Characterization of Discharge Areas of Radionuclides Originating From Nuclear Waste Repositories
If leakages in nuclear waste repositories located in crystalline bedrock arise, radionuclides will reach the biosphere and cause a risk of radiological impact. The extent of the radiological impact depends on in which landscape elements the radionuclides emerge. In this study, we investigate if there are certain landscape elements that generally will act as discharge areas for radionuclides leaking from subsurface deposits. We also characterize the typical properties that distinguish these areas from others. In humid regions, landscape topography is the most important driving force for groundwater flow. Because groundwater is the main transporting agent for migrating radionuclides, the topography will determine the flowpaths of leaking radionuclides. How topography and heterogeneities in the subsurface affect the discharge distribution of the radionuclides is therefore an important scope of this study. To address these issues, we developed a 3-D transport model. Our analyses are based on site-specific data from two different areas in Sweden, Forsmark, Uppland, and Oskarshamn, Småland. The Swedish Nuclear Waste Management Company (SKB) has selected these two areas as candidate areas for a deep repository of nuclear waste and the areas are currently subject to site investigations. Our results suggest that there are hot-spots in the landscape i.e. areas with high probability of receiving large amounts of radionuclides from a leaking repository of nuclear waste. The hot-spots concentrate in the sea, streams, lakes and wetlands. All these elements are found at lower elevations in the landscape. This pattern is mostly determined by the landscape topography and the locations of fracture zones. There is a relationship between fracture zones and topography, and therefore the importance of the topography for the discharge area distribution is not contradicted by the heterogeneity in the bedrock. The varieties of landscape elements which have potential for receiving significant amounts of radionuclides are limited. To limit the radiological dose assessment, analyses should be focused to and more detailed in such landscape areas in which doses are expected to be high. Due to the similarities among deep groundwater discharge areas, one can make site-specific analyses of those areas, which have a broad applicability for migration of radionuclides originating from a nuclear waste repository.
Conceptual-Model-Driven Characterization of the Culebra Dolomite at the Waste Isolation Pilot Plant
The Culebra Dolomite Member of the Rustler Formation would be the primary groundwater transport pathway for radionuclides released from the Waste Isolation Pilot Plant (WIPP) by inadvertent human intrusion. The WIPP, located in southeastern New Mexico, is the U.S. Department of Energy's underground repository for transuranic and mixed wastes. Characterization of the Culebra began in the late 1970s and showed that the Culebra is highly heterogeneous with transmissivity varying over ten orders of magnitude. Although multiple generations of groundwater flow models had been developed for the Culebra by the time the WIPP was first licensed in 1999, the Environmental Protection Agency (a WIPP regulator) felt that a coherent conceptual model relating transmissivity variations to their cause(s) was lacking. A conceptual model relating Culebra transmissivity to geologic factors was developed and combined with a monitoring well network optimization study to identify locations where new wells would be of high value. Since 2003, 16 new wells have been sited, drilled, and tested. The wells provided confirmation of expected geologic conditions, local head and transmissivity measurements, and transient response data during large-scale pumping tests. The information provided by these new wells has been used in the development of a new groundwater flow model for the WIPP that provides the best representation yet achieved of measured heads and transient responses observed during large-scale pumping tests at the site. This model will be used in future performance assessment calculations for the WIPP.
The Long-term Management of Used Nuclear Fuel in Canada: A Geoscientific Prespective
The Nuclear Waste Management Organization (NWMO) is responsible for implementing Adaptive Phased Management, the approach selected by the Government of Canada for long-term management of used nuclear fuel waste generated by Canadian nuclear reactors. In support of this objective, NWMO is pursuing an active technical research and development program in areas such as repository engineering, repository geoscience and repository safety. The geoscience work program is designed to develop a geoscientific basis for understanding long-term geosphere barrier performance, as well as building confidence in deep geological repository safety in both sedimentary and crystalline settings. This is achieved through a multidisciplinary approach involving the coordinated effort of research groups drawn from universities, consultants, and international nuclear waste management organizations. The main objectives of the program are to: develop tools and methods to improve NWMO's geosphere characterization capabilities and develop readiness for evaluating potential candidate sites in willing host communities; advance the understanding of long-term physical and geochemical evolution of the geosphere at time scales relevant to repository safety; and improve numerical methods to assess the geosphere evolution and its response to long-term perturbations. The paper provides an overview of the geoscience issues and challenges associated with the development of deep geological repositories and key activities that the NWMO is pursuing to address them.
Ontario Power Generation's Proposed Deep Geologic Repository, Tiverton, Ontario, Canada
Ontario Power Generation is proposing to develop a Deep Geologic Repository (DGR) for the long-term management of its Low and Intermediate Level Radioactive Waste (L&ILW) at the Bruce site located near Tiverton, Ontario, 225 km northwest of Toronto. The shaft accessed repository, as envisioned, would accommodate 200,000 m3 (as packaged) of L&ILW in emplacement rooms excavated at a depth of 680 m within the Ordovician age argillaceous limestone Cobourg Formation. The Bruce site is underlain by an approximate 860 m thick Paleozoic sedimentary sequence comprised of near horizontally bedded carbonates, shales, evaporates and sandstones, Devonian to Cambrian in age, overlying crystalline basement rocks. Regional and site-specific geoscientific studies to verify the suitability of the Bruce site to host the DGR were initiated in 2006. The focus for the geoscientific investigations has been on gathering data to develop and test an understanding of the evolution and stability of the geologic, hydrogeologic, hydrogeochemical and geomechanical environ as it relates to demonstrating repository safety. Scheduled for completion in 2010, the interim results, which have included the drilling, coring and testing of 4 deep boreholes, are providing evidence of a predictable geosphere with a deep seated (>400 m), low permeability (K < 10-13 m sec-1), low porosity (0.01-0.08), saline (TDS > 250 gm l-1) groundwater regime that is ancient and resilient to external perturbations (e.g. glaciation). Work program activities in this regard have included, among others, detailed studies of rock core lithology, mineralogy and petrophysics, rock matrix pore fluid and groundwater characterisation, in-situ rock mass hydraulic testing, geomechanical rock core testing, 2-D seismic reflection surveys and long-term hydraulic borehole instrumentation. These data, in addition to regional and site-scale hydrogeologic modelling of the sedimentary sequence that among other aspects is examining groundwater system evolution through an understanding of long-term environmental tracer migration and observed abnormally elevated and depressed formation pore pressures, are to be integrated as part of a Geosynthesis document for the project supporting a case for safety. This presentation will provide an overview of the DGR program and the role of geoscience as it is contributing to an understanding of far-field barrier performance and long-term DGR safety.
Geoscientific Characterization of the Bruce Site, Tiverton, Ontario
Ontario Power Generation is proposing a Deep Geologic Repository (DGR) for the long-term management of its Low and Intermediate Level Radioactive Waste (L&ILW) within a Paleozoic-age sedimentary sequence beneath the Bruce site near Tiverton, Ontario, Canada. The concept envisions that the DGR would be excavated at a depth of approximately 680 m within the Ordovician Cobourg Formation, a massive, dense, low- permeability, argillaceous limestone. Characterization of the Bruce site for waste disposal is being conducted in accordance with a four year multi-phase Geoscientific Site Characterization Plan (GSCP). The GSCP, initially developed in 2006 and later revised in 2008 to account for acquired site knowledge based on successful completion of Phase I investigations, describes the tools and methods selected for geological, hydrogeological and geomechanical site characterization. The GSCP was developed, in part, on an assessment of geoscience data needs and collection methods, review of the results of detailed geoscientific studies completed in the same bedrock formations found off the Bruce site, and recent international experience in geoscientific characterization of similar sedimentary rocks for long-term radioactive waste management purposes. Field and laboratory work related to Phase 1 and Phase 2A are nearing completion and have focused on the drilling, testing and monitoring of four continuously cored vertical boreholes through Devonian, Silurian, Ordovician and Cambrian bedrock to depths of about 860 mBGS. Work in 2009 will focus on drilling and testing of inclined boreholes to assess presence of vertical structure. The available geological, hydrogeological and hydrogeochemical data indicate the presence of remarkably uniform and predictable geology, physical hydrogeologic and geochemical properties over well separation distances exceeding 1 km. The current data set including 2-D seismic reflection surveys, field and lab hydraulic testing, lab petrophysical and diffusion testing, lab porewater and field groundwater characterization, and field head monitoring confirm the anticipated favourable characteristics of the Bruce site for long-term waste management. These favourable characteristics include a tight geomechanically stable host formation that is overlain and underlain by thick, massive, very low permeability shale and argillaceous limestone formations where radionuclide transport appears to be very limited and dominated by diffusion.
Diffusion Measurements in Low-Permeability Ordovician Sedimentary Rocks From Southern Ontario
Effective diffusion coefficients for tritiated water (HTO) and iodide tracers, defined as: De = φ δ D0 / τ2 where: De = solute-specific effective diffusion coefficient (m2s-1); D0 = solute-specific free-water diffusion coefficient (m2s-1); φ = diffusion accessible porosity (-); τ = tortuosity (- ), and δ = constrictivity (-) were measured for samples of Ordovician sedimentary rocks from southern Ontario using through-diffusion (TD) and X-ray radiography techniques. Water-loss porosities were also measured using a gravimetric technique that involves drying to 105 oC. The work was conducted as part of on-going investigations related to Ontario Power Generation's proposed DGR at the Bruce site, near Tiverton, Ontario. The radiography method was used effectively for samples with porosity > 0.03 (3%) and TD was used for the low-porosity limestone. In a comparison exercise, diffusion measurements on paired samples using the radiography and TD methods provide consistent results. In addition, results from diffusion measurements conducted independently in three different labs compare favourably. The water-loss porosity values for the Ordovician shales range from 0.053 to 0.10 (5.3 to 10%), and the water-loss porosities from the Ordovician limestones range from 0.005 to 0.021 (0.5 to 2.1%). The De values for iodide tracer in shale samples from the Bruce site range from 4.1 x 10-13 to 6.3 x 10-12 m2/s, and values for iodide tracer in limestone samples from the Bruce site range from 7.8 x 10-14 to 1.2 x 10-12. De values for HTO tracer in shale samples from the Bruce site range from 4.3 x 10- 13 to 4.8 x 10-12, and values for HTO tracer in limestone samples from the Bruce site range from 1.0 x 10-13 to 2.3 x 10-12. With increasing depth, there is a general trend to lower De values from the top of the Ordovician shales (approximately 450 mbgs) to the top of the Ordovician limestones (approximately 650 mbgs), with no trend evident in De values from the limestones. Anisotropy was investigated with paired sub-samples, oriented parallel and normal to bedding. For iodide tracer, the De values parallel to bedding are 0.4 to 5.5 times larger (mean = 2.2) than those obtained normal to bedding. For HTO tracer, the De values parallel to bedding are 1.3 to 8.2 times larger (mean = 3.2) than those obtained normal to bedding. Values of De obtained with HTO are up to 2.5 times greater than De values obtained with iodide.
Regional and Site-Scale Hydrogeologic Analyses of a Proposed Canadian Deep Geologic Repository for Low and Intermediate Level Radioactive Waste
A Deep Geologic Repository (DGR) for Low and Intermediate Level radioactive waste has been proposed by Ontario Power Generation for the eastern edge of the Michigan Basin at the Bruce site, near Tiverton, Ontario, Canada. The DGR is to be constructed within the argillaceous Ordovician limestone of the Cobourg Formation at a depth of about 680 m below ground surface. This paper describes a regional-scale and linked site-scale geologic conceptual model for the DGR site and analyzes flow system evolution using the FRAC3DVS-OPG flow and transport model. The work illustrates the factors that influence the predicted long-term performance of the geosphere barrier and provides a framework for the assembly and integration of site-specific geoscientific data. The structural contours at the regional and site scale of the 31 sedimentary strata that may be present above the Precambrian crystalline basement rock were defined by the Ontario Petroleum Institute's Oil, Gas and Salt Resources Library borehole logs covering Southern Ontario and by site-specific data. The regional- scale domain encompasses an 18,500 km2 region extending from Lake Huron to Georgian Bay. The site- scale spatial domain encompasses an area of approximately 361 km2 with the repository at its centre. Its boundary conditions are determined using both the nested model approach and an embedment approach. The groundwater zone below the Devonian is characterized by units containing pore fluids with high concentrations of total dissolved solids that can exceed 300 g/l. Site-specific data indicate that the Ordovician is under-pressured relative to the surface elevation while the Cambrian is over-pressured. The computational sequence for the analyses involves the calculation of steady-state density independent flow that is used as the initial condition for the determination of pseudo-equilibrium for a density-dependent flow system that has an initial TDS distribution developed from observed data. Sensitivity analyses can be computationally intensive, particularly for large-scale dynamic problems that couple energy, flow and mass transport. Important in the sensitivity analysis is the selection of the performance measure used to evaluate the system. The traditional metric of average water particle travel time is inappropriate for geologic units such as the Ordovician and lower Silurian where solute transport is diffusion dominant. The use of life expectancy and groundwater age is a more appropriate metric for such a system. The mean life expectancy for the DGR and base-case parameters has been estimated to be in excess of 8 million years. The analyses support the conclusion that solute transport in the Ordovician sediments is diffusion dominant.
Paleoclimate Impact on a Proposed Canadian Deep Geologic Repository for Low and Intermediate Level Radioactive Waste
A Deep Geologic Repository (DGR) for low and intermediate level radioactive waste has been proposed by Ontario Power Generation (OPG) for the Bruce site near Tiverton, Ontario Canada. As envisioned, the DGR is to be constructed at a depth of about 680 m below ground surface within the argillaceous Ordovician limestone of the Cobourg Formation. Within the geologic setting of southern Ontario, the Bruce site is located west of the Algonquin Arch within the Bruce Megablock, positioned along the eastern edge of the Michigan Basin. It is clear that to credibly address the long-term safety of a deep geologic repository, long-term climate change and in particular a glaciation scenario, must be incorporated into performance assessment modelling activities. In addition, by simulating flow system responses to the last Laurentide (North American) glacial episode, insight is gained into the role of significant past stresses (mechanical, thermal and hydrological) on determining the nature of present flow system conditions, and by extension, the likely impact of similar, future boundary condition changes on long-term flow system stability. The last Laurentide glacial episode was characterized by the following: occurred over a 120 000 year time period; included numerous cycles of glacial advance and retreat, with maximum ice thickness over a typical Ontario site reaching nearly 3 km; included extensive periods of transient, peri-glacial conditions during which permafrost could impact the subsurface, depending on location, to several hundreds of metres; and was accompanied by significant basal meltwater production near the end of the glacial episode. The impact of glaciation and deglaciation on density-dependent groundwater flow was investigated using results from the deterministic University of Toronto Glacial Systems Model (GSM) of continental ice-sheet evolution. The 18,500 km2 regional-scale domain extends from Lake Huron to Georgian Bay and includes 31 sedimentary strata that may be present above the Precambrian crystalline basement rock. The effects of long- term climate change on the groundwater flow system are investigated by modifying the permeability of rock within the permafrost zone, by changing the surface boundary conditions to reflect a glacial scenario, and depending on the one-dimensional loading efficiency, by the inclusion of a pressure modifying term in the flow equation. At the DGR site, two glaciation events were predicted to occur over the regional-scale domain with the first event spanning a period from approximately -62.5 kyr to -56 kyr and the most recent event occurring in the period from approximately -24 kyr to -13 kyr. Permafrost occurred approximately 12 kyr to 14 kyr prior to the onset of glaciation and was fully absent approximately 1 kyr after onset. The model results indicate that basal meltwater does not penetrate below the units of the Salina at the DGR site. The most significant consequence of glacial loading is the generation of elevated transient pore pressures throughout the rock column, with the level dependent on the compressibility of the rock and the one-dimensional loading efficiency. The analyses clearly show that glacial impact is very sensitive to local scale lithology and stratigraphy.