The need to address radiological impacts from radiocarbon released to the biosphere has been recognized for some time. In 2011, the Swedish Radiation Safety Authority (SSM) commissioned a study to develop a 14C model of the soil-plant-atmosphere system that would provide them with an independently developed assessment capability. This paper summarizes that study, which comprised a review of contemporary models, the development of a new conceptual model, SSPAM14C, and the application of SSPAM14C to a set of experimental data relating to the atmospheric exposure of cabbages.
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.
This paper reviews the application of a freely accessible on-line database of generic Features, Events and Processes (FEPs), designed to support the analysis of geological CO2 storage systems during performance assessments. The Generic CO2 FEP Database was established by Quintessa in 2004 through international collaboration under the auspices of the Weyburn project. Subsequently, development of the database has continued and its use has become widespread, with over 1000 people having registered to access the database. Most commonly, the database has been used as an audit tool to help build confidence that a systems analysis covers all relevant FEPs and to document transparently those FEPs that are not being considered. In other applications the generic FEP database has been screened to identify relevant FEPs that are then used directly to build conceptual models. As a generic resource, the Generic CO2 FEP Database covers the range of FEPs that might be relevant to assessments, from those associated with the storage formation and cap rock to potential impacts on humans and the environment. The range of applications to date demonstrates its use in support of different scales of assessment for different components of the system. Examples include total-systems models, assessments focusing solely on potential loss of containment from the storage formation and natural analogue studies of potential impacts. Over the past five years the use of the Generic CO2 FEP database has helped to build confidence in assessments relating to long-term geological storage. Additionally, the database represents a knowledge base relating to the potential performance and safety of storage systems. The experience gained from application of the database to date helps to inform the way in which it can be applied in future. The database continues to be developed, based on experience gained in its application. Recently references and links have been updated and a targeted review has revised descriptions and FEPs relating to the marine environment. Further targeted reviews and updates are planned. For the database to continue to structure the latest knowledge and understanding relating to geological storage, on-going feedback from its user base is sought.
This paper presents a structured qualitative approach to analysing the varied kinds of information from a CO2 storage site, so as to produce scenarios that are amenable to numerical analysis. The approach is illustrated by application to an industrial scale CCS project at Krechba, In Salah, in Algeria. A structured approach is needed to support assessments of the likely performance of CCS systems over operational, monitoring and longer term time-frames. Very varied information concerning such systems' engineered and environmental components must be obtained and evaluated to attain sufficient confidence that performance will be acceptable. Computer simulations and risk assessment models are needed to help understand the behaviour of CO2 and place plausible bounds on the temporal evolution of all aspects of the system. The outcomes will be uncertain, even if underpinning data sets are of good quality The approach included identification of the important the Features, Events and Processes (FEPs) that together describe the Krechba system and its likely evolution. An 'expected evolution' scenario was then identified by systematically evaluating existing knowledge. Scenarios describing potential situations that could involve alternative evolution mechanisms were also identified; these included consideration of mechanisms that could in principal lead to containment failure. These scenarios need to be analysed to show that they are either unlikely to occur and/or will be of limited impact and so do not represent threats to adequate performance. After audit against Quintessa's freely available generic online CO2 FEP database to ensure and demonstrate comprehensiveness, the site-specific scenarios identified and the associated list of remaining uncertainties, were used to prioritise future (e.g. systems modelling) work. The outcomes of this and other data analysis and modelling programmes will be used to update the FEP and scenario descriptions.
Ontario Power Generation (OPG) is proposing to build a Deep Geologic Respository (DGR) for Low and Intermediate Level Waste (L&ILW) near the existing Western Waste Management Facility at the Bruce site in the Municipality of Kincardine, Ontario. The Nuclear Waste Management Organization (NWMO), on behalf of OPG, is currently preparing an Environmental Impact Statement (EIS) and Preliminary Safety Report (PSR) for the proposed repository. This involves investigation of the site’s geological and surface environmental characteristics, conceptual design of the DGR, and technical studies to demonstrate the operational and long-term safety of the proposed facility. A preliminary postclosure safety assessment (SA) was undertaken in 2008 and 2009. Consistent with the guidelines for the preparation of the EIS for the DGR and the regulatory guide on assessing the long-term safety of radioactive waste management, the SA evaluated the DGR’s performance and its potential impact on human health and the environment through pathway analysis of contaminant releases, contaminant transport, receptor exposure and potential effects. Consideration was given to the expected long-term evolution of the repository and site following closure (the Normal Evolution Scenario) and four disruptive (“what if”) scenarios (Human Intrusion, Severe Shaft Seal Failure, Open Borehole, and Extreme Earthquake), which considered events with uncertain or low probability that could disrupt the repository system. Conceptual and mathematical models were developed and then implemented in a range of software tools including AMBER, to provide estimates of impacts such as dose, FRAC3DVS, for detailed 2D and 3D groundwater flow and transport calculations, and T2GGM, a code that couples the Gas Generation Model (GGM) and TOUGH2 and models the generation of gas in the repository and its subsequent 2D transport through the geosphere. Calculations have been undertaken to assess the impact of radionuclides on human and non-human biota and the impact of non-radioactive species on humans and the environment. The results indicate that the DGR system provides a high level of postclosure safety.
Latera is a volcanic area north of Rome where deep, naturally-produced carbon dioxide migrates to the surface and is released to the atmosphere. The region provides an environment in which many processes relevant to the geological storage of carbon dioxide can be studied. This paper describes system-level modelling studies using Quintessa's QPAC–CO2 code, which includes a novel soil–plant model that represents both fertilization and toxicity effects from elevated carbon dioxide concentrations. Good comparisons have been obtained between model calculations of surface venting patterns, soil gas concentrations and plant responses, giving confidence that the key features of the system are well understood.
Underground CO2 storage facilities are designed to contain CO2 permanently and the expected performance scenario is that no CO2 leakage from the storage complex will occur. However, it is necessary to assess the possible environmental impacts of CO2 leakage in the unlikely event that a storage system evolves differently from the design aim. The RISCS project is being undertaken under the European Union's 7th Framework Programme to research these potential impacts. To help focus RISCS research and aid communication of the results, a set of reference terrestrial and marine European receptor environments and associated impact scenario descriptions were defined systematically by evidence-based expert elicitation.
Recent developments in the modelling of key radionuclides in long-timescale assessments of the safety of geological disposal of spent fuel and other radioactive wastes emphasise the influence of the redox conditions of the soil column. Models with higher spatial resolution than typically employed in standard modelling approaches have been shown to capture important features of experimental observations that are not otherwise manifested. Furthermore, models with monthly, rather than annually, averaged parameters and with dynamic transfers between soil and plant have been shown to lead to key differences compared with standard models employing soil–plant concentration ratios.
Radiocarbon is present in solid radioactive wastes arising from the nuclear power industry, in reactor operating wastes, and in graphite and activated metals that will arise from reactor decommissioning. Its half-life of 5730 yr, among other factors, means that 14 C may be released to the biosphere from radioactive waste repositories. These releases may occur as 14 C-bearing gases, especially methane, or as aqueous species, and enter the biosphere from below via natural processes or via groundwater pumped from wells. Assessment of radiation doses to humans due to such releases must take account of the major role of carbon in biological processes, requiring specific 14 C assessment models to be developed. Therefore, an intercomparison of 5 14 C assessment models was organized by the international collaborative forum, BIOPROTA. The intercomparison identified significantly different results for the activity concentrations in the soil, atmosphere, and plant compartments, based upon the different modeling approaches. The major source of uncertainty was related to the identification of conditions under which mixing occurs and isotopic equilibrium is established. Furthermore, while the assumed release area plays a role in determining the calculated atmospheric 14 C concentrations, the openness of the plant canopy and the wind profile in and above the canopy are the key drivers. The intercomparison has aided understanding of the processes involved and helped to identify areas where further research is required to address some of the uncertainties.
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