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Batch adsorption, batch diffusion, and flow-through column experiments were conducted using groundwater and fractured rock collected in unsaturated zone to increase our understanding of sorption and transport behavior of radionuclides. Increasing Kd values were observed in the sequence 90Sr, 99Tc, and 3H regardless of the geological media tested. For all sorbing radionuclides, Kd values for the fracture-filling/coating material were observed to be higher than those for without fracture-filling/coating material regardless of the groundwater. These higher Kd values are the result of zeolite mineral in filling/coating material of fractured rock. The batch diffusion and flow-through column experiments were also conducted using the same fractured rock sample, and the results of diffusion and column experiments showed similar trend of radionuclide sorption and transport to sorption experiment. In this study, sorption Kd of radionuclide was determined and used to increase our understanding of radionuclide retardation through fracture-filling/coating materials.

In order to assess the potential radiological risk to humans and the environment from a geological repository for radioactive waste, a safety assessment must be performed. This implies that the release and transfer of radionuclides from the repository into the surface environment are calculated and that the effects in the biosphere are evaluated for an assessment period up to one hundred thousand years according to Swedish regulations. This paper discusses the challenges associated with the modelling of surface ecosystems over such long time scales, using the recently completed assessment for the extension of the existing repository for the low- and intermediate-level nuclear waste (called SFR) in Forsmark, Sweden as an applied example.In the assessment, natural variation and uncertainties in climate during the assessment period were captured by using a set of climate cases, primarily reflecting different expectations on the effects of global warming. Development of the landscape at the site, due to post-glacial isostatic rebound, was modelled, and areas where modelling indicated that radionuclides could discharge into the biosphere were identified. Transfers of surface water and groundwater were described with spatially distributed hydrological models. The projected release of radionuclides from the bedrock was then fed into a biosphere radionuclide transport model, simulating the transport and fate of radionuclides within and between ecosystems in the landscape. Annual doses for human inhabitants were calculated by combining activity concentrations in environmental media (soil, water, air and plants) with assumptions on habits and land-use of future human inhabitants. Similarly, dose rates to representative organisms of non-human biota were calculated from activity concentrations in relevant habitats, following the ERICA methodology.In the main scenario, the calculated risk for humans did not exceed the risk criteria or the screening dose rate for non-human biota, indicating that the repository design is sufficient to protect future populations and the environment. Although the combination of radionuclides, land-uses/habitats, type of most exposed population and area of exposure that contribute most to the total dose shifts over time, the total calculated dose shows limited variability. Significant reductions in the dose only occur during submerged periods and under periglacial climate conditions. As several different water and food pathways were equally important for endpoint results, it is concluded that it would be difficult to represent the biosphere with one or a set of simplified models. Instead, we found that it is important to maintain a diversity of food and water pathways, as key pathways for radionuclide accumulation and exposure partly worked in parallel.

Repository of Low and Intermediate Level Solid Radioactive Wastes (ILRW) is accepted to deal with the ILRW produced from fast development of nuclear power industry and other fields. In performance assessment of the potential geologic repository, modeling plays a very important role in addition to experimental inverstigations. In addition to experimental inverstigations modeling has also to be used for these purposes. In the paper, stochastic mathematical theories were used for calculating the radionuclide transport in fractured rock masses at Repository of Low and Intermediate Level Solid Radioactive Wastes. To better understand about the dangers of public for radiation and the nuclides transport in fractured rock masses at low and intermediate level solid radioactive waste disposal, the paper builds three dimensional stochastic seepage grid model to describe radionuclide transport in fractured rock masses based on three-dimensional directional seepage theory that is built into percolation mechanics, sets up a rigorous computer simulating system to calculate the radionuclide transport and safety performance of the Repository of Low and Intermediate Level Solid Radioactive Wastes based on computing technology. Finally, To illustrate the precision and validity of this model that is built into the stochastic mathematical theories or computing technology, the simulation experiments are carried out to understand law of radionuclide transport.

Submitted by Jairo Amaro (jairo.amaro@sibi.ufrj.br) on 2019-07-01T17:40:45Z No. of bitstreams: 1 2012_COTTA_WBV_v3_p1-15-min.pdf: 334299 bytes, checksum: a5e6fc02385b163af492325dfd18eddf (MD5) Made available in DSpace on 2019-07-01T17:40:45Z (GMT). No. of bitstreams: 1 2012_COTTA_WBV_v3_p1-15-min.pdf: 334299 bytes, checksum: a5e6fc02385b163af492325dfd18eddf (MD5) Previous issue date: 2012-08-19 Indisponível. A detailed environmental impact assessment is required in many countries when considering the disposal of wastes containing radioactive materials. In this paper we present hybrid numerical-analytical solutions for the subsurface transport of radioactive contaminant decay chains that may be used for such an assessment. The model involves the advective–dispersive transport of multiple radionuclide species within separate but coupled saturated and unsaturated soil domains. The resulting partial differential equations were solved using the Generalized Integral Transform Technique to yield analytical expressions for the concentration distributions versus distance, and analytical or numerical solutions as a function of time. The potential of the hybrid modeling approach is illustrated by means of an environmental impact assessment of an uranium milling liquid waste pond near Caetité, Brazil. Calculated radionuclide concentration distributions were for this purpose used in subsequent radiation dose calculations. Several scenarios were analyzed, including operational, human intrusion, farmer/road builder, and leaching scenarios. Numerical results show that for all scenarios analyzed the total dose is less than the Brazilian regulatory dose limit of 0.3 mSv/year above natural background for a period of 10,000 years.

The degradation of organic wastes and the corrosion of metallic wastes and steel containers in low and intermediate level radioactive waste (LLW/ILW) repositories are important processes that affect repository geochemistry and the speciation and transport of radionuclides. Gas is generated in association with these degradation processes and this has the potential to overpressure the repository, which can promote transport of groundwater and gas, and consequently radionuclide transport. Microbial activity plays an important role in organic degradation, corrosion and gas generation through the mediation of reduction–oxidation reactions.A large-scale gas generation experiment has been established at the LLW/ILW repository, Olkiluoto, Finland to examine gas generation from LLW in waste drums disposed of in the operational VLJ Repository (VLJ is a Finnish acronym which translates to “reactor operating waste”). The experiment has monitored, for a period of 9 a, the rate and composition of gas generated, and the aqueous geochemistry and microbe populations present at various locations within the experiment. There is considerable heterogeneity within the experiment, such that pH is observed to vary from pH 5.5 to pH 10 between organic-rich waste and water associated with concrete. The heterogeneity results in competing anaerobic processes occurring together in the experiment but within different niches. Microbial activity initially dominant in organic waste has after 7 a reduced the alkalinity of the concrete influenced regions.The experiment has been modelled using a biogeochemical reaction-transport code (GRM) using a blind testing approach. Using independent data, the model was able to reproduce, within a factor of two, the rate of gas production. In addition, the model represented the main anaerobic microbial processes leading to methanogenesis and the observed spatial and temporal variations in aqueous and gaseous species. In order to model the experiment, its heterogeneity was considered such that individual waste containers were represented and assumptions were made concerning transport rates of chemical species. Cellulose waste and H2 produced by corrosion provide microbial substrates for reduction processes and CH4 generation. However, gas generation is a complex interaction of waste degradation processes. Simple repository gas generation models that consider corrosion and cellulose degradation in isolation will tend to overestimate H2 content and gas generation. The GRM model is more realistic and utilises information concerning SO42-, NO3- and other oxidised species present in LLW/ILW to consider the competition between microbial groups for electron donors that reduce the extent of H2 and CH4 generation. Models such as GRM could be applied to other repository systems, such as for high level waste and spent nuclear fuel, to evaluate how H2 gas generation from corrosion and radiolysis may be affected by microbial activity. However, this will require estimation of appropriate microbial kinetic parameters for these more extreme environments.

Geological repository designs employ a multi-barrier approach. The materials, which include wasteforms, backfill and host rock, are typically porous quasi-brittle. Mechanical damage (e.g. nucleation and growth of microcracks) can result in significant changes in permeability. A knowledge of how the permeability is affected is critical to accurate modelling of radionuclide transport. This work proposes a novel 3D lattice-type model for the damage evolution in such materials, referred to as the site-bond model. Its advantages over previous models are that the shape of the lattice cell is physically realistic and that any macroscopic elastic response can be represented, including those of cementitious and geological materials. Damage accumulates as bonds fail upon reaching prescribed failure strengths. These are dictated by a predefined pore size distribution. Concrete is used as a study material. It is demonstrated that the model can predict the macroscopic stress–strain response under unconfined tension and compression with emergent non-linearity due to damage evolution. Ongoing work on the prediction of permeability changes with damage is discussed. This is based on the interaction between the model proposed here and a lattice model of the pore space.

Submitted by Jairo Amaro (jairo.amaro@sibi.ufrj.br) on 2019-07-04T17:16:11Z No. of bitstreams: 1 2010_COTTA_JHM_v174_p648-655-min.pdf: 674029 bytes, checksum: de75090ec8c0b0decf7321cbbf7a33a3 (MD5) Made available in DSpace on 2019-07-04T17:16:11Z (GMT). No. of bitstreams: 1 2010_COTTA_JHM_v174_p648-655-min.pdf: 674029 bytes, checksum: de75090ec8c0b0decf7321cbbf7a33a3 (MD5) Previous issue date: 2009-10-09 Indisponível. This paper investigates the environmental fate of radionuclide decay chains (specially the 238U and 232Th series) being released from a conventional mining installation processing ore containing natural occurring radioactive materials (NORMs). Contaminated waste at the site is being disposed off in an industrial landfill on top of a base of earth material to ensure integrity of the deposit over relatively long geologic times (thousands of years). Brazilian regulations, like those of many other countries, require a performance assessment of the disposal facility using a leaching and off-site transport scenario. We used for this purpose the HYDRUS-1D software package to predict long-term radionuclide transport vertically through both the landfill and the underlying unsaturated zone, and then laterally in groundwater. We assumed that a downgradient well intercepting groundwater was the only source of water for a resident farmer, and that all contaminated water from the well was somehow used in the biosphere. The risk assessment was carried out for both a best-case scenario assuming equilibrium transport in a fine-textured (clay) subsurface, and a worst-case scenario involving preferential flow through a more coarse-textured subsurface. Results show that preferential flow and soil texture both can have a major effect on the results, depending upon the specific radionuclide involved.

This paper presents 3D saturated flow and transport calculations performed using numerical methods developed within the MELODIE software in the framework of assessing the performance of a radioactive waste geological repository. This type of computational modeling is a challenging task due to the presence of strong physical, chemical and hydrogeological heterogeneities, as well as the very different geometrical scales (from dm to km) to be handled simultaneously. First, the software is briefly described, with a particular focus on the specific features that help for dealing with such contrasted system, notably the finite volume finite element and the Schwarz domain decomposition methods. IRSN calculations based on the French "spent fuel/iron canister/clay" concept designed by Andra in the framework of its feasibility study of a deep geological repository are then presented. Through sensitivity analyses based on various evolution scenarios, these calculations aim at assessing the possible influence of design features, waste degradation mechanisms or radionuclide transfer properties of repository components on the behavior of the containment system as a whole. The results of the simulation have shown the transport regime in the drifts depends on the efficiency of the seals. When seals are efficient, velocities are low and transfer regime in the drifts is dominated by diffusion, whereas advection is dominant when seals are poorly sealed. However, whatever the dominant transport regime (diffusion or advection) in the drifts, the activity released out from the canisters was shown to be mainly transferred by diffusion to the host rock exits. This transfer through clayey components (bentonite and host rock) highlights the importance of chemical properties of the three types of long-lived radionuclides considered in the present study. The sensitivity analysis has shown that strongly sorbed and/or weakly soluble radionuclides are less influenced by activity release mechanisms and by seal efficiency. Concerning the strongly sorbed radionuclide 94Nb, the activity is hardly transferred beyond the vicinity of the disposal cells: the sorption properties allow limiting and delaying efficiently radionuclide migration. Concerning the weakly soluble radionuclide 79Se, a part of the activity is instantaneously precipitated in the bentonite surrounding the canisters and the activity release in the system is controlled by the dissolution of this precipitated activity. Such radionuclides are thus hardly concerned with canister dissolution mechanisms. On the contrary, the migration and release at the host rock exits of 129I, which is a non-sorbed and soluble radionuclide, is shown to be influenced by the radionuclide source term, i.e. dissolution mechanism and instantaneous released fraction (IRF). The IRF, which corresponds to 8% of the total activity contained in the canisters impacts significantly the activity released out of the host rock when UO2 matrix dissolution controlled by uranium solubility lasts up to several million years, whereas its contribution is far less marked when matrix dissolves rapidly by alpha-radiolysis. © 2008 Elsevier Ltd. All rights reserved.

The Dutch national research programme into the feasibility of retrievable storage of radioactive waste (CORA Programme Phase I; CORA: Comité Opslag Radioactief Afval = Committee on Radioactive Waste Disposal) examined the suitability of Tertiary clay deposts for such storage. Long-term isolation - up to 1 million years - of high-level radioactive waste under varying conditions is essential. A key concern is the hydro-mechanical response of the clay deposits in which radioactive waste might possibly be stored, in particular during glacial climate conditions as has happened repeatedly in the Netherlands during the Pleistocene. To evaluate this possibility hydro-mechanical computer simulations and mechanical laboratory experiments have been performed to analyse the effects of glacial loading by a thousand-metri-thick ice sheet on the permeability characteristics, fluid flow rates and the associated migration of radio-nuclides both within and out of Tertiary clays. Glacial loading causes the expulsion of pore water from deeply buried clay deposits into adjoining aquifers. The rates and duration of the consolidation-driven outflow of water from the clay deposit, are very sensitive to the permeability of the clay and the dynamics of the advancing ice sheet. The maximum outflow rate of pore water is 1 mm per year. This rate is approximately three times faster than the flow rate of water in clay prior to ice loading. These preliminary simulation studies also indicate that cyclic loading can result in more rapid migration of radio-nuclides in clays. In clay deposits that are covered by a thick ice sheet, the contribution of dispersed transport relative to the total transport by diffusion amounts to 14%, assuming that there is no absorption of radio-nuclides by the clays and a longitudinal dispersivity of 50 m.