interface of the Bryansk region of Russia and the Gomel and Mogilev regions of Belarus. The ground depositions of caesium-137 in the most highly contaminated areas in this spot were comparable to the levels in the Central spot and reached 5,000 kBq/m2 in some villages (Ba93).

Figure 4. Main spots of caesium-137 contamination
See also Figure 4 at end of file

Figure 5. Central spot of caesium-137 contamination
See also Figure 5 at end of file

The Kaluga-Tula-Orel spot in Russia, centered approximately 500 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 28-29 April. However, the levels of deposition of caesium-137 were lower, usually less than 600 kBq/m2 (Ba93).

In addition, outside the three main hot spots in the greater part of the European territory of the former Soviet Union, there were many areas of radioactive contamination with caesium-137 levels in the range 40 to 200 kBq/m2. Overall, the territory of the former Soviet Union initially contained approximately 3,100 km2 contaminated by caesium-137 with deposition levels exceeding 1,500 kBq/m2; 7,200 km2 with levels of 600 to 1,500 kBq/m2; and 103,000 km2 with levels of 40 to 200 kBq/m2 (US91).

Outside the former Soviet Union

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. The most radiologically important radionuclides detected outside the Soviet Union were iodine-131, tellurium/iodine-132, caesium-137 and caesium-134.

Figure 6. Areas covered by the main body of the radioactive cloud on various days during the release

Most countries in Europe experienced some deposition of radionuclides, mainly caesium-137 and caesium-134, 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. For example, the estimated average depositions of caesium-137 in the provinces of Upper Austria, Salzburg and Carinthia in Austria were 59, 46 and 33 kBq/m2 respectively, whereas the average caesium-137 deposition in Portugal was 0.02 kBq/m2 (Un88). 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 (Un88).

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.

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