| 1. |
- Cera, E., et al.
(författare)
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Modelling experimental results on radiolytic processes at the spent fuel water interface. II. Radionuclides release
- 2006
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Ingår i: Mater Res Soc Symp Proc. - : Springer Science and Business Media LLC. - 1558998896 - 9781558998896 ; , s. 537-546
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Konferensbidrag (refereegranskat)abstract
- Experimental and modelling efforts in the last decade in the frame of nuclear waste management field have been focused on studying the role of the UO2 surfaces in poising the redox state of solid/water systems as well as the radionuclides release behaviour. For this purpose, an experimental programme was developed consisting on dissolution experiments with PWR spent fuel fragments in an anoxic environment and by using different solution compositions. Some of the collected data has been previously published [1], specifically those data concerning radiolysis products and dissolution of the matrix. The results and the modelling tasks indicated an overall balance of the generated radiolytic species and that uranium dissolution was controlled by the oxidation of the spent fuel matrix in 10mM bicarbonate solutions while in the tests carried out at lower or without carbonate concentrations uranium in the aqueous phase was governed by the precipitation of schoepite. This paper is the continuation of a series accounting for the data and modelling work related to investigating the release behaviour of minor radionuclides from the spent fuel. Uranium concentrations as a function of time showed an initial increase until reaching a steady stale, indicating a matrix dissolution control. The same behaviour is observed for neptunium, caesium, strontium, technetium and molybdenum indicating a congruent release of these elements with the major component of the fuel matrix. On the other hand, no clear tendency is observed for plutonium data where additional solubility limiting mechanisms may apply. Kinetic modelling of the trace elements: caesium, strontium, technetium and molybdenum is based on the congruent release of these elements with the major component of the fuel matrix. Rate constants have been determined. Kinetic modelling of neptunium data took also into account the. subsequent precipitation as Np(IV) hydroxide. Finally, measured Pu concentrations may be explained by the precipitation of Pu(IV) and/or Pu(III) solid phases.
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| 2. |
- Nilsson, K. -F, et al.
(författare)
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A probabilistic methodology to determine acceptance criteria and failure probabilities for the KBS-3 ductile cast iron inserts
- 2006
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Ingår i: Mater Res Soc Symp Proc. - : Springer Science and Business Media LLC. - 1558998896 - 9781558998896 ; , s. 275-282
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Konferensbidrag (refereegranskat)abstract
- The Swedish KBS-3 copper-cast iron canisters for geological disposal are in an advanced stage of its development. This reports deals with the cast iron insert that provides the mechanical strength of the canister. The ductility of manufactured inserts often fall below the initial requirements [1]. An issue then was what consequences low tensile properties and scatter may have on the long-term failure probability of the canisters. This paper describes an approach to assess the failure probabilities for given actual material data and how to derive acceptance criteria for manufactured canisters. A statistical test programme was implemented using three inserts to determine the tensile, compression and fracture properties. Tested specimens were investigated by micro-structural analysis to determine the microstructure and categorize and size defects. It was found that the material scatter and low ductility was caused by many defect types, but with slag defects as the most important ones. These data were then used to compute defect distributions and as direct input to FE-calculations of KBS-3 canisters. A large number of FE-analyses were performed at the maximum design load (44 MPa) covering distributions of material parameters and geometrical features of the canisters. The computed probabilities were very low for both failure modes even for material data with poor ductility.
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| 3. |
- André, Magnus, et al.
(författare)
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Determining sorption coefficients in intact rock using an electrical potential gradient as a driving force for migration
- 2006
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Ingår i: Scientific Basis for Nuclear Waste Management XXIX. - WARRENDALE, PA : MATERIALS RESEARCH SOCIETY. - 1558998896 ; , s. 975-982
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Konferensbidrag (refereegranskat)abstract
- The transport of radionuclides in indigenous rock is greatly affected by the sorption of cations in the porous rock matrix. For the determination of sorption coefficients, batch experiments have traditionally been used. For these experiments, the rock sample is crushed into fine particles to reduce the experimental time. However, this procedure increases the specific surface area of the sample and the new surfaces created could have different sorption qualities than the naturally occurring surfaces, which may impair the results of sorption coefficient determinations. A new method for determining sorption coefficients in intact rock is being developed, using electromigration as a means to speed up the transport process, thereby allowing for faster equilibration between the rock sample and the tracer solution. Here, we report results from preliminary experiments, using cesium as a sorbing tracer, showing a consistent difference between sorption coefficients obtained using electromigration methods on intact rock samples and traditional batch experiments on crushed samples.
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| 4. |
- Crawford, James, et al.
(författare)
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Epistemic and aleatory uncertainty in recommended, generic rock K-d values used in performance assessment studies
- 2006
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Ingår i: Scientific Basis for Nuclear Waste Management XXIX. - WARRENDALE, PA : MATERIALS RESEARCH SOCIETY. - 1558998896 ; , s. 251-258
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Konferensbidrag (refereegranskat)abstract
- Over the past decade or so there has been an explosion in the number of sorption modelling approaches and applications of sorption modelling for understanding and predicting solute transport in natural systems. The most widely used and simplest of all models, however, is that employing a constant distribution coefficient (K-d) relating the sorbed concentration of a solute on a mineral surface and its aqueous concentration. There are a number of reasons why a constant partitioning coefficient is attractive to environmental modellers for predicting radionuclide retardation, and in spite of all the shortcomings and pitfalls associated with such an approach, it remains the leitmotif of most performance assessment transport modelling. This paper examines the scientific basis underpinning the K-d-approach and its broad defensibility in a performance assessment framework. It also examines sources of epistemic and aleatory uncertainty that undermine confidence in K-d-values reported in the open literature. The paper focuses particularly upon the use of so-called "generic" data for generalised rock types that may not necessarily capture the full material property characteristics of site-specific materials. From the examination of recent literature data, it appears that there are still a number of outstanding issues concerning interpretation of experimental laboratory data that need to be considered in greater detail before concluding that the recommended values used in performance assessments are indeed conservative.
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| 5. |
- Moreno, Luis, et al.
(författare)
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Modelling of transport in fractures with complex matrix properties
- 2006
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Ingår i: Scientific Basis for Nuclear Waste Management XXIX. - WARRENDALE, PA : MATERIALS RESEARCH SOCIETY. - 1558998896 ; , s. 267-274
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Konferensbidrag (refereegranskat)abstract
- In the ongoing Swedish site investigations it has been found that the rock matrix adjacent to many open fractures has been altered. The alteration can extend from mm to several cm. The altered rock can have different sorption and diffusion properties compared to the undisturbed rock and this may influence the retardation of the nuclides. The paper presents how the Channel Network model has been adapted to handle diffusion into a matrix composed of several layers with different properties in addition to the infinite undisturbed matrix. For one channel, the solution for the Residence Time Distribution, RTD, may be found in the Laplace-plane. For the transport in the Channel Network, a particle tracking technique is used to determine the paths followed by solute particles. The RTD for this path is obtained using convolution, which in the Laplace-plane means multiplication of the transfer functions for each channel. The inversion to the time-plane is made by numerical inversion of the Laplace transforms for each path. The method has been tested with data from the TRUE (Tracer Retention Understanding Experiments) project, Task 6F, fluid flow and solute transport in two features in 100 in scale where a complex matrix was modelled. The model was used to predict the transport of the tracers (I-129), Cs-137, and Am-241) over some 20 m. The paper also addresses how the RTD is influenced by the different retardation mechanisms under Site Characterisation (SC) as well as Performance Assessment (PA) conditions.
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| 6. |
- Sidborn, Magnus, et al.
(författare)
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Corrosion of copper canisters through microbially mediated sulphate reduction
- 2006
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Ingår i: Scientific Basis for Nuclear Waste Management XXIX. - : Springer Science and Business Media LLC. - 1558998896 ; , s. 861-868
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Konferensbidrag (refereegranskat)abstract
- A biogeochemical model was developed with an aim to illustrate the transport and reaction processes that may be involved in copper canister corrosion by sulphide in a deep repository for nuclear waste. Sulphide concentrations in the groundwaters in Sweden are relatively low and are generally considered to be of little importance for corrosion of the copper canisters. Sulphate, however, is present in relatively large amounts (50-700 mg/L) and may be reduced to sulphide by reaction with organic matter. Mediated by microbes, the reduction rate may be increased considerably compared to abiotic reduction. Microbially mediated sulphate reduction occurs in many natural environments, provided that reactive organic matter is available. Groundwater analyses indicate relatively high concentrations of dissolved methane (up to 16 mg/L) which thermodynamically is a suitable reducing agent. This processes could occur in fractures in the bedrock provided that substrates are continuously supplied and that there is a sink for the reaction products. In the repository, the copper canisters may provide a sink for sulphide that yields a favourable environment for the microbe population. The model domain includes a canister with surrounding backfill material intersected by a hydraulically conductive fracture. Transport of substrates and reaction products in the fracture and backfill as well as microbially mediated reaction at the fracture opening is included in the model. Results indicate that microbially produced sulphide could potentially contribute to canister corrosion.
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