Modelling the effects of ionizing radiation on survival of animal population: acute versus chronic exposure.

The objective of the present paper was application of a model, which was originally developed to simulate chronic ionizing radiation effects in a generic isolated population, to the case of acute exposure, and comparison of the dynamic features of radiation effects on the population survival in cases of acute and chronic exposure. Two modes of exposure were considered: acute exposure (2-35 Gy) and chronic lifetime exposure with the same integrated dose. Calculations were made for a generic mice population; however, the model can be applied for other animals with proper selection of parameter values. In case of acute exposure, in the range 2-11 Gy, the population response was in two phases. During a first phase, there was a depletion in population survival; the second phase was a recovery period due to reparation of damage and biosynthesis of new biomass. Model predictions indicate that a generic mice population, living in ideal conditions, has the potential for recovery (within a mouse lifetime period) from acute exposure with dose up to 10-11 Gy, i.e., the population may recover from doses above an LD50 (6.2 Gy). Following acute doses above 14 Gy, however, the mice population went to extinction without recovery. In contrast, under chronic lifetime exposures (500 days), radiation had little effect on population survival up to integrated doses of 14-15 Gy, so the survival of a population subjected to chronic exposure was much better compared with that after an acute exposure with the same dose. Due to the effect of "wasted radiation", the integrated dose of chronic exposure could be about two times higher than acute dose, producing the same effect on survival. It is concluded that the developed generic population model including the repair of radiation damage can be applied both to acute and chronic modes of exposure; results of calculations for generic mice population are in qualitative agreement with published data on radiation effects in mice.

Radiation effects in generic populations inhabiting a limiting environment.

A generic population model is formulated for radiation risk assessment of natural biota. The model demonstrates that effects of radiation on the population survival do not follow directly the effects on individual organisms. Dose rates resulting in population extinction can be analytically calculated. Besides individual radiosensitivity, two key parameters were found to determine the survival potential of a population under chronic radiation stress: the ratio "biomass losses/biomass synthesis," and the lump amount of limiting resource in the environment. A benchmark scenario "Population response to chronic irradiation" developed within the IAEA Programme EMRAS II was calculated for generic populations of mice, hare/rabbit, wolf/wild dog, and deer/goat chronically exposed to different levels of ionizing radiation. In the conditions of the benchmark scenario, model populations survived normally (>90% of the control value) at dose rates below the following levels: 3 mGy day(-1) for wolf/wild dog; 10 mGy day(-1) for deer/goat; 14 mGy day(-1) for hare/rabbit; and 20 mGy day(-1) for mice. The model predictions showed a relatively high survival potential of short-lived and productive species such as mice. At the same time, populations of long-lived animals with slow and radiosensitive reproduction such as wolf/wild dog were candidates to extinction at chronic exposures above 5 mGy day(-1). Recovery of short-lived and productive species took a much shorter time compared with long-lived and slow reproductive species.

Modelling the radioactive contamination of commercial fish species in the Barents Sea following a hypothetical short-term release to the Stepovogo Bay of Novaya Zemlya.

A dynamic modelling of radionuclides accumulation in commercial species in the Barents Sea is performed for hypothetical SCR accident with the dumped submarine K-27 at the Stepovogo Bay. Box radioecological model is employed for calculating the radionuclides dynamics in water, bottom sediments and marine biota. The model takes into account the seasonal fish migrations in the Barents Sea. The model allows predicting the dynamic effects of the radionuclide transfer in fish in case of an accidental water contamination. Maximum 137Cs activity concentrations in bottom sediments from the eastern part of the Barents Sea could be reached as late as 5-6 years after the accidental release. Based on the results of model calculations, assessment was made of doses to humans from consumption of seafood contaminated after a hypothetical SCR accident with K-27 at the Stepovogo Bay. The peak activity concentration of the released 137Cs in fish from the Stepovogo Bay is calculated to be 109 Bq∙kg-1, 90Sr - 12 Bq∙kg-1; both estimates are below the permissible activity concentrations of these radionuclides in commercial fish. Predicted maximum annual dose from consumption of fish from the Stepovogo Bay is 47 ± 18 µSv∙year-1, the Barents Sea fish - less than 3∙10-6 µSv∙year-1.137Cs is the major dose contributor; it provides more than 99% to the annual dose Radiation risks from consumption of the Barents Sea commercial fish are evaluated to be negligible in case of a hypothetical SCR accident with the submerged submarine K-27 in Stepovogo Bay of Novaya Zemlya.

Manifestation of radiation effects in cold environment: data review and modeling.

The peculiarities of radiation response in animals at low environmental temperatures are analyzed in the context of radiation safety of the Arctic/Northern wildlife. The paper includes a data review on radiation effects in cold environments based on international and Russian publications since 1948, which forms a supplement to the EPIC and FREDERICA data collections. In homoiothermic and heterothermic animals, imbalances in thermoregulation caused by ionizing radiation are discussed, which increase energy loss of animals, and decrease their fitness to the Arctic/Northern climate. In poikilothermic animals, both radiation damage and recovery are temperature dependent, their rates being slow in the cold environment. At low temperatures, radiation damage of biological tissues is conserved in hidden form; when the temperature of poikilothermic animal rises to a normal level, radiation injury is developed rapidly similar to acute dose response. Additionally, a mathematical model is described, demonstrating the combined effects of chronic radiation exposures and seasonal temperature variations on a fish population. Computer simulations show that at the same level of irradiation, the overall radiation damage to Arctic/Northern poikilothermic fish is higher than that to the fish from warm climate. Considering the peculiarities of radiation effects in the cold climate, the Arctic/Northern fauna might be expected to be more vulnerable to chronic radiation stress compared to temperate fauna. In the case of acute radiation exposure during winter periods, hibernation of heterothermic and cooling of poikilothermic animals may provide temporary protection from acute radiation effects.

[The reconstruction of radiation dose and risk assessment for population living near the Enisei River from the long-term period of exploitation of the Krasnoyarsk mining and chemical enterprise (1975-2000)].

Radiation doses and risks for population living near the Yenisei River have been reconstructed for the long-term period of radioactive discharges from Krasnoyarsk NCC to the river (1975-2000). The analysis of multiple pathways show that the consumption of local fish was the major contributor to the radiation dose to population living near the Yenisei River (more than 90%). 32P was the most important contributor among the discharged radionuclides at the distances 0-100 km downstream the Krasnoyarsk NCC. The maximum annual dose to critical group of population at the nearest area of the Krasnoyarsk NCC was estimated to be 0.95 mSv/year (in 1977), at the same time in settlement located 800 km downstream the discharge source it was 0.2 mSv/year. Since 1993, the doses to population do not exceed 0.02 mSv/year even for nearest area the Krasnoyarsk NCC (0-100 km downstream the discharge source).

Determination of radionuclide composition of the Russian NPPs atmospheric releases and dose assessment to population.

Original data on radionuclide activities in air emissions of Russian NPPs are presented based on direct on-site measurements in vent stacks of NPPs during an extensive survey in 2017-2018. For the first time, the detailed inventories of radionuclides in the air releases were directly measured for all types of nuclear reactors, employed in the Russian NPPs. Based on the results of measurements, annual doses for local populations around the Russian NPPs were estimated. The technogenic radionuclides, detected in the air emissions, were ranked according to their contribution in the annual dose. 14C is the major contributor for the annual dose from the atmospheric releases of LWGR reactors - almost 98% for Bilibino NPP's EGP-6 reactor, and up to 86% for RBMK-1000 reactors (Smolensk NPP). For PWR reactors (VVER) contribution to the annual dose from atmospheric releases is formed mostly by tritium, 14C and noble gases. Estimated annual doses for local populations in the vicinity of the Russian NPPs do not exceed 10-5 Sv/year. Atmospheric releases of 60Co, 134Cs, 137Cs and other aerosols, involved in the routine monitoring programs of Russian NPPs, do not contribute significantly to the annual dose. Based on these results, 14C and tritium will be included in the monitoring programs of atmospheric release control at all Russian NPPs; maximal permissible levels of annual releases for 14C and tritium will be established since 2019.

Assessment of permissible levels of radionuclides in soil for different types of land-use.

Permissible levels of radionuclides in soil were estimated for different human dose criteria. A variety of land-use scenarios, which could lead to human exposure, was considered and all reasonably likely human exposure pathways were analyzed. The obtained estimates are potentially useful for supporting the decision-making process on the possible use or remediation of contaminated territories.

Model testing using data on 131I released from Hanford.

The Hanford test scenario described an accidental release of 131I to the environment from the Hanford Purex Chemical Separations Plant in September 1963. Based on monitoring data collected after the release, this scenario was used by the Dose Reconstruction Working Group of BIOMASS to test models typically used in dose reconstructions. The primary exposure pathway in terms of contribution to human doses was ingestion of contaminated milk and vegetables. Predicted mean doses to the thyroid of reference individuals from ingestion of 131I ranged from 0.0001 to 0.8 mSv. For one location, predicted doses to the thyroids of two children with high milk consumption ranged from 0.006 to 2 mSv. The predicted deposition at any given location varied among participants by a factor of 5-80. The exercise provided an opportunity for comparison of assessment methods and conceptual approaches, testing model predictions against measurements, and identifying the most important contributors to uncertainty in the assessment result. Key factors affecting predictions included the approach to handling incomplete data, interpretation of input information, selection of parameter values, adjustment of models for site-specific conditions, and treatment of uncertainties.

Model testing using data on 137Cs from Chernobyl fallout in the Iput River catchment area of Russia.

Data collected for 10 years following the Chernobyl accident in 1986 have provided a unique opportunity to test the reliability of computer models for contamination of terrestrial and aquatic environments. The Iput River scenario was used by the Dose Reconstruction Working Group of the BIOMASS (Biosphere Modelling and Assessment Methods) programme. The test area was one of the most highly contaminated areas in Russia following the accident, with an average contamination density of 137Cs of 800,000 Bq m-2 and localized contamination up to 1,500,000 Bq m-2, and a variety of countermeasures that were implemented in the test area had to be considered in the modelling exercise. Difficulties encountered during the exercise included averaging of data to account for uneven contamination of the test area, simulating the downward migration and changes in bioavailability of 137Cs in soil, and modelling the effectiveness of countermeasures. The accuracy of model predictions is dependent at least in part on the experience and judgment of the participant in interpretation of input information, selection of parameter values, and treatment of uncertainties.

Current and potential doses from Arctic seafood consumption.

Current collective and individual dose rates to humans are estimated from the consumption of seafood harvested in the Arctic Seas. Statistical data on catches are used for the dose assessment, as well as observed data (1991-1994) on the radioactivity of marine biota. The actual collective dose rates to the world population are estimated to be: 2.7-4.5 manSv/year due to consumption of seafood from the Barents Sea, and 0.03 manSv/year-due to seafood from the Kara Sea. The contribution of 137Cs to the collective dose rate is about 90%. Current individual dose rates to high-rate consumers are estimated to be: 2.6 x 10(-6) Sv/year due to seafood from the Barents Sea; and 4.2 x 10(-6) Sv/year-due to seafood from the Kara Sea. The future radiological impact of the radioactive waste (RW) disposals in the Kara Sea is simulated for the period over 1000 years, using the regional box model of the Arctic Seas. The model predictions are made for three hypothetical scenarios of long-term radionuclide releases, prepared within the framework of the International Arctic Seas Assessment Project. The potential collective dose to world population truncated to 3000 AD is shown to be not higher than 0.13 manSv. The maximum individual dose rates from Arctic seafood consumption are estimated to be about 1.2 x 10(-7) Sv/year. The predicted doses are much smaller than the actual doses due to the current radioactive contamination of the Arctic Seas.

Using a bank of predatory fish samples for bioindication of radioactive contamination of aquatic food chains in the area affected by the Chernobyl accident.

From the analysis of experimental data on radioactive contamination of various fish, it is suggested that predatory fish specimens can be used as bioindicators of radionuclide accumulation in reservoir food chains of the Chernobyl emergency area. The increased content of cesium radionuclides were detected in the muscle tissue of predatory fish collected in various regions of the Chernobyl emergency area. In most of the water bodies studied, maximum contamination levels of predatory fish by radionuclides of cesium occurred in 1987-1988, whereas in 'nonpredatory' fish the concentration of cesium was maximum, as a rule, in the first year following the accident. The exposure doses of fish of various ecological groups and ages are estimated. The exposure doses of various population groups, using fish from contaminated water bodies, are also estimated. When forming the environmental data bank for the Chernobyl accident zone it is suggested that perch, pike-perch and pike be used as bioindicators of radioactive contamination of food chains.

International Arctic Seas Assessment Project.

The International Atomic Energy Agency responded to the news that the former Soviet Union had dumped radioactive wastes in the shallow waters of the Arctic Seas, by launching the International Arctic Seas Assessment Project in 1993. The project had two objectives: to assess the risks to human health and to the environment associated with the radioactive wastes dumped in the Kara and Barents Seas; and to examine possible remedial actions related to the dumped wastes and to advise on whether they are necessary and justified. The current radiological situation in the Arctic waters was examined to assess whether there is any evidence for releases from the dumped waste. Potential future releases from the dumped wastes were predicted, concentrating on the high-level waste objects containing the major part of the radionuclide inventory of the wastes. Environmental transport of released radionuclides was modelled and the associated radiological impact on humans and the biota was assessed. The feasibility, costs and benefits of possible remedial measures applied to a selected high-level waste object were examined. Releases from identified dumped objects were found to be small and localised to the immediate vicinity of the dumping sites. Projected future annual doses to members of the public in typical local population groups were very small, less than 1 microSv--corresponding to a trivial risk. Projected future doses to a hypothetical group of military personnel patrolling the foreshore of the fjords in which wastes have been dumped were higher, up to 4 mSv/year, which still is of the same order as the average annual natural background dose. Moreover, since any of the proposed remedial actions were estimated to cost several million US$ to implement, remediation was not considered justified on the basis of potentially removing a collective dose of 10 man Sv. Doses calculated to marine fauna were insignificant, orders of magnitude below those at which detrimental effects on fauna populations might be expected to occur. Remediation was thus concluded not to be warranted on radiological grounds.

Model testing using Chernobyl data: II. Assessment of the consequences of the radioactive contamination of the Chernobyl Nuclear Power Plant cooling pond.

The "Cooling Pond" scenario is designed to test models for radioactive contamination of aquatic ecosystems, based on data for contamination of different aquatic media and biota due to fallout of radionuclides into the cooling pond of the Chernobyl Nuclear Power Plant. Input data include characteristics of the cooling pond ecosystem (hydrological, hydrochemical, and hydrobiological conditions) and estimates of the amounts of 137Cs in the cooling pond. Predictions are requested in two stages: (1) calculations for 137Cs concentrations for comparison against actual measurements, including activities of 137Cs in the cooling pond water, in sediment layers, and in fish; and (2) calculations for which actual measurements are not available, including dose and risk estimates for aquatic biota and for humans following hypothetical consumption of contaminated biota. The latter calculations are intended to provide an opportunity for intercomparison among modelers of their results for a simulated assessment problem.

Environmental contamination and assessment of doses from radiation releases in the Southern Urals.

The Southern Urals in Russia was contaminated by radioactive discharges into the Techa River (1949-1956), the Kyshtym accident (1957), and the current releases and discharges from the Mayak Nuclear Materials Production Complex. In this paper, the consequences of radioactive contamination of the Ural region are analyzed. The current content of 90Sr in the components of food chains is as follows (Bq kg(-1) wet weight): potatoes, 0.2-6.7; grain, 0.5-12.6; milk, 0.2-6.3; beef, 0.2-1.7; lake water, 0.12-1.0; river water, 0.2-8.5; fish, 7-480; mushrooms, 400-1,100; and berries, 700-16,000. The content of 137Cs is as follows: potatoes, 0.5-3.8; grain, 0.3-2.9; milk, 0.2-4.5; beef, 0.3-2.6; lake and river water, 0.002-0.019; fish, 2-32; mushrooms, 110-1,600; and berries, 150. A major fraction of the dose to humans comes from the consumption of local food products, including natural ones, which have higher contamination levels than agricultural products. The average annual dose rates in contaminated areas are (0.5-4) x 10(-4) Sv y(-1), which is lower by a factor of 10(2)-10(4) than in the periods of "acute" exposure (1950-1951 and 1957-1958). Natural organisms received very high doses up to 200-800 Gy resulting from radioactive discharges into the Techa River and the radiation accident in 1957. In all cases, including the "acute" exposure followed by the chronic irradiation, the doses to biota were by a factor of 10-10(3) higher than those to humans.

EPIC database on the effects of chronic radiation in fish: Russian/FSU data.

The paper presents the extraction of data from the EPIC database, outlining the effects of chronic radiation exposure in fish. The EPIC database 'Radiation effects on aquatic biota' is compiled as part of the current EC Project EPIC (Environmental Protection from Ionizing Contaminants in the Arctic). The EPIC database is based on information from publications in Russian (Russian/former Soviet Union data). The data are focused on the effects in fish at relatively low doses of chronic radiation exposure. The effects are grouped by three key endpoints: morbidity, reproduction, and mortality/life shortening. A preliminary scale of dose-effects relationships for fish has been constructed.

Comparative analysis of doses to aquatic biota in water bodies impacted by radioactive contamination.

Comparative analysis of doses to the reference species of freshwater biota was performed for the following water bodies in Russia or former USSR: Chernobyl NPPs cooling pond, Lakes Uruskul and Berdenish located in the Eastern Urals Radioactive Trace, Techa River, Yenisei River. It was concluded that the doses to biota were considerably different in the acute and chronic periods of radioactive contamination. The most vulnerable part of all considered aquatic ecosystems was benthic trophic chain. A numerical scale on the "dose rate - effects" relationships for fish was formulated. Threshold dose rates above which radiation effects can be expected in fish were evaluated to be the following: 1 mGy d(-1) for appearance of the first morbidity effects in fish; 5 mGy d(-1) for the first negative effects on reproduction system; 10 mGy d(-1) for the first effects on life shortening of fish. The results of dose assessment to biota were compared with the scale "dose rate - effects" and the literature data on the radiobiological effects observed in the considered water bodies. It was shown that in the most contaminated water bodies the dose rates were high enough to cause the radiobiological effects in fish.

Dynamics of radiation exposure to marine biota in the area of the Fukushima NPP in March-May 2011.

Estimates of radiation dose rates are presented for marine biota in March-May 2011 in the coastal zone near Fukushima NPP, and in the open sea. Calculations of fish contamination were made using two methods: a concentration factor approach, and a dynamic model. For representative marine organisms (fish and molluscs) the radiation dose rates did not exceed the reference level of 10 mGy/day. At a distance 30 km from the NPP, in the open sea the radiation doses for marine biota were much lower than those in the coastal zone near the NPP. Comparative estimates are presented for radiation doses to aquatic organisms in the exclusion zones of the Eastern Urals Radioactive Trail, and the Chernobyl NPP.

Modelling of effects due to chronic exposure of a fish population to ionizing radiation.

A dynamic model was developed for description of radiation effects in an isolated fish population chronically exposed at different dose rates. The induced effects were predicted based on damage created by the radiation, recovery by means of repair mechanisms, and natural growth of the population. Three types of radiation effects (umbrella endpoints) were simulated--decrease of population size, decrease of reproductive capacity, and effects on the morbidity of the population. The influence of ecological interactions on the irradiated fish population was simulated using the combined action of radiation and parasite infestation as an example (ecological interaction "host-parasite"). The model calculations demonstrate that influence of ecological interactions can considerably aggravate the effects of radiation to an exposed population. It was concluded that development of standards for wildlife protection against ionizing radiation requires consideration of possible ecological interactions and to take into account the ecological effects of radiation.

Mathematical simulation of dose-effect relationships for fish eggs exposed chronically to ionizing radiation.

A mathematical model which simulates the observed dose-effect relationships for fish eggs exposed to chronic irradiation is presented. The model assumes that the exposed fish eggs may exist in one of the following states: normally developing, reversibly damaged, and lethally damaged. Reversible damages may be recovered by repairing mechanisms which are spent for the repairing processes. The model was applied to describe the observed differences in effects of chronic exposure for quickly (2 weeks) and slowly (up to 20 weeks) developing fish eggs. Calculations were performed for dose rates of chronic irradiation ranging from 10 to 300 mGy/day. Two types of radiation effects were considered-the effect on eggs survival (percentage of survived eggs at time t), and the depletion of the repairing pool (in percentage of its maximal value). The model predictions have been compared with the experimental data from the EPIC database. This comparison showed that the model adequately describes the radiation effects in fish eggs of different species, within a wide range of chronic radiation exposures.