8.2.MAJOR SOURCES AND PATHWAYS OF HUMAN EXPOSURE IN THE DNIEPER RIVER BASIN

Radiation exposure of the population of the Dnieper River basin is caused both by naturally occurring radionuclides (40K, radionuclides of 238U, 235U and 232Th decay chains, etc.) and human-made radionuclides, mainly fission products (especially 137Cs and 90Sr). The pathways of human exposure include external exposure from deposited gamma emitting radionuclides and internal exposure via ingestion of contaminated food and drinking water as well as inhalation of airborne radionuclides. The major pathways of human exposure from environmental radioactivity are schematically presented in Fig. 8.1.

The concentrations of natural radionuclides in the Dnieper River basin, and associated human exposure levels, are generally close to average worldwide levels. However, in uranium mining and milling areas in the Dnipropetrovsk region of Ukraine, concentrations of uranium and its daughter radionuclides are significantly elevated in river water due to releases to the Zheltaya River

and leakage from tailings into Dnieper tributaries and ultimately into the Dnieper River itself. If river water is used for drinking and/or irrigation, elevated levels of uranium compounds and its daughter radionuclides may enter the human body. Ingestion is the major pathway of human exposure, due to past and present operations of the uranium industry. However, in the immediate vicinity of uranium tailings, a person could be subjected to external exposure from gamma radiation and to internal exposure via inhalation of radon and its daughter products, and possibly tailings dust.

Whereas natural radiation has accompanied the whole of human history, significant environmental contamination with human-made radionuclides occurred during two time periods. The first period of exposure started in the 1950s, and increased in the 1960s, as the result of global stratospheric fallout from worldwide nuclear weapon testing. The second period was in 1986, when a large radioactive release occurred during the Chernobyl accident. The largest population doses occurred in the Dnieper River basin, and residual radionuclides remain a source of radiation exposure.

	Airborne effluent	Desposition
	discharge
	Source	Direct	irradiation
	Surface deposits		Resuspension
	Liquid effluent
	discharge	External
	Run-off			irradiation

Original cloud of contaminated air

Inhalation

External irradiation

Deposition on skin/clothing

Deposition Deposition

Water bodies

External irradiation

Sedimentation

External irradiation

	Uptake	Sand and sediment
		Decom-	Uptake
	n	position		s
				m
	o
	i			s
	t
	i			i
	s			n
Uptake	o			a
	p			g
	m			r
	o			O
	c			c
	e	Aquatic plants		i
	D			t
				h
				n
Water				e
			Uptake	B
purification

Food preparation

Aquatic animals

Plants and crops

		Resuspension
		rain-splash
Ingestion		Root
		uptake
External irradiation	Irrigation	Topsoil
Food		Subsoil
preparation
	Uptake
Food		Irrigation
preparation

Food and drink

Animals

Drinking

FIG. 8.1. Main environmental pathways of human radiation exposure [8.7].

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Compared with the Chernobyl related environmental contamination of the Dnieper River basin with 137Cs and 90Sr, the historical global fallout of the same radionuclides, which happened 20– 30 years earlier, can be neglected in the present dose calculations (see Ref. [8.6] for maps of the distribution of fallout from global weapons testing and the Chernobyl accident).

For completeness of public environmental dose assessment, the levels of dose caused by regular discharges of Ukrainian nuclear power plants should be considered. Reference [8.8] used the PC CREAM computer code [8.9] to calculate dose rates for conditions of actual radioactive discharges in 1988–1998 from four Ukrainian nuclear power plants located in the Dnieper River basin (Chernobyl, Khmelnitski, Rovno and Zaporozhe). For these calculations, the minimum distance between operating units and settlements was taken to be 2 km. The results show that the appropriate annual doses range from 0.07 μSv for the Khmelnitski nuclear power plant to 0.4 μSv for the Zaporozhe nuclear power plant; these doses rapidly decrease with increasing distance between the source (the nuclear power plant) and the target (the settlement). The radionuclides that contribute most to the dose are 14C (long term exposure), 131I, inert radioactive gases and tritium.

In the Russian Federation, gas–aerosol releases of radionuclides to the environment from RBMK reactors (the Kursk and Smolensk nuclear power plants) are higher than those from WWER units. Nevertheless, the population radiation doses resulting from releases in the areas around these plants are much lower than the site specific dose constraints. Over the first 16 years of operation of the Kursk nuclear power plant, the radiation dose to the critical group was 40 μSv or, on average, 2.5 μSv/ a [8.10]. The main contributor to this dose (about 95%) was exposure due to radionuclides in airborne plumes.

From comparison of the above exposure levels from nuclear power plants with the background exposure (see Section 8.1), it is obvious that they are negligible and therefore are not considered further. The only radiological issue of concern with regard to nuclear power plants arises from possible exposure following a nuclear accident. The Chernobyl accident in 1986 was an example of a most severe nuclear power plant accident. Possible levels of human exposure in the event of an accident at a Ukrainian nuclear power plant on the Dnieper River are considered in Section 9.

8.3.MODELS OF EXTERNAL AND INTERNAL EXPOSURE

8.3.1.Model for external exposure

In this analysis we consider quasi-stationary conditions when the dose rate both in the open air and in buildings changes slowly during the year, mainly because of the presence or absence of snow cover. In the Dnieper River basin, such conditions are applicable to long lived, naturally occurring radioactive material (NORM) and fission products, from both global fallout and the Chernobyl accident. In the case of the Chernobyl accident, the dominant radionuclide for external dose calculations is 137Cs, which has a half-life of 30 years and a half-life for dose reduction due to soil redistribution processes of about 50 years [8.3, 8.11]. The effective half-life of 137Cs/137mBa gamma radiation dose rate reduction in open air due to both mechanisms is about 20 years.

In order to assess external doses to humans caused by natural radiation in temperate climate conditions, Ref. [8.3] used a simple model of humans spending 80% of their time indoors and 20% outdoors. This model can be applied to areas slightly contaminated with Chernobyl fallout. For the most significantly Chernobyl affected areas, more precise models have been developed to justify countermeasures in the early period, and to support remediation and epidemiological studies in the later period.

Deterministic models of exposure of different age and social groups of the population residing in the Dnieper River basin have been developed for the Chernobyl accident on the basis of numerous experimental investigations [8.12, 8.13]. These studies included measurements of the dose rate in different periods after the accident above virgin soil and in typical plots of settlements (including residential, industrial and recreational buildings), inhabitants’ surveys about their mode of behaviour during different seasons, radionuclide analyses in profiles of virgin soil and over 10 000 measurements of individual doses using the thermoluminescence method [8.11–8.13].

According to the deterministic model presented in Fig. 8.2, the average annual effective dose Ek in the kth group of a settlement’s inhabitants depends on: the absorbed dose rate in

·

air, D(t), at a height of 1 m above an open plot of virgin soil in this settlement and its vicinity; the location factor, LFi, which is equal to the ratio of

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Dose rate		Location
over soil		factor
D(t)		LFi

External dose to a human Ek

Ek = ∫ D(t)dt ∑ LFi OFik CFik

i

CFik		OFik
Dose conversion		Occupancy
factor		factor

FIG. 8.2. Model of external exposure of the kth occupational group of the population (i: location index).

dose rate at the ith typical plot in the settlement to

·

D(t); the occupancy factor, OFik, which is equal to the fraction of time spent during a year at the ith plot; and a conversion factor, CFik, which converts the absorbed dose rate in the air to the effective dose. Numerical values of the model parameters both for the Russian Federation and Ukraine can be found in Refs [8.11-8.13].

Table 8.4 presents the generic dose conversion parameters needed in order to reconstruct the past, assess the present and forecast the future average effective external doses to the adult population of a settlement located in the intermediate (100 km < D < 1000 km) zone of Chernobyl contamination based

on experimental data and models developed in the Russian Federation and Ukraine [8.12, 8.13]. The values for indicated time periods for the population of a settlement are given separately for the urban and rural populations as the ratios of the mean external dose (E) to the mean 137Cs soil deposition in a settlement, as of 1986 (σ137) (μSv·kBq–1·m–2). From Table 8.4, one can conclude that the urban population has been exposed to a lower dose by a factor of 1.5 to 2 compared with the dose to the rural population living in areas with similar levels of radioactive contamination. This arises because of the better shielding features of urban buildings and different occupational habits.

The parameters obtained from independent sets of Russian and Ukrainian data are in reasonable agreement. Some differences can be explained as being due to the different compositions of radionuclide deposition that occurred in different parts of the Chernobyl affected areas and to various human habits. Multiplication of the parameters presented in Table 8.4 by the mean 137Cs soil deposition (as of 1986) gives an estimate of the external dose caused by gamma radiation from all the deposited radionuclides.

Depending on occupation and type of dwelling, the average doses to different social and age groups of people living in the same Russian settlement differ by a factor of 1.7 from the mean value for a settlement [8.14] — Table 8.5.

8.3.2.Model for internal exposure

The structure of a simple, Chernobyl related model of internal exposure of a person located in an area contaminated with radionuclides is presented in Fig. 8.3. The main pathways of radionuclide intake into the body of a person of kth age and

TABLE 8.4. RECONSTRUCTION AND PROGNOSIS OF THE AVERAGE EFFECTIVE EXTERNAL DOSE TO THE ADULT POPULATION IN THE INTERMEDIATE (100 km < D < 1000 km) ZONE OF CHERNOBYL CONTAMINATION

	Population		E/σ137 (μSv·kBq–1·m–2 of 137Cs)a
		1986	1987–1995	1996–2005	2006–2056	1986–2056
Russian Federation [8.11, 8.12]	Rural	14	25	10	19	68
	Urban	9	14	5	9	37
Ukraine [8.13]	Rural	24	36	13	14	88
	Urban	17	25	9	10	61
a σ137 is given as for 1986.

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TABLE 8.5. RATIO OF THE AVERAGE EXTERNAL EFFECTIVE DOSES IN SOME POPULATION GROUPS TO THE MEAN DOSE IN A SETTLEMENT [8.14]

Type of dwelling	Indoor workers	Outdoor workers	Herders, foresters	Schoolchildren
Wooden	0.8	1.2	1.7	0.8
One to two storey, brick	0.7	1.0	1.5	0.9
Multistorey	0.6	0.8	1.3	0.7

gender group are considered: viz. inhalation with average inhalation rate IRk (m3/d) of air with time dependent concentration of rth radionuclide ACr (Bq/m3) and ingestion of the set of fth food products, including drinking water, with consumption rate CRfk (kg/d) with time dependent specific activity SAfr (Bq/kg). The model is also applicable to the intake of NORMs.

Data on the radionuclide content in the air, drinking water and agricultural and natural food products are obtained from current radiation monitoring and radioecological studies. The rates of air inhalation by persons of different ages and genders for different activities are well known from physiological studies [8.15]. The consumption rate of different food products varies significantly, depending both on age and gender and on local technologies of agricultural production, collection of natural food, dietary habits, etc. For internal dose estimation after the Chernobyl accident, these data were obtained by population surveys [8.16] and analysis of statistical data. Age dependent dose coefficients for inhalation and ingestion of different

Air

Inhalation

Inhalation dose

concentration

rate

coefficient

ACr (t)

IRk

DCrk

Inhalation

Ek = IRk ∑ DC rk ∫ ACr (t) dt

r

Ingestion

Ek

= ∑ CDrk

∑ CR fk ∫ SAfr (t) dt

r

f

SAfr (t)

CRfk

CDrk

Food specific

Food

Ingestion dose

activity

consumption rate

coefficient

FIG. 8.3. Model of internal exposure of the kth age and gender group of the population. r: radionuclide index; f: food product index.

types of radioactive material are usually taken from ICRP publications [8.15, 8.17, 8.18].

Table 8.6 presents the generic dose conversion parameters needed in order to broadly reconstruct the past, assess the present and forecast the future average effective internal dose of the adult rural population of a settlement located in the intermediate (100 km < D < 1000 km) zone of Chernobyl contamination based on experimental data and models developed in the Russian Federation and Ukraine [8.16]. The values for each indicated time period for the population of a settlement are given separately for various soil types as the ratios of the mean internal dose (E) to the mean 137Cs soil deposition in a settlement as of 1986 (σ137) (μSv·kBq-1·m-2). From Table 8.6 one can conclude that people living in areas with a higher clay content (e.g. black soil) obtained a lower internal dose because of slower radionuclide transfer from soil to plants.

The urban population of the affected areas has been exposed to lower internal doses compared with the doses to the rural population living in areas with similar levels of radioactive contamination, because of consumption of foodstuffs from noncontaminated areas and different dietary habits. The parameters obtained from independent sets of Russian and Ukrainian data significantly differ for some soil types and time periods (see Table 8.6). Some of these discrepancies can be explained by the different meteorological conditions (mainly dry in Ukraine and wet in the Russian Federation) of radionuclide deposition that occurred in different parts of the Chernobyl affected areas and different food consumption habits.

Multiplication of the parameters presented in Table 8.6 by the mean 137Cs soil deposition (as of 1986) gives an estimate of the internal effective dose caused by radiation from 137Cs and 134Cs (for the Russian Federation, also from 90Sr and 89Sr). Dose estimates are given for conditions when countermeasures against internal exposure were not applied. Thyroid doses caused by intake of iodine radionuclides in the immediate aftermath of

133