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.

An international model validation exercise on radionuclide transfer and doses to freshwater biota.

Under the International Atomic Energy Agency (IAEA)'s EMRAS (Environmental Modelling for Radiation Safety) programme, activity concentrations of (60)Co, (90)Sr, (137)Cs and (3)H in Perch Lake at Atomic Energy of Canada Limited's Chalk River Laboratories site were predicted, in freshwater primary producers, invertebrates, fishes, herpetofauna and mammals using eleven modelling approaches. Comparison of predicted radionuclide concentrations in the different species types with measured values highlighted a number of areas where additional work and understanding is required to improve the predictions of radionuclide transfer. For some species, the differences could be explained by ecological factors such as trophic level or the influence of stable analogues. Model predictions were relatively poor for mammalian species and herpetofauna compared with measured values, partly due to a lack of relevant data. In addition, concentration ratios are sometimes under-predicted when derived from experiments performed under controlled laboratory conditions representative of conditions in other water bodies.

Model testing of radioactive contamination by 90Sr, 137Cs and 239,240Pu of water and bottom sediments in the Techa River (Southern Urals, Russia).

This paper presents results of testing models for the radioactive contamination of river water and bottom sediments by (90)Sr, (137)Cs and (239,240)Pu. The scenario for the model testing was based on data from the Techa River (Southern Urals, Russia), which was contaminated as a result of discharges of liquid radioactive waste into the river. The endpoints of the scenario were model predictions of the activity concentrations of (90)Sr, (137)Cs and (239,240)Pu in water and bottom sediments along the Techa River in 1996. Calculations for the Techa scenario were performed by six participant teams from France (model CASTEAUR), Italy (model MARTE), Russia (models TRANSFER-2, CASSANDRA, GIDRO-W) and Ukraine (model RIVTOX), all using different models. As a whole, the radionuclide predictions for (90)Sr in water for all considered models, (137)Cs for MARTE and TRANSFER-2, and (239,240)Pu for TRANSFER-2 and CASSANDRA can be considered sufficiently reliable, whereas the prediction for sediments should be considered cautiously. At the same time the CASTEAUR and RIVTOX models estimate the activity concentrations of (137)Cs and (239,240)Pu in water more reliably than in bottom sediments. The models MARTE ((239,240)Pu) and CASSANDRA ((137)Cs) evaluated the activity concentrations of radionuclides in sediments with about the same agreement with observations as for water. For (90)Sr and (137)Cs the agreement between empirical data and model predictions was good, but not for all the observations of (239,240)Pu in the river water-bottom sediment system. The modelling of (239,240)Pu distribution proved difficult because, in contrast to (137)Cs and (90)Sr, most of models have not been previously tested or validated for plutonium.

Non-parametric estimation of thresholds for radiation effects in vertebrate species under chronic low-LET exposures.

Databases on effects of chronic low-LET radiation exposure were analyzed by non-parametric statistical methods, to estimate the threshold dose rates above which radiation effects can be expected in vertebrate organisms. Data were grouped under three umbrella endpoints: effects on morbidity, reproduction, and life shortening. The data sets were compiled on a simple 'yes' or 'no' basis. Each data set included dose rates at which effects were reported without further details about the size or peculiarity of the effects. In total, the data sets include 84 values for endpoint "morbidity", 77 values for reproduction, and 41 values for life shortening. The dose rates in each set were ranked from low to higher values. The threshold TDR5 for radiation effects of a given umbrella type was estimated as a dose rate below which only a small percentage (5%) of data reported statistically significant radiation effects. The statistical treatment of the data sets was performed using non-parametric order statistics, and the bootstrap method. The resulting thresholds estimated by the order statistics are for morbidity effects 8.1 x 10(-4) Gy day(-1) (2.0 x 10(-4)-1.0 x 10(-3)), reproduction effects 6.0 x 10(-4) Gy day(-1) (4.0 x 10(-4)-1.5 x 10(-3)), and life shortening 3.0 x 10(-3) Gy day(-1) (1.0 x 10(-3)-6.0 x 10(-3)), respectively. The bootstrap method gave slightly lower values: 2.1 x 10(-4) Gy day(-1) (1.4 x 10(-4)-3.2 x 10(-4)) (morbidity), 4.1 x 10(-4) Gy day(-1) (3.0 x 10(-4)-5.7 x 10(-4)) (reproduction), and 1.1 x 10(-3) Gy day(-1) (7.9 x 10(-4)-1.3 x 10(-3)) (life shortening), respectively. The generic threshold dose rate (based on all umbrella types of effects) was estimated at 1.0 x 10(-3) Gy day(-1).

[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.

[Modelling of fish contamination with 90Sr in relation to the calcium concentrations in water].

A set of data on the equilibrium concentration factors of 90Sr in fish in relation to the Ca concentrations in water was collected and analyzed. An empiric relationship was obtained on the basis of statistical processing of this dataset: CF(90Sr, Bq/kg w.w.) = 3940(1770-6110)/[Ca, mg/L]water. The range of observed data on the concentration factors of 90Sr in fish is from 20 to 4000 L/kg (about 200 times). The statistical relationship obtained in the paper allows one to reduce this uncertainty by more than 50 times. The formula adequately describes equilibrium concentration factors of 90Sr both in freshwater and in marine fish. A dynamics model approach is described for the cases of accidental contamination of water bodies, when the equilibrium approaches are not appropriate.

90Sr in fish: a review of data and possible model approach.

Data on the relationship between the equilibrium concentration factor of (90)Sr in fish and concentration of Ca in water have been collected and analyzed. The dataset contains 115 values of the concentration factors at different environmental concentrations of Ca. The data included in the dataset are based primarily on publications in Russian and relate to the whole-body concentration factors. An empirical formula was derived to estimate the equilibrium concentration factor of (90)Sr in fish as a inverse relationship of Ca: CF((90)Sr, Bq/kg w.w.)=3940/[Ca, mg/l](water). The derived relationship adequately accounts for the equilibrium concentration factor of (90)Sr in freshwater and marine fish species. A dynamic model was proposed to calculate the (90)Sr bioaccumulation of fish for non-equilibrium situations, for instance, immediately after accidental contamination of water bodies. The model was applied to reconstruct the dynamics of (90)Sr in fish from highly contaminated lake in the Southern Ural (Lake Uruskul). A comparison with data of observations showed that the model is well suited for estimating the dynamic characteristics of (90)Sr bioaccumulation by fish.

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.

[Model for calculation of fish contamination by 137Cs and its application for Lake Kozhanovskoe (Bryansk Region)].

A dynamic model was developed for evaluation of the fish contamination by 137Cs. The dynamics of the accumulation of the radionuclide and the elimination is calculated taking into account the ecological and physiological characteristics of the fish, such as growth and metabolic rates, its position in the food chains of the aquatic ecosystem. The biological elimination of 137Cs from the freshwater fish is proportional to the metabolic rate of the fish. The dimensionless coefficient of the proportionality is equal to epsilonA = 0.3 +/- 0.1. The model was applied for the reconstruction of the long-term dynamics of 137Cs in the food chain "goldfish-pike" from Lake Kozhanovskoe (Bryansk Region, Russia). In 1990-1999 the specific activity of 137Cs in goldfish (generation of 1990) varied in range about 5-20 kBq/kg, whereas the specific activity of 137Cs in pike (generation of 1989) varied in range about 20-40 kBq/kg. The comparison with the data of the observations confirmed that the presented method allows to evaluate adequately the levels and the dynamics of 137Cs accumulation by the freshwater fish.

Evaluation of the biological transfer of 32P, 137Cs and 65Zn by fish in the Yenisei River.

The biological transfer of three radionuclides ((32)P, (137)Cs, (65)Zn) by fish in the Yenisei River (Central Siberia, Russia) was evaluated using a radioecological model. The modelling is based on the general ECOMOD methodology, where radionuclide behavior in an aquatic organism is linked with the processes of growth and metabolism, also with the concentrations of stable analogous elements in the organism, its food and the environment. The model was applied to explain the peculiarities of (32)P, (137)Cs and (65)Zn accumulation in different ecological groups of fish, including non-migratory and migratory fish, non-predatory and predatory fish species. The highest activity concentrations in non-migratory fish from the Yenisei River were found for (32)P. The accumulation of (32)P by fish was shown to depend on the fish size (age, weight); however, it did not depend on the trophic status of fish. The modelling approach was developed to evaluate the biological transfer of radionuclides by the migratory fish, which spend the most part of life in the Yenisei delta, inlet or bay, and go upstream the Yenisei River for spawning. The results of the ECOMOD model calculations are in good agreement with available measurement data.

Model reconstruction of 90Sr concentrations in fish from 16 Ural lakes contaminated by the Kyshtym accident of 1957.

The paper outlines the results of the ECOMOD model application to simulate 90Sr dynamics in fish dwelling in Southern Ural lakes and contaminated as a result of the Kyshtym accident. The detailed reconstruction of 90Sr behavior in fish for the 40-year period (1957-1997) was made for the most contaminated lakes (Uruskul and Berdenish). The adapted ECOMOD model was applied for the other 14 Ural lakes to explain the peculiarities in 90Sr accumulation by fish in lakes with different concentrations of Ca2+ ions in lake water.

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.

[Modeling of different types of size-effect in accumulation of 137Cs in fish from the cooling pond of the Chernobyl Power Plant]. / Modelirovanie razlichnykh tipov razmernogo éffekta v nakoplenii 137Cs ryboi vodoema-okhladitelia Chernobyl'skoi AES.

All known types of size-effect in accumulation of radiocesium in fish are described on the basis of the dynamic model of distribution and migration of the radionuclide in aquatic ecosystem. The time-dependent activity of 137Cs in different age classes of carp and pike-perch from the Chernobyl Nuclear Power Plant Cooling Pond is calculated. It is shown that during the first year after the accident the activity of 137Cs in the young generations of fish was higher (negative size-effect) owing to higher feeding rate of young fish. During the following years the increase of activity in the older fish generations takes place (positive size-effect). It is shown that size-dependence is more clear for the predatory species.

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.