5. HUMAN EXPOSURE LEVELS
5.2. External exposure
5.2.2. Input data for the estimation of effective external dose
Numeric values for the parameters listed above have been determined from long term dosimetric investigations in the most highly contam- inated regions after the Chernobyl accident.
5.2.2.1. Dynamics of external gamma dose rate over open undisturbed soil
Immediately after the accident, external gamma exposure rates were relatively high, and contributions from many short lived radionuclides were important. Thus in the contaminated areas outside the Chernobyl nuclear power plant boundaries the initial dose rate over lawns and meadows ranged between 3 and 10 µGy/h in areas
contaminated at about 37 kBq/m2 (1 Ci/km2) of
137Cs and up to 10 000 µGy/h within the CEZ with
higher deposition levels. Exposure rates decreased rapidly, due to the radioactive decay of short lived radionuclides, as shown in Fig. 5.3.
Owing to different isotopic compositions of radionuclide fallout in different geographical areas [5.8, 5.13, 5.14], the contribution of short lived radionuclides to the overall dose rate was highly variable. In the CEZ, 132Te + 132I, 131I and 140Ba +
140La dominated during the first month and then
95Zr + 95Nb for another half year before 137Cs and 134Cs became dominant (Fig. 5.4). In contrast, in the
far zone only the radioiodine isotopes dominated during the first month; afterwards 137Cs and 134Cs
dominated, with a moderate contribution from
103Ru and 106Ru (Fig. 5.5). Since 1987 more than
=
∑
OF CF∫
LF dt E i k LF Location factor over soil( )
t D Dose conversion factor ik OF Occupancy factor ik CF =∑
∫
E i i ik ik i Dose rate t DExternal dose to humans
D
D
( )
t( )
tFIG. 5.2. Model of external exposure of the kth population group (i is a location index) [5.9].
90% of the dose rate in air has come from the gamma radiation of long lived 137Cs and 134Cs. Thus
the radionuclide composition of the deposited activity was a major factor in determining the external exposure of the population in the early period of time after the accident. Model estimates of the gamma dose rate in free air (90% confidence interval) based on the radionuclide composition of
the deposited activity agree well with the measured values during the first month after deposition (see Fig. 5.6).
The influence of radionuclide migration into soil on the gamma dose rate has been determined using gamma spectrometric analyses of over 400 soil samples taken during 1986–1999 in the contami- nated areas of Germany (Bavaria), the Russian Time after the accident (days)
0.1 1.0 10.0 100.0 1000.0 1 10 100 1000 10 000
North-west direction, CEZ South direction, CEZ Far zone (>100 km)
Absorbed dose rate in air (nGy/h per 1 kBq/m
2 of
137
Cs)
FIG. 5.3. Dynamics of standardized dose rate in air over undisturbed soil after the Chernobyl accident in different geographical areas [5.12].
Time after the accident (days) North-west direction, CEZ
Contribution to absorbed dose rate in air 1 m above gr
ound (%) Cs-137; Cs-134; Cs-136 Zr-95 + Nb-95 Te-132 + I-132 I-131; I-133 Ba-140 + La-140 Ru-103; Ru-106 1 10 100 100 80 60 40 20 0
FIG. 5.4. Relative contribution of gamma radiation from individual radionuclides to the external gamma dose rate in air during the first year after the Chernobyl accident (north-west direction, CEZ) [5.12].
Time after the accident (days)
0 20 40 60 80 100 1 10 100 Cs-137; Cs-134; Cs-136 Te-132 + I-132 Ru-103; Ru-106 Ba-140 + La-140 I-131; I-133 Zr-95 + Nb-95 Far zone (>100 km)
Contribution to absorbed dose rate in air 1 m above gr
ound (%)
FIG. 5.5. Relative contribution of gamma radiation from individual radionuclides to the external gamma dose rate in air during the first year after the Chernobyl accident (far zone — more than 100 km from the Chernobyl nuclear power plant) [5.12].
Time after the accident (days)
0 1 2 3 4 5 6 0 5 10 15 20 25 30 35 Mean 5% 95%
Dose rate (relative units)
FIG. 5.6. Dose rate in air during the first days after the accident in several rural settlements in the Bryansk and Tula regions of the Russian Federation (normalized to the dose rate on 10 May 1986). Points indicate dose rate meas- urements and curves represent calculated values according to the isotopic composition [5.7].
Federation, Sweden and Ukraine [5.7, 5.8, 5.15]. The analysis also included data on the 137Cs distri-
bution in soil at sites in the north-east region of the USA, whose contamination was attributed to nuclear tests at the Nevada test site [5.16], and in Bavaria (Germany), where contamination was due to global fallout. The last two data sets were obtained 20 to 30 years after deposition; this allows for long term predictions to be applied to the Chernobyl depositions. The measurement sites were considered to be representative of reference sites (i.e. open, undisturbed fields).
For a few years after the accident, the dose rate over open plots of undisturbed soil decreased by a factor of 100 or more compared with the initial level (see Fig. 5.3). At that time, the dose rate was mainly determined by gamma radiation of caesium radionuclides (i.e. 137Cs (half-life 30 years) and 134Cs
(half-life 2.1 years), and later, one decade and more after the accident, mainly the longer lived 137Cs).
Long term studies of external gamma exposure rates during the past 17 years have shown that the external gamma exposure rate is decreasing faster than that due to radioactive decay alone. Golikov et al. [5.7] and Likhtarev et al. [5.8] have calculated a reference function for 137Cs gamma radiation dose
rate that has 40–50% of the exposure rate decreasing with an ecological half-life of 1.5–2.5 years and the remaining 50–60% decreasing with an ecological half-life of 40–50 years, as indicated in Fig. 5.7. The latter value is rather uncertain. It corresponds to an effective half-life of 17–19 years that takes into account both the radioactive decay of 137Cs and its gradual deepening in soil.
5.2.2.2. Dynamics of external gamma dose rate in anthropogenic areas
In settlements in urban and rural areas, the characteristics of the radiation field differ consid- erably from those over an open plot of undisturbed land, which is used as the reference site and starting point for calculation of external dose to people from deposited activity. These differences are attrib- utable to varying source distributions as a result of deposition, runoff, weathering and shielding. All such effects can be summarized by the term ‘location factors’.
Location factors for typical western European buildings have been assessed [5.11, 5.17, 5.18]. Gamma spectrometric measurements performed in Germany and Sweden [5.19–5.22] allowed the
determination of location factors in urban environ- ments and their variation with time over several years after the Chernobyl accident. The character- istic feature, and advantage, of these investigations is that they began immediately after the accident, whereas systematic investigations of location factors in the contaminated areas of Belarus, the Russian Federation and Ukraine began two to three years after the accident. The results of one such later investigation in Novozybkov (in the Bryansk region of the Russian Federation) are presented in Fig. 3.12 (Section 3).
5.2.2.3. Behaviour of people in the radiation field
The influence of the behaviour of different social population groups on the level of exposure can be taken into account if the frequency with which people of the kth population group remain at the location of the ith type is known. The times spent in various types of location (indoor, outdoors on streets or in yards, etc.) by members of different population groups have been assessed on the basis of responses to a questionnaire. Data collected included age, sex, occupation, information about dwelling, etc. An example of the results is shown in Table 5.3, where values of occupancy factors for the summer period are presented for different groups of the rural populations of Belarus, the Russian Federation and Ukraine [5.15].
Time after the accident (years)
0.0 0.2 0.4 0.6 0.8 1.0 0 5 10 15 20 25 30 35 95% 5% Median ‘Chernobyl’ caesium
Bryansk region (Russian Federation)
Caesium from Nevada test site
(north-west USA)Global falloutfrom Bavaria (Germany)
r(t) = 0.38 exp(–0.693 t/2.4y) + 0.39 exp(-0.693 t/37y)
Dose rate (r
elative units)
FIG. 5.7. Reduction of the 137Cs gamma dose rate in air due to caesium migration in undisturbed soil relative to the dose rate caused by a plane source on the air–soil interface (from Ref. [5.7]).
5.2.2.4. Effective dose per unit gamma dose in air
Mean values of conversion factors CFk, which convert the gamma dose rate in air to the effective dose rate in a member of population (age) group k, were obtained for the three groups of population by use of phantom experiments [5.15] and Monte Carlo calculations [5.23]. The values were 0.75 Sv/ Gy for adults, 0.80 Sv/Gy for schoolchildren (7– 17 years) and 0.90 Sv/Gy for pre-school children (0– 7 years). For the calculation of effective doses,
conversion factors CFk were used that are
independent of the location and time after the accident.