Joint U.S./Russian Studies of Population Exposures Resulting From Nuclear Production Activities in the Southern Urals
Napier, Bruce A.*
*Environmental Assessment Group, Earth Systems Science Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K3-54, Richland, WA 99352.
The author declares no conflicts of interest.
For correspondence contact the author at the above address, or email at Bruce.Napier@pnnl.gov.
Supplemental Digital Content is available in the HTML and PDF versions of this article on the journal’s Web site (www.health-physics.com).
(Manuscript accepted 5 September 2013)
Abstract: Beginning in 1948, the Soviet Union initiated a program for production of nuclear materials for a weapons program. The first facility for production of plutonium was constructed in the central portion of the country east of the southern Ural Mountains, about halfway between the major industrial cities of Ekaterinburg and Chelyabinsk. The facility, now known as the Mayak Production Association, and its associated town, now known as Ozersk, were built to irradiate uranium in reactors, separate the resulting plutonium in reprocessing plants, and prepare plutonium metal in the metallurgical plant. The rush to production, coupled with inexperience in handling radioactive materials, led to large radiation exposures, not only to the workers in the facilities, but also to the surrounding public. Fuel processing started with no controls on releases, and fuel dissolution and accidents in reactors resulted in release of ∼37 PBq of 131I between 1948 and 1967. Designed disposals of low- and intermediate-level liquid radioactive wastes, and accidental releases via cooling water from tank farms of high-level liquid radioactive wastes into the small Techa River, caused significant contamination and exposures to residents of numerous small riverside villages downstream of the site. Discovery of the magnitude of the aquatic contamination in late 1951 caused revisions to the waste handling regimes, but not before over 200 PBq of radionuclides (with large contributions of 90Sr and 137Cs) were released. Liquid wastes were diverted to tiny Lake Karachay (which today holds over 4 EBq); cooling water was stopped in the tank farms. In 1957, one of the tanks in the tank farm overheated and exploded; over 70 PBq, disproportionately 90Sr, was blown over a large area to the northeast of the site. A large area was contaminated and many villages evacuated. This area today is known as the East Urals Radioactive Trace (EURT). Each of these releases was significant; together they have created a unique group of cohorts with their chronic, low dose-rate radiation exposure. The 26,000 workers at Mayak were highly exposed to external gamma and inhaled plutonium. A cohort of individuals raised as children in Ozersk is under evaluation for their exposures to radioiodine. The Techa River Cohort consists of over 30,000 people who were born before the start of exposure in 1949 and lived along the Techa River. The Techa River Offspring Cohort consists of ∼21,000 persons born to one or more exposed parents of this group, many who also lived along the contaminated river. The EURT Cohort consists of ∼18,000 people who were evacuated from the EURT soon after the 1957 explosion and another 8,000 who remained. These groups together are the focus of dose reconstruction and epidemiological studies funded by the United States, Russia, and the European Union to address the question, “Are doses delivered at low dose rates as effective in producing health effects as the same doses delivered at high dose rates?”
Introduction of Joint U.S. and Russian Studies of Population Exposures (Video 2:13, http://links.lww.com/HP/A28)
THE JOINT Coordinating Committee for Radiation Effects Research (JCCRER) was established in 1994 through a bilateral U.S./Russian agreement to support research and exchange information on radiation health effects.† The U.S. member agencies include the Department of Energy (DOE), Nuclear Regulatory Commission, Department of Health and Human Services, Department of Defense, National Aeronautics and Space Administration, and the Environmental Protection Agency. The DOE is the lead U.S. agency; the Federal Medical Biological Agency (FMBA) is the lead Russian agency. The goal of the program is to better define the relationship between the health effects and the chronic low dose and dose-rate exposure, these data being essential to validate current radiation protection standards and practices. The current primary areas of JCCRER research include dose reconstruction, epidemiologic health studies, molecular epidemiology/biodosimetry, and creation of tissue banks (Fountos and Rabovsky 2007). The bulk of the joint scientific work is conducted in Russia, with U.S. researchers supplementing expertise available in Russia. Projects conducted under the auspices of the JCCRER are reviewed by independent U.S. and Russian Scientific Review Groups. The primary purpose of the Scientific Review Groups is to review and evaluate critically the technical progress reports and proposals and recommend research priorities to DOE and FMBA.
The focus of the JCCRER research is on the workers from the Mayak Production Association in the Southern Urals and on the neighboring populations along the Techa River exposed to contamination from the plant. Mayak began operations in 1948. At various times, Mayak accidentally or intentionally released large quantities of radioactive materials into the environment from uranium-graphite production reactors and its radiochemical processing plant. These releases resulted in significant exposures of ionizing radiation for protracted periods of time to workers, their families, people living along the Techa River downstream from the complex, and in an area now known as the East Urals Radioactive Trace (EURT). A summary of the primary projects related to the Mayak radiation exposures supported by the JCCRER since its inception is presented in Table 1. Currently, about 15 U.S. scientists work with over 150 Russian scientists and technical personnel at Mayak, the Southern Urals Biophysics Institute, and the Urals Research Center for Radiation Medicine (URCRM) on these projects. These teams interact with other scientists from organizations such as the International Science and Technology Center and the European Union’s Southern Urals Radiation Risk Research and Epidemiological Studies of Exposed Southern Urals Population projects.
This overview describes the major Mayak facilities and the releases that resulted from their operations, describes the cohorts of individuals exposed to radiation by Mayak operations, outlines the radiation dose reconstruction activities established for each of these cohorts, and summarizes the results of several epidemiological studies performed on the basis of the collected information.
HISTORY OF MAYAK AND ITS MAJOR RELEASES
Information in this section is based on Kruglov (1995). On 9 April 1946, a decree of the USSR government on construction of what would become the Mayak Production Association was adopted. Construction of the production facility was started in early 1947 east of the Ural Mountains about halfway between the cities of Ekaterinburg and Chelyabinsk. The A Reactor, the first production reactor, was run to designed capacity on 19 June 1948. On 22 December 1948, the first batch of product from A Reactor was delivered to the radiochemical Plant B, and in January 1949 reprocessing started.
In February 1949, construction was finished, and the chemical-metallurgical Plant V was brought into operation; the USSR’s first nuclear weapon was manufactured by August 1949.
In the end of 1948, construction of the AV-1 Reactor was started, and the reactor was ready for startup in the beginning of 1950. On 30 March 1951, the AV-2 Reactor was brought into operation. Construction of Reactor Complex AI was finished in 1951. Reactor AV-3 started up on 15 September 1952. Two heavy-water reactors began operations in 1951 and 1955.
A new radiochemical Plant, “DB” (“double B”—essentially two times the size of the original Plant B), entered operation on 15 September 1959. In 1955, a radioisotope laboratory was organized at AI Reactor. In 1957, construction of the “RI” Plant was started for fabrication of ionizing radiation sources. In 1977, the RT-1 Fuel Processing Facility was brought into operation. It was the only plant in Russia for reprocessing of spent fuel from marine and research reactors. Additional light- and heavy-water reactors were constructed in 1979 and 1987. A mixed-oxide fuel manufacturing plant was prepared. A waste vitrification facility started operation in 1987.
In 1987, the fabrication of weapons-grade plutonium ceased. On 16 June 1987, Reactor A was shut down, and on 25 May 1987, Reactor AI and research facilities were shut down, followed by Reactor AV-1 on 12 August 1989, Reactor AV-2 on 14 July 1990, and reactor AV-2 and Reactor AV-3 on 1 November 1991.
Today Mayak Production Association has become one of the largest Russian State Atomic Energy Corporation ROSATOM facilities. This complex incorporates seven main plants and 16 auxiliary divisions with 14,000 employees.
The Mayak plutonium production reactors consisted of graphite stacks with vertical aluminum process tubes. The graphite stacks were ventilated initially with air and in later years with nitrogen. In the course of reactor operation, large amounts of noble radioactive gases (activation products, mainly 41Ar, and fission products isotopes of krypton and xenon) were released into the atmosphere. Argon radioisotopes were generated as a result of activation of stable argon isotopes in ventilation gas by thermal neutrons. In the early period of reactor operations, process upsets resulted in fuel becoming jammed in the process tubes, and in some instances, these stuck fuel elements were drilled out of the core. This resulted in immediate release of fission gases from the removal operations and in the contamination of the graphite stack with uranium particles, which continued to fission and release noble gases during subsequent reactor operation. During the first years of operation, there were no holdup systems on the reactor cover gas ventilation systems, so releases were routed directly to the atmosphere (Rovny and Mokrov 2006a). Analysis of the reconstructed results (Rovny and Mokrov 2006b) shows that in 1948–1990, ∼83 EBq (2.2 GCi) of 41Ar was released from reactor stacks into the atmosphere, and of that, the major part (over 80%) was released before 1955, in the period when reactor cores were ventilated with atmospheric air prior to use of nitrogen cover gas. The total release of fission noble gas activity for the entire period of reactor plant operation is estimated to be ∼89 EBq (2.4 GCi). The largest components of the release are 59 Ebq (1.6 GCi) of 89Kr, 1.8 EBq (48 MCi) of 138Xe, and 1.7 EBq (46 MCi) of 87Kr.
Of greater dosimetric importance than the noble gases is 131I. Mokrov et al. (2008) indicate that the primary determinants of the atmospheric release of iodine from the reprocessing plants are the amount of fuel processed, its period of cooling between reactor discharge and dissolution, and the type and effectiveness of filtration. The amount of fuel increased significantly between 1949 and 1953, but the cooling period also increased from 30–80 d or more.
Starting in December 1948, silica-gel columns with silver nitrate for entrapping 131I were installed in the dissolution facilities behind refluxers in the gas supply lines leading from the B Plant dissolvers. Due to low working temperatures of the absorbent, the purification coefficient of the silica-gel columns decreased from 366 to 4 over the operation period. There was no gas purification equipment on other process and ventilation exhausts, and discharges were unfiltered. During 1952–1953, devices for steam removal of aerosols were installed in process exhausts of the radiochemical plant. In September 1959, a Venturi scrubber was put into operation at the radiochemical plant to clean process releases. At the DB Plant prior to May 1963, steam devices were added for all the process lines. Ion exchange columns were installed at the dissolution facility of the DB Plant in December 1966. The sorbent in the ion exchange columns was alumogel saturated with silver iodide. The columns were operated only during the acid dissolution of uranium and the output of nitric acid solutions. The DB Plant stopped weapons-grade plutonium production in 1987.
A summary of the releases of 131I from the B and DB Plants is shown in Fig. 1. Very large releases are indicated in the first few years of operations. The increase in cooling time, the application of exhaust gas cleaning equipment, and other incremental improvements dramatically decreased the releases during the operating period of the Mayak Facility. The total integrated release is ∼37 PBq of 131I. There is significant uncertainty in the daily values from which this total is derived, but the overall uncertainty in the integrated release value is less than a factor of two. Releases from the RT-1 Fuel Processing Facility, which began operation in 1977, are negligible in comparison.
Because of leaks to the discharged water (discussed below), cooling was turned off to high-level waste storage tanks. As a result, one of the tanks overheated, eventually reaching a temperature of ∼350 °C. The subsequent explosion in September 1957 corresponded to ∼25–29 tons of TNT (Avramenko et al. 1997) – the “Kyshtym explosion.” About 740 PBq of material were ejected from the tank; ∼90% of the ejected material fell out in close proximity. The remaining 74 PBq were distributed over a large area by 25 km h−1 winds, forming the EURT. Table 2 gives a recent estimate of the isotopic distribution of the distributed material.
Low- and medium-level radioactive liquid wastes were disposed of in Lake Karachay, a small pond on the Mayak Site. This pond is highly contaminated. During a drought in the mid-1960s, the level of the pond surface lowered, exposing the contaminated shoreline. During a high-wind event in 1967, ∼22 TBq of 137Cs were resuspended from the shoreline and distributed in the general regions of the Mayak Facility (Degteva et al. 1996).
Surface water releases
Radiochemical processing in Plant B resulted in large volumes of liquid radioactive waste of different specific activities. The absence of reliable waste management and waste storage technologies during the first period of Mayak operation resulted in significant radioactive contamination of the Techa River. A special project was established that included both experts from Mayak and the JCCRER (Anspaugh et al. 2006) to evaluate the magnitude and timing of the releases. Upon project implementation, scientists from Mayak were given full access to documents, workbooks, and files within the Mayak archives. Important documents regarding the early operation of Mayak were declassified and shared with project participants. These historical data on the Techa River contamination have been reviewed in English (Degteva et al. 2008). A complete re-evaluation of the source term was prepared by Degteva et al. (2012).
At Plant B, high-level wastes were formed during the first stages of radiochemical processing, and special high-level waste storage tanks were built. Low-level wastes formed in the final stages of plutonium production were planned to be released into the Techa River. Other types of low-level and medium-activity waste were planned to be released into closed water bodies located near Plant B (e.g., Lake Karachay). It should be noted that wastes formed in the final stages of uranium production were anticipated to be low level. Also, creation of large amounts of two other types of medium-activity waste was not anticipated; these were condensate from the main stack and so-called “desorption waters” formed after periodic washing of radioactive contamination from the equipment and canyons. At the time Plant B began operation, the technology of plutonium and uranium production had not been completely developed. Also, methods for processing and storing of liquid wastes were not fully normalized. Thus, there were ongoing technological changes during the first period of operation that influenced the amount and radionuclide composition of the releases to the Techa River.
High-level waste storage included three separate underground canyons. Inside the canyons were stainless steel tanks; the tanks were cooled by a continuous water supply in the gap between the tank walls and concrete canyon walls. Cooling lines were individual for each tank, and water was supplied alternately to each tank. Water was pumped out of the canyons and discharged into the Techa River. According to design, all tanks were placed below the level of groundwater. Because of damaged waterproofing, groundwater flowed into the canyons and mixed with cooling water. As a result, partially full tanks floated up from the canyon floors, leading to damaged or broken pipes for transfer of solutions. The damage resulted in leakage of high-level waste into the canyons. Because the design did not consider potential radioactive contamination of the cooling waters, there was no monitoring of the discharges, and contamination was not revealed in a timely manner. A monitoring system was installed in 1951, which revealed very large, episodic releases, particularly in late October 1951. It should be noted that starting on 28 October 1951, if significant radioactive contamination appeared in effluents from the tank cooling, the effluents were to be temporarily switched to Karachay Lake. However, additional accidental releases were observed on 2 and 12 November 1951.
At the end of 1949, a facility was started for concentrating fission products. A method of alkaline concentration of stable impurities (iron and chromium) was used, and uranium, plutonium, and fission products adsorbed on these impurities. Aluminum cladding solutions were also processed with the same method. This allowed volume reduction of high-level waste by a factor of 30–40 but created problems with medium-activity wastes formed as a result of this step. This waste was processed in the so-called “chromate hole,” which was a wooden reservoir with walls made of boards buried in soil to a depth of 2 m. Suspensions of barium chromate and barium carbonate were drained into the hole for settling. Originally the solution in the hole drained through the ground and much of the radioactive material was adsorbed on soil; however, the filtering capacity of the hole decreased due to plugging of its inner surface with barium chromate. Starting in autumn 1950, the radioactive solution was released into the Techa River after settling in the hole. In spring 1951, a second ground hole without inner covering was dug near the existing hole, so that the filtered solution flowed from the old hole into the new one. Thus, the first hole served as a settling tank and the second hole as a filter. However, use of the second hole ended on 17 September 1951 because of its malfunction, and releases into the Techa River from the first hole were resumed.
A complete evaluation of all of these processes and waste streams was developed by Degteva et al. (2008) and reported in Degteva et al. (2012). The total estimated releases into the Techa River are shown in Fig. 2. The total routine release was ∼52 PBq; the total release from accidental overflows from the tank farms was ∼63 PBq. There is significant uncertainty in the daily release values, but when integrated over the entire period, the overall uncertainty in the release is less than a factor of two.
The release was made up of a large number of radionuclides. Each waste stream had a different radionuclide composition, which also varied in time. The total release spectrum has been estimated for the purposes of dose reconstruction, based on the theoretical content of irradiated fuel stored for various effective decay periods (Degteva et al. 2012).
THE COHORTS OF EXPOSED INDIVIDUALS AND THEIR DOSIMETRY
Each of the releases discussed above, as well as the actual operations, has resulted in groups of people being exposed to radiation and radioactive materials. The main groups investigated by the JCCRER are discussed here.
The Mayak Worker Cohort was established in the 1980s and currently includes 25,757 workers hired during the period from 1948–1982 in one of the main plants (nuclear reactors, radiochemical plant, plutonium production facility) or in auxiliary plants involved with water treatment or mechanical repair (Gilbert et al. 2013). Cohort members from the main plants (nuclear, radiochemical, and plutonium) number 22,349 individuals. Nearly all workers have dosimeter records of external exposure; workers in the radiochemical and plutonium production facilities had potential for plutonium exposure. Radiation doses to Mayak workers were much higher than those to workers from similar operations in other countries, especially during the early period of operations (1948–1958). Thus, study of these workers offers a unique opportunity to evaluate health effects from both protracted external exposure and to plutonium.
Estimates of individual annual external and internal (plutonium) doses are being developed as a collaborative effort of Russian and U.S. dosimetrists. An early version of this system was named DOSES-2005 and is documented in a series of articles (Smetanin et al. 2007a and b; Vasilenko et al. 2007a and b). Film badge dosimeter readings provide the basis for most external gamma radiation doses with adjustments to convert the originally recorded doses to organ doses, as well as to account for limitations in the ability of dosimeters to respond accurately to all radiation energies and to radiation coming from all directions. External dose was primarily from whole-body gamma radiation and in most cases was protracted over many years. Internal doses were caused by inhalation of mixtures of oxides and nitrates of plutonium. An updated version of the dosimetry system, MWDS-2008, is described by Khokhryakov et al. (2013). An ongoing advance in the dosimetry that will provide estimates of uncertainty is underway as part of the JCCRER, with scheduled completion in 2013, incorporating a two-dimensional Monte Carlo approach using the Weighted Likelihood Monte Carlo Sampling Method, which will allow dose estimates based upon simultaneous use of urinalysis and autopsy data.
External doses to the workers as recorded on film badges were very high in the early period of Mayak operations. The external dose estimates from DOSES-2005 (Vasilenko et al. 2007a) and MWDS-2008 are similar. During the early years of Mayak operations, worker exposures were particularly high in comparison with later years of operation and in comparison with current radiation protection safety guidelines. This is illustrated in Fig. 3, in which the average reconstructed badge doses for workers in the reactor, radiochemical, and plutonium facilities are plotted.
Internal doses from inhalation of plutonium aerosols were also quite high for some portions of the cohort. Annual and cumulative individual doses for all organs and tissues were calculated for each worker with at least one biophysical examination. Individual cumulative exposure doses in the primary organs of plutonium deposition for Mayak Production Association workers had a wide range. For example, the doses in lungs were up to ∼21 Gy; in liver up to ∼36 Gy; and up to ∼140 Gy for bone surface cells. The greatest cumulative lung absorbed doses were observed in workers who started employment in the initial years of Mayak operations (Khokhryakov et al. 2013.)
The cohort of Ozersk residents exposed as children to 131I as a component of unmonitored gaseous discharges during the early years of operation of the Mayak Production Association is not as well developed as some of the others. In the mid-1990s, the Southern Urals Biophysics Institute created a registry of ∼72,000 individuals born between 1934 and 1988 who lived in Ozersk for at least 1 y before their 15th birthday. This registry, which is known as the Ozersk Offspring Cohort [often referred to as the Mayak Children’s Cohort], includes offspring of Mayak workers and other children meeting the age and residential history criteria. Criteria and methods used in the development of the Ozersk Offspring Cohort have been published by Koshurnikova et al. (2003). Details for these individuals on residence history, food supplies, and other lifestyle characteristics have not yet been developed; vital status follow-up and cause of death are ascertained actively using address bureau records and vital statistics data (including death certificates and autopsy reports). Only initial estimates of dose for reference persons are currently available. Initial scoping studies performed for the JCCRER indicate that thyroid doses to children in Ozersk could be as high as 2 Gy. A JCCRER dosimetry system for reference individuals is in progress.
Techa River residents
The Techa River Cohort (TRC) consists of individuals who were born before 1950 and lived in any of the 41 villages situated along the Techa River from 1950 to 1960, plus ∼5,000 persons who migrated to the villages after the period of high exposure but before 1960; the late entrants in the TRC have been restricted to those born before 1949 as for the original TRC. Extensive review of official documents (including taxation books, vital statistics, and medical records) between the late 1960s and the 1980s to identify cohort members was conducted by URCRM. As of October 2010, the TRC numbered 29,730 persons, ∼58% of whom were women. Approximately 40% of the cohort was first exposed before 20 y of age, 28% at 20–40 y of age, and 32% after 40 y of age. The cohort is 80% Slav and 20% Tatar and Bashkir. The cohort is now relatively old; the youngest member is over 60 y of age (Schonfeld et al. 2013).
In addition, the URCRM Registry includes data on 29,700 persons exposed in utero and/or the progeny of exposed parents. Twelve thousand (12,000) such persons who are the children of TRC members and who were born in the catchment area after 1 January 1950 have been identified as the Techa River Offspring Cohort. The Offspring Cohort has the potential to provide direct data on radiogenic health effects in progeny from exposure of a general population to chronic low dose-rate radiation (Kossenko et al. 2002).
Internal dose computation with use of the Techa River Dosimetry System TRDS-2009D gives estimates of doses for all cohort members who resided on the Techa River (Degteva et al. 2009). The initial estimates are based on village average-intake functions and external dose rates with consideration of an individual’s residence history, age, gender, and the date of vital status or migration from the catchment area (the area of epidemiologic follow-up). Doses are highest to red bone marrow because the dominant radionuclide is 90Sr. Significant variations from settlement to settlement (Fig. 4) are explained mainly by different sources of drinking water (Techa River, wells, or both sources). The levels have a tendency to decrease with distance from the release site, which is explained by reduction of 90Sr concentration in the river water due to dilution. The highest doses are in the range of 2 Gy to red bone marrow. For individuals with whole-body counts, these generic village average doses are modified to account for the person’s actual measurements and sometimes household-average internal exposure estimates for persons living in the same household.
External dose estimates are made based on accumulation of gamma-emitting radionuclides in the bottom sediments and flooded soils (Fig. 5), combined with assumptions on amounts of time spent near the river, outdoors in villages, and indoors (Degteva et al. 2009). As can be seen from Fig. 5, external doses decreased sharply with distance from the release site because of sorption and decay of shorter-lived isotopes.
An additional potentially confounding factor is related to the fact that TRC members have been provided medical monitoring at the URCRM for many decades, including extensive radiographic examinations. Six thousand four-hundred fourteen (6,414) members of the TRC (22% of the entire cohort) were exposed to x-rays for diagnostic purposes. Additions to individual doses in different organs of these persons were 30–35 mGy on average and 650–750 mGy at most. Therefore, this source of confounding exposure is also being addressed.
Additional dosimetric modeling is also underway. Enhancements in the existing TRDS-2009 dosimetry system will include better individualization of subjects’ internal and external doses and a modular, two-dimensional Monte-Carlo approach separating shared and unshared uncertainties, as described by the National Council on Radiation Protection and Measurements (NCRP 2009).
East Urals radioactive trace residents
Following the tank explosion in 1957, residents of the most highly contaminated villages were evacuated. About 1,100 of the evacuations took place within 7–14 d because of high levels of external exposure rate. Relocation of other settlements took place later, but due to high levels of 90Sr-contamination density (74–148 kBq km−2 of 90Sr), 2,280 persons were evacuated 250 d after the accident. Later (within 330–670 d after the accident) another 8,300 persons were removed from territories with contamination density >74 kBq m−2 of 90Sr (Shagina et al. 2012).
In addition to the evacuated residents, ∼8,000 additional people continued to live in the less-contaminated fringes of the EURT footprint.
In addition to exposure on the Techa River, TRDS-2009D includes doses due to residence on the EURT area; 4,695 members of the TRC (16% of the entire cohort) were exposed on the EURT. Nevertheless, additions to individual red bone marrow doses for these persons were low: 5.5 mGy on average and 44 mGy at most (Shagina et al. 2012).
Studies supported by the JCCRER have made numerous estimates of the effects of radiation exposure on the various cohorts. Some of the more recent are summarized here.
Mayak workers have been exposed to high doses of low linear energy transfer (LET) external gammas and a significant subcohort to additional high doses of high-LET internal plutonium alpha-particles. In addition to showing radiation responses from external irradiation, the Mayak Worker Cohort provides the only human data that are adequate for estimating cancer risks from plutonium exposure.
Sokolnikov et al. (2008), in analyses corrected for external exposure, indicate a highly significant dose response for lung, liver, and bone cancer from inhaled plutonium. For lung and liver cancer, the dose response is well described by a linear function; there is no evidence of nonlinearity to doses <0.2 Gy. The excess relative risk per gray (ERR Gy−1) of exposure for males and females is presented in Table 3. All are statistically significant, with the exception of bone cancer in males, which is marginally nonsignificant mostly because of the small number of cases.
The lung cancer risk estimates for Mayak workers have recently been updated by Gilbert et al. (2013) using the updated MWDS-2008 dosimetry system. In analyses that were adjusted for external radiation dose and smoking, the plutonium lung cancer ERR Gy−1 values were similar to those found earlier by Sokolnikov et al. (2008), declined with attained age, and were higher for females than for males. The ERR Gy−1 for males at 60 y of age was 7.4 [95% confidence interval (CI): 5.0–11] compared to 7.1, while that for females was 24 (95% CI: 11–56) compared to 15. Adjustment for smoking reduced some of the major differences by gender, but large differences remained. When analyses were restricted to plutonium doses <0.2 Gy, the ERR Gy−1 for males at 60 y of age were similar: 7.0 (95% CI: 2.5–13); results in this format were not reported for females. Gilbert et al. (2013) also performed an interesting comparison with the Radiation Effects Research Foundation’s Life-Span Study (LSS), in which they compared the risks of lung cancer on the basis of effective dose by multiplying the Mayak absorbed lung dose by a radiation weighting factor. If the Mayak estimate is expressed as Sievert with a weighting factor of 20 as recommended by the International Commission on Radiological Protection (ICRP 2003), it becomes 0.35 (95% CI: 0.24–0.50), very similar to the low-LET LSS-based estimates (Preston et al. 2007), indicating that the assumed equivalence of risk from internal and external radiation dose may be appropriate. However, given the large uncertainty of the LSS-based estimates, a wide range of weighting factors would also be consistent with these data.
Schonfeld et al. (2013) evaluated the association between in utero radiation exposure and risk of solid cancer and leukemia mortality among 8,000 offspring, born from 1948–1988, of female workers at Mayak. Among 3,226 offspring exposed in utero, the mean estimated in utero dose was highest for those born before 1954 (mean 113.4 mGy) and lowest for offspring born after 1969 (mean 2.7 mGy). The initial results provide no evidence that low-dose gamma in utero radiation exposure increases solid cancer or leukemia mortality risk, but neither are the data inconsistent with such an increase.
Azizova et al. (2010a and b) performed interesting analyses of noncancer radiation effects in Mayak workers. There were statistically significant increasing trends with both total external gamma-ray dose and internal liver dose in incidence of ischemic heart disease (Azizova et al. 2010a). The trend with external dose in ischemic heart disease mortality was not statistically significantly greater than zero but was consistent with the corresponding trend in incidence. There was also a statistically significant increasing trend in acute myocardial infarction (heart attack) incidence. There were statistically significant increasing trends in cerebrovascular disease incidence, but not mortality, with both total external gamma-ray dose and internal liver dose (Azizova et al. 2010b). The risk estimates for ischemic heart disease in relation to external radiation are generally compatible with those from other large occupational studies and the Japanese atomic bomb survivors; the risk estimates for cerebrovascular disease imply somewhat higher risk estimates compared to those from the atomic bomb survivors.
Although dosimetry is lacking, Koshurnikova et al. (2012) have performed an ecological epidemiological analysis of thyroid cancers in individuals who were born or moved before the age of 15 y to the cities of Ozersk and Kyshtym near Mayak. As a control, the analysis used the population of the regional capital city of Chelyabinsk, located ∼100 km from Mayak. The analysis shows that the relative risk of thyroid cancer incidence averaged over age in the Ozersk and Kyshtym Cohorts is >1.5 times greater than in the Chelyabinsk population. The greatest relative occurrence of thyroid cancers occurred among individuals who established residence in the cities prior to 1965. Histological verification of the cancer cases diagnosed for individuals who lived in the city as children until 1965 indicated that tumors were of a papillary type (spontaneous tumors are usually follicular). No increase in thyroid cancer incidence was found for people who currently are children; iodine releases currently are much lower than historical releases. The average age of thyroid cancer patients was 40.2 ± 3.3 in Ozersk and 35.9 ± 3.1 in Kyshtym; it is conjectured that the prolonged latent period of thyroid cancer development in Ozersk and Kyshtym residents is caused by the prolonged nature of the exposure.
Techa River residents
Schonfeld et al. (2013) recently estimated the dose-response relationship for solid cancer mortality using dose estimates from the TRDS-2009D dosimetry system. About 928,000 person-y have accumulated in the Techa River Mortality Cohort, and the 2,303 deaths from solid cancers represent an excess of ∼50 cases. The linear ERR Gy−1 is 0.61 (95% CI: 0.04–1.27). It is estimated that ∼2% of solid cancer deaths were associated with the radiation exposure (an excess of ∼50); this is an insufficient number to allow estimation of organ-specific cancer risks. The data were well described by a linear dose-response model. The results support an increased risk of solid cancer mortality following protracted radiation exposure from the Techa River contamination. Preliminary analyses of medical exposure at URCRM produced extremely large ERR estimates with respect to solid cancer mortality, suggesting that medical diagnostic dose could be more of an indication of ill health than a cause of subsequent cancer risk in this population.
Incidence of leukemia has been investigated in the TRC (Krestinina et al. 2009). Between 1953 and 2005, 93 first primary cases of leukemia, including 23 cases of chronic lymphatic leukemia, were ascertained among the cohort members. A significant linear dose-response relationship was seen for leukemias other than chronic lymphatic leukemia, using the older TRDS-2000 dosimetry. The estimated ERR Gy−1 is 4.9 (95% CI: 1.6, 14.3) for leukemias other than chronic lymphatic leukemia. Cohort members identified as being of Tatar or Bashkir ethnicity appear to have higher rates than those identified as Slavs. The estimate of the non-chronic lymphocytic leukemia ERR at 1 Gy is similar to that seen in the atomic bomb survivors who received acute high-dose-rate exposures. There is practically no difference between leukemia risk values for the TRC calculated using both a linear and those using a linear-quadratic model.
As with the Mayak workers, investigators are evaluating the potential for cardiovascular and cerebrovascular impacts of radiation. A study by Krestinina et al. (2013), not adjusted for smoking and alcohol consumption, found that for all circulatory diseases, the estimated excess relative risk of mortality per 100 mGy with a 15-y lag period was 3.6% (95% CI: 0.2–7.5%), and for ischemic heart disease it was 5.6% (95% CI: 0.1–11.9%). A linear ERR model provided the best fit. There was no evidence of an increased mortality risk from cerebrovascular disease.
The potential for effects to members of the Techa River Offspring Cohort were evaluated by Kossenko et al. (2002). Doses to children born during the first years of releases from parental gonad, in utero, and childhood exposures were evaluated using the TRDS-2000 dosimetry. The dose estimates considered both external and internal sources. Paternal gonad doses ranged from 0–1,172 mSv (mean value 114.4 mSv), and doses to maternal gonads ranged from 0–1,169 mSv (mean value 105.3 mSv). The mean dose to the fetus was 20.6 mSv (ranging from 0–334 mSv). Crude incidence rates were used to assess gender and ethnic differences. The number of person-y under observation amounted to 309,045 for the total cohort and to 125,734 for individuals whose exposure originated in the prenatal period. The overall cancer incidence rate was 24.3 per 100,000. This comparatively low morbidity rate can be attributed to the fact that the highest age attained was 45 y, with only 26% over 40 y of age. No statistically significant gender difference was seen. This cohort needs to be followed for a longer period before statistically significant impacts may start to appear.
The JCCRER studies of the Mayak populations are valuable because radiation effects (cancer and noncancer) are evident in these chronically exposed groups. Radiation effects are commensurate with the Japanese LSS (Preston et al. 2007) and with the 15-Country Study (Cardis et al. 2007). Radiation effects appear to be linear with dose to <0.1 Gy. A significant result of most of the studies is that internal doses protracted over many years seem to be just as important as instantaneous external doses. Long-term chronic doses appear to have essentially the same effect as instantaneous acute doses; within the limits of the observations of these studies, the dose and dose-rate effectiveness factor is approximately equal to one, which has not been widely anticipated. Because the dose reconstructions for each cohort are still ongoing, these results may change in the future.
The studies do confirm that radiation is a weak carcinogen. The fraction of cancers in the Mayak and TRCs attributable to radiation is small.
Finally, events from the 1940s, 1950s, and 1960s still disrupt the lives of regional inhabitants. These populations deserve additional study and open communication about the results.
The author would like to recognize the nearly 20-y efforts of U.S. Department of Energy staff members who have served on the JCCRER Executive Committee; previous Program Managers Mohandas Bhat, Elizabeth White, and Claudia Beach; previous Subject Matter Experts Eleanor Melamed, Ruth Neta, Mohandas Bhat, and Libby White; current Subject Matter Expert Joel Rabovsky; and current Program Manager Barrett Fountos. The JCCRER has been sustained by the continuing contributions by dozens of scientists at the Mayak Production Association, Southern Urals Biophysics Institute, and Urals Research Center for Radiation Medicine. Funding for JCCRER activities over the years has been supplied by the U.S. Department of Energy, U.S. Nuclear Regulatory Commission, U.S. Environmental Protection Agency, and National Aeronautics and Space Administration and by the Russian Ministry of Health, ROSATOM, and Federal Medical Biological Agency. Other work briefly described here has also been funded at various times by the Commission of European Communities, the International Science and Technology Center, and the International Science Foundation. Much of the epidemiological work has been independently supported by the National Cancer Institute in close collaboration with the JCCRER.
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†Agreement on Cooperation in Research on Radiation Effects for the Purpose of Minimizing the Consequences of Radioactive Contamination on Health and the Environment, 14 January 1994. Cited Here...
dose assessment; epidemiology; National Council on Radiation Protection and Measurements; radiation effects
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