Search

Americium-241 is a significant radiation hazard at facilities that handle or reprocess spent nuclear fuels. An inhalation intake model for 241Am was developed using autopsy data obtained from former workers at the Radiochemical and Plutonium Production Plants at the Mayak Production Association (Mayak PA), Ozyorsk, Russia. Accumulation of 241Am in the body can occur though direct exposure to 241Am (termed here "exogenous" exposures), usually as an inhaled aerosol, or though exposure to 241Pu that decays inside the body to 241Am (termed here "endogenous" exposures). Metabolism of endogenous and exogenous 241Am can differ, with endogenous 241Am being initially related to the behavior of 241Pu. For the model, it was assumed that intakes of 241Am and 241Pu were functionally associated with intakes of 239Pu. The current Mayak Worker Dosimetry System model (MWDS-2008) was used to describe metabolism of plutonium and americium in the respiratory tract. The ICRP-30 model was used for the gastrointestinal tract, the ICRP-67 model was used for metabolism after absorption into the blood for americium, and the "Leggett modification" of the ICRP-67 model for plutonium was used for systemic, non-pulmonary organs. The proposed inhalation intake model for americium provides estimates for internal doses from 241Am from both exogenous and endogenous sources.

A new modification of the prior human lung compartment plutonium model, Doses-2005, has been described. The modified model was named "Mayak Worker Dosimetry System-2008" (MWDS-2008). In contrast to earlier models developed for workers at the Mayak Production Association (Mayak PA), the new model more correctly describes plutonium biokinetics and metabolism in pulmonary lymph nodes. The MWDS-2008 also provides two sets of doses estimates: one based on bioassay data and the other based on autopsy data, where available. The algorithm of internal dose calculation from autopsy data will be described in a separate paper. Results of comparative analyses of Doses-2005 and MWDS-2008 are provided. Perspectives on the further development of plutonium dosimetry are discussed.

Workers at the Mayak nuclear facility in the Russian Federation offer the only adequate human data for evaluating cancer risks from exposure to plutonium. Risks of mortality from cancers of the lung, liver and bone, the organs receiving the largest doses from plutonium, were evaluated in a cohort of 17,740 workers initially hired 1948-1972 using, for the first time, recently improved individual organ dose estimates. Excess relative risk (ERR) models were used to evaluate risks as functions of internal (plutonium) dose, external (primarily gamma) dose, gender, attained age and smoking. By December 31, 2003, 681 lung cancer deaths, 75 liver cancer deaths and 30 bone cancer deaths had occurred. Of these 786 deaths, 239 (30%) were attributed to plutonium exposure. Significant plutonium dose-response relationships (p < 0.001) were observed for all 3 endpoints, with lung and liver cancer risks reasonably described by linear functions. At attained age 60, the ERRs per Gy for lung cancer were 7.1 for males and 15 for females; the averaged-attained age ERRs for liver cancer were 2.6 and 29 for males and females, respectively; those for bone cancer were 0.76 and 3.4. This study is the first to present and compare dose-response analyses for cancers of all 3 organs. The unique Mayak cohort with its high exposures and well characterized doses has allowed quantification of the plutonium dose-response for lung, liver and bone cancer risks based on direct human data. These results will play an important role in plutonium risk assessment.

Thyroid cancer incidence was studied in the cohort of residents of Ozyorsk and Kyshtym, the nearest upwind cities to the Mayak Production Association (Mayak PA), Russia's first plutonium production facility, which has been in operation since 1948. Radioactive contamination of areas around the Mayak PA were from unmonitored releases of inert gases produced by industrial reactors and also from the release of uranium fission products from a radiochemical plant stack where irradiated uranium blocks were refined. Iodine-131 (131I) was the main contributor to the technogenic dose from atmospheric releases. Routine monitoring of gaseous releases began in the mid-1960s, when a gas purification system was perfected. Children were a critical group due to their higher radiosensitivity and specific diet (dairy products and vegetables). Both cities maintain Registries containing over 100,000 individuals born from 1934-2006. Among this group, more than 100 cases of thyroid cancer were registered during 1948-2009. The relative risk of thyroid cancer incidence is 1.5 times higher than in the Chelyabinsk.

The Mayak Production Association (MPA) is located in the northern part of Chelyabinsk Oblast. Operating areas are about 10 km from the town of Ozyorsk, the largest populated area nearby, but other nearby populated areas include Novogorny Village, New Metlino Village, and Kyshtym Town. The long-term objective of this (unfunded) project is reconstruction of the time-dependent individual radiation doses to residents of Ozyorsk and the surrounding area from atmospheric releases of radionuclides from the facilities of the Mayak Production Association (MPA). The time period is from 1948 to the present. This information could be used in several epidemiologic studies of the regional population. Two pilotscale studies of thyroid disease among residents of Ozyorsk have found an increase in thyroid nodules among exposed persons compared to unexposed persons and an increase in thyroid carcinoma in Ozyorsk. The success of follow-on studies would depend upon the availability of thyroid doses proposed to be provided. The availability of credible thyroid doses would allow the quantification of risk of thyroid disease and the evaluation of factors such as host susceptibility, age and time effects, and gender differences. Perhaps more importantly, studies of the Ozyorsk residents would not be encumbered with the complications associated withmore » previous early detection screening, as in the Chernobyl studies, or previous medical conditions, as in the I-131 medical studies. The releases to the atmosphere from MPA stacks are a source of exposure to other populations that are the subject of epidemiologic investigation; these populations include the Extended Techa River Cohort (JCCRER Direction 1), the MPA workers (JCCRER Direction 2), and proposed studies of the East Urals Radioactive Trace (EURT) cohort. The doses received by these cohorts from atmospheric releases at the MPA represent a confounding variable that cannot be considered without the information proposed to be provided.« less