Repository For Disposal Of Nuclear Waste Research Articles

Reactive transport model of interactions of corrosion products and bentonite

Carbon-steel and compacted bentonite have been proposed as candidate materials for overpack and buffer, respectively, in deep geological repositories for nuclear waste disposal. Here, we present 1-D and 2-D axisymmetric water flow and multicomponent reactive solute transport models to simulate canister corrosion, the interactions of corrosion products with bentonite and the long-term hydrochemical evolution of porewater composition in the near field of a repository in fractured granite. Proton surface complexation is highly effective in buffering pH in bentonite porewater which increases due to canister corrosion. Other mechanisms such as calcite dissolution/precipitation, Fe exchange and dissolution/precipitation of Fe minerals are much less effective in buffering pH. Magnetite is the main corrosion product. Most of the Fe diffuses from canister into bentonite where it precipitates. Siderite precipitation is two orders of magnitude smaller than magnetite precipitation due to the limited availability of dissolved carbonates. The amounts of dissolved, sorbed and exchanged iron are in turn two orders of magnitude smaller than precipitated siderite. Bentonite porosity decreases due to magnetite precipitation. The maximum decrease which occurs at the canister–bentonite interface is 0.1 and is insufficient to clog bentonite pores. Apparent distribution coefficient, K d, of corrosion products derived from computed dissolved and exchanged Fe concentrations increases strongly with time, indicating that the use of a constant K d for corrosion products is largely unrealistic.

Harmonisation of Site Characterisation and Performance Assessment Modelling - The Relative Importance of Surface Sorption and Matrix Interaction Phenomena

ABSTRACTIn the context of geological repositories for nuclear waste disposal, the goal of site characterisation (SC) is to obtain input data that can later be used as a basis for performance assessment calculations (PA). Performance assessment is required to give an indication as to whether the repository will behave as intended over the geological timescales relevant for risk analysis.Processes that may be important in a PA setting for constraining radionuclide transport may not necessarily be dominating, or indeed may not even be observable during SC investigations.In this paper it is shown that the migration of sorbing tracers is governed largely by surface sorption phenomena in typical SC-type field experiments whereas in a PA scenario, matrix interaction instead can be expected to play an overwhelmingly important role. This study uses data and settings that are representative for fractured rocks in Sweden, but the method used and the conclusions may have more general applicability.

Modeling stress-dependent permeability for anisotropic fractured porous rocks

Most of deep geological engineered structures, such as rock caverns, nuclear waste disposal repositories, metro rail tunnels, multi-layer underground parking, are constructed within hard crystalline rocks because of their high quality and low matrix permeability. In such rocks, fluid flows mainly through fractures. Quantification of fractures along with the behavior of the fluid flow through them, at different scales, becomes quite important. Earlier studies have revealed the influence of sample size on the confining stress–permeability relationship and it has been demonstrated that permeability of the fractured rock mass decreases with an increase in sample size. However, most of the researchers have employed numerical simulations to model fluid flow through the fracture/fracture network, or laboratory investigations on intact rock samples with diameter ranging between 38 mm and 45 cm and the diameter-to-length ratio of 1:2 using different experimental methods. Also, the confining stress, σ3, has been considered to be less than 30 MPa and the effect of fracture roughness has been ignored. In the present study, an extension of the previous studies on “laboratory simulation of flow through single fractured granite” was conducted, in which consistent fluid flow experiments were performed on cylindrical samples of granitoids of two different sizes (38 mm and 54 mm in diameters), containing a “rough walled single fracture”. These experiments were performed under varied confining pressure (σ3 = 5–40 MPa), fluid pressure (fp ≤ 25 MPa), and fracture roughness. The results indicate that a nonlinear relationship exists between the discharge, Q, and the effective confining pressure, σeff., and Q decreases with an increase in σeff.. Also, the effects of sample size and fracture roughness do not persist when σeff. ≥ 20 MPa. It is expected that such a study will be quite useful in correlating and extrapolating the laboratory scale investigations to in-situ scale and further improving theoretical/numerical models associated with fluid flow through rock masses.

Locational Conflict and the Siting of Nuclear Waste Disposal Repositories: An International Appraisal

The industrialized nations of the world have begun to plan for the storage and eventual disposal of their increasing volumes of nuclear wastes. In this paper the authors inventory the progress made by these nations in planning for nuclear waste disposal. A typology based on the adoption of spent-fuel reprocessing programs and of progress toward selection of permanent disposal sites is developed, and the world's nuclear nations are located within this typology. However, those countries which have been active in determining and constructing disposal sites have faced considerable internal and international political conflict. Conflict over nuclear waste disposal site-selection is both a geographical and a political problem. There is a review of various factors which contribute to the articulation and resolution of political conflict, including public participation in siting decisions, veto power of local governments, and prospects for international cooperation in siting decisions. It is concluded that decentralized democratic procedures, which emphasize public participation in siting decisions, have the strongest potential for conflict resolution.

The impact of a high level nuclear waste repository on the regional ground water flow

Of all the alternatives suggested for the disposal of the high level nuclear waste, that of the deep underground burial in geologic formations may prove to be the most attractive. Within the constraints of economy and feasibility, the primary concern in deep underground burial is the selection of a geologic formation that will contain and isolate the wastes so that radioactive nuclides from the waste do not enter the biosphere in amounts that will endanger public health and safety. It is recognized that the primary mechanism for the likely introduction of the radioactive elements into the biosphere is that of ground water transport. It is thus imperative that the effect of nuclear waste disposal repository on ground water flow be fully explored and understood. This paper examines: (1) the problem of defining reliable hydrogeological properties of a rock mass, (2) the prediction of thermally induced ground flow on a regional scale for a crystalline rock formation. It is concluded that in the absence of major geologic discontinuities the deep disposal of radioactive waste may be technically feasible. However, the need for further field and theoretical studies is highlighted by the paucity of reliable data and the inadequacies of existing methodologies.