Effective doses due to external irradiation from the Chernobyl accident for different population groups of Ukraine

A Methodology for Estimating External Doses to Individuals and Populations Exposed to Radioactive Fallout from Nuclear Detonations

ABSTRACT

A methodology of assessment of the doses from external irradiation resulting from the ground deposition of radioactive debris (fallout) from a nuclear detonation is proposed in this paper. The input data used to apply this methodology for a particular location are the outdoor exposure rate at any time after deposition of fallout and the time-of-arrival of fallout, as indicated and discussed in a companion paper titled "A Method for Estimating the Deposition Density of Fallout on the Ground and on Vegetation from a Low-yield Low-altitude Nuclear Detonation." Example doses are estimated for several age categories and for all radiosensitive organs and tissues identified in the most recent ICRP publications. Doses are calculated for the first year after the detonation, when more than 90% of the external dose is delivered for populations close to the detonation site over a time period of 70 y, which is intended to represent the lifetime dose. Modeled doses in their simplest form assume no environmental remediation, though modifications can be introduced. Two types of dose assessment are considered: (1) initial, for a rapid but only approximate dose estimation soon after the nuclear detonation; and (2) improved, for a later, more accurate, dose assessment following the analysis of post-detonation measurements of radiation exposure and fallout deposition and the access of information on the lifestyle of the exposed population.

Estimation of radiation gonadal doses for the American-Ukrainian trio study of parental irradiation in Chornobyl cleanup workers and evacuees and germline mutations in their offspring

Radiation doses of parents exposed from the Chornobyl accident as cleanup workers or evacuees were estimated in the National Cancer Institute-National Research Center for Radiation Medicine trio (i.e. father, mother, offspring) study aimed at investigating the radiation effects on germlinede novomutations in children as well as other outcomes. Paternal (testes) and maternal (ovaries) gonadal doses were calculated along with associated uncertainty distributions for the following exposure pathways: (a) external irradiation during the cleanup mission, (b) external irradiation during residence in Pripyat, and (c) external irradiation and (d) ingestion of radiocesium isotopes, such as¹³⁴Cs and¹³⁷Cs, during residence in settlements other than Pripyat. Gonadal doses were reconstructed for 298 trios for the periods from the time of the accident on 26 April 1986 to two time points before the child's date of birth (DOB): 51 (DOB-51) and 38 (DOB-38) weeks. The two doses, DOB-51 and DOB-38 were equal (within 1 mGy) in most instances, except for 35 fathers where the conception of the child occurred within 3 months of exposure or during exposure. The arithmetic mean of gonadal DOB-38 doses was 227 mGy (median: 11 mGy, range 0-4080 mGy) and 8.5 mGy (median: 1.0 mGy, range 0-550 mGy) for fathers and mothers, respectively. Gonadal doses varied considerably depending on the exposure pathway, the highest gonadal DOB-38 doses being received during the cleanup mission (mean doses of 376 and 34 mGy, median of 144 and 7.4 mGy for fathers and mothers, respectively), followed by exposure during residence in Pripyat (7.7 and 13 mGy for mean, 7.2 and 6.2 mGy for median doses) and during residence in other settlements (2.0 and 2.1 mGy for mean, 0.91 and 0.81 mGy for median doses). Monte Carlo simulations were used to estimate the parental gonadal doses and associated uncertainties. The geometric standard deviations (GSDs) in the individual parental stochastic doses due to external irradiation during the cleanup mission varied from 1.2 to 4.7 (mean of 1.8), while during residence in Pripyat they varied from 1.4 to 2.8 (mean of 1.8), while the mean GSD in doses received during residence in settlements other than Pripyat was 1.3 and 1.4 for external irradiation and ingestion of radiocesium isotopes, respectively.

Impact of decontamination on individual radiation doses from external exposure among residents of Minamisoma City after the 2011 Fukushima Daiichi nuclear power plant incident in Japan: a retrospective observational study

Following the Fukushima incident, radiation doses from external exposure accounted for the majority of the total doses. Although countermeasures are being implemented, with the aim of reducing radiation exposure, little information is available on the effects of decontamination on individual doses among the residents of radioactively contaminated areas. To evaluate the effectiveness of the decontamination measures in reducing individual doses, and to examine the influence of the timing of decontamination and the district, data were analysed for 18 392 adults and 3 650 children in Minamisoma City, Fukushima, who participated in a voluntary screening programme using individual radiation dosimeters (Glass Badge) between June 2013 and September 2016. The dose reduction rates (DRR) were calculated for one year by comparing the first and last three-month measurement results between areas with and without decontamination. Using a regression approach and Monte Carlo simulation, the dose reduction rate by decontamination eliminating the effect of physical decay (DRRd') was also estimated as a function of the timing of the decontamination and the dose at the time of starting the decontamination. The annual DRR in areas with decontamination for both adults and children were significantly higher than those in areas without decontamination, depending on the timing of decontamination: 31%-36% for 2013-14 for adults in decontamination areas and 33%-35% for children in decontamination areas, compared to 12%-23% and 13%-23% for adults and children in areas without decontamination, respectively. There was a positive correlation between DRRd' and individual doses, and DRRd' was estimated at 30%-40% for adults and children with doses of 3 mSv y^-1 in 2013 and 2014. This study demonstrated that decontamination does lower individual doses from external exposure. The higher the dose at the time of starting the decontamination, the greater the dose reduction rate by decontamination, regardless of the timing of the decontamination. Our study confirms that decontamination was useful for high-dose areas in the later phases of the incident.

METHODOLOGY OF RECONSTRUCTION OF INDIVIDUALIZED EXPOSURE DOSES FOR PERSONS RESIDING AT RADIOACTIVELY CONTAMINATED TERRITORIES OF UKRAINE

OBJECTIVE

Development of methodology for reconstruction of individualized exposure doses for persons residing atradioactively contaminated after Chornobyl accident territories.Materials and methods of research. The methodology is based on the data of radio-ecological (ground, meal) anddosimetric (WBC measurements) monitoring held in Ukraine in 1986-2013, the results of which are saved in databases of Central Ecological and Dosimetric Register of Radiation Protection Laboratory of NRCRM. It is presentedfour levels of individualization of exposure doses. Each subsequent level of the model is based on the previous oneand takes into account additional parameters, and as a result a model of the next level comprises more number ofexposure situations and provides more accurate estimates. Using the methodology it is necessary to select a modelof such level that enables to solve in the best way the assignments posed before a researcher. For the population ofradioactively contaminated territories, two main ways of exposure are considered: (a) external gamma-exposurefrom radionuclides in the ground, and (b) internal exposure from consumption of radioactively contaminated meal.The dose is evaluated from all the main radionuclides of radioactive accident deposition, and in case of internalexposure after 1986 - only from radio-isotopes of cesium (134Cs and 137Cs).

RESULTS

Individualized doses are evaluated for different age groups of population residing in 10 raions of the mostcontaminated after Chornobyl accident Oblasts of Ukraine: Zhytomyr, Kyiv, Rivne and Chernihiv Oblasts. Dose esti-mates are presented weighted by the number of residents in settlements of specific raions. Among them are: thedose for 1986, doses accumulated during the first 15 years after the accident (1986-2000), and during the periodof 32 years after the accident (1986-2017).

CONCLUSIONS

Average raion dose reconstructed for adult residents of the most radioactively contaminatedNarodychy Raion of Zhytomyr Oblast in 1986 was 18 mSv, and accumulated during the whole period after the acci-dent was 51 mSv. The lowest doses among abovementioned 10 raions were obtained by persons residing at the ter-ritory of Kozelets and Ripky Raions of Chernihiv Oblast. Average dose for them in 1986 constituted 1.3 and 1.7 mSv,and the one accumulated during 32 years was 6.1 and 6.2, respectively.

Individual external doses below the lowest reference level of 1 mSv per year five years after the 2011 Fukushima nuclear accident among all children in Soma City, Fukushima: A retrospective observational study

After the 2011 Fukushima Daiichi nuclear power plant accident, little information has been available on individual doses from external exposure among residents living in radioactively contaminated areas near the nuclear plant; in the present study we evaluated yearly changes in the doses from external exposure after the accident and the effects of decontamination on external exposure. This study considered all children less than 16 years of age in Soma City, Fukushima who participated in annual voluntary external exposure screening programs during the five years after the accident (n = 5,363). In total, 14,405 screening results were collected. The median participant age was eight years. The geometric mean levels of annual additional doses from external exposure attributable to the Fukushima accident, decreased each year: 0.60 mSv (range: not detectable (ND)–4.29 mSv), 0.37 mSv (range: ND–3.61 mSv), 0.22 mSv (range: ND–1.44 mSv), 0.20 mSv (range: ND–1.87 mSv), and 0.17 mSv (range: ND–0.85 mSv) in 2011, 2012, 2013, 2014, and 2015, respectively. The proportion of residents with annual additional doses from external exposure of more than 1 mSv dropped from 15.6% in 2011 to zero in 2015. Doses from external exposure decreased more rapidly than those estimated from only physical decay, even in areas without decontamination (which were halved in 395 days from November 15, 2011), presumably due to the weathering effects. While the ratios of geometric mean doses immediately after decontamination to before were slightly lower than those during the same time in areas without decontamination, annual additional doses reduced by decontamination were small (0.04–0.24 mSv in the year of immediately after decontamination was completed). The results of this study showed that the levels of external exposure among Soma residents less than 16 years of age decreased during the five years after the Fukushima Daiichi nuclear power plant accident. Decontamination had only limited and temporal effects on reducing individual external doses.

Doses for post-Chernobyl epidemiological studies: are they reliable?

On 26 April 2016, thirty years will have elapsed since the occurrence of the Chernobyl accident, which has so far been the most severe in the history of the nuclear reactor industry. Numerous epidemiological studies were conducted to evaluate the possible health consequences of the accident. Since the credibility of the association between the radiation exposure and health outcome is highly dependent on the adequacy of the dosimetric quantities used in these studies, this paper makes an effort to overview the methods used to estimate individual doses and the associated uncertainties in the main analytical epidemiological studies (i.e. cohort or case-control) related to the Chernobyl accident. Based on the thorough analysis and comparison with other radiation studies, the authors conclude that individual doses for the Chernobyl analytical epidemiological studies have been calculated with a relatively high degree of reliability and well-characterized uncertainties, and that they compare favorably with many other non-Chernobyl studies. The major strengths of the Chernobyl studies are: (1) they are grounded on a large number of measurements, either performed on humans or made in the environment; and (2) extensive effort has been invested to evaluate the uncertainties associated with the dose estimates. Nevertheless, gaps in the methodology are identified and suggestions for the possible improvement of the current dose estimates are made.

Radiation-induced leukemia among children aged 0-5 years at the time of the Chernobyl accident

This case-control study was conducted to estimate the radiation-induced risk of acute leukemia during the period from 1987 to 1997 among residents 0-5 years of age at the time of the Chernobyl accident in the most radioactively contaminated territories of the Ukraine (Rivno, Zhytomyr, Chernihiv and Cherkasy regions). Data were collected from 246 leukemia cases diagnosed between 1 January, 1987, and 31 December, 1997. Each case was verified and interviewed. Verified cases were compared to 492 randomly selected controls matched by age, sex, type of settlement (rural, semirural and urban) and administrative region of residency. The cumulative level of radiation exposure from the time of the Chernobyl accident to the date of diagnosis was assessed for each case and corresponding controls. Four dose-range groups were selected for statistical analysis (0-2.9, 3-9.9, 10-99.9 and 100-313.3 mGy). The risk of leukemia was significantly increased (-2.4 [95%CI: 1.4-4.0]) among those with radiation exposure doses higher than 10 mGy (p = 0.01). The association between radiation exposure and risk was stronger among males (-2.8 [95%CI: 1.4-5.5, p = 0.01]), and for cases of acute leukemia that were diagnosed during the period from 1987 to 1992 (-2.5 [95%CI: 1.2-5.1, p = 0.05]), particularly acute myeloid leukemia (-5.8 [95%CI: 1.4-24.6, p = 0.05]). The influence of possible confounders and methods of selecting controls on the leukemia risk assessment was analyzed. The evaluated risk per unit dose is discussed.

Breast cancer in Belarus and Ukraine after the Chernobyl accident

An increase in breast cancer incidence has been reported in areas of Belarus and Ukraine contaminated by the Chernobyl accident and has become an issue of public concern. The authors carried out an ecological epidemiological study to describe the spatial and temporal trends in breast cancer incidence in the most contaminated regions of Belarus and Ukraine, and to evaluate whether increases seen since 1986 correlate to radiation exposure from the Chernobyl accident. The authors investigated the trends through age-cohort-period-region analyses of district-specific incidence rates of breast cancer for Gomel and Mogilev regions of Belarus and Chernigiv, Kyiv and Zhytomir regions of Ukraine. Dose-response analyses were based on Poisson regression, using average district-specific whole body doses accumulated since the accident from external exposure and ingestion of long-lived radionuclides. The study demonstrated increases in breast cancer incidence in all areas following the Chernobyl accident, reflecting improvements in cancer diagnosis and registration. In addition, a significant 2-fold increase in risk was observed, during the period 1997-2001, in the most contaminated districts (average cumulative dose of 40.0 mSv or more) compared with the least contaminated districts (relative risk [RR] in Belarus 2.24, 95% confidence interval [CI] 1.51-3.32 and in Ukraine 1.78, 95% CI=1.08-2.93). The increase, though based on a relatively small number of cases, appeared approximately 10 years after the accident, was highest among women who were younger at the time of exposure and was observed for both localised and metastatic diseases. It is unlikely that this excess could be entirely due to the increased diagnostic activity in these areas.

Review of dynamical models for external dose calculations based on Monte Carlo simulations in urbanised areas

After an accidental release of radionuclides to the inhabited environment the external gamma irradiation from deposited radioactivity contributes significantly to the radiation exposure of the population for extended periods. For evaluating this exposure pathway, three main model requirements are needed: (i) to calculate the air kerma value per photon emitted per unit source area, based on Monte Carlo (MC) simulations; (ii) to describe the distribution and dynamics of radionuclides on the diverse urban surfaces; and (iii) to combine all these elements in a relevant urban model to calculate the resulting doses according to the actual scenario. This paper provides an overview about the different approaches to calculate photon transport in urban areas and about several dose calculation codes published. Two types of Monte Carlo simulations are presented using the global and the local approaches of photon transport. Moreover, two different philosophies of the dose calculation, the "location factor method" and a combination of relative contamination of surfaces with air kerma values are described. The main features of six codes (ECOSYS, EDEM2M, EXPURT, PARATI, TEMAS, URGENT) are highlighted together with a short model-model features intercomparison.

Radioactivity concentrations of 40K, 134Cs, 137Cs, 90Sr, 241Am, 238Pu and 239+240Pu radionuclides in Jordanian soil samples

In November 2000, surface and core soil samples were collected from different regions of Jordan. The samples were analyzed by direct gamma spectrometry and combined radiochemical separation procedure to quantify (40)K, (134)Cs, (137)Cs, (90)Sr, (241)Am, (238)Pu and (239+240)Pu radioactivity. Concentrations (Bq.kg(-1) dry weight) have been observed to vary in the range 1.5-2.6 for (134)Cs, 2.8-11.4 for (90)Sr, and 0.13-0.48 for (241)Am, 0.016-0.062 for (238)Pu, 0.28-1.01 for (239+240)Pu and 155-543 for (40)K. The typical concentration of (137)Cs found in topsoils (0-2 cm) ranged in 7.5-576 Bq.kg(-1), dry weight. These values were greater than those observed in samples taken at greater depths (up to 32 cm). Activity ratios of (134)Cs/(137)Cs, (90)Sr/(137)Cs, (239+240)Pu/(137)Cs, (238)Pu/(137)Cs, (241)Am/(137)Cs, (239+240)Pu/(238)Pu and (241)Am /(238)Pu have mean values of 0.0049 (R=1), 0.29 (R=0.76), 0.41 (R=0.90), 0.39 (R=0.85), 0.41 (R=0.88), 7.72 (R=0.97) and 16.66 (R=0.98), respectively. The underlying concentrations were correlated and relatively higher than those reported in neighboring countries. One moss sample, as a biomonitor indicator, was measured and evaluated along with the soil samples. Its data showed higher concentrations of all measured radionuclides due to accumulations over years. The depth distribution of the fission product (137)Cs and the total deposition (Bq.m(-2)) were also studied in selected samples. Estimations of the annual effective dose equivalent due to (137)Cs-soil contamination showed values up to more than 200 microSv.

Measuring the external exposure dose in the contaminated area near the Chernobyl nuclear power station using the thermoluminescence of quartz in bricks

We collected bricks from buildings in the heavily contaminated evacuated area of Belarus in a 30-km zone around the Chernobyl nuclear power station and the Gomel-Bryansk area of 150-250 km from Chernobyl and estimated the cumulative radiation dose caused by the reactor accident by measuring the thermoluminescence (TL) of the bricks. The annual dose at each location was measured using glass dosimeters and thermoluminescence dosimeters (TLD). The dose rate was measured using an energy-compensated NaI scintillation survey meter. The soil contamination near the location of each brick was measured using a germanium semiconductor detector. The main purpose of the project was to extrapolate the relation between the cumulative external dose and the present dose rate or contamination level to the lower contaminated areas. The results of the glass dosimeter, TLD, and survey meter determinations were almost identical. For a determination of the annual dose higher than 10 mGy y(-1), the cumulative dose by TL (TL dose) was roughly proportional to the annual dose and about 1.5 times larger than the cumulative dose calculated from the annual dose and 137Cs half life. The difference is expected due to the contribution of short-lived nuclides immediately after the accident or localized heavy contamination of the ground surface with 137Cs that migrated afterwards. For annual dose smaller than 10 mGy y(-1), the proportionality was not observed and most of the locations facing indoors showed TL doses very much larger than that expected from the proportionality. The cumulative dose outdoors by TL was also roughly proportional to the regional 137Cs contamination level and the proportional constant is about 10(-1) mGy per GBq km(-2), and is about 250 times larger than the present annual internal dose derived from published results. The correlation between the present dose rate where the brick was sampled and the average 137Cs concentration in the ground soil near the point is not clear, possibly because of the large spatial fluctuation in the 137Cs concentration in the soil.

Radiation-induced leukemia risk among those aged 0-20 at the time of the Chernobyl accident: a case-control study in the Ukraine

A case-control study was conducted to estimate the radiation-induced acute leukemia risk for the period 1987-1997 among residents aged 0-20 at the time of the Chernobyl accident in the most radioactively contaminated territories of the Ukraine (Rivno and Zhytomir regions). Data were collected on 272 leukemia cases diagnosed between 1 January 1987 and 31 December 1997. Of these, 98 cases were verified and interviewed. Verified cases were compared to 151 randomly selected controls matched by age, gender and type of settlement. The mean value of the estimated accumulated equivalent dose to the bone marrow was 4.5 mSv, and the maximum value was 101 mSv. A statistically significant increased risk of leukemia was found among males whose estimated radiation exposure was higher than 10 mSv. This association was statistically significant for acute leukemia cases that occurred in the period 1993-1997, particularly for acute lymphoblastic leukemia. A similar association was found for acute myeloid leukemia, diagnosed in the period 1987-1992.

Chernobyl accident: retrospective and prospective estimates of external dose of the population of Ukraine

Following the Chernobyl accident many activities were conducted in Ukraine in order to define the radiological impact. Considered here are gamma spectrometric analyses of soil-depth-profile samples taken in the years 1988-1999, gamma spectrometric measurements of radionuclide concentration in soil samples taken in 1986, and measurements of external gamma-exposure rate in air. These data are analyzed in this paper to derive a "reference" radionuclide composition and an attenuation function for the time-dependent rate of external gamma exposure that changes due to the migration of radiocesium into the soil column. An attenuation function for cesium is derived that consists of two exponential functions with half lives of 1.5 and 50 y. The dependencies of attenuation on direction and distance from the Chernobyl Nuclear Power Plant are also demonstrated. On the basis of these analyses the average individual and collective external gamma doses for the population of Ukraine are derived for 1986, 1986-2000, and 1986-2055. For the 1.4 million persons living in rural areas with 137Cs contamination of >37 kBq m(-2), the collective effective dose from external exposure is estimated to be 7,500 person-Sv by the end of 2000. A critical group of 22,500 persons who received individual doses of >20 mSv is identified for consideration of increased social and medical attention.

Design and analysis of epidemiological studies of excess cancer among children exposed to Chernobyl radionuclides

Within the last decade, a substantial amount of attention has been devoted to etiological research on the association between exposure to fallout radionuclides from the Chernobyl accident and radiation-induced late effects (cancer) among children. A majority of the studies completed to date have been of the descriptive type, which only correlate average population exposure with average rate of cancer incidence as a function of calendar period. Since individual dosimetry is not performed in descriptive studies, it is unclear whether exposure precedes the development of cancer and a final decision cannot be made regarding the association between radiation exposure and cancer. This paper reviews the background epidemiology and outlines an analytical study design that is needed to clarify the unclear association between Chernobyl fallout exposure and childhood cancer. We discuss the essential elements of an analytical case-control design such as genetic predisposition, vital statistics, sample size and power determinations, ascertainment of cases and controls, and phenomenological dose modeling to establish individual doses. Examples such as cytogenetic biodosimetry, medical radiation dosimetry, and cytogenetic characterization of leukemia to minimize exposure and diagnostic misclassification are provided. We recommend the analytical methods described in this paper for studying the role of Chernobyl radionuclides and development of childhood cancer.