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.
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
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
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).
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
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.
Chapter II
THE RELEASE, DISPERSION AND DEPOSITION
OF RADIONUCLIDES
Chemical and physical forms
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.
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 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).
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.
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.