THIRD CASE STUDY - SITE AND DESIGN DESCRIPTION

Report No: 06819-REP-01200-10124-R00

March 2004

Key Words: Third Case Study, safety assessment, repository, used fuel

For further information contact: Ontario Power Generation, Nuclear Waste Management Division 700 University Avenue, Toronto, Ontario, Canada, M5G 1X6

Fax: (416) 592-7336

P. Gierszewski M. Jensen P. Maak A. Vorauer Ontario Power Generation

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ABSTRACT

Report Number: 06819-REP-01200-10124-R00 Title: THIRD CASE STUDY - SITE AND DESIGN DESCRIPTION Author(s): P. Gierszewski, M. Jensen, P. Maak and A. Vorauer Company: Ontario Power Generation Date: March 2004 Abstract Disposal in a deep geologic repository is one approach for the long-term management of used CANDU fuel. Geological disposal isolates the wastes from people and the environment. Two major Canadian postclosure safety assessments of this concept have been previously completed – the Environmental Impact Statement case study in 1994 and the Second Case Study in 1996. The Third Case Study is a new postclosure safety assessment of the deep geologic repository concept for used CANDU fuel in the Canadian Shield. It differs from the two previous studies in that it considers a revised repository design and a new hypothetical site. The hypothetical reference site is located within an 84 km2 watershed with a surface topography and surface water features characteristic of those of a Canadian Shield setting. A surface lineament analysis was completed using site-specific information typical of that which would be available during the initial stages of a site investigation. Using the results of this analysis and a novel geostatistic approach, one realization of an explicit discrete fracture network was defined across the watershed, remaining consistent with the surface expression of large scale discontinuities. Other geosphere characteristics were defined based on general Canadian Shield data. The specific values selected were intermediate within the range of available data relevant to siting. The site is considered to be representative of a potential Canadian Shield setting, but neither the best nor worst possible case. The reference design concept is a 2 km2 single-level repository located at about 670 m depth. The repository design is based on current Canadian, multi-barrier, design concepts. Approximately 11,200 containers would be emplaced horizontally in tunnels excavated within the crystalline rock. The container concept is based on a copper-shell corrosion barrier with steel inner-vessel support structure. Each container holds 324 bundles of used CANDU fuel, for a total capacity of 3.6 million fuel bundles. The containers are surrounded by a highly compacted buffer jacket, buffer blocks, backfill blocks and a light backfill seal adjacent to the rock in the emplacement rooms. After a room is filled, it is sealed from the access tunnel by a concrete and clay bulkhead. It is estimated that the emplacement of the containers will take about 30 years. It is assumed that the repository will be actively monitored for another 70 years, and then all remaining access shafts and tunnels would be sealed and the repository fully closed. This report provides a description of the site and engineering design for the hypothetical repository. These were developed in sufficient detail to support the Third Case Study safety assessment.

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TABLE OF CONTENTS

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ABSTRACT............................................................................................................................... iii

1. INTRODUCTION................................................................................................. 1 1.1 BACKGROUND.................................................................................................. 1 1.2 REPORT OUTLINE............................................................................................. 3

2. REFERENCE SITE ............................................................................................. 4 2.1 SITE CHARACTERIZATION............................................................................... 4 2.1.1 Surface Features ................................................................................................ 5 2.1.2 Fractures and Permeability ................................................................................. 6 2.1.3 Geosphere Properties ......................................................................................... 9 2.1.4 Groundwater flow................................................................................................ 9 2.2 REPOSITORY LOCATION ............................................................................... 12 2.2.1 Site Screening................................................................................................... 12 2.2.2 Particle tracking ................................................................................................ 13 2.2.3 Reference Vault Location .................................................................................. 16

3. WASTE FORM.................................................................................................. 18

4. CONTAINER..................................................................................................... 19

5. REPOSITORY LAYOUT ................................................................................... 22

6. SURFACE BIOSPHERE................................................................................... 26

7. SUMMARY........................................................................................................ 27

REFERENCES......................................................................................................................... 28

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LIST OF TABLES Page

Table 2.1: Site screening technical factors applied to the subregional area ............................ 12 Table 3.1: Used fuel wasteform parameters............................................................................ 18 Table 4.1: Container internal parameters ................................................................................ 19 Table 4.2: Container external parameters ............................................................................... 19 Table 5.1: Emplacement room parameters ............................................................................. 23

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LIST OF FIGURES Page

Figure 1.1: Illustration of the multi-barrier deep geologic repository concept considered in the

Third Case Study. A human figure is shown for scale........................................................ 2 Figure 1.2: Report structure for the Third Case Study. .............................................................. 3 Figure 2.1: Perspective view of hypothetical 84 km2 subregional area; the repository location

(not shown) is near the southeastern corner. .................................................................... 5 Figure 2.2: Perspective view of 84 km2 subregional area showing: a) intersection of major

fractures with the surface; and b) all discrete fractures and surface water features. ......... 7 Figure 2.3: Range of permeability variation with depth considered in the Third Case Study: a)

compared to data from the Whiteshell and Atikokan areas of the Canadian Shield (Stevenson et al. 1996, Ophori and Chan 1996); and b) compared to profiles from the AECL EIS and Second Case Studies. .............................................................................. 8

Figure 2.4: (a) Fracture distribution and (b) groundwater linear velocities at an elevation of -300 mASL for the Case 2 (intermediate permeability) rock. ................................................... 11

Figure 2.5: Subregional area showing the projected location of 5 candidate repository locations (A to E). ............................................................................................................................ 13

Figure 2.6: Advective time-of-transit contours at (a) -100 mASL and (b) -300 mASL for Case 3 (higher permeability) rock (roughly 520 and 670 m depth, respectively). Squares show candidate repository locations. ......................................................................................... 14

Figure 2.7: Minimum advective transit time from vault to surface for five candidate repository locations, as a function of vault depth below ground surface. ........................................... 15

Figure 2.8: Perspective view of the area around the repository location, showing the discrete fracture model in more detail. ........................................................................................... 16

Figure 2.9: Hydraulic conductivity variation around repository (Case 2 permeability profile).... 17 Figure 2.10: Particle tracks from repository location to surface for Case 2 (intermediate)

permeability rock. ........................................................................................................... 17 Figure 4.1: Container design showing copper outer shell, inner steel vessel, and fuel

assemblies inside support tubes (Russell and Simmons 2003). ....................................... 20 Figure 4.2: Illustration of fuel bundle support basket and bundles. Three of these baskets are

placed in each container................................................................................................... 21 Figure 5.1: Perspective view of section of emplacement room showing in-room emplacement

arrangement with two rows of containers in an elliptical room cross-section. ................. 23 Figure 5.2: Cross-sectional view of an emplacement room showing materials and dimensions.

All dimensions are in mm.................................................................................................. 24 Figure 5.3: Plan view of vault showing the emplacement rooms, access tunnels and general

facility support areas. All dimensions are in m. ................................................................ 25 Figure 6.1: Illustration of Canadian Shield surface biosphere similar to those assumed at

reference site.................................................................................................................... 26

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1. INTRODUCTION 1.1 BACKGROUND Two major Canadian case studies have been completed on the postclosure safety assessment of the deep geologic repository concept for disposal of used CANDU fuel in the Canadian Shield - the Environmental Impact Statement (EIS) case study (AECL 1994; Goodwin et al. 1994) and the Second Case Study (SCS) (Goodwin et al. 1996). The Third Case Study (TCS) considers a third variation of the deep geologic repository concept, as shown in Figure 1.1. The previous EIS study considered titanium alloy containers with 72-fuel-bundle capacity placed vertically into boreholes along the vault rooms, and assumed the repository was located in sparsely-fractured granitic rock with very low permeability. The SCS considered 72-bundle copper containers placed horizontally within the vault rooms, and assumed the repository was located in granitic rock with substantially higher permeability. In contrast, the TCS uses an updated copper container with 324-bundle capacity, placed horizontally within the vault rooms. It assumes the repository is located in granitic rock that is characterized by an intermediate permeability and a geostatistically-generated discrete fracture network. The main objective of the Third Case Study is to assess key aspects of the postclosure safety of a deep geologic repository based on a current Canadian design concept. A second objective is to test and evaluate current assessment methods and tools, with an aim to determine where future development may be important. The study results should be considered within the following context. • The Third Case Study focuses on three key scenarios - design base, defective container

(groundwater transport) and inadvertent human intrusion - and it evaluates radiological doses to humans. Prior studies have identified these as important aspects of postclosure safety. An assessment of a real site would also consider other scenarios and impacts.

• The Third Case Study is based on a specific repository design and hypothetical site

concept. The geosphere is considered representative of a potential Canadian Shield setting, and not necessarily to be representative of the best (nor worst) features of a potential site.

• The analyses are appropriate for the level of information that would typically be available for

a candidate site during the site evaluation stage but prior to exploratory drilling.

• The Third Case Study incorporates a geostatistical approach for the creation of a geosphere model. The safety assessment uses the same hydrogeological numerical code as was used in the development of the site model by the geoscience group. This approach represents a significant improvement in how the geosphere model and its uncertainty could be incorporated into the assessment.

The results of this study provide a basic test of postclosure safety for this repository concept and hypothetical site, and a basis for future iterations in which progressively more topics can be addressed. The results of a similar study for a real site would provide information on whether the expected safety margins are sufficient to proceed with exploratory drilling as part of a staged site evaluation program.

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Figure 1.1: Illustration of the multi-barrier deep geologic repository concept considered in the Third Case Study. A human figure is shown for scale.

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1.2 REPORT OUTLINE The present report is one of several documents supporting the Third Case Study postclosure safety assessment, as illustrated in Figure 1.2. These reports are intended for a technical audience familiar with nuclear waste disposal topics. An important aspect of the present study was the transfer of a realistic site conceptual model from the geoscience group to the safety assessment group. This was accomplished in part through the adoption of a common computer model for reference flow and transport analyses. This report describes the geologic setting and the engineering design concept for the hypothetical repository considered in the Third Case Study. These were developed in sufficient detail to support the Third Case Study safety assessment. The report is organized as follows: • Section 2 describes the site-specific geologic setting; • Section 3 describes the characteristics of the used fuel waste form; • Section 4 describes the used fuel container; and • Section 5 describes the repository layout.

Figure 1.2: Report structure for the Third Case Study.

Third Case StudyPostclosure Safety Assessment Report

OPG 06819-REP-01200-10109

Site & Design DescriptionOPG 06819-REP-01200-10124

Features, Events and ProcessesOPG 06819-REP-01200-10125

Reference Data and CodesOPG 06819-REP-01200-10107

Defective Container ScenarioOPG 06819-REP-01200-10126

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2. REFERENCE SITE A key aspect of the Third Case Study is the approach used to develop a hypothetical site with characteristics and attributes that closely match those of a typical Canadian Shield crystalline rock site. Fundamental in this approach is a demonstration of how critical features of the geosphere model (boundary conditions, discrete fracture network) are supported by surface-based data that would likely be available at the early stage of a site investigation, as well as results of supporting regional-scale numerical modelling. The characteristics of the geologic setting as needed for purposes of this postclosure safety assessment are described in two parts: • characteristics of the surface and subsurface geologic setting around the reference site; • location, depth and orientation of the repository within this geologic setting. Note that consideration of the hypothetical site does not include social, economic or environmental factors since the focus of this study is on illustrating the technical aspects of postclosure safety. 2.1 SITE CHARACTERIZATION A reference subregional area for the hypothetical repository was to be selected based on the following guidelines: • subregional area of 25-100 km2 in size; • minimize topographic gradients to 0.005 or less (i.e., relatively flat surface); • relatively few surface water bodies (i.e., less than 25% of surface area); • subregional area defined by flow divides as predicted through regional flow simulations; • allows repository to be positioned away from boundaries (e.g., prefer square area). In order to provide the hypothetical repository site with a realistic representation of surface topography and hydraulic boundary conditions, the subregional model domain was chosen from among several candidate sites within the 5700 km2 regional model domain investigated by Sykes et al. (2003) as part of the Regional Scale Groundwater Flow study. In this study, Sykes et al. used a Digital Elevation Model (DEM) and Digital National Topographic Service (NTS) maps representative of a Canadian Shield watershed to determine the elevation and water features of the top layer of the regional model. Various aspects of regional groundwater flow were investigated with the large-scale, horizontally-layered model, including the influence of anisotropic and spatially-correlated permeability fields, groundwater salinity distributions, and alternative glacial hydraulic boundary conditions on groundwater flow paths and rates. An important conclusion was that regional flow did not occur in the domain and that groundwater did not underflow the main rivers and their tributaries at any depth in the domain. Piezometric heads in all model layers were highly correlated to surface topography. Four candidate subregions were identified within the regional domain and their attributes, as listed above, were summarized (Sykes et al. 2004). Although any of the four subregions could have been selected, ultimately subregion 2 was selected for the Third Case Study based on water feature attributes and surface water boundary conditions. The characteristics of this subregional domain are described in the following sections.

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2.1.1 Surface Features The hypothetical 84 km2 subregional area and its surface topography are shown in Figure 2.1. Figure 2.1 shows the boundaries of the subregion, and surface water bodies (wetlands, streams, lakes) representing approximately 5% of the active surface area. The boundaries are hydrogeological divides, identified from the larger regional groundwater model. That is, there is no groundwater flow across these boundaries. The southern boundary is defined by a river, the eastern and western boundaries are defined by streams, and the northern boundary is defined by a topographic high. The surface elevation is seen to be gently dropping from north to south, with a variation across the region of about 50 m, which represents a topographic gradient of approximately 0.005. Note that elevations are referenced to metres above sea level (mASL).

Figure 2.1: Perspective view of hypothetical 84 km2 subregional area; the repository location (not shown) is near the southeastern corner. This figure shows the relatively flat topography typical of the Canadian Shield, drained here by various streams to a river along the southern boundary. Lakes and rivers are dark blue; wetlands are light blue. The surface elevation is defined in metres above sea level (mASL). Contour lines are at 5 m intervals.

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2.1.2 Fractures and Permeability For the hypothetical Third Case Study site, Srivastava (2002a) completed a surface lineament analysis across the entire subregional area. This analysis identified major fracture patterns that were mainly coincident with surface drainage features. Through an approach described in Srivastava (2002a), lineament realisations honouring location, strike, trace length and area density characteristics could be extrapolated across the flow domain, particularly into areas of poor bedrock exposure. The resulting surface fracture pattern is shown in Figure 2.2a. The discrete fracture network for the hypothetical site was generated using a geostatistical fracture propagation procedure, the results of the TCS surface lineament analysis and lineament/fracture statistics for the Whiteshell Research Area (Srivastava 2002b). The surface lineaments were extended into a self-consistent set of fractures to depths of 1500 m using propagation conditions that represent both sensible and geomechanically plausible fracture behaviour (Srivastava 2002a). Of note is that the methodology preserved the surface fracture orientations and lengths, as well as fracture density distribution statistics. The resulting discrete fracture network, consisting of 553 curve-planar fractures, is illustrated in Figure 2.2b. This figure illustrates the complexity of the fracture network at the subregional level. Although not clear from these figures, the network consists of a large number of intersecting features within the first few hundred metres, and much fewer, larger and more vertical features extending to greater depths. Given that the fracture propagation procedure is geostatistical, the network illustrated in Figure 2.2b represents one realization of an ensemble of feasible discrete fracture networks that can be generated for the subregional domain. The methodology described in Srivastava (2002a) would allow alternative fracture sets to be generated to provide a basis for assessing the impact of fracture distribution uncertainty on the safety assessment. For the Third Case Study, such an iteration was not carried out and the one fracture set is assumed as reference. For a real candidate site, it is expected that during subsequent phases of the site characterization program, subsurface data from boreholes would become available, and the ensemble of plausible fracture networks would be iteratively refined using the same geostatistical methodology. Three potential permeability-vs-depth profiles were defined for the Third Case Study, based on the range of values that were obtained from Canadian Shield research sites. The permeability profiles are isotropic and horizontally layered. These simple profiles are consistent with the information that would be available at an early stage of site characterization. Also, regional scale groundwater flow modelling of a Canadian Shield setting (Sykes et al. 2003) indicated that the addition of anisotropic and spatially-correlated permeability fields to a horizontally-layered model did not significantly change the hydraulic heads in steady-state. The profiles of isotropic permeability for the bulk rock mass are shown in Figure 2.3, along with data from two Canadian Shield research sites (Whiteshell and Atikokan) and with the profiles considered in the EIS and Second Case Studies. The Case 1 and Case 3 permeability profiles can be seen to approximate the upper limits of the Whiteshell and Atikokan data respectively, and to span the range between the very tight rock considered in the EIS, and the more permeable rock considered in the Second Case Study. In all three cases, at repository depths of 500 to 1000 m, the host rock between major fractures would be considered as sparsely-fractured rock with permeabilities less than approximately 5 x10-18 m2. (Sykes et al. 2003

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Figure 2.2: Perspective view of 84 km2 subregional area showing: a) intersection of major fractures with the surface; and b) all discrete fractures and surface water features. The figure shows many near-surface fractures but relatively few deep fractures, consistent with a typical Canadian Shield setting.

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(b) Figure 2.3: Range of bulk rock permeability variation with depth considered in the Third Case Study: a) compared to data from the Whiteshell and Atikokan areas of the Canadian Shield (Stevenson et al. 1996, Ophori and Chan 1996); and b) compared to profiles from the AECL EIS and Second Case Studies. Permeabilities at 1000 m were projected to the 1500 m depth of the model domain.

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considered a similar range of permeabilities.) Case 2 represents an intermediate permeability case, and will be subsequently used as the reference case for analyses. All the major fractures generated for the TCS hypothetical subregion were assumed to be water conducting and to have a uniformly high permeability of 10-13 m2. This assumption is considered conservative; smaller values would be observed in a real setting at repository depth. 2.1.3 Geosphere Properties Since the Third Case Study site is hypothetical, the geosphere property data were obtained from the EIS and Second Case Studies. These data were based largely on field and laboratory studies of the granitic rock from the Lac du Bonnet batholith in the Canadian Shield. The specific parameter values are described in the TCS Reference Data and Codes report. The assumed site database includes: • host rock and fracture infilling mineralogy, • porosity, • groundwater chemistry, • colloid concentration, • sediment and overburden thickness, • diffusion coefficients, and • sorption properties. During an actual site characterization program, site-specific geosphere property data would be obtained during progressively more detailed phases and used to update the geosphere conceptual model. 2.1.4 Groundwater Flow The regional model study of Sykes et al. (2003) was used to define the lateral boundaries of the subregional domain. The northern model boundary was chosen based on a topographic divide, while the eastern, southern and western model boundaries were chosen coincident with rivers (Figure 2.2a). An implied assumption from the use of rivers as model boundaries is that water does not underflow these rivers (Sykes et al. 2004). The side and bottom boundaries of the subregional area flow model were selected as no-flow boundaries based on the results from the regional scale groundwater flow model. The surface boundary for the flow model considered two options. First, a fixed piezometric head boundary condition was assumed where the water table was set to follow the surface topography, at 1 m below surface. Second, variable recharge rates were assumed across the top of the domain to improve the correlation between calculated surface water discharge locations and those identified from the surface topography. Numerical tests on these options indicated that groundwater flow (except very near surface) was not strongly affected by plausible surface recharge rate assumptions. Consequently, for the purpose of the TCS, it was judged sufficient to use a fixed water table boundary condition that followed the topographical surface. As part of an actual site characterization program, detailed hydrological information would be available to support a refined groundwater flow model. In the present study, the groundwater flow model does not consider the effects of groundwater salinity. It is well established that the groundwater at depth in the Canadian Shield is quite

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saline (> 35 g/L TDS). The coupling of these higher density fluids with the low permeability rock results in sluggish flow systems that are diffusion dominated (Sykes et al. 2003). Therefore, we expect that the absence of salinity is a conservative assumption. That is, we expect more flow at depth in the Third Case Study model than would occur if salinity effects were included. Figure 2.4 provides an example of the steady-state groundwater flow regime at an elevation of -300 mASL in the subregional area (a depth of about 670 m at the repository location), based on the Case 2 (intermediate) rock permeabilities. The flow was calculated using FRAC3DVS (see TCS Reference Data and Code report), with freshwater properties and an ambient temperature of 20oC. In this figure, it can be seen that the predicted vertical groundwater velocities are very low (< 0.4 mm/a) in the sparsely fractured rock between the fracture zones and that the direction of flow within the fractures can be upward or downward (positive or negative values). Also, it can be seen by inspection that there are several locations at this depth horizon where a 2 km2 repository could be placed without crossing fractures.

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Figure 2.4: (a) Fracture distribution and (b) groundwater linear velocities at an elevation of -300 mASL for the Case 2 (intermediate permeability) rock (roughly 670 m depth at repository location). The arrows show the horizontal flow direction, while the colour contours show the (grid element average) vertical flow magnitude. The vault location is indicated as a square. The groundwater velocities are very low between the major fractures. Also note that the flow in fractures is not necessarily upwards.

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2.2 REPOSITORY LOCATION 2.2.1 Site Screening The hypothetical 84 km2 subregional area was analyzed in order to select a specific location - site, depth and orientation - for the deep geologic repository. Davison et al. (1994) identify 6 technical factors to assist with site screening: rock mass stability, mineral resources, geological setting, hydrology, degree of fracturing, and surface environment. The available information on the subregional area is briefly compared with these factors as described below in Table 2.1. In the absence of site specific information, most of these factors were assumed to be satisfied across the hypothetical subregional area. A key requirement of siting is the ability to maximize the distance between the repository and major fractures. From visual inspection of the subregional area, five locations were identified where a square, 2 km2 repository could be located at a depth of 500 to 1000 m with clear separation from the nearest fractures. The surface projection of these candidate locations is shown in Figure 2.5.

Table 2.1: Site screening technical factors applied to the subregional area

Criterion Comment Rock mass stability, seismicity and seismic risk

Stable rock and low seismicity are assumed for the sub-regional area.

Mineral resources and alternative use potential

No mineral resources are assumed within the subregional area.

Geological setting Plutonic rock extending close to the surface is assumed to exist everywhere within the subregional area.

Hydrology and hydrogeological setting

Surface topography is relatively flat across the subregional area. Groundwater chemistry is assumed to be similar across the area.

Degree of fracturing in the rock

The bedrock is considered to be sparsely-fractured rock at plausible vault depths, with major fractures distributed across the subregional area as described in Section 2.1.2.

Surface environment Environmental criteria are assumed to be satisfied everywhere within the subregional area, except the large wetlands in the south and north are assumed to be sensitive areas and should be avoided.

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Figure 2.5: Subregional area showing the projected location of 5 candidate repository locations (A to E).

2.2.2 Particle tracking In order to identify a reference location, particle tracking was used to evaluate the five candidate locations. Specifically, groundwater flow calculations were carried out across the subregional area using FRAC3DVS. From the groundwater velocities, Time-Of-Transit (TOT) contour maps were constructed using particle tracking based on MODPATH. These maps show the time a particle would take to reach the surface if released into the groundwater flow system at that location and depth. This particle tracking only follows advective pathways (i.e., no sorption, no diffusion), so is only an indication of how dissolved radionuclide contaminants would move. However, long particle tracking times are expected to indicate long contaminant travel times. Fractures were represented as blocks of equivalent transmissivity with dimensions of around 50 m. TOT contour plots were prepared for three representative repository depths - approximately 500, 650 and 900 m below surface. A depth of 1000 m was not considered since it did not appear to be necessary to go this deep to find a suitable location. (Note that calculations were made at a defined planar height relative to sea level, rather than depth below surface, since a "plane" of constant depth would undulate with the surface topography.) The Time-Of-Transit results are shown for the subregional area in Figure 2.6 at two depths for the Case 3 (higher) permeability profile. The transit times were longer for the other two cases with less permeable rock, so the Case 3 results were used for siting purposes, respecting the uncertainties that could exist at a real site, particularly early in the site characterization phase.

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Figure 2.6: Advective time-of-transit contours at (a) -150 mASL and (b) -300 mASL levels (roughly 520 and 670 m depth, respectively) for Case 3 (higher permeability) rock. Squares show candidate repository locations.

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The Time-Of-Transit results shown in Figure 2.6 show that there is considerable variation in transit time across the subregional area (3 orders of magnitude), reflecting the influence of the fractures and surface topography on the groundwater transport. Each of the five candidate locations encompasses areas of very long transit times, with some portion of shorter transit times. In some of the candidate vault locations as shown in Figure 2.6, the minimum time to surface applies only to a small fraction of the vault near a fast-flowing fracture. If these vaults were shifted slightly from their candidate locations, or their shape were changed from square to rectangular, then in some cases there could be a significant increase in the vault footprint exhibiting minimum transit time. However, for simplicity and consistent with the intent to choose an intermediate quality site, this option was not explored. For the five candidate vault locations, the minimum advective time-of-transit varies with depth, as shown in Figure 2.7. This figure illustrates the observation that much of the fracturing across the subregional area is within the top few hundred metres. Since fewer fractures extend below 500 m, larger areas of sparsely-fractured rock are available for siting the hypothetical repository. Therefore, there is a significant improvement in transit times below around 600 m depth. Figure 2.7: Minimum advective transit time from vault to surface for five candidate repository locations, as a function of vault depth below ground surface.

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2.2.3 Reference Vault Location As described above, candidate repository locations were identified within the subregional area where a square vault could be located without intersecting major fracture zones. The advective transit time was found to increase significantly at depths below 600 m. Since the Third Case Study is hypothetical, the selection of a candidate site location is somewhat arbitrary. Therefore, Location A at -300 mASL was selected as the reference vault location. The surface over the repository varies from 360 to 380 m, so the average repository depth is approximately 670 m. The vault is oriented around this location so as to align it away from the adjacent fracture zones. Specifically, the vault center is located at coordinates 7400 m Easting and 4500 m Northing, and the vault axis is oriented at 50o clockwise relative to the North direction (lower left location in Figures 2.5 and 2.6b). Within the range of repository locations possible across the subregional area, Location A is intermediate in features with respect to average TOT and proximity to fractures - neither the best possible nor the most pessimistic. Figure 2.8 provides a closer view of the fracture network in the vicinity of Location A, while Figure 2.9 shows the hydraulic conductivity profile (Case 2 – intermediate) used for groundwater velocity calculations. Particle tracks from the reference vault to the surface are shown in Figure 2.10. These provide an indication of the geometry of the advective transport paths in the vicinity of the vault, and of the surface discharge locations associated with water bodies. Figure 2.8: Perspective view of the area around the repository location, showing the discrete fracture model in more detail.

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Figure 2.9: Hydraulic conductivity variation around repository (Case 2 permeability profile).

Figure 2.10: Particle tracks from repository location to surface for Case 2 (intermediate) permeability rock. The surface discharge points (pink dots) occur primarily at surface water features (blue lines), notably to a lake (east of repository), to a river (south), and to a stream (northwest).

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3. WASTE FORM The reference waste form (Table 3.1) is a natural-uranium CANDU fuel bundle from a commercial nuclear power reactor in Canada. The repository is designed with the capacity to hold 3.6 million fuel bundles (Russell and Simmons 2003). There are presently a few variants on a CANDU fuel bundle in use or storage in Canada, notably the 28- or 37-element bundle, the standard or long length bundle, and bundles with or without CANLUB. Sensitivity studies by Tait et al. (2000) indicated that the radionuclide inventory per unit mass of fuel is not sensitive to these factors, and so the standard 37-element (Bruce) fuel bundle was selected as reference. The design of this bundle is summarized in Tait et al. (2000). The age of the fuel at emplacement in the repository will vary. For example, the earliest CANDU fuel dates to 1970, whereas the repository is unlikely to be open before 2035, leading to some fuel being over 60 years old at time of emplacement. The older the fuel, the lower its residual thermal power and radiation fields. For design purposes, the used fuel containers are assumed to have a mix of fuel bundles with an age of 30 years at time of emplacement. The irradiation history of a used fuel bundle can be characterized by its average power rating and its burnup. The reference fuel is assumed to have a power rating of 455 kW/bundle (Tait et al. 2000). (Higher power rating generally means more short-lived radionuclides.) The average burnup of existing fuel bundles from all OPG reactors is about 200 MWh/kgU (initial U). However, on a station-specific basis, the used fuel from Pickering Nuclear Generating Station has the highest average burnup of 220 MWh/kgU, and a 95% percentile burnup of 280 MWh/kgU (Tait et al. 2000). (Higher burnup generally means more long-lived radionuclides.) The reference used fuel disposal container holds 324 bundles. For a given container, and certainly for the vault as a whole, nuclide inventories will approach those for the overall average burnup. However, because containers are likely to be filled with bundles from one station, the average bundle burnup in some containers may approach 220 MWh/kgU. Therefore, for the Third Case Study, the reference fuel burnup is set to 220 MWh/kgU.

Table 3.1: Used fuel wasteform parameters

Parameter Value Comments Waste form 37-element fuel

bundle Standard CANDU fuel bundle used in Bruce and Darlington stations

Total number of bundles

3.6 million Sufficient capacity for life of present Canadian CANDU power reactors

Initial U-235 0.72 wt% Natural uranium is used to manufacture CANDU fuel.

Burnup 220 MWh/kgU Highest station-average burnup Fuel age at emplacement

30 years E.g. 10 years in pools, 20 years in dry storage

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4. CONTAINER For the Third Case Study, the used fuel container design illustrated in Figures 4.1 and 4.2 has been adopted as the reference (Russell and Simmons 2003, Maak 2003). Its main properties relevant to safety assessment are summarized in Tables 4.1 and 4.2. The container is made from oxygen-free low-phosphorous copper, which is durable under the conditions expected in the deep geologic repository. The main structural support is provided by a thick steel inner vessel. Inside this vessel are baskets holding used fuel bundles. Each container can hold 324 bundles. The containers are designed to retain their integrity for a minimum of 100,000 years (Maak 2001).

Table 4.1: Container internal parameters

Parameter Value Comments Bundles/layer 54 Hexagonal close-packed layer Number layers 6 Total number of fuel bundles

324

Mass U in container

6.22 MgU Initial mass (not burnup corrected)

Inner vessel outside and inside diameter

1116 and 924 mm

IV-324-hex design from Russell and Simmons (2003)

Inner vessel outside and inside length

3708 and 3390 mm

IV-324-hex design from Russell and Simmons (2003)

Inner vessel inside volume

2.2 m3 The geometric inner volume is about (π⋅0.462)⋅(3.4) = 2.3 m3, minus an allowance for filleting at the lids

Internal void volume

1.3 m3 Inner vessel contains about 0.8 m3 of UO2/Zircaloy and 0.1 m3 of internal basket steel

Table 4.2: Container external parameters

Parameter Value Comments Container outer diameter

1168 mm Oxygen-free phosphorous doped copper shell, IV-324-hex configuration

Container length 3867 mm Including lifting lugs Copper shell thickness

25 mm

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1168mm

Copper Head

Steel Head (159mm) Steel Bolt

Fuel Bundle (102mm x 495mm) Steel Basket Tubes

Steel Shell (96mm)

Copper Shell (25mm)

Copper Bottom (32mm)

Lifting Ring

3867mm

Figure 4.1: Container design showing copper outer shell, inner steel vessel, and fuel assemblies inside support tubes (Russell and Simmons 2003).

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Figure 4.2: Illustration of fuel bundle support basket and bundles. Three of these baskets are placed in each container.

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5. REPOSITORY LAYOUT The Canadian deep geologic repository development program is considering several conceptual designs for the repository design. Simmons and Baumgartner (1994) provide a good background description of factors to be considered for repository engineering in Canadian Shield. Both in-floor borehole and in-room container emplacement methods are under development. An in-room geometry has been adopted for the Third Case Study, similar to that used in the Second Case Study (Baumgartner et al. 1996) but modified for use with larger containers (Russell and Simmons 2003). Figure 5.1 shows a three-dimensional perspective of the room layout, and Figure 5.2 provides dimensions. Within the emplacement room, the containers are surrounded by a minimum of 0.5 m of buffer (as bentonite and bentonite-sand layers) and 0.5 m of backfill (as dense backfill and light backfill layers). The emplacement room parameters are listed in Table 5.1. The spacing between containers is based in part on maintaining a container surface temperature of less than 100oC. The initial design-basis calculations leading to the room dimensions in Table 5.1 were made with assumed constant thermal conductivities for the buffer and backfill. However, scoping studies with coupled thermal-hydraulic models have indicated that the moisture content of these materials, and therefore their thermal properties, will change. Thus, more detailed thermal analyses will likely result in some modification to the repository layout. However, for purposes of this Third Case Study safety assessment, it will be assumed that the present design meets the 100oC temperature limit. The emplacement rooms are placed in an overall vault layout that can be constructed safely, meets thermal and mechanical constraints, and provides reasonable access for movement of equipment and ventilation air. Ultimately, the vault layout must also be compatible with the site characteristics. For the hypothetical geosphere defined for the Third Case Study, a reference square vault has been sited between the major fractures. The configuration of this vault is shown in Figure 5.3. This square layout is consistent with the layout used in the EIS and SCS studies, and with recent engineering design studies (Russell and Simmons 2003).

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Table 5.1: Emplacement room parameters

Parameter Value Comment Number of containers in vault

11,232 3.6 million bundles, 324 bundles/container, with margin for incompletely filled containers.

Number of rooms 4 x 26 Containers per room 108 Two rows of 54 containers Container spacing 1.25 m Space between containers Room height/width 4.2 m/

7.14 m Based on minimum buffer/backfill thicknesses, sufficient space for emplacement, and 1.7 aspect ratio for minimizing rock stresses

Room length 278.5 m Length of room from concrete bulkhead is 54*(3.87+1.25 m) containers + 2*(1 m) buffer plugs

Room spacing 45 m Center-to-center

Figure 5.1: Perspective view of section of emplacement room showing in-room emplacement arrangement with two rows of containers in an elliptical room cross-section. The containers are surrounded by layers of buffer and backfill sealing materials.

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Figure 5.2: Cross-sectional view of an emplacement room showing materials and dimensions. All dimensions are in mm.

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Figure 5.3: Plan view of vault showing the emplacement rooms, access tunnels and general facility support areas. All dimensions are in m.

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6. SURFACE BIOSPHERE The Third Case Study biosphere is based on central Canadian Shield conditions. The topography is relatively flat, with a general slope from north to south. The granitic rock of the Canadian Shield extends close to surface throughout the site, with some sedimentary layers in the low-lying areas underneath the streams and lakes. The local watershed is bounded by streams on the east and west, and a larger river on the south (Figure 2.1). There are small lakes and wetlands in the general vicinity of the repository, notably a lake near the eastern corner. The water table is assumed to be fairly close to the surface. The site is not directly adjacent to either Hudson Bay or the Great Lakes. The present climate is assumed to be typical of a cool continental location. The annual average surface temperature is +5oC. Winds and rainfall are typical for the Canadian Shield, about 2.36 m/s annual average and 0.8 m/a. The surface ecosystem is temperate boreal. This is a relatively stable ecosystem for present climatic conditions. It is possible that global warming could cause changes to the climate, and therefore biosphere, over the next 1000 years. On a longer time scale (>10,000 years), another glaciation cycle could begin, starting with about 60,000 years of progressively cooler climate (Peltier 2002). After each glaciation, it is expected that a period with similar conditions would again occur. A more complete description of the characteristics of the Shield biosphere is provided in Davis et al. (1993), p.26. For the present study, the average Shield properties identified in Davis et al. (1993) and Zach et al. (1996) are assumed. The specific values are summarized in the TCS Reference Data and Codes report.

Figure 6.1: Illustration of Canadian Shield surface biosphere similar to those assumed at reference site.

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7. SUMMARY The Third Case Study is a preliminary postclosure safety assessment of the deep geologic repository concept for used CANDU fuel, based on a hypothetical site on the Canadian Shield. This report provides a description of the site and engineering design for the hypothetical repository. These were developed in sufficient detail to support the Third Case Study postclosure safety assessment. The hypothetical reference site located within a 84 km2 watershed with a surface topography and surface hydrology consistent with broad areas of the Canadian Shield. An explicit discrete fracture network was assumed for this watershed taking into account the surface topography as well as other characteristics of fractures on the Canadian Shield. Other geosphere characteristics (such as permeability) were defined based on general Canadian Shield data. Specific values were selected such that the site was intermediate in features relevant to safety siting, rather than the best possible. The design concept is a 2 km2 single-level repository located at about 670 m depth. Approximately 11,200 containers would be emplaced horizontally in tunnels excavated within the crystalline rock. The container is based on a copper-shell corrosion barrier with steel inner-vessel support structure. Each container holds 324 bundles of used CANDU fuel, for a total capacity of 3.6 million fuel bundles. The average burnup of the fuel bundles is 220 MWh/kgU, and they are 30 years old at the time of emplacement. The containers are surrounded by a highly compacted buffer jacket, buffer blocks, backfill blocks and a light backfill seal adjacent to the rock in the emplacement rooms. After a room is filled, it is sealed from the access tunnel by a concrete and clay bulkhead. It is estimated that the emplacement of the containers will take about 30 years. It is assumed that the repository will be actively monitored for another 70 years, and then all remaining access shafts and tunnels would be sealed and the repository fully closed. ACKNOWLEDGEMENTS S. Normani, R. McLaren, E. Sudicky and J. Sykes (U. Waterloo) kindly provided relevant model data from their Regional Scale Groundwater Flow numeric model. The subregional flow and particle track calculations were carried out by J. Avis and N. Calder (Intera Engineering).

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