Mike Thorne

A review of the behaviour of U-238 series radionuclides in soils and plants.

The U-238 series of radionuclides is of relevance in a variety of environmental contexts ranging from the remediation of former uranium mining and milling facilities to the deep geological disposal of solid radioactive wastes. Herein, we review what is known concerning the behaviour of radionuclides from the U-238 decay chain in soils and plants. This review is intended to provide a single comprehensive source of information to anyone involved in undertaking environmental impact assessment studies relating to this decay chain. Conclusions are drawn relating to values and ranges of distribution coefficients appropriate to uranium, thorium, radium, lead and polonium in different soil types and under various environmental conditions. Similarly, conclusions are drawn relating to plant:soil concentration ratios for these elements for different plant and soil types, and consideration is given to the distribution of these elements within plants following both root uptake and foliar application.

Regulating exposure of the lens of the eye to ionising radiations

The International Commission on Radiological Protection (ICRP) has reviewed recent epidemiological evidence suggesting that, for the lens of the eye, the threshold in absorbed dose for the induction of deleterious health effects is about 0.5 Gy. On this basis, the Commission recommends that for occupational exposure in planned exposure situations, the equivalent dose limit for the lens of the eye should be 20 mSv in a year, averaged over defined periods of 5 yr, with exposure not exceeding 50 mSv in any single year. This paper summarises the data that have been taken into account by the ICRP and critically examines whether the proposed downward revision of the dose limit is justified. Overall, it is concluded that the accumulating radiobiological and epidemiological evidence makes it more appropriate to treat cataract induction as a stochastic rather than a deterministic effect. Within this framework, it is illogical to have the same dose limit for the lens of the eye as for the whole body irradiated uniformly. This could be addressed either by removing the special dose limit for the lens of the eye, assigning it an appropriate tissue weighting factor and including it in the computation of the effective dose, or through a composite approach involving the use of a tissue weighting factor for effective dose computations together with a special limit on the equivalent dose to the lens of the eye to ensure that no individual was subject to an unacceptably high risk of induction of clinically significant cataracts.

An impulse response function for the "long tail" of excess atmospheric CO2 in an Earth system model

Author(s): Lord, NS; Ridgwell, A; Thorne, MC; Lunt, DJ | Abstract: ©2015. The Authors. The ultimate fate of (fossil fuel) CO2 emitted to the atmosphere is governed by a range of sedimentological and geological processes operating on timescales of up to the ca. hundred thousand year response of the silicate weathering feedback. However, how the various geological CO2 sinks might saturate and feedbacks weaken in response to increasing total emissions is poorly known. Here we explore the relative importance and timescales of these processes using a 3-D ocean-based Earth system model. We first generate an ensemble of 1 Myr duration CO2 decay curves spanning cumulative emissions of up to 20,000 Pg C. To aid characterization and understanding of the model response to increasing emission size, we then generate an impulse response function description for the long-term fate of CO2 in the model. In terms of the process of carbonate weathering and burial, our analysis is consistent with a progressively increasing fraction of total emissions that are removed from the atmosphere as emissions increase, due to the ocean carbon sink becoming saturated, together with a lengthening of the timescale of removal from the atmosphere. However, we find that in our model the ultimate CO2 sink - silicate weathering feedback - is approximately invariant with respect to cumulative emissions, both in terms of its importance (it removes the remaining excess ~7% of total emissions from the atmosphere) and timescale (~270 kyr). Because a simple pulse-response description leads to initially large predictive errors for a realistic time-varying carbon release, we also develop a convolution-based description of atmospheric CO2 decay which can be used as a simple and efficient means of making long-term carbon cycle perturbation projections. Key Points An ensemble of CO2 pulse emissions are modeled using an Earth system model Our impulse response function projects the atmospheric lifetime of emitted CO2 We characterize how the marine CO2 sinks tend to saturate at very high emissions.

A mathematical model for the behaviour of Se-79 in soils and plants that takes account of seasonal variations in soil hydrology

Se-79 is a long-lived radionuclide of potential radiological significance in relation to the deep geological disposal of solid radioactive wastes. In the context of release to the terrestrial environment, its main radiological impact is delivered through food chain pathways. Therefore, its accumulation in soils and uptake by plants is an important consideration in post-closure safety assessment studies. However, representation of its behaviour in the soil-plant system requires consideration of the multiple valence states that it can exhibit under different redox conditions and its susceptibility to volatilisation. A simple model is described that includes seasonal variations in soil hydrology and their effects on the mobility and root uptake of Se-79. Illustrative calculations are undertaken with the model, to demonstrate its capabilities for interpreting experimental data on the behaviour of Se-79 in soils and plants, and for making projections on the long-term behaviour of Se-79 transported to soils.

Generally applicable limits on intakes of uranium based on its chemical toxicity and the radiological significance of intakes at those limits.

Uranium is chemically toxic and radioactive, and both considerations have to be taken into account when limiting intakes of the element, in the context of both occupational and public exposures. Herein, the most recent information available on the chemical toxicity and biokinetics of uranium is used to propose new standards for limiting intakes of the element. The approach adopted allows coherent standards to be set for ingestion and inhalation of different chemical forms of the element by various age groups. It also allows coherent standards to be set for occupational and public exposures (including exposures of different age groups) and for various exposure regimes (including short-term and chronic exposures). The proposed standards are more restrictive than those used previously, but are less restrictive than the Minimal Risk Levels proposed recently by the US Agency for Toxic Substances and Disease Registry. Having developed a set of proposed limits based solely on chemical toxicity considerations, the radiological implications of exposure at those proposed limits are investigated for natural, depleted and enriched uranium.

Biosphere studies supporting the disposal system safety case in the UK

Abstract Higher activity radioactive wastes remain hazardous for extremely long timescales, of up to hundreds of thousands of years. Disposing of such wastes deep underground presents the internationally accepted best solution for isolating them from the surface environment on associated timescales. Geological disposal programmes need to assess potential releases from such facilities on long timescales to inform siting and design decisions and to help build confidence that they will provide an adequate degree of safety. Assessments of geological disposal include consideration of the wastes, the engineered facility, the host geology and the surface and near-surface environment including the biosphere. This paper presents an overview of recent post-closure biosphere assessment studies undertaken in support of the Nuclear Decommissioning Authority Radioactive Waste Management Directorate disposal system safety case for geological disposal of the United Kingdom’s higher activity radioactive wastes. Recent biosphere studies have included: (1) ensuring that the United Kingdom’s approach to consideration of the biosphere in safety case studies continues to be fit for purpose, irrespective of which site or sites are considered in the United Kingdom’s geological disposal programme; (2) updating projections of global climate and sea level, together with consideration of the potential importance of transitions between climate states; (3) considering geosphere-biosphere interface issues and their representation, including redox modelling and catchment-scale hydrological modelling; and (4) identifying key radionuclides and developing a series of reports describing their behaviour in the biosphere together with an evaluation of associated implications for post-closure assessment calculations.

Is Yucca Mountain a long-term solution for disposing of US spent nuclear fuel and high-level radioactive waste?

On 26 January 2012, the Blue Ribbon Commission on Americas Nuclear Future released a report addressing, amongst other matters, options for the managing and disposal of high-level waste and spent fuel. The Blue Ribbon Commission was not chartered as a siting commission. Accordingly, it did not evaluate Yucca Mountain or any other location as a potential site for the storage or disposal of spent nuclear fuel and high-level waste. Nevertheless, if the Commissions recommendations are followed, it is clear that any future proposals to develop a repository at Yucca Mountain would require an extended period of consultation with local communities, tribes and the State of Nevada. Furthermore, there would be a need to develop generally applicable regulations for disposal of spent fuel and high-level radioactive waste, so that the Yucca Mountain site could be properly compared with alternative sites that would be expected to be identified in the initial phase of the site-selection process. Based on what is now known of the conditions existing at Yucca Mountain and the large number of safety, environmental and legal issues that have been raised in relation to the DOE Licence Application, it is suggested that it would be imprudent to include Yucca Mountain in a list of candidate sites for future evaluation in a consent-based process for site selection. Even if there were a desire at the local, tribal and state levels to act as hosts for such a repository, there would be enormous difficulties in attempting to develop an adequate post-closure safety case for such a facility, and in showing why this unsaturated environment should be preferred over other geological contexts that exist in the USA and that are more akin to those being studied and developed in other countries.

Climate change and landscape development in post-closure safety assessment of solid radioactive waste disposal: Results of an initiative of the IAEA

The International Atomic Energy Agency has coordinated an international project addressing climate change and landscape development in post-closure safety assessments of solid radioactive waste disposal. The work has been supported by results of parallel on-going research that has been published in a variety of reports and peer reviewed journal articles. The project is due to be described in detail in a forthcoming IAEA report. Noting the multi-disciplinary nature of post-closure safety assessments, here, an overview of the work is given to provide researchers in the broader fields of radioecology and radiological safety assessment with a review of the work that has been undertaken. It is hoped that such dissemination will support and promote integrated understanding and coherent treatment of climate change and landscape development within an overall assessment process. The key activities undertaken in the project were: identification of the key processes that drive environmental change (mainly those associated with climate and climate change), and description of how a relevant future may develop on a global scale; development of a methodology for characterising environmental change that is valid on a global scale, showing how modelled global changes in climate can be downscaled to provide information that may be needed for characterising environmental change in site-specific assessments, and illustrating different aspects of the methodology in a number of case studies that show the evolution of site characteristics and the implications for the dose assessment models. Overall, the study has shown that quantitative climate and landscape modelling has now developed to the stage that it can be used to define an envelope of climate and landscape change scenarios at specific sites and under specific greenhouse-gas emissions assumptions that is suitable for use in quantitative post-closure performance assessments. These scenarios are not predictions of the future, but are projections based on a well-established understanding of the important processes involved and their impacts on different types of landscape. Such projections support the understanding of, and selection of, plausible ranges of scenarios for use in post-closure safety assessments.

An investigation into the upward transport of uranium-series radionuclides in soils and uptake by plants.

The upward migration of radionuclides in the (238)U decay series in soils and their uptake by plants is of interest in various contexts, including the geological disposal of radioactive waste and the remediation of former sites of uranium mining and milling. In order to investigate the likely patterns of behaviour of (238)U-series radionuclides being transported upward through the soil column, a detailed soil-plant model originally developed for studying the behaviour of (79)Se in soil-plant systems has been adapted to make it applicable to the (238)U series. By undertaking a reference case simulation and a series of sensitivity studies, it has been found that a wide variety of behaviour can be exhibited by radionuclides in the (238)U decay chain in soils, even when the source term is limited to being a constant flux of either (238)U or (226)Ra. Hydrological conditions are a primary factor, both in respect of the overall advective flow deeper in the soil, which controls the rate of upward migration, and in the influence of seasonally changing flow directions closer to the soil surface, which can result in the accumulation of radionuclides at specific depths irrespective of changes in sorption between the oxic and anoxic regions of the soil. However, such changes in sorption can also be significant in controlling the degree of accumulation that occurs. This importance of seasonally varying factors in controlling radionuclide transport in soils even in very long-term simulations is a strong argument against the use of annually averaged parameters in long-term assessment models. With a water table that was simulated to fluctuate seasonally from a substantial depth in soil to the surface soil layer, the timing of such variations in relation to the period of plant growth was found to have a major impact on the degree of uptake of radionuclides by plant roots. In long-term safety assessment studies it has sometimes been the practice to model the transport of (226)Ra in soil, but to assume that both (210)Pb and (210)Po can be treated as being present in secular equilibrium with the (226)Ra. This simplification is not always appropriate. Where geochemical conditions are such that the (226)Ra migrates upward in the soil column faster than (210)Pb and (210)Po, disequilibrium is not a significant issue, as the (226)Ra supports (210)Pb and (210)Po at concentrations somewhat below those estimated on the basis of assumed secular equilibrium. However, for low, but realistic, values of the distribution coefficients for (210)Pb and (210)Po and high, but realistic, distribution coefficients for (226)Ra, the (210)Pb and (210)Po can reach the surface soil in high concentrations that are not locally supported by (226)Ra. This means that models based on the assumption of secular equilibrium should not be employed without a careful consideration of the hydrological and hydrochemical situation of interest.