Natural background radiation: problem of radionuclide migration and biological effects

Natural radiation background is the main contributor to radiation dose delivered to plants, animals, and man. That is why its effects are of increasing interest in radiobiology, radioecology, and radiation hygiene. The following problems are discussed: migration of main natural radionuclides in biosphere, dose formation, biological role of natural radiation background on the Earth, standards for natural radiation background, and problems of natural radiation background and biosphere evolution. The tasks of further radiobiological research in evaluating the role of natural radiation background are outlined.     

The cytogenetic consequences of chronic irradiation in rodent populations inhabiting the Eastern Urals Radioactive Trace zone

In bone marrow cells of rodents (Apodemus (Sylvaemus) uralensis Pall., 1811, Apodemus agrarius Pall., 1771) inhabiting the Eastern Urals Radioactive Trace (EURT) zone (Kyshtym radiation accident 1957) and adjacent areas of Urals, the chromosome instability and 90Sr accumulation in bones were investigated. Intensive mutagenic process in both species from impact plots (the soil pollution by 90Sr 2322-16690 kBq/m2) was found. Significant positive correlation of aberrant cells frequencies and 90Sr was shown. Possible causes of the lack of resistance to long-term mutagenic factor (over 100 generations since 50 years from the accident) such as migration of animals and specific configuration of the EURT zone (narrow extended territory with sharply falling gradient of radionuclide pollution), which considerably decrease the probability that certain changes will be fixed and inherited in a series of generations of rodents, are discussed.     

Ecosystem remediation in radioactively polluted areas: the Chernobyl experience

The ecosystems of Chernobyl nuclear accident zone, having been deeply transformed by normal long-term agricultural and forestry activity, have been damaged by irradiation after the nuclear accident of 1986 and then influenced by post-accident countermeasures. Ecosystem remediation is being undertaken in this area, including maintenance of the forests, artificial afforestation, and processes of spontaneous self-restoration of the natural coenoses. Both natural and artificial remediation of the landscapes and ecosystems at the abandoned territory improves their capacity to retain radionuclides and their ability to stabilise the processes of redistribution of radionuclides and their spread in the environment. The Chernobyl zone is a territory of great importance for studying ecosystem remediation processes and elaborating on ecological engineering technologies to correct serious environmental problems arising from radionuclides release.     

Reconstruction of radionuclide intakes for the residents of East Urals Radioactive Trace (1957–2011)

The East Urals Radioactive Trace (EURT) was formed after a chemical explosion in the radioactive waste-storage facility of the Mayak Production Association in 1957 (Southern Urals, Russia) and resulted in an activity dispersion of 7.4?×?10¹⁶ Bq into the atmosphere. Internal exposure due to ingestion of radionuclides with local foodstuffs was the main factor of public exposure at the EURT. The EURT cohort, combining residents of most contaminated settlements, was formed for epidemiological study at the Urals Research Center for Radiation Medicine, Russia (URCRM). For the purpose of improvement of radionuclide intake estimates for cohort members, the following data sets collected in URCRM were used: (1) Total β-activity and radiochemical measurements of ⁹⁰Sr in local foodstuffs over all of the period of interest (1958–2011; n?=?2200), which were used for relative ⁹⁰Sr intake estimations. (2) ⁹⁰Sr measurements in human bones and whole body (n?=?338); these data were used for average ⁹⁰Sr intake derivations using an age- and gender-dependent Sr-biokinetic model. Non-strontium radionuclide intakes were evaluated on the basis of ⁹⁰Sr intake data and the radionuclide composition of contaminated foodstuffs. Validation of radionuclide intakes during the first years after the accident was first carried out using measurements of the feces β-activity of EURT residents (n?=?148). The comparison of experimental and reconstructed values of feces β-activity shows good agreement. ⁹⁰Sr intakes for residents of settlements evacuated 7–14 days after the accident were also obtained from ⁹⁰Sr measurements in human bone and whole body. The results of radionuclide intake reconstruction will be used to estimate the internal doses for the members of the EURT cohort.     

Cytogenetic consequences of chronic irradiation in rodent populations inhabiting the Eastern Ural Radioactive Trace zone

Chromosome instability in bone-marrow cells and ⁹⁰Sr accumulation in the bone tissue are studied in rodents (Apodemus (Sylvaemus) uralensis Pall., 1811 and Apodemus agrarius Pall., 1771)* inhabiting the Eastern Ural Radioactive Trace (EURT) zone (Kyshtym radiation accident 1957) and adjacent areas of the Urals. Intensive mutagenic process in both species from impact plots (soil pollution by ⁹⁰Sr, 2322–16690 kBq/m²) is found. Significant positive correlation of the frequency of aberrant cells and the concentration of ⁹⁰Sr is shown. Possible causes of the lack of resistance to long-term mutagenic factors (over 100 generations in the 50 years since the accident), such as migration of animals and the specific configuration of the EURT zone (an extended narrow territory with a sharply falling gradient of radionuclide pollution), which considerably decrease the probability that certain changes will be fixed and inherited in a series of generations of rodents, are discussed.     

The effect of Chernobyl accident on the development of non malignant diseases

The early medical complications of Chernobyl accident include post radiation disease, which were diagnosed in 134 subjects affected by ionizing radiation. 28 persons died during the first 100 days after the event. The increase occurrence of coronary heart disease, endocrine, haematological, dermatological and other diseases were observed after disaster in the contaminated territories. We also discussed the impact of ionizing radiation from Chernobyl accident on pregnancy and congenital defects occurrence. Changes following the Chernobyl accident, as the inhabitants migration from contaminated regions, political and economic conversions, led to depression, anxiety, and even to "epidemic" of mental diseases. Increased suicide rate, car accidents, alcohol and drug abuse have been observed in this population. Nowadays vegetative neurosis is more often diagnosed in Ukrainian children. Epidemiological studies were conducted on the ionising radiation effect on the health and on the dose of received radiation after Chernobyl accident face numerous problems as the absence of reliable data regarding diseases in the contaminated territories.     

Chernobyl and the environment

Analyzed information which describes a wide spectrum of the consequences of radioactive contamination of natural ecosystems by the Chernobyl accident. The main regularities of radionuclide migration in the environment was been considered and scales of radiation damage to biota assessed. The area where signs of direct damage to biota are visible is shown to be noticeably smaller in size that the area where radionuclide concentration in environmental objects is above the permissible levels. The importance of the impact of radioactive contamination of natural ecosystems is assessed in terms of formation of exposure doses to the population. The conclusion was made that the detriment from all the factors is larger for humans (direct irradiation, limitation of the economic activity, etc.) than for biota.     

Reduced vitamin A (retinol) levels indicate radionuclide exposure in Streaked Shearwaters (Calonectris leucomelas) following the 2011 Fukushima nuclear accident

The Fukushima nuclear accident in 2011 released significant amounts of radionuclides into the marine environment. Exposure to radiation reduces levels of antioxidants such as carotenoids and vitamins A and E within exposed individuals. Such reductions can cause teratogenic or mutagenetic effects leading to reduced reproductive viability and fitness. Reduced antioxidant levels therefore may be used as an indicator of radionuclide contamination and to infer individual or population level impacts; however, the taxa-specific responses of marine organisms, such as seabirds, are poorly understood. As top predators, seabirds are ideal bio-indicators of the prevalence of contaminants and pollutants in marine ecosystems. At-sea foraging distributions of Streaked Shearwaters (Calonectris leucomelas) from Mikura Island (MKR), Japan during the post egg-laying period coincide with the Fukushima nuclear plume while the breeding colony on Birou Island (BRU) lies outside the affected zone. We examined the physiological responses of Streaked Shearwater chicks at MKR and BRU to possible radiation exposure during the 2011 breeding season, four to seven months after the Fukushima nuclear accident. Fledging mass did not differ between islands but fledglings from MKR displayed significantly reduced vitamin A levels. Available information suggests these depletions most likely result from radiation exposure due to the Fukushima nuclear accident, implying that the risk of radionuclide contamination is considerably elevated for Streaked Shearwaters on MKR, where more than 60% of the world's population breeds. While additional negative impacts are expected due to delayed effects of radionuclide transport via biomagnification in the food chain, this study highlights the potential immediate and worrisome consequences of the Fukushima nuclear accident for marine wildlife.     

Computational study of radiation chemical processing in comet nuclei.

Cometary nuclei have been exposed to high levels of ionizing radiation since their formation. We present here some results of a computer model calculation of the effect of ionizing radiation on cometary material. The external (cosmic rays) and internal (embedded radionuclides) contributions in the processing of cometary nuclei are considered. As a first approximation we have used the available kinetic data of the liquid water system to model the radiation effects in a frozen cometary environment. Our [correction of out] data suggest that massive radiation chemical processing due to cosmic rays may have taken place only in the outer layers of comets. The internal contribution of radionuclides to the radiation processing of comet cores seems to be modest. Therefore, comets could be carriers of intact homochiral biomolecules.     

Adaptation of small mammals to radiocontaminated environment: Oxidative metabolism and radioresistance

Differences in radioresistance and oxidative metabolism parameters in rodents of different ecological specialization in the East Ural radioactive trace (EURT) area are shown. Northern mole voles (radiosensitive species) exhibit a pronounced resistance to chronic exposure to radiation, living in a series of generations in a radioactive environment, whereas pigmy wood mice (radioresistant species) are less resistant in the EURT area due to their migration activity. Thus, under the EURT area conditions, northern mole voles realize the genotypic radioadaptation, whereas pigmy wood mice realize the physiological radioadaptation, the key role in which is played by the ecological and physiological characteristics and the lifestyle of animals.     

Cytogenetic effects in plant populations from the East Urals Radioactive Track

Mutation dynamics in generations was studied in natural populations of plants from the East Ural Radioactive Track (EURT, Kyshtym accident). The experiments were aimed at the investigation of the formation of genetic load in natural plant populations under conditions of prolonged radiation exposure. The main approach used in the study was the analysis of cytogenetic alteration, chlorophyll and gene mutations in plants exposed for more than 40 years to chronic beta-irradiation and in their progeny.     

Computational study of radiation chemical processing in comet nuclei

Cometary nuclei have been exposed to high levels of ionizing radiation since their formation. We present here some results of a computer model calculation of the effect of ionizing radiation on cometary material. The external (cosmic rays) and internal (embedded radionuclides) contributions in the processing of cometary nuclei are considered. As a first approximation we have used the available kinetic data of the liquid water system to model the radiation effects in a frozen cometary environment. Out data suggest that massive radiation chemical processing due to cosmic rays may have taken place only in the outer layers of comets. The internal contribution of radionuclides to the radiation processing of comet cores seems to be modest. Therefore, comets could be carriers of intact homochiral biomolecules.Part of this work was carried out during a leave at the Laboratory of Chemical Evolution.     

Biological consequence of nuclear versus cytoplasmic decays of 125I: cysteamine as a radioprotector against Auger cascades in vivo

When the radionuclide 125I is localized in mouse testis as 125I-iododeoxyuridine (an analogue of thymidine) and incorporated into the DNA of spermatogonial cells, the cytocidal effects are as severe as those due to densely ionizing alpha particles. In contrast, 125I confined to the cytoplasm of these cells is much less radiotoxic, the efficacy being the same as for selective irradiation of the testis with sparsely ionizing external X rays. The biological effects, in both cases, are strongly mitigated upon pretreatment of the testes with very small amounts (0.75 microgram) of cysteamine, a radioprotector. These findings suggest an important role for such chemical agents in radiation protection and in understanding the mechanisms of radiation damage involving radionuclides incorporated in tissue.     

The dose estimation of woody plants in the long-term after the Chernobyl NPP accident

Dosimetric models have been developed to estimate the exposure doses of woody plants growing in the area contaminated by long-lived radionuclides. The models are parameterized based on the data obtained from the experimental plots in the south-west districts of the Bryansk region affected by radioactive fallout of the Chernobyl NPP accident. Doses are estimated to generative organs of pine trees from these plots. The contribution from various sources and types of ionizing radiation to the absorbed dose formation for these objects is determined.     

Genotoxicity and toxicity assay of water sampled from the underground nuclear explosion site in the north of the Perm region (Russia)

The results of our study revealed a local biologically relevant surface water contamination in the radionuclide anomaly in the north of Russia (Perm region) by means of Allium shoenoprasum L. the anaphase-telophase chromosome aberration assay. This radionuclide anomaly was formed in 1971 as a result of an underground nuclear explosion with soil excavation. Specific activities of main dose-forming radionuclides in all examined reservoirs are below intervention levels officially adopted in Russia for drinking water. We found that 90Sr significantly contribute to induction of cytogenetic disturbances. Our previous and described here data suggest that metal ions and radionuclides combined exposure on the various biota species (with the dose below permissible exposure limits for human) may cause substantial biological effects in part be due to synergic response. The findings described here indicated that development of a new concept of radiation protection for humans and biota should be based on the clear understanding of biological effects of low doses of radiation in chronic exposure to multi-pollutant mixtures.     

Microbial interaction with and tolerance of radionuclides: underlying mechanisms and biotechnological applications

Radionuclides (RNs) generated by nuclear and civil industries are released in natural ecosystems and may have a hazardous impact on human health and the environment. RN-polluted environments harbour different microbial species that become highly tolerant of these elements through mechanisms including biosorption, biotransformation, biomineralization and intracellular accumulation. Such microbial–RN interaction processes hold biotechnological potential for the design of bioremediation strategies to deal with several contamination problems. This paper, with its multidisciplinary approach, provides a state-of-the-art review of most research endeavours aimed to elucidate how microbes deal with radionuclides and how they tolerate ionizing radiations. In addition, the most recent findings related to new biotechnological applications of microbes in the bioremediation of radionuclides and in the long-term disposal of nuclear wastes are described and discussed.     

Radiation dose rates estimation and allozyme variability in the population of the northern red-backed vole (Clethrionomys rutilus) from the East-Ural Radioactive Trace

The paper estimates the external (due to radionuclides accumulated in the soil) and internal (due to incorporated radionuclides) exposure of rodents that live in the head of the East Urals Radioactive Trace (EURT). It is shown that in the last decades the dose rates were lower than the values that lead to a doubling the number of mutations in the allozyme loci in mammals. The variability of the eight allozyme loci in populations of northern red-backed voles from the EURT zone and their neighboring plots, as well as the territories of the Urals and Trans-Urals with background levels of radioactive contamination, are analyzed. No differences in the pattern and frequency of allozymes that would distinguish the EURT samples from a number of other populations of the Urals, were found. In the control sample “Sysert’,” “unique” for the Ural populations of northern red-backed voles, alleles of the loci Got and Sod were marked, conspecific to a closely related species-the bank vole. This fact can be regarded as evidence of recent cross-species hybridization.     

The biological effects in animals related to the accident at the Chernobyl Atomic Electric Power Station. 1. The experimental model. The radiation loads on animals kept continuously under external and internal radiation exposure in the area of the Chernobyl Atomic Electric Power Station]

Irradiation conditions in which laboratory animals were kept in experimental laboratories of Chernobyl and Kiev after the accident at the Chernobyl A.P.S. are described. The data are presented on the spectral structural and activity of radionuclides in the diet as well as in the organs and tissues of the animals. The radiation loads have been estimated with regard to an external gamma component and the internal one contributed by the incorporated radionuclides. It has been shown that radiation doses received by the animals during their lifetime due to these contributions do not exceed units of cGy.     

Estimating the biological half-life for radionuclides in homoeothermic vertebrates: a simplified allometric approach

The application of allometric, or mass-dependent, relationships within radioecology has increased with the evolution of models to predict the exposure of organisms other than man. Allometry presents a method of addressing the lack of empirical data on radionuclide transfer and metabolism for the many radionuclide–species combinations which may need to be considered. However, sufficient data across a range of species with different masses are required to establish allometric relationships and this is not always available. Here, an alternative allometric approach to predict the biological half-life of radionuclides in homoeothermic vertebrates which does not require such data is derived. Biological half-life values are predicted for four radionuclides and compared to available data for a range of species. All predictions were within a factor of five of the observed values when the model was parameterised appropriate to the feeding strategy of each species. This is an encouraging level of agreement given that the allometric models are intended to provide broad approximations rather than exact values. However, reasons why some radionuclides deviate from what would be anticipated from Kleiber’s law need to be determined to allow a more complete exploitation of the potential of allometric extrapolation within radioecological models.     

Fungi and ionizing radiation from radionuclides

Radionuclides in the environment are one of the major concerns to human health and ecotoxicology. The explosion at the Chernobyl nuclear power plant renewed interest in the role played by fungi in mediating radionuclide movement in ecosystems. As a result of these studies, our knowledge of the importance of fungi, especially in their mycorrhizal habit, in long-term accumulation of radionuclides, transfer up the food chain and regulation of accumulation by their host plants was increased. Micro-fungi have been found to be highly resilient to exposure to ionizing radiation, with fungi having been isolated from within and around the Chernobyl plant. Radioresistance of some fungal species has been linked to the presence of melanin, which has been shown to have emerging properties of acting as an energy transporter for metabolism and has been implicated in enhancing hyphal growth and directed growth of sensitized hyphae towards sources of radiation. Using this recently acquired knowledge, we may be in a better position to suggest the use of fungi in bioremediation of radioactively contaminated sites and cleanup of industrial effluent.