Radioactive Aerosols of Chernobyl Accident - Aerosols: Measurement, Dosimetry, and Health Effects - page 497

(HOYaT-2) at a 2 km distance south-east of the “Shelter.” There, the stumps were pulled, ground
was leveled by scrapers, foundation ditches were excavated, and ground irradiated by radionu-
clides was carried out. A part of the radioactive substances came to the area of AU-3 as a result
of the dust rising to air.
A road was constructed along the southern fence of the local zone for delivery of hardware and
automobiles to the berm, which was to be partially destroyed in order to obtain place for the con-
struction of a new and safer containment. In that time period the concentrations of 137 Cs and 241 Am
increased 10-fold in the area of AU-3 (see Figure 18.18). The maximal content of 137 Cs reached
0.05 Bq/m 3 in samples taken from August 7-22.
Σβ of 12 and 15 mBq/m 3 was registered in February and March 2007, in the area of AU-2, which
are approximately 10 times higher than average for samples of that year. Most probably the surge
of concentrations was caused by dismantling of the concrete protection wall and of the site for top
assembly of steel trusses.
Distinctive minimums apart from peaks of radioactive substance contents in the air of the local
zone can be noted in Figure 18.18. They practically always coincide with the cold season of the
year, for example, in 1995, 1998, 2000, 2002, 2003, and 2006. One can conclude from the ear-
lier discussion that dust elevation from underlying terrain was decreasing during a rainy season,
moreover, in the presence of snow, and consequently quantity of radioactive aerosols in the air was
decreasing, too.
18.11 CONCENTRATIONS OF RN, TN, AND THEIR DAUGHTER PRODUCTS
INSIDE AND OUTSIDE OF THE “SHELTER” IN 2002-2009
Radon ( 222 Rn) and thoron ( 220 Rn) daughters occupy a special place among the radioactive aerosols
presented in the “Shelter”. They affect radiation environment at the premises of the “Shelter” and
detection of radioactive aerosols of Chernobyl genesis. Both gases emanate from the concrete con-
structions of both the former fourth unit and the new elements of the “Shelter” (cascade and separa-
tion walls, materials of the reactor illing from helicopters in 1986 and others) containing natural
radionuclides 226 Ra и 232 Th. Radon and thoron are also coming from the ground (mainly sand), into
which the foundation and underneath premises of the Chernobyl NPP are embedded. Some quantity
of thoron may appear from the irradiated fuel left in the ruins of the fourth unit as a result of decay
of 232 U that had accumulated within 2 years of the reactor operation before the catastrophe. A chain
of three subsequent α-decays of 232 U brings about appearance of thoron, out of which 216 Po and
212 Pb appear later on. As the half-life period of 232 U is 72 years, then the emanation of thoron from
it will last long.
DPs of radon and thoron represent an obstructive factor while monitoring radioactive aerosols in
the “Shelter” as they aggravate radiometry of samples. One should hold ilters with the samples for
6 h to decrease radon DP content approximately 1000-fold to determine concentration of the aerosol
products of Chernobyl accident. Around 4 days are necessary to make the same decrease of DPs
for thoron. But there is also a positive moment: DPs of radon and thoron are sui generis markers of
submicron aerosols. One can use them, for example, to evaluate the eficiency of iltering installa-
tions' and respirators' operation.
Long-term monitoring of radon and thoron DPs was conducted for the irst time at room
No. 207/5 of the “Shelter” in December 2000 [44]. Systematic sampling from ventilation low com-
ing to the “Bypass” system started in 2002, radon and thoron DPs being captured with a package
of ilters simultaneously with aerosol products of Chernobyl accident (see Chapter 17). The results
of determination of volumetric activities of 212 Pb in 2002-2008 are presented in Figure 18.19. As
a rule, they stayed in the range of 0.5-9 Bq/m 3 . That is 50-100 times higher than at simultaneous
sampling conducted by the same technique in March-May 2007, in the territory of the “Shelter”
object. It followed from this fact that the thoron source was situated inside the “Shelter” and 212 Pb
originated at the same place and did not come with the outside air.