Environmental Engineering Reference
In-Depth Information
18.12 CONCLUSION
The accident in the reactor of the fourth block of Chernobyl NPP resulted in the ejection of radio-
active materials with a summary activity of 3 × 10
18
Bq without taking inert gases into account.
As a result of the fallout of radioactive gas-aerosol products of the accident on the earth's surface,
extensive territories not only of the former USSR but also of other countries were polluted.
Ejection from the reactor was not stable both in terms of concentration and radioactive com-
position. Usually, the composition of aerosol particles did not correspond to the composition of
irradiated nuclear fuel at the moment of the accident. Transport of activity from the reactor at long
distances took place on particle carriers of submicrometer range.
Volatile compounds of I, Te, Ru, and Cs for a very long period of time were found in both aerosol
and gaseous form. The largest portion of the gaseous component was for radioactive iodine.
After stopping ejection from the destroyed reactor, radioactivity at the ground level was deter-
mined by secondary raising of aerosol particles from polluted surfaces. Aerosol concentration
depends on many natural and technogene factors. Variations of concentration under typical condi-
tions were in the range of an order of magnitude. A decrease in concentration took place substan-
tially faster than according to the law of radioactive decay. Radionuclides were disposed on the
same carriers, and sizes of the particles were in the micrometer range.
Fires on the territory, polluted by radionuclides of Chernobyl origin, resulted in the sharp growth
of aerosol concentrations of radiocesium both in the zone close to the source and in the far zone at
a distance of hundreds of kilometers. Its transport from the ire zone takes place on submicrometer
carriers and the water-soluble forms of Cs in aerosols of ire origin are increased substantially.
“Shelter” was built above the reactor in order to prevent pollution of the environment from
radioactive materials. Only some percent of permissible levels for 1 GWt capacity nuclear object
ejection go through cracks in the walls of the “Shelter.” Inside this construction, aerosol concentra-
tion depends on the type and intensity of provided work and changes in the range of some orders
of magnitude. The composition of particles usually corresponds to the composition of nuclear fuel,
and long-lived α- and β-nuclides are disposed on particles of the same sizes of micrometer and
sometimes submicrometer range.
Radon and thoron in premises of the “Shelter” is a negative factor that has not been taken into
account before 2000. The main dangers are the DPs of radon and thoron. First, some of them radiate
α-particles. Secondly, being placed on submicron aerosols they intrude the lowest parts of lungs—
bronchi and alveolus. Due to the inhalation of aerosol carriers of DPs of natural noble gases the
average radiation dose for personnel of the “Shelter” object may reach 20 mSv/g, that is, the value
of the effective radiation dose limit.
The dispersity of carriers of accident products was in the range of AMAD 0.5-8 μm. Such
a wide range of sizes was probably because the aerosols were produced from different sources,
with different mechanisms of formation. DPs of radon and thoron had the AMAD in the range of
0.02-0.8 μm, the 50% interval falling to the AMAD = 0.15 μm. Consequently, radio nuclide carriers
of the Chernobyl genesis had appeared as a rule because of dispersion and DPs of radon and thoron
as a result of condensation.
The physicochemical characteristics of radioactive aerosols of Chernobyl origin are still being
studied very extensively. At the present time, the 30 km zone around Chernobyl NPP and “Shelter”
is a unique experimental testing area, where it is possible to conduct a wide spectrum of studies,
particularly studies of the behavior of disperse systems, and the testing of new methods of measure-
ments and analysis of radioactive aerosols.
ACKNOWLEDGMENT
Chapter translated from original Russian by Lev S. Ruzer and Natalya A. Tchuksina.
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