Modelling of long-term behaviour of caesium and strontium radionuclides in the Arctic environment and human exposure

Caesium-137 in mountain flora with emphasis on reindeer's diet - Spatial and temporal trends

The present study summarizes three decades of studies on ¹³⁷Cs transfer to various species of lichens, graminoids, herbs and woody plants across a ~3000 km² area used as mountain pasture for reindeer and other ruminants. The investigation comprised of field studies covering the period 2011-2016, and a compilation of studies and data for the preceding period (1986-2010). Altogether, more than 700 individual vegetation samples were considered. For lichens, relatively fast decrease in contamination levels was observed during the first decade after the Chernobyl fallout (ecological half-time of about 3 years). For later years there seems to be a continuous re-contamination which results in a "steady state" where time-trends are mainly governed by physical decay of ¹³⁷Cs. For green plants, decline in transfer factors (TF) (i.e. the ratio between activity concentration in vegetation and activity density in soil) during the period 1986-2012 was not as pronounced as for lichens: Some species showed significant decrease with time, while others did not. 25-30 years after the Chernobyl accident, ¹³⁷Cs levels in lichens and green plants were significantly dependent on the levels in soil (R² between 0.53 and 0.57), but there were also some significant differences in transfer between sampling sites. Moreover, marked variability in TFs was found between different plant species growing at the same site, whereas such differences were not found for reindeer lichens.

Impacts on the terrestrial environment in case of a hypothetical accident involving the recovery of the dumped Russian submarine K-27

Objects containing radioactivity have been routinely dumped in Arctic waters near NW Russia up until the 1990s. One of the most radioactive objects in this region, the nuclear submarine K-27, was dumped in Stepogovo Fjord and contained spent nuclear fuel (SNF). Although the two K-27 submarine reactors were mothballed before dumping, concerns about the potential long term risks of contamination remain and plans to retrieve and decommission K-27 exist. In this article, human dose and environmental impact aseessments are presented for two possible future scenarios involving: (1) an ingress of water into a reactor in situ leading to a spontaneous chain reaction (SCR) and (2) an on-board fire when SNF is being removed at the mainland decommissiong site at Gremhika Bay on the Kola Peninsula. Assessments have been completed using conservative assumptions, focusing on possible effects to Norwegian territory. Atmospheric transport and deposition of radioactivity was modelled near field and regionally, using appropriate models, whilst human doses and environmental exposures were modelled using a standard IAEA approach and the ERICA tool, respectively. Results indicate that large areas of Norwegian territory could be affected by fallout from the Gremhika scenario, especially in the north, though at levels two orders of magnitude lower than those observed after the Chernobyl accident. Potential doses, primarily due to ground shine, to a critical group of personnel on-site at Stepogovo resulting from a SCR could require preventative measures based on ICRP recommendations (20-100 mSv). Doses to non-human biota in Norway for the Gremhika scenario would be negligible, typical of background dose rates for terrestrial organisms.

Global searches for microalgae and aquatic plants that can eliminate radioactive cesium, iodine and strontium from the radio-polluted aquatic environment: a bioremediation strategy

The Fukushima 1 Nuclear Power Plant accident in March 2011 released an enormously high level of radionuclides into the environment, a total estimation of 6.3 × 10¹⁷ Bq represented by mainly radioactive Cs, Sr, and I. Because these radionuclides are biophilic, an urgent risk has arisen due to biological intake and subsequent food web contamination in the ecosystem. Thus, urgent elimination of radionuclides from the environment is necessary to prevent substantial radiopollution of organisms. In this study, we selected microalgae and aquatic plants that can efficiently eliminate these radionuclides from the environment. The ability of aquatic plants and algae was assessed by determining the elimination rate of radioactive Cs, Sr and I from culture medium and the accumulation capacity of radionuclides into single cells or whole bodies. Among 188 strains examined from microalgae, aquatic plants and unidentified algal species, we identified six, three and eight strains that can accumulate high levels of radioactive Cs, Sr and I from the medium, respectively. Notably, a novel eustigmatophycean unicellular algal strain, nak 9, showed the highest ability to eliminate radioactive Cs from the medium by cellular accumulation. Our results provide an important strategy for decreasing radiopollution in Fukushima area.

Concentrations of (137)Cs in summer pasture plants that reindeer feed on in the reindeer management area of Finland

Samples of summer pasture plants that reindeer feed on were collected in order to study (137)Cs concentrations in different plant species and in species nested in certain site types, and to study the regional distribution of (137)Cs in the Finnish reindeer management area. Plant species were categorized by the site types of mineral soil forest (xeric heath forest and mesic heath forest) and peatland. A third category called 'other plant species' included plants with various site types, poorly determined species and species with poor statistics. The (137)Cs concentrations in different site types differed significantly. The mean (137)Cs concentrations of the whole reindeer management area in the xeric heath forest plant species was 44 ± 27 Bq/kg dw, in the mesic heath forest plant species 75 ± 59 Bq/kg dw and in the peatland plant species 219 ± 150 Bq/kg dw. The peatland species uptake (137)Cs more efficiently than plant species of mineral soil forests. A particularly efficient collector of (137)Cs was Trichophorum sp. It is suggested that Trichophorum sp. could be used as an indicator species for reindeer summer fodder plants. The highest concentrations of (137)Cs were found in Southern Lapland and the lowest in Northern Lapland. Today, the concentrations of (137)Cs in summer pasture plants that reindeer feed on in Finland are at such a level that there is no need to avoid any plant species. In the case of future nuclear fallout, reindeer grazing in peatlands would increase concentrations of (137)Cs in reindeer meat.

Transfer parameter values in temperate forest ecosystems: a review

Compared to agricultural lands, forests are complex ecosystems as they can involve diverse plant species associations, several vegetative strata (overstorey, shrubs, herbaceous and other annual plant layer) and multi-layered soil profiles (forest floor, hemi-organic and mineral layers). A high degree of variability is thus generally observed in radionuclide transfers and redistribution patterns in contaminated forests. In the long term, the soil compartment represents the major reservoir of radionuclides which can give rise to long-term plant and hence food contamination. For practical reasons, the contamination of various specific forest products has commonly been quantified using the aggregated transfer factor (T(ag) in m(2)kg(-1)) which integrates various environmental parameters including soil and plant type, root distribution as well as nature and vertical distribution of the deposits. Long lasting availability of some radionuclides was shown to be the source of much higher transfer in forest ecosystems than in agricultural lands. This study aimed at reviewing the most relevant quantitative information on radionuclide transfers to forest biota including trees, understorey vegetation, mushrooms, berries and game animals. For both radiocaesium and radiostrontium in trees, the order of magnitude of mean T(ag) values was 10(-3)m(2)kg(-1) (dry weight). Tree foliage was usually 2-12 times more contaminated than trunk wood. Maximum contamination of tree components with radiocaesium was associated with (semi-)hydromorphic areas with thick humus layers. The transfer of radionuclides to mushrooms and berries is high, in comparison with foodstuffs grown in agricultural systems. Concerning caesium uptake by mushrooms, the transfer is characterized by a very large variability of T(ag), from 10(-3) to 10(1)m(2)kg(-1) (dry weight). For berries, typical values are around 0.01-0.1 m(2)kg(-1) (dry weight). Transfer of radioactive caesium to game animals and reindeer and the rate of activity reduction, quantified as an ecological half-life, reflect the soil and pasture conditions at individual locations. Forests in temperate and boreal regions differ with respect to soil type and vegetation, and a faster decline of muscle activity concentrations in deer occurs in the temperate zone. However, in wild boar the caesium activity concentration shows no decline because of its special feeding habits. In the late phase, i.e. at least a few months since the external radionuclide contamination on feed plants has been removed, a T(ag) value of 0.01 m(2)kg(-1) (fresh weight) is common for (137)Cs in the muscles of adult moose and terrestrial birds living in boreal forests, and 0.03 m(2)kg(-1) (fresh weight) for arctic hare. Radiocaesium concentrations in reindeer muscle in winter may exceed the summer content by a factor of more than two, the mean T(ag) values for winter ranging from 0.02 to 0.8 m(2)kg(-1) (fresh weight), and in summer from 0.04 to 0.4m(2)kg(-1). The highest values are found in the year of initial contamination, followed by a gradual reduction. In waterfowl a relatively fast decline in uptake of (137)Cs has been found, with T(ag) values changing from 0.01 to 0.002 m(2)kg(-1) (fresh weight) in the three years after the contaminating event, the rate being determined by the dynamics of (137)Cs in aquatic ecosystems.

Modelling radiocaesium transfer and long-term changes in reindeer

A dynamic model on (137)Cs in reindeer is presented, taking into consideration short- and long-term mechanisms, including the effect of ground deposition, transfer to vegetation, reindeer diet, feed intake, absorption and depletion of radiocaesium in the reindeer body. The model was optimised to fit measured activity concentrations in Swedish reindeer after the Chernobyl fallout. For comparison, regression analyses were made and aggregated transfer factors and effective ecological half-lives were estimated. The fit of the simulated model to observed activity concentrations was slightly better than the fit obtained by linear regressions. Improved knowledge about radiocaesium in vegetation would make the model more accurate for predictive purposes. Presently, the use of Tag and T(eff) is probably better for predictions, provided that their temporal and geographical limitations are taken into consideration. The dynamic model describes mechanisms better and may explain how changes in the system influence on activity concentrations of radiocaesium in the animal.

137Cs and 90Sr in live and dead reindeer lichens (genera Cladonia) from the "Kraton-3" underground nuclear explosion site

The contents of 137Cs and 90Sr have been determined in 29 samples of live and dead reindeer lichens (genera Cladonia) collected at the "Kraton-3" underground nuclear explosion site (65.9 degrees N 112.3 degrees E, event year--1978) in Yakutia, Russia in 2002. The area contamination was within the range of 0.36-700 and 0.13-770 kBq m(-2) for 137Cs and 90Sr, respectively. The dead organisms were on average much more contaminated than the live ones. Vertical fractionation of the live lichen carpet demonstrated maximal activity concentrations of both radionuclides in the lower older section of the plants, while for the dead lichens the maximal activity concentrations of 137Cs were detected in the upper part. The vertical distribution of 90Sr was more or less homogeneous in the cushions of dead lichens. Elevated levels of 137Cs and 90Sr activity concentrations were also detected in the re-establishing young lichens growing over the residua of some dead lichens.