GEOSCIENTIFIC SITE CHARACTERIZATION PLAN FOR THE DEEP GEOLOGIC REPOSITORY AT THE BRUCE SITE, KINCARDINE, ONTARIO

Transcription

1 GEOSCIENTIFIC SITE CHARACTERIZATION PLAN FOR THE DEEP GEOLOGIC REPOSITORY AT THE BRUCE SITE, KINCARDINE, ONTARIO Kenneth Raven, Sean Sterling, John Avis and Richard Jackson Intera Engineering Ltd., Ottawa, Ontario, Canada Mark Jensen Ontario Power Generation Inc., Toronto, Ontario, Canada ABSTRACT Ontario Power Generation Inc. is proposing the development of a Deep Geologic Repository (DGR) for low and intermediate level radioactive waste at Bruce site, located near Tiverton, Ontario. The DGR will be constructed as an engineered facility at a depth of about 680 m within the Paleozoic argillaceous limestones of the Cobourg Formation. This paper describes the geoscientific site characterization program developed to acquire the necessary information to support preparation of DGR environmental and safety assessments and site preparation/construction license applications. RÉSUMÉ Ontario Power Generation Inc. propose le développement d un dépôt géologique profond (DGP) pour des déchets radioactifs de niveau bas à intermédiaire, au site de Bruce, situé près de Tiverton, Ontario. Le DGP soit construit comme une service ingénié, en profondeur d approximativement 680 m dans les calcaires argillacées Paléozoïques de la formation de Cobourg. Cet article décrit le programme de caractérisation d emplacement développé pour acquérir l information géoscientifique nécessaire pour soutenir la préparation de l évaluation environnementale et sauveté de DGP et des application de permis pour la préparation/construction du site. 1 INTRODUCTION Ontario Power Generation Inc. (OPG) has proposed the construction of a Deep Geologic Repository (DGR) at the Bruce site near Tiverton, Ontario. This engineered facility will be constructed at depth of approximately 680 m within very low permeability Paleozoic sedimentary bedrock of the Michigan Basin. The facility will be designed for the long-term management of low and intermediate level radioactive (L&ILW) waste produced by OPG-owned nuclear generating stations. Figure 1 shows an artist s rendition of the DGR concept. Ontario. Based on this understanding, the DGR will be constructed within the competent, low permeability argillaceous limestone of the Cobourg Formation at a depth of about 680 m, below 200 m of low permeability shale of the Queenston, Georgian Bay and Blue Mountain Formations, and about 200 m above the Precambrian basement. The DGR concept relies, in part, on the long-term geologic isolation provided by the host, overlying and underlying bedrock formations. The DGR project (OPG, 2005) includes the preparation, construction, operation and long-term performance of above- and below-ground facilities for the long-term management of low and intermediate level radioactive waste. The underground repository excavations will comprise a series of waste emplacement rooms, perimeter access tunnels, various underground service areas and vertical access and vent raise shafts. 2 EXISTING DESCRIPTIVE SITE GEOSPHERE MODEL The geological, hydrogeological, and geomechanical setting of the DGR has been summarized and described by Golder Associates Ltd. (2003) based on available geoscientific data for the Bruce site, the surrounding area and from elsewhere in southern Figure 1. Bruce site. Artist s rendition of DGR concept at the 628

2 The existing descriptive site geosphere model for the Bruce site suggests the following favourable geoscience attributes for demonstrating the long-term safety of the DGR. 1. The deep horizontally-layered shale and argillaceous limestone sedimentary sequence that will overlie and host the DGR is geologically stable, geometrically simple and predictable, relatively undeformed and of large lateral extent. 2. Active faulting and seismicity at and near the site are very limited. 3. The deep argillaceous formations that will host the DGR will provide stable and practically dry openings. 4. The regional stress regime (horizontally compressive) is favourable with respect to sealing of any vertical fractures or faults. 5. The deep shale and argillaceous limestones are thick and of very low permeability, providing a very tight 200 m thick bedrock horizon enclosing the repository, and an overlying very tight 200 m thick barrier to upward migration. 6. Mass transport in the deep shales and limestones is diffusion dominated. The deep groundwater system in the shales and limestones is saline (about g/l TDS), stable and ancient, showing no evidence of either glacial perturbations or, cross formational flow or mixing. 7. The shallow water supply aquifer in the upper carbonate bedrock is hydrogeologically isolated and protected from the stagnant deep saline groundwater system. Detailed geoscientific characterization of the Bruce site is recognized as an essential part of the work necessary to confirm these site conditions and to support DGR safety and environmental assessments and license applications. 3 GSCP OBJECTIVE, SCOPE AND STRATEGY The objective of the Geoscientific Site Characterization Plan (GSCP) for the Bruce DGR is to provide information necessary to develop a comprehensive descriptive site geosphere model that: provides a geoscientific understanding of the current condition of the site, its past evolution and likely future natural evolution over the period of interest for safety; establishes a baseline for detecting potential short-term and long-term environmental impacts caused by the construction, operation and closure of the facility; and provides the necessary geoscience data and information to design the facility and perform safety assessments and optimizations (i.e., for environmental assessment and/or licensing). The primary focus of the GSCP is subsurface characterization completed through surface-based investigations. Underground characterization work is contemplated but will be undertaken following excavation for DGR construction. The GSCP (Intera Engineering Ltd., 2006) is a 5 year, three phase program for the development, testing and refinement of the descriptive site geosphere model for the Bruce site. It is based on internationally accepted geoscience attributes considered necessary for understanding technical site suitability and has been subject to peer review by OPG stakeholders, regulatory agencies and an independent Geoscience Review Group. The GSCP has also been integrated with ongoing regional geologic, hydrogeologic and geomechanical studies of southwestern Ontario initiated and supported by OPG. The GSCP has been developed based on inclusion of international (e.g. Swiss, French, United States) experience in investigation of deep sedimentary formations for longterm radioactive waste management purposes. Selection and scheduling of GSCP activities have been conducted in a manner specifically designed to minimize GSCP and DGR project risk and cost. 4 GEOSCIENCE DATA NEEDS The GSCP was developed based on the identification of geoscience data needs, provision of a rationale for including such data needs, and the selection of preferred tools and methods for collecting data to meet identified needs. Table 1 summarizes the master table of key geoscience data needs developed in the GSCP. Table 1. Master table of key geoscience data needs Geoscience Discipline Geological setting & framework Geomechanical setting & rock properties Hydraulic properties & state Diffusion & sorption properties Groundwater & porewater characterization Seismicity Geoscience Data Need Existing geological information, existing geophysical information, stratigraphic sequence, formation thicknesses and attitudes, structural framework, bedrock petrology and mineralogy Existing geomechanical information, in situ stress regime, rock material properties, rock mass properties Existing hydrogeologic information, rock mass hydraulic properties, hydraulic heads, total & effective rock matrix porosities, fracture/fault hydraulic properties, gas-brine flow properties, groundwater densities Effective diffusion coefficients, effective diffusion porosities, sorption parameters Existing hydrogeochemcial information, major ion & trace element chemistry, isotope chemistry, dissolved gases, redox state, water physical properties Map significant faults, local seismographic monitoring Data needs and rationale were driven in large part by the existing descriptive site geosphere model and the characteristics and features considered important to 629

3 OttawaGeo2007/OttawaGéo2007 safety assessment. Preferred methods for data collection were selected based on application of six screening criteria: practicality, demonstrated effectiveness, accuracy, compatibility, quality assurance and cost-effectiveness. 5 ultracentrifugation, stepped heating/vacuum distillation, core out-diffusion and diffusional equilibration. 5.2 Phase 1 Geological Characterization Geological characterization in Phase 1 of the GSCP includes the following major field investigative activities: 2-D seismic reflection surveys (Figure 3). Drilling and casing of two deep boreholes: DGR-1 & DGR-2 (Figure 4). Logging and photography of core. Collection of preserved core samples for geological laboratory testing. Borehole geophysical logging. GEOSCIENTIFIC SITE CHARACTERIZATION PLAN The GSCP is structured into an initiation task, three phases of site investigation and a parallel geosynthesis activity intended to support and enhance the field data collection work and site characterization effort. The primary focus of GSCP work is the deep (>460m) Ordovician shales and argillaceous limestones that will host, overlie and underlie the DGR. A secondary focus is the intermediate depth ( m) dolostones and shales that would provide a transport pathway for an contaminants diffusing from the underlying Ordovician sediments. The upper 100 m of dolostones which form a regional drinking water supply aquifer also require characterization and long-term monitoring. Approximately 20 km of 2-D seismic reflection surveys in nine lines were completed at the Bruce site October, Phase 1 site investigations are designed to answer basic questions about geologic, hydrogeologic and geomechanical characteristics and properties of the site. More complex investigations (e.g., in situ stress measurement, laboratory study of radionuclide migration, inclined and sub-horizontal drilling), will be addressed in Phase 2 and 3 investigations. Since the GSCP is iterative, details of Phase 2 and 3 investigations will only be available following the completion of Phase 1 work. 5.1 Requirements for GSCP Initiation Prior to the start of site characterization work several activities were initiated and/or completed to facilitate site characterization work. These activities included development of a Project Quality Plan, establishment of a project data warehouse/gis, refinement of laboratory porewater extraction and testing methods, assembly of precedent geoscientific data, definition of scientific terminology, assessment of GSCP against argillaceous limestone geoscience attributes, preparation of Phase 1 test plans, and establishment of site infrastructure (on-site core storage facility, office space, drill site and drill site services). Figure 2. Location of possible DGR footprint, 2-D seismic lines, US-series and DGR-1 & 2 boreholes The refinement of laboratory testing methods was undertaken because of the lack of experience in extracting porewater from the anticipated very low porosity (1-4%) and low permeability (~10 m ) of intact shale and argillaceous limestone formations and the recognition that characterization of groundwater in such deep formations must rely heavily on extraction and testing of porewater. This GSCP initiation activity involved the collection and testing of samples of Queenston shale and Cobourg argillaceous limestone. Porewater extraction and testing methods being evaluated include crush and leach, forced advection, Figure 3. 2-D seismic surveys at Bruce site with Vibroseis trucks used for generating seismic sources. 630

4 OttawaGeo2007/OttawaGéo2007 Final processing and interpretation of the 2-D seismic surveys are awaiting completion of the deep bedrock drilling to the Precambrian and the completion of vertical seismic profiling (VSP) surveys within borehole DGR US-4 DGR-1 DGR Deep bedrock drilling is underway and involves the collection of continuous core of 76 mm diameter from two closely spaced 155 mm diameter boreholes using telescoping casing concepts and brine-based drilling fluids to mitigate drilling fluid - formation interactions. DGR-1 has been completed into the Queenston shale and DGR-2 will be cased to the Queenston shale and completed into the Precambrian basement. The casing of the upper sections of DGR-2 was undertaken to minimize cross formational fluid flow effects to the deeper low permeability parts of DGR-2, which is the primary focus of the Phase 1 site characterization work. 150 Amherstburg Lmst 100 Bois Blanc Dlst 50 Bass Island Dlst 0 50 Salina F-member Shale Salina E-member Dlst Salina C-member Shale Salina B-member Dlst/Anhydrite 100 Elevation Meters Salina A2-member Dlst 150 Salina A1-member Shale 200 Guelph Dlst 250 Cabot Head Shale Manitoulin Dlst 300 Queenston Shale 350 Georgian Bay Shale Blue Mountain Shale Cobourg Lmst Repository Horizon Sherman Fall Lmst 550 Kirkfield Lmst 600 Coboconk Lmst Gull River Lmst 650 Figure 4. DGR-1 and DGR-2 drill site. 700 All bedrock drilling is being conducted in accordance with the requirements of Ontario Ministry of Natural Resources, Oil, Gas and Salt Resources of Ontario, Provincial Operating Standards. This includes use of blow-out prevention equipment, and special casing cementing and testing and reporting requirements for all deep drilling operations. Shadow Lake Sltst/Sst Cambrian Sst Precambrian Basement Grouted Steel Casing Open Borehole Figure 5 Bedrock stratigraphy and boreholes DGR-1 and DGR Figure 5 shows the interpreted deep bedrock stratigraphic sequence at the Bruce site based on the completion of DGR-1 and information from off-site oil and well exploration wells. Figure 5 shows stratigraphy following the subsurface nomenclature of Armstrong and Carter (2006). Phase 1 Hydrogeological Characterization Hydrogeological characterization in Phase 1 of the GSCP includes the following major field investigative and testing activities: Re-establishment of shallow bedrock monitoring wells (US-series wells) Tracing and monitoring of drilling fluids Collection of opportunistic samples of groundwater during drilling where drilling and core conditions indicate likelihood of obtaining representative samples. Collection of preserved cores for porewater extraction and characterization, and diffusion and petrophysical testing Completion of fluid electrical conductivity (hydrophysical) logging. Rock quality encountered during drilling of DGR-1 has been moderate to excellent with poorer quality rock encountered in the upper Devonian and Silurian limestone and dolostone units above the Salina Formation. Core quality within the Salina, middle and lower Silurian Guelph, Cabot Head and Manitoulin Formations (see Figures 5 and 6) and the upper part of the Queenston shales (maximum depth of drilling in DGR-1) was typically excellent with RQD greater than 90%. 631

5 OttawaGeo2007/OttawaGéo2007 Hydrophysical testing identifies permeable zones of inflow for subsequent straddle packer testing. Borehole straddle-packer hydraulic testing. TM Installation of Westbay MP-55 multi-level monitoring systems in DGR-1 and DGR-2. Groundwater sampling and pressure monitoring in shallow bedrock and deep bedrock monitoring wells. Figure 7. Straddle-packer hydraulic testing in borehole DGR-1. Figure 6. Intact core recovered from Guelph Formation dolostone in DGR-1 at m depth. The straddle packer testing program includes both slug and drill stem testing for zones of moderate to low hydraulic conductivity (10 to 10 m/s) and pulse tests for zones of lower hydraulic conductivity. The straddlepacker testing tool (Figure 7) was specifically constructed for the DGR testing program to cover the expected range of formation hydraulic conductivities. It includes a downhole shut-in valve and pulse piston with downhole pressure measurement within, and below the testing interval, and within and outside of the drill tubing used to position the tool in the borehole. Downhole temperature measurement is made within and below the test interval. Pressures are measured using four Paroscientific Series 8CB High Pressure Intelligent Depth Sensors, with accuracy of 0.01% full scale (approximately 1.4 kpa). Several shallow (80 to 120 m deep) bedrock monitoring wells (US-series) exist at the Bruce site from investigations of the shallow carbonate bedrock aquifer completed in the late 1980s. These existing wells (see TM Figure 2) will be refurbished with new Westbay MP38 multi-level casing systems to act as long-term shallow bedrock monitoring wells. Because of the anticipated low to very low permeability of the bedrock formations, special precautions are being taken to maximize the quality of groundwater and porewater samples selected for characterization. This includes tracing of all drilling fluids and other fluids introduced to the boreholes using a field-detectable tracer (Na Fluoroscein) and lab-detectable tracer (tritium). Core samples selected for laboratory porewater extraction and characterization are preserved in the field within 30 minutes of core recovery using double vacuum sealed aluminum foil bags following nitrogen gas flushing. An archive of preserved cores is maintained to address unforeseen core testing needs. Following the completion of borehole hydraulic testing, boreholes DGR-1 and DGR-2 will be completed with TM Westbay MP-55 multi-level monitoring equipment. The proposed monitoring systems will include pumping ports and pressure profiling ports for DGR-1 and pumping ports and dedicated pressure transducer systems for the anticipated very low permeability units intersected in DGR-2. Pressure profiling (in DGR-1 and US-series wells) will be undertaken on quarterly Borehole hydraulic testing includes both hydrophysical testing and straddle-packer hydraulic testing. 632

6 basis and groundwater sampling will be performed routinely on a semi-annual basis. Confirmatory hydraulic testing of test intervals will also be undertaken as necessary within the completed Westbay TM multi-level casings. 5.4 Phase 1 Geomechanical Characterization Geomechanical characterization in Phase 1 of the GSCP includes the following major field investigative and testing activities: Installation of local seismograph stations Field lab testing of intact core for point load strength, P & S wave velocities, and slake durability. Fracture and RQD logging of core for assessment of rock mass properties Logging of borehole breakouts for indirect information on state of stress. Three new seismograph stations are being installed within 50 km of the Bruce site. These new stations will increase the ability for observation of low-level seismicity (M 1), and identification of possible structural discontinuities contributing to such low-level activity in the vicinity of Bruce site. Field geomechanical testing - including diametral and axial point load testing, P & S wave testing and slake durability testing - are conducted on a regular basis during drilling. P & S wave testing is also completed in the field and in the laboratory on all samples selected for uniaxial compression testing to identify any deterioration in sample quality between collection and laboratory testing. Standard fracture and RQD logging of recovered core is undertaken to assist in assessing the rock mass properties for geotechnical design purposes. Measurement of in situ stresses using hydraulic fracturing or overcoring techniques is not part of Phase 1 geomechanical characterization but will be attempted in Phase 2. Indirect evidence for state of in situ stress will be inferred from analysis of core disking and any borehole breakouts or other borehole deformation as measured with borehole caliper and acoustic televiewer logs. 5.5 Phase 1 Laboratory Testing Program Laboratory testing of recovered core, porewater extracted from core and groundwater collected from monitoring wells is a major component of the GSCP. Table 2 lists the major elements of the Phase 1 laboratory testing program. This testing program is being undertaken by Canadian universities (University of New Brunswick - iodine diffusion testing, University of Ottawa - porewater extraction, environmental isotopes, radioisotopes, noble gases, University of Waterloo tritium analyses, University of Western Ontario - free swell testing, Laurentian University - abrasiveness testing), government labs (CANMET uniaxial compression testing), private labs (Activation Laboratories, Ontario solid core analyses and porewater/groundwater analyses, Core Laboratories, Texas petrophysical testing), and international research facilities (University of Bern, Switzerland porewater extraction, noble gases, core analyses and Paul Scherrer Institut, Switzerland radioiodine diffusion under anoxic conditions). Table 2. Summary of Phase 1 GSCP laboratory testing programs Media and Geoscience Discipline Core - Geological Core - Petrophysical Core - Hydrogeological Core - Geomechanical Core Porewater Extraction Porewater & Groundwater Laboratory Tests Mineralogy (XRD), pore structure (SEM, EDS), petrography (thin sections), lithogeochemistry Effective diffusion coefficients and porosities (through diffusion and X- ray radiography), gas permeability, effective & total porosities, fluid saturations, bulk density, capillary entry pressure, pore size distribution (Hg injection). Cation exchange capacity, sorbed cation populations, noble gases, BET + exchange isotherms Uniaxial compression (with P & S wave velocity & acoustic emission), free swelling, CHERCHAR abrasiveness index. Crush and leach, forced advection, ultracentrifugation, sequential heating/vacuum distillation, core out diffusion, diffusional equilibration Master variables and major ions, fluid density, trace elements and environmental isotopes ( 18 O, 2 H, 3 H, 87 Sr), radioisotopes ( 129 I, 36 Cl, 14 C, 4 He), gases and noble gases (Rn, He, Ar, Ne, N 2, CH 4), and drill water tracers (Na Fluoroscein, 3 H) 5.6 Descriptive Site Models and Geosynthesis The data collected from all Phases of the GSCP investigations will be used to develop descriptive geological, hydrogeological and geomechanical models of the DGR site and surrounding area. The geologic site model will describe the 3-D spatial distribution of all important geological formations and structural features within the Paleozoic and upper Precambrian bedrock units and will provide the basic framework for the development of descriptive hydrogeological and geomechanical site models. Figure 8 illustrates a simple representation of the current geological site model for the DGR. 633

7 680 m Limestones Upper Dolostones Shales Figure 8. Simplified descriptive geological site model for DGR. Descriptive hydrogeological and geomechanical site models will describe the 3-D spatial distribution of hydrogeological and geomechanical properties and conditions. The hydrogeological site model will describe the distribution of groundwater flow and radionuclide transport and attenuation properties and processes of the bedrock units that will host and overlie/underlie the proposed DGR. The geomechanical site model will describe the distribution of relevant geomechanical parameters including in situ stresses, rock peak and residual strength, elastic parameters, joint characteristics, swell and creep parameters, rock mass classifications, etc. In short, the descriptive hydrogeological and geomechanical site models will provide the information necessary to support safety assessment, environmental assessment and repository engineering design functions. Geosynthesis is the overall integration of all project data, descriptive geoscientific site models, supporting numerical analyses and complementary off-site and regional geoscientific studies necessary to support preparation of the DGR environmental assessment and regulatory license applications. Geosynthesis is an essential component in the development of a basis to understand the long-term performance of the DGR concept. It is an activity that is conducted throughout the entire site characterization work program and involves coordinated and collaborative efforts of specialists from all relevant disciplines. 6 PHASE 2 AND PHASE 3 GSCP ACTIVITIES Following the completion of Phase 1 GSCP work and assuming favourable results, Phase 2 and Phase 3 investigations will be defined and completed. Current plans for Phase 2 investigations include the drilling of two deep vertical boreholes to the Precambrian basement outside of the proposed DGR footprint to triangulate the attitude of sedimentary formations. During the drilling of these boreholes opportunistic groundwater sampling will be completed and recovered core will be logged, photographed, preserved and tested in accordance with procedures established in Phase 1. Major geological, hydrogeological and geomechanical field and laboratory activities undertaken in Phase 1 will be conducted in Phase 2. In addition, stress testing using overcore and hydraulic fracturing methods will be undertaken in one of the deep boreholes drilled in Phase 2. An enhanced laboratory testing program will address issues of radionuclide migration and more complete characterization of geomechanical properties of the DGR host rock including standard index tests, triaxial strength and deformation properties, and creep parameters. Phase 3 GSCP investigations will focus on the drilling of two to three deep inclined or deviated boreholes to investigate potential sub-vertical structural features in the Ordovician shales and argillaceous limestones that may be identified by 2-D seismic reflection surveys and Phase 2 work. Similar to Phase 2, major field and laboratory testing programs in Phase 3 will include the elements described for Phase 1 investigations (e.g., borehole geophysical logging, borehole hydraulic testing, multi-level casing installation and monitoring, laboratory geological, hydrogeological, geomechanical and petrophysical testing, etc.). ACKNOWLEDGEMENTS The ongoing review and advice provided by the Geoscience Review Group (Dr. Joe Pearson, Groundwater Geochemistry; Dr. Andreas Gautschi, Swiss National Cooperative for Disposal for Radioactive Waste; Dr. Derek Martin, University of Alberta; and Mr. Jacques Delay, Agence Nationale pour la Gestion de Déchets Radioactifs), are gratefully acknowledged. Dr. Dougal McCreath, Laurentian University, Dr. Ian Clark, University of Ottawa and Mr. Richard Beauheim, Sandia National Laboratories, provided guidance and direction on the development of the geomechanical, geochemical and borehole hydraulic testing programs, respectively, in the GSCP. REFERENCES Armstrong, D.K. and Carter, T.R., An updated guide to the subsurface Paleozoic stratigraphy of southern Ontario, Ontario Geological Survey, Open File Report Golder Associates Ltd., LLW Geotechnical Feasibility Study, Western Waste Management Facility, Bruce Site, Tiverton, Ontario, Report , January. 634

8 Intera Engineering Ltd., Geoscientific Site Characterization Plan, OPG s Deep Geologic Repository for Low and Intermediate Level Waste, Report, OPG Report # REP R00, April. Ontario Power Generation Inc., Project Description, Deep Geologic Repository for Low and Intermediate Level Radioactive Wastes, OPG Report # REP , November. 635