Search

A large excess risk of thyroid cancer was observed among Belarusian/Russian/Baltic Chornobyl cleanup workers. A more recent study of Ukraine cleanup workers found more modest excess risks of thyroid cancer. Dose errors in this data are substantial, associated with model uncertainties and questionnaire response. Regression calibration is often used for dose-error adjustment, but may not adequately account for the full error distribution. We aimed to examine the impact of exposure-assessment uncertainties on thyroid cancer among Ukrainian cleanup workers using Monte Carlo maximum likelihood, and compare with results derived using regression calibration. Analyses assessed the sensitivity of results to various components of internal and external dose. Regression calibration yielded an excess odds ratio per Gy (EOR/Gy) of 0.437 (95% CI - 0.042, 1.577, p = 0.100), compared with the EOR/Gy using Monte Carlo maximum likelihood of 0.517 (95% CI - 0.039, 2.035, p = 0.093). Trend risk estimates for follicular morphology tumors exhibited much more extreme effects of full-likelihood adjustment, the EOR/Gy using regression calibration of 3.224 (95% CI - 0.082, 30.615, p = 0.068) becoming ~ 50% larger, 4.708 (95% CI - 0.075, 85.143, p = 0.066) when using Monte Carlo maximum likelihood. Results were sensitive to omission of external components of dose. In summary, use of Monte Carlo maximum likelihood adjustment for dose error led to increases in trend risks, particularly for follicular morphology thyroid cancers, where risks increased by ~ 50%, and were borderline significant. The unexpected finding for follicular tumors needs to be replicated in other exposed groups.

This original study aims to quantify the human factor uncertainties in radiation doses for Chernobyl cleanup workers that are associated with errors in direct or proxy personal interviews due to poor memory recall a long time after exposure. Two types of doses due to external irradiation during cleanup mission were calculated independently. First, a "reference" dose, that was calculated using the historical description of cleanup activities reported by 47 cleanup workers shortly after the completion of the cleanup mission. Second, a "current" dose that was calculated using information reported by 47 cleanup workers and respective 24 proxies (colleagues) nominated by cleanup workers during a personal interview conducted more recently, as part of this study, i.e., 25-30 years after their cleanup missions. The Jaccard similarity coefficient for reference and current doses was moderate: the arithmetic mean ± standard deviation was 0.29 ± 0.18 (median = 0.31) and 0.23 ± 0.18 (median = 0.22) for the cleanup worker's and proxy's interviews, respectively. The agreement between two doses was better if the cleanup worker was interviewed rather than his proxy: the median ratio of current to reference dose was 1.0 and 0.56 for cleanup workers and proxies, respectively. The present study has shown that human factor uncertainties lead to underestimation or overestimation of the "true" reference dose for most cleanup workers up to 3 times. In turn, the potential impact of these errors on radiation-related risk estimates should be assessed.

This article presents a methodology for assessing the radiation doses in an urban environment due to external irradiation from radionuclides deposited on the ground and other surfaces as well as from a passing radioactive cloud. The approach was developed and applied to assess individual doses of residents of the town of Pripyat who were evacuated shortly after the Chernobyl accident. Typically, the so-called location factor is defined as the ratio of the dose rate at a point of exposure and the dose rate at an undisturbed lawn far from any buildings. The present study used a new definition of the location factor as a regular four-dimensional grid of ratios of air kerma rates indoors and outdoors distributed in space and time. The location factors were calculated for two scenarios: outdoor and indoor values for typical apartments and buildings in Pripyat. Indoor location factors varied within two orders of magnitude depending on the floor of residence and place of staying inside the apartment. Values of the indoor location factor differed during the daytime and night by a factor of 30–40 depending on the behaviour of an individual within the apartment. Both, outdoor and indoor location factors decreased with decreasing distances between buildings. It was shown that during the first 4 days after the accident, air kerma rates in Pripyat were governed by the radionuclides deposited on the ground surface, and not by radionuclides in the cloud. Specifically, the contribution of the radioactive cloud to air kerma rate was maximal (i.e., 2.3%) on the morning of 28 April 1986. The methodology and results of this study are currently being used to reconstruct the radiation gonadal dose for the subjects of the American–Ukrainian study of parental irradiation in Chernobyl cleanup workers and evacuees for investigating germline mutations in their offspring.

This article presents a method for assessing the radionuclide surface contamination density (SCD) on open sites and in premises of a radiation hazardous facility based on measurements of the ambient dose equivalent rate (ADER). The method is intended for use at the initial stage of the assessment of the radiation environment at facilities. The assessed SCD at a given location on the surface can differ from the directly measured SCD at that location, since sources located on the surface and distributed by the depth contribute to the ADER value. The method makes it possible to estimate SCD with reasonable accuracy without increasing the number of measurements, and thus avoid additional occupational exposure and the use of additional resources. SCD and ADER as spatial variables have different support of measurement data. For ADER, measured at a height of 1 m, the support of measurement data can be taken to be a circle in the centre of which a gamma-ray detector is located, with a radius of several tens of meters. In contrast, SCD has the support of measurement data, close to the overall dimensions of the beta detector (100 cm2). To solve the problem of SCD calculation on the basis of ADER measurements, the method of conversion coefficients (MCC) is usually applied, based on the use of conversion factors; however, this method provides an adequate estimate only under conditions of an SCD with low gradient over the surface. The method proposed in this article is applicable for an arbitrary distribution of SCD, and designed to deal with heterogeneous contamination patterns. The developed method is based on the numerical solution of the Fredholm equation of the first kind. The measurement data always contain an error, therefore, the task of the SCD calculation is an ill-posed problem, and the Tikhonov regularisation method (ridge regression) was used to solve it. The article presents the method developed and examples of use. Validation of the method was performed using 38 measurements of the radioactive contamination from 137Cs in soil. It is shown that the method proposed in the article demonstrates a significant superiority in comparison with the MCC method, because it allows more accurate localisation of areas contaminated with radionuclides and is applicable for an arbitrary distribution of SCD.

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 germline de novo mutations 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 as 134Cs and 137Cs, 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.

Radiation doses received by workers during their movement within areas contaminated as a result of events and activities, leading to emergency or existing exposure situations, may provide a substantial contribution to total external exposure during remediation work. This paper describes an approach to minimise worker external exposure in these circumstances, based on graph theory. The paper describes several tasks, including: searching for a route with the lowest dose, searching for an optimal bypass with a given set of control points and searching for the optimal road network coverage. Classical graph theory algorithms have been used (Dijkstra's algorithm, Chinese postman problem and travelling salesman problem). Algorithms for solving the above mentioned problems have been developed and were included in the information-analytical system for radiation safety. This software has been applied for optimisation of protection during remediation work at the Andreeva Bay site of temporary storage for spent fuel and radioactive waste in the Kola Peninsula, both in the context of existing exposure situations and improving the preparedness for emergency exposure situations.

This paper deals with classification of dose distributions of nuclear workers based on antikurtosis (Q) and entropy coefficients (K) and their relationship presented in QK-diagrams. It is shown that determination of the most appropriate distribution to adopt, for a specific data set of a wide range of input data, requires building and analysing QK-diagrams for distributions of logarithms of individual doses. Actual dose distributions for emergency and occupational exposure situations were then considered, as well as doses for one day of work during clean-up and routine activities. It is shown that, in all cases, three types of distributions of logarithms of individual doses were present: normal, Weibull and Chapeau. The location of the representation point of a dose distribution reflects the degree of dose control of the group of workers whose individual doses are collectively displayed in the QK-diagram. The more the representation point of the analysed distribution of the logarithms of the individual dose of a given contingent of workers deviates from the point of the lognormal distribution, the more there was intervention in the process of individual dose accumulation. Thus, QK-diagrams could be used to develop a dose control function. It is shown that the hybrid lognormal distribution, which is widely used in the field of radiation safety, for the purpose of approximation of real dose distributions, is unable to satisfactorily describe many dose distributions arising in aftermath operations and occupational exposure.

The Coastal Technical Base (CTB) №569 at Andreeva Bay was established in the early 1960s and intended for the refueling of nuclear submarine reactors and temporary storage of spent nuclear fuel (SNF) and radioactive waste (RW). In 2001, the base was transferred to the Russian Ministry for Atomic Energy and the site remediation began. The paper describes in detail the radiation situation change at the technical site in Andreeva Bay from 2002-2016, the period of preparation for the most critical phase of remedial work: removal of spent fuel assemblies. The analysis of aggregated indicators and data mining were used. The article suggests the best number and location of checkpoints needed to ensure sufficient accuracy of the radiation situation description. The fractal properties of the radiation field are studied using the Hurst index. The relationship between checkpoints was assessed using the method of searching for checkpoint communities. The decrease in the integral of the ambient dose equivalent rate (ADER) at the technical site was evaluated by the method of time series decomposition. Three components of time series were identified: trend, seasonal and residual. The trend of the ADER integral over the technical site is a monotonic decreasing function, where the initial and final values differ tenfold. Taking into account that 137Cs dominates the radiation situation on-site, it is clear that the ADER due to the radionuclide decay will have decreased by 1.4 times. It is estimated that only a small proportion of 137Cs has migrated off-site. Therefore, approximately a sevenfold decrease in dose rate is mainly due to remediation activities of personnel. During the year, the seasonal component varies the ADER integral by a factor of two, due to snowfall. The residual component reflects the uncertainty of the ADER integral calculation and phases of active SNF and RW management. The methods developed are used to support the optimization of remediation work as well as regulatory supervision of occupational radiation protection.