External dose reconstruction in tooth enamel of Techa riverside residents.

This study summarizes the 20-year efforts for dose reconstruction in tooth enamel of the Techa riverside residents exposed to ionizing radiation as a result of radionuclide releases into the river in 1949-1956. It represents the first combined analysis of all the data available on EPR dosimetry with teeth of permanent residents of the Techa riverside territory. Results of electron paramagnetic resonance (EPR) measurements of 302 teeth donated by 173 individuals living permanently in Techa riverside settlements over the period of 1950-1952 were analyzed. These people were residents of villages located at the free-flowing river stream or at the banks of stagnant reservoirs such as ponds or blind river forks. Cumulative absorbed doses measured using EPR are from several sources of exposure, viz., background radiation, internal exposure due to bone-seeking radionuclides (89Sr, 90Sr/90Y), internal exposure due to 137Cs/137mBa incorporated in soft tissues, and anthropogenic external exposure. The purpose of the present study was to evaluate the contribution of different sources of enamel exposure and to deduce external doses to be used for validation of the Techa River Dosimetry System (TRDS). Since various EPR methods were used, harmonization of these methods was critical. Overall, the mean cumulative background dose was found to be 63 ± 47 mGy; cumulative internal doses due to 89Sr and 90Sr/90Y were within the range of 10-110 mGy; cumulative internal doses due to 137Cs/137mBa depend on the distance from the site of releases and varied from 1 mGy up to 90 mGy; mean external doses were maximum for settlements located at the banks of stagnant reservoirs (~500 mGy); in contrast, external doses for settlements located along the free-flowing river stream did not exceed 160 mGy and decreased downstream with increasing distance from the site of release. External enamel doses calculated using the TRDS code and derived from the EPR measurements were found to be in good agreement.

Analysis of EPR and FISH studies of radiation doses in persons who lived in the upper reaches of the Techa River.

Waterborne radioactive releases into the Techa River from the Mayak Production Association in Russia during 1949-1956 resulted in significant doses to about 30,000 persons who lived in downstream settlements. The residents were exposed to internal and external radiation. Two methods for reconstruction of the external dose are considered in this paper, electron paramagnetic resonance (EPR) measurements of teeth, and fluorescence in situ hybridization (FISH) measurements of chromosome translocations in circulating lymphocytes. The main issue in the application of the EPR and FISH methods for reconstruction of the external dose for the Techa Riverside residents was strontium radioisotopes incorporated in teeth and bones that act as a source of confounding local exposures. In order to estimate and subtract doses from incorporated (89,90)Sr, the EPR and FISH assays were supported by measurements of (90)Sr-body burdens and estimates of (90)Sr concentrations in dental tissues by the luminescence method. The resulting dose estimates derived from EPR to FISH measurements for residents of the upper Techa River were found to be consistent: The mean values vary from 510 to 550 mGy for the villages located close to the site of radioactive release to 130-160 mGy for the more distant villages. The upper bound of individual estimates for both methods is equal to 2.2-2.3 Gy. The EPR- and FISH-based dose estimates were compared with the doses calculated for the donors using the most recent Techa River Dosimetry System (TRDS). The TRDS external dose assessments are based on the data on contamination of the Techa River floodplain, simulation of air kerma above the contaminated soil, age-dependent lifestyles and individual residence histories. For correct comparison, TRDS-based doses were calculated from two sources: external exposure from the contaminated environment and internal exposure from (137)Cs incorporated in donors' soft tissues. It is shown here that the TRDS-based absorbed doses in tooth enamel and muscle are in agreement with EPR- and FISH-based estimates within uncertainty bounds. Basically, this agreement between the estimates has confirmed the validity of external doses calculated with the TRDS.

Age and gender specific biokinetic model for strontium in humans.

A biokinetic model for strontium in humans is necessary for quantification of internal doses due to strontium radioisotopes. The ICRP-recommended biokinetic model for strontium has limitations for use in a population study, because it is not gender specific and does not cover all age ranges. The extensive Techa River data set on (90)Sr in humans (tens of thousands of measurements) is a unique source of data on long-term strontium retention for men and women of all ages at intake. These, as well as published data, were used for evaluation of age- and gender-specific parameters for a new compartment biokinetic model for strontium (Sr-AGe model). The Sr-AGe model has a similar structure to the ICRP model for the alkaline earth elements. The following parameters were mainly re-evaluated: gastrointestinal absorption and parameters related to the processes of bone formation and resorption defining calcium and strontium transfers in skeletal compartments. The Sr-AGe model satisfactorily describes available data sets on strontium retention for different kinds of intake (dietary and intravenous) at different ages (0-80 years old) and demonstrates good agreement with data sets for different ethnic groups. The Sr-AGe model can be used for dose assessment in epidemiological studies of general populations exposed to ingested strontium radioisotopes.

Reconstruction of the contamination of the Techa River in 1949-1951 as a result of releases from the "Mayak" Production Association.

More accurate reconstruction of the radioactive contamination of the Techa River system in 1949-1951 has been made on the basis of refined data on the amounts and the rate of discharge of radionuclides into the Techa River from the Mayak Production Association; this has led to the development of a modified Techa River model that describes the transport of radionuclides through the up-river ponds and along the Techa River and deposition of radionuclides in the river-bottom sediments and flooded areas. The refined Techa River source-term data define more precisely the time-dependent rates of release and radionuclide composition of the releases that occurred during 1949-1951. The Techa River model takes into account the time-dependent characteristics of the releases and considers (a) the transport of radionuclides adsorbed on solid particles originally contained in the discharges or originating in the up-river ponds as a result of stirring up of contaminated bottom sediments and (b) the transport of radionuclides in soluble form. The output of the Techa River model provides concentrations of all source-term radionuclides in the river water, bottom sediments, and floodplain soils at different distances from the site of radioactive releases for the period of major contamination in 1950-1951. The outputs of the model show good agreement with historical measurements of water and sediment contamination. In addition, the river-model output for (90)Sr concentration in the river water is harmonized with retrospective estimates derived from the measurements of (90)Sr in the residents of the Techa Riverside villages. Modeled contamination of the floodplain soils by (137)Cs is shown to be in agreement with the values reconstructed from late measurements of this radionuclide. Reconstructed estimates of the Techa River contamination are being used for the quantification of internal and external doses received by residents of the Techa Riverside communities.

The global impact of the Chernobyl reactor accident.

Radioactive material was deposited throughout the Northern Hemisphere as a result of the accident at the Chernobyl Nuclear Power Station on 26 April 1986. On the basis of a large amount of environmental data and new integrated dose assessment and risk models, the collective dose commitment to the approximately 3 billion inhabitants is calculated to be 930,000 person-gray, with 97% in the western Soviet Union and Europe. The best estimates for the lifetime expectation of fatal radiogenic cancer would increase the risk from 0 to 0.02% in Europe and 0 to 0.003% in the Northern Hemisphere. By means of an integration of the environmental data, it is estimated that approximately 100 petabecquerels of cesium-137 (1 PBq = 10(15) Bq) were released during and subsequent to the accident.

Enhancements in the Techa River Dosimetry System: TRDS-2016D Code for Reconstruction of Deterministic Estimates of Dose From Environmental Exposures.

Waterborne releases to the Techa River from the Mayak plutonium facility in Russia during 1949-1956 resulted in significant doses to persons living downstream. The dosimetry system Techa River Dosimetry System-2016D has been developed, which provides individual doses of external and internal exposure for the members of the Techa River cohort and other persons who were exposed to releases of radioactive material to the Southern Urals. The results of computation of individual doses absorbed in red bone marrow and extraskeletal tissues for the Techa River cohort members (29,647 persons) are presented, which are based on residence histories on the contaminated Techa River and the East Urals Radioactive Trace, which was formed in 1957 as a result of the Kyshtym Accident. Available Sr body-burden measurements and available information on individual household locations have been used for refinement of individual dose estimates. Techa River Dosimetry System-2016D-based dose estimates will be used for verification of risk of low-dose-rate effects of ionizing radiation in the Techa River cohort.

Contribution of internal exposures to the radiological consequences of the Chernobyl accident.

The main pathways leading to exposure of members of the general public due to the Chernobyl accident were external exposure from radionuclides deposited on the ground and ingestion of contaminated terrestrial food products. The collective dose to the thyroid was nearly 1.5 million man Gy in Belarus, Russia and Ukraine with nearly half received by children and adolescents. The collective effective dose received in 1986-2005 by approximately five million residents living in the affected areas of the three countries was approximately 50,000 man Sv with approximately 40% from ingestion. That contribution might have been larger if countermeasures had not been applied. The main radionuclide contributing to both external and internal effective dose is 137Cs with smaller contributions of 134Cs and 90Sr and negligible contribution of transuranic elements. The major demonstrated radiation-caused health effect of the Chernobyl accident has been an elevated incidence of thyroid cancer in children.

An approach to reduction of uncertainties in internal doses reconstructed for the Techa River population.

A methodology was developed for reduction of uncertainties in estimates of internal dose for residents of the Techa Riverside communities, who were exposed as a result of releases of radionuclides from the Mayak plutonium production facility in 1949-56. The 'Techa River Dosimetry System' (TRDS) was specifically elaborated for reconstruction of doses. A preliminary analysis of uncertainty for doses estimated using the current version of the TRDS showed large ranges in the uncertainty of internal absorbed dose and led to suggestions of methods to reduce uncertainties. The new methodological approaches described in this paper will allow for significant reduction of uncertainties of 90Sr-dose. The major sources of reduction are: making use of individual measured values of 90Sr and through development of a Household Registry to associate unmeasured persons with measured persons living in the same household(s).

Development of an improved dose reconstruction system for the Techa River population affected by the operation of the Mayak Production Association.

The Techa River Dosimetry System (TRDS) has been developed to provide estimates of dose received by approximately 30,000 members of the Extended Techa River Cohort (ETRC). Members of the ETRC were exposed beginning in 1949 to significant levels of external and internal (mainly from (90)Sr) dose but at low to moderate dose rates. Members of this cohort are being studied in an effort to test the hypothesis that exposure at low to moderate dose rates has the same ability to produce stochastic health effects as exposure at high dose rates. The current version of the TRDS is known as TRDS-2000 and is the subject of this paper. The estimated doses from (90)Sr are supported strongly by approximately 30,000 measurements made with a tooth beta-particle counter, measurements of bones collected at autopsy, and approximately 38,000 measurements made with a special whole-body counter that detects the bremsstrahlung from (90)Y. The median doses to the red bone marrow and the bone surface are 0.21 and 0.37 Gy, respectively. The maximum doses to the red bone marrow and bone surface are 2.0 and 5.2 Gy, respectively. Distributions of dose to other organs are provided and are lower than the values given above. Directions for future work are discussed.

Measurement of 129 I and 137 Cs in soils from Belarus and reconstruction of 131I deposition from the Chernobyl accident.

I and Cs have been measured in a large number of soil samples collected throughout the country of Belarus to support efforts for thyroid-dose reconstruction following the Chernobyl accident. Samples of soil consisting of multiple 30-cm-deep cores per site were sampled following a selection process to ensure sites were undisturbed and representative. Samples were measured by accelerator mass spectrometry (AMS) for I, gamma spectrometry for Cs, and gas chromatography (GC) for total iodine. Results show that both I and Cs are retained firmly in the top approximately 15 to 20 cm of the soil. Our results also suggest that the correlation between I and Cs deposition across the country of Belarus is poor; hence, I is a better surrogate for I than is Cs. It was also noted that total iodine concentrations in topsoil from Belarus are low compared with other regions of the world where radiogenic thyroid cancer has been studied.

Evaluation of age and gender dependences of the rate of strontium elimination 25-45 years after intake: analysis of data from residents living along the Techa river.

The Mayak Production Association released large amounts of 90Sr into the Techa River with peak amounts in 1950-1951. Residents near the Techa River ingested an average of approximately 3,000 kBq of 90Sr. The affected people have been followed by scientists at the Urals Research Center for Radiation Medicine. The whole-body content of 90Sr of approximately 15,000 individuals has been measured over a period of 24 years (1974-1997) using a special whole-body counter. This report evaluates the gender and age dependences of individual rates of strontium elimination. Data on persons who had been measured 12 or more times were selected for study. There were 108 men and 81 women older than 30 years who met this criterion. Individual measurement results were fitted to an exponential function and grouped mean averages of the rate of strontium elimination as a function of age for each sex were derived. For men, a significant increase (from 2.8% year(-1) to 3.2% year(-1)) in the rate of strontium elimination after age 55 years is seen. For women, the increase in the rate of elimination was significant at age 45 and reached 5.8% year(-1) after the age of 60. The results may be used to develop a gender- and age-dependent model of strontium metabolism.

Chernobyl-related thyroid cancer in children of Belarus: a case-control study.

The accident at the Chernobyl nuclear power plant on April 26, 1986, released approximately 2 EBq of 131I and other radioiodine isotopes that heavily contaminated southern Belarus. An increase in thyroid cancer reported in 1992 and attributed to the Chernobyl accident was challenged as possibly the result of intensive screening. We began a case-control study to test the hypothesis that the Chernobyl accident caused the increase in thyroid cancer. Records of childhood thyroid cancer in the national therapy centers in Minsk in 1992 yielded 107 individuals with confirmed pathology diagnoses and available for interview. Pathways to diagnosis were (1) routine endocrinological screening in 63, (2) presentation with enlarged or nodular thyroid in 25 and (3) an incidental finding in 19. Two sets of controls were chosen, one matched on pathway to diagnosis, the other representing the area of heavy fallout, both matched on age, sex and rural/urban residence in 1986. The 131I dose to the thyroid was estimated from ground deposition of 137Cs, ground deposition of 131I, a data bank of 1986 thyroid radiation measurements, questionnaires and interviews. Highly significant differences were observed between cases and controls (both sets) with respect to dose. The differences persisted within pathway to diagnosis, gender, age and year of diagnosis, and level of iodine in the soil, and were most marked in the southern portion of the Gomel region. The case-control comparisons indicate a strong relationship between thyroid cancer and estimated radiation dose from the Chernobyl accident.

Transfer of aged 239+240Pu, 238Pu, 241Am, and 137Cs to cattle grazing a contaminated arid environment.

In this paper, estimates are obtained of the fraction of ingested 239+240Pu, 238Pu, 241Am and 137Cs transferred to blood, muscle, liver, kidney, femur, vertebra, and gonads of a reproducing herd of 17 beef cattle, individuals of which grazed within fenced enclosures for up to 1064 days under natural conditions with no supplemental feeding at an arid site contaminated 16 years previously with transuranic radionuclides. The estimated geometric mean (GM) GI-to-blood fractional transfer of 238Pu (0.0001) was about 20 times larger than the estimated transfer of 239+240Pu (0.000005), while the estimated transfer of 241Am (0.00001) was about 2 times larger than that of 239+240Pu. These GM GI-to-blood transfers were smaller than the GI-to-blood transfer value of 0.001 recommended by the International Commission on Radiological Protection (ICRP) for humans exposed via food chains or occupationally from unknown mixtures or compounds of plutonium and americium. Statistical tests indicated significantly (p less than 0.05) larger GI-to-tissue transfers of (1) 238Pu as compared to 239+240Pu for all tissues examined, (2) of 238Pu as compared to 241Am for muscle, liver, femur, and vertebra, and (3) of 241Am as compared to 239+240Pu for blood serum, femur, and kidney. The estimated GM fractional transfers of 137Cs from GI to muscle and liver were 0.03 (n = 8) and 0.001 (n = 3), respectively, assuming a 50-day biological half-time of 137Cs in cattle tissue.

Bases for secondary standards for residual radionuclides in soil and some recommendations for cost-effective operational implementation.

The future use of land contaminated with radionuclides depends upon scientifically defensible bases for setting limits for radionuclides in soil. The purpose of this work is to develop such bases for establishing "posting criteria" to protect nonradiological workers at the Nevada Test Site and to provide a rationale for cost-effective measurements to readily determine the boundary conditions. The analysis begins with a mandated limit on total effective dose equivalent (1 mSv y(-1)) via all pathways. The possible pathways of exposure are external gamma exposure, inhalation of resuspended material, and incidental soil ingestion. These pathways are evaluated for each radionuclide of interest on the Nevada Test Site, and the results are used to define for each radionuclide the deposition-density limits for each pathway of exposure. The minimum deposition-density limits are noted to occur via the external gamma-exposure pathway for most radionuclides; exceptions are incidental soil ingestion for 90Sr/90Y and inhalation for 238Pu, 239,240Pu, and 241Am. The limiting values of deposition density or average concentration in soil are then determined appropriately by combining all pathways. Procedures are developed for dealing with mixtures of many radionuclides and to apply the principles developed so that even a simple measurement of external gamma-exposure rate may be used to define the boundary conditions in the field, provided that the relative abundance of the radionuclide mixture is known and that the defining level of exposure rate is sufficiently above background.

Historical estimates of external gamma exposure and collective external gamma exposure from testing at the Nevada Test Site. I. Test series through HARDTACK II, 1958.

In 1959, the Test Manager's Committee to Establish Fallout Doses calculated estimated external gamma exposure at populated locations based upon measurements of external gamma-exposure rate. Using these calculations and estimates of population, we have tabulated the collective estimated external gamma exposures for communities within established fallout patterns. The total collective estimated external gamma exposure is 85,000 person-R. The greatest collective exposures occurred in three general areas: Saint George, UT; Ely, NV; and Las Vegas, NV. Three events, HARRY (19 May 1953), BEE (22 March 1955), and SMOKY (31 August 1957), accounted for more than half the total collective estimated external gamma exposure. The bases of the calculational models for external gamma exposure of "infinite exposure," "estimated exposure," and "1-yr effective biological exposure" are explained.

ORERP (Off-Site Radiation Exposure Review Project) internal dose estimates for individuals.

A method was developed to reconstruct the internal radiation dose to off-site individuals who were exposed to fallout from nuclear weapons tests at the Nevada Test Site (NTS). By this method, committed absorbed doses can be estimated for 22 target organs of persons in four age groups and for selected organs of the fetus. Ingestion doses are calculated by combining age-group dose factors and intakes specific for age group, test event, and location as calculated by the PATHWAY food-chain model. Inhalation doses are calculated by combining age-group dose factors and breathing rates, and time-integrated air concentrations that are derived from the ORERP Air-Quality Data Base. Dose estimates are calculated for the radionuclides that contribute significantly to the total dose; these number 20 via the ingestion pathway and 46 via the inhalation pathway. Internal doses to nonspecified individuals and nonspecified fetuses are being reconstructed for each location in the ORERP Town Data Base for which exposure rates and cloud-arrival times are listed. Examples of reconstructing internal dose are presented. This method will also be adapted to reconstruct internal doses from NTS fallout to specific individuals in accordance with the person's age, past residence, life-style, and living pattern.

Ingestion of Nevada Test Site fallout: internal dose estimates.

This paper summarizes individual and collective dose estimates for the internal organs of hypothetical yet representative residents of selected communities that received measurable fallout from nuclear detonations at the Nevada Test Site. The doses, which resulted from ingestion of local and regional food products contaminated with over 20 radionuclides, were estimated with use of the PATHWAY food-chain-transport model to provide estimates of central tendency and uncertainty. The thyroid gland received much higher doses than other internal organs and tissues. In a very few cases, infants might have received thyroid doses in excess of 1 Gy, depending on location, diet, and timing of fallout. 131I was the primary thyroid dose contributor, and fresh milk was the main exposure pathway. With the exception of the thyroid, organ doses from the ingestion pathway were much smaller (< 3%) than those from external gamma exposure to deposited fallout. Doses to residents living closest to the Nevada Test Site were contributed mainly by a few fallout events; doses to more distantly located people were generally smaller, but a greater number of events provided measurable contributions. The effectiveness of different fallout events in producing internal organ doses through ingestion varied dramatically with seasonal timing of the test, with maximum dose per unit fallout occurring for early summer depositions when milk cows were on pasture and fresh, local vegetables were used. Within specific communities, internal doses differed by age, sex, and lifestyle. Collective internal dose estimates for specific geographic areas are provided.

Estimating internal dose due to ingestion of radionuclides from Nevada Test Site fallout.

The U.S. Department of Energy initiated the Radiation Exposure Review Project to provide a critical reexamination of radiation doses to people resulting from testing nuclear devices at the Nevada Test Site. One part of this effort focused on the dose resulting from the ingestion of contaminated food. The PATHWAY radionuclide transport model was developed to provide estimates of food concentrations for 20 radionuclides for each of 86 test events and 15 agricultural scenarios. These results were then used as input to the Human Ingestion model to provide dose estimates for individuals and populations in 9 western states. The model considered the life-style and age of the people, and accounted for the transport of milk between locations. Estimates of uncertainty were provided for all doses using Monte Carlo simulation techniques. Propagation of uncertainty between the PATHWAY model and the Human Ingestion model required the development of special strategies to ensure that the inherent correlations between concentrations of the radionuclides in foods were handled properly. In addition, the size of the input data base (60 megabytes), the number of cases to consider (over 30,000), and the number of Monte Carlo simulations (over 6 million) required the development of efficient and reliable methods of data access and storage while running simulations concurrently on up to 14 UNIX workstations. The problems encountered in this effort are likely to be typical of any dose reconstruction involving geographically heterogeneous environmental conditions. This paper documents the methods used to disaggregate the system to achieve computation efficiency, the methods used to propagate uncertainty through the model system, and the techniques used to manage data in a distributed computing environment. The radionuclide- and age-specific dose factors used in the analysis are also provided.

Overview of the Department of Energy's Off-Site Radiation Exposure Review Project (ORERP).

The Off-Site Radiation Exposure Review Project (ORERP) was established by the U.S. Department of Energy to (1) collect, preserve, and disseminate historical data related to radioactive fallout and health effects from nuclear testing, and (2) reconstrut, insofar as possible, the exposures to the off-site public from nuclear testing at the Nevada Test Site and doses to individuals resulting from these exposures. The goals, methods, and example results of the ORERP are presented.

Ground-based air-sampling measurements near the Nevada Test Site after atmospheric nuclear tests.

Historical air-sampling data measured within 320 km (200 mi) of the Nevada Test Site (NTS) have been reviewed for periods following atmospheric nuclear tests, primarily in the 1950s. These data come mostly from high-volume air samplers, with some from cascade-impactor samplers. Measurements considered here are for beta radiation from gross fission products. The resulting air-quality data base is comprised of almost 13,000 samples from 42 sampling locations downwind of the NTS. In order to compile an accurate air-quality data base for use in estimating exposure via inhalation, raw data values were sought where possible, and the required calculations were performed on a computer with state-of-the-art algorithms. The data-processing procedures consisted of (1) entry and error checking of historical data; (2) determination of appropriate background values, air-sampling volumes, and net air concentrations; and (3) calculation of integrated air concentration (C) for each sample (considering fallout arrival times). Comparing C values for collocated high-volume and cascade-impactor samplers during the Upshot-Knothole series showed similar lognormal distributions, but with a geometric mean C for cascade impactors about half that for the high-volume air samplers. Overall, the uncertainty in C values is about a factor of three. In the past, it has been assumed that C could be related to ground deposition by a constant having units of velocity. In our data bases, simultaneous measurements of air concentration and ground deposition at the same locations were not related by a constant; indeed, there was a great amount of scatter. This suggests that the relationship between C and ground deposition in this situation is too complex to be treated adequately by simple approaches.