Individual annual doses and lifetime doses to the population

In analysing the radiation dose to the population, three periods may be distinguished:

· The first year following the accident is the most significant from the point of view of the radiological impact. The dose in this first year is complex to assess because of the duration and the non-uniform character of the release and fallout, fast changes in isotopic and radiation composition because of radionuclide decay, and the need to take into account the effectiveness of protective measures.

· During the period 1987–1991, the radiation impact was largely determined by the ¹³⁷Cs and ¹³⁴Cs external and internal exposures, by the official limitation and self-limitation of the population's activities, and by extensive countermeasures, including resettlement and decontamination, especially as they affected agricultural and forest production.

· The period 1992–1995 is essentially similar to the second period but the extent of protective action was reduced.

2.4.1. Doses to the thyroid

Until June 1986, ¹³¹I was one of the major radionuclides contributing to the irradiation of the population. A large number of measurements of ¹³¹I activity in the thyroid have shown wide variations (about two orders of magnitude) between individual doses, even within the same settlement. The highest dose values were found in children. Fig. 14 is a map showing mean thyroid doses to children in Ukraine [12]. Radioiodine contamination of milk was the most common cause of thyroid exposure. According to a 1993 publication [30], the coefficient of ¹³¹I interception in pasture grass, due to various weather conditions during the fallout period, seemed higher in the less contaminated territories than in the more contaminated ones. This observation has been confirmed by further studies.

The average dose to the thyroid in young children from the most contaminated rural settlements was 3 Gy, and in children evacuated from some settlements in Belarus the dose even exceeded 10 Gy.

Fig. 14. Geographical distribution of the mean thyroid dose in Ukraine for people exposed in childhood due to radiological release.

2.4.2. External dose

During the first post-accident year, two main sources were responsible for the external exposure of population: the radioactive cloud and radioactive fallout onto ground litter, vegetation and buildings. The assessment, carried out in several countries, of the external dose due to the g-emitters in passing radioactive cloud indicates the small contribution of the cloud to external exposure. According to the highest estimates, it accounted for approximately 3% of the external dose during the first year. The external dose was evaluated using three types of data:

· characteristics of the g-radiation field;

· characteristics of human behaviour in this field;

· coefficients of conversion from the g-radiation field to the dose to individuals.

Models for dose assessment took account of such factors as the radioactive deposition on soil, the radioactive decay of the radionuclides, the vertical migration of long lived

radionuclides in the upper soil layer, the presence of snow, and the modification of these factors in an inhabited environment [31–33].

In addition, reconstruction of external exposure was carried out from 1986 to 1995 in settlements with varying ¹³⁷Cs deposition (40–4000 kBq.m^-2). This external dose reconstruction included the measurement of individual doses by thermoluminescent dosimeters, and the measurement of local dose rates in streets, courtyards, gardens, and houses, and on fallow land.

The results obtained with these two methods are quite similar, but the second approach makes it possible to determine radiation doses to subgroups of the population more accurately, and also the influence of people's life-style on dose. Table XIII shows some results of measurements of external radiation dose to the population of contaminated settlements in the Bryansk region of the Russian Federation. The analysis of these data shows a good correlation with ¹³⁷Cs surface contamination [34].

The critical group were people working in the forests and in agriculture. For urban population, a marked variability in individual dose estimates could be observed, depending on living conditions. The effective external dose to the residents of multi-storey buildings was less than half of that to the residents of single-storey houses located in city suburbs.

On the basis of the analysis of all the data available, a model was created to reconstruct annual radiation doses to the population during the first post-accident decade, and to arrive at reliable assessments. The external doses per unit of ¹³⁷Cs surface contamination are presented in Table XIV [31, 35].

2.4.3. Internal dose from long lived radionuclides

Since the summer of 1986, the internal dose has been mainly caused by ingesting foodstuffs contaminated by ¹³⁴Cs and ¹³⁷Cs and ⁹⁰Sr, and also by inhaling plutonium and ²⁴¹Am with dust particles.

The retrospective assessment of internal doses received by inhabitants of contaminated areas of the Russian Federation in 1986–1995 [31] has been carried out using:

· ¹³⁷Cs and ¹³⁴Cs whole-body counting data (about 1 000 000 measurements);

· the results of ¹³⁷Cs, ¹³⁴Cs and ⁹⁰Sr activity concentration measurements in local food products;

· the results of calculations based on the transfer coefficients of radionuclides in different types of soils prevailing in agricultural land;

· the results of measurements of internal contamination, by radiochemical analyses of urine.

In 1986, the internal dose (E_int) to individuals consisted of two components:

· E_s — the dose to individuals due to surface contamination of vegetation with radionuclides and subsequent consumption of foodstuffs contaminated directly (leafy vegetables) or indirectly (milk, meat); and

· E_r — the dose to individuals due to root uptake of radionuclides deposited in the soil by vegetation and consumption of contaminated foodstuffs. Since the autumn of 1986, the root uptake pathway has been prevalent.

TABLE XIII. MEAN EFFECTIVE EXTERNAL DOSES RECEIVED BY ADULT INHABITANTS OF SOME CONTAMINATED LOCALITIES OF THE BRYANSK REGION, RUSSIAN FEDERATION, 1993–1995

Locality

137Cs soil deposition (kBq.m^-2)

Monthly effective external dose (mSv)

1993 1994 1995

v. Novye Bobovichi 1030 – 160/140 – v. StaryeBobovichi 980 – 190/160 – v. Dobrodeevka 1020 – 220/190 – v. StaryVyshkov 1250 300/260 – – v. Kuznets 850 170/160 – – v. Makarichi 660 150/140 – – v. Yalovka 2600 390/370 – – t. Novozybkov 680 – – 69/64/57 Note: In this table, the first figure is data for inhabitants living in single-storey wooden houses, the second figure, in single-storey stone houses and the third figure (when provided) in a multistorey stone building.

TABLE XIV. STANDARDIZED EFFECTIVE EXTERNAL DOSES TO RURAL AND URBAN POPULATIONS IN VARIOUS POST-ACCIDENT PERIODS (mSv per kBq.m^-2 of ¹³⁷Cs)

Years

Country 1986 1987–1995 1996–2056 1986–2056

rural urban rural urban rural urban rural urban Belarus 32* – 36* – 33* – 100* – Russian

Federation

15 11 21 13 31 16 67 40 Ukraine 16 6 26 11 32** 13** 74** 32**

*) Effective external average doses in the Bragin, Khojniki and Narovlya districts.

**) Preliminary assessments.

It should be noted that in many areas the surface component (E_s), formed during the first post-accident months, was the major source of the internal dose to the population in the ten years from 1986 to 1995. The root component (E_r) in this period and the dose forecast for the future show significant variations depending on the type of soil prevailing in a given region. For example, in the Ukrainian Polessye and Bryansk region, characterized mainly by poor soddy-podzolic sandy and sandstone soils with high soil-to-plant transfer coefficients, doses are the highest, while in chernozem areas (Tula and Orel regions) they are significantly lower (Tables XV, XVI and XVII).

The contribution of ⁹⁰Sr to the internal dose caused mainly by ingestion of ¹³⁷Cs with food is currently negligible but it is predicted to increase in the future, and could reach tens of a per cent on chernozem soils. The internal radiation dose due to ⁹⁰Sr was evaluated by its concentration in major diet components and by the results of measurement of its specific activity in urine samples.

TABLE XV. MEAN EFFECTIVE INTERNAL DOSE TO THE RURAL POPULATION IN VARIOUS POST-ACCIDENT PERIODS IN BELARUS AND THE RUSSIAN FEDERATION (mSv per kBq.m^-2 of ¹³⁷Cs)

Country Region or district

137Cs soil

deposition Years

(kBq.m^-2) 1986 1987–1995 1996–2056 1986–2056

Bragin >555 8 13 7 28

Belarus <555 10 18 9 37

Khojniki >555 8 13 7 28

<555 9 16 8 33

Narovlya >555 9 16 8 33

<555 8 15 8 31

Bryansk >555 10 (0.6%) 11 (1.0%) 9 (7.1%) 30 (2.9%) Russian <555 36 (0.5%) 48 (1.2%) 9 (7.1%) 93 (1.5%) Federation* Tula <555 15 (1.3%) 6 (3.6%) 1.8 (35%) 23 (4.6%) Orel <555 15 (2.0%) 8 (4.1%) 2.4 (3.3%) 25 (5.6%)

* In brackets — the assessment of the ⁹⁰Sr contribution to the internal dose.

TABLE XVI. EFFECTIVE ACCUMULATED INTERNAL DOSE TO THE RURAL POPULATION IN VARIOUS POST-ACCIDENT PERIODS IN UKRAINE (mSv per kBq.m^-2 of ¹³⁷Cs) [37]

Soil–milk transfer

coefficient in 1991 Year

(Bq.L^-1/kBq.m^-2) 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995

<1 8 15 21 25 29 31 34 35 37 38 1–5 42 76 103 124 141 154 165 173 180 185 5–10 94 170 230 277 315 345 369 387 402 414

>10 175 315 425 513 583 638 682 717 744 766

TABLE XVII. ANNUAL INTERNAL AND EXTERNAL EFFECTIVE DOSES TO INHABITANTS OF SOME UKRAINIAN SETTLEMENTS WITH VARIOUS SOIL–MILK TRANSFER COEFFICIENTS

* Dose estimations based on whole body counting data.

** For each settlement, the upper line presents annual effective external dose and the lower line presents annual effective internal dose.

An assessment of annual effective internal dose levels from plutonium in the population of the contaminated regions of the Russian Federation between 1986 and 1992 [36], shows that this component amounted to 25 mSv. The levels of effective dose in the critical group (machine operators) are approximately 5–6 times higher. For the inhabitants of Kiev, the upper estimate of lifetime dose from plutonium isotopes and ²⁴¹Am is 18 mSv [12].