WM’03 Conference, February 23-27, 2003, Tucson, AZ DEVELOPMENT OF THE SWEDISH DEEP REPOSITORY FOR SPENT NUCLEAR FUEL IN CRYSTALLINE HOST ROCK Stig Pettersson, Eva Widing Swedish Nuclear Fuel and Waste Management Co (SKB) Box 5864, SE-102 40 Sweden Phone: +46 8 459 8528, Fax: +46 8 667 3648 ABSTRACT The Swedish Nuclear Fuel and Waste Management Company, SKB, has developed a system that ensures the safe handling of all kinds of radioactive waste from the Swedish nuclear power plants for a long time period ahead. The keystones of this system are: • A transport system with the ship M/S Sigyn which has been in operation since 1983. • A central interim storage facility for spent nuclear fuel, CLAB, in operation since 1985. • A final repository for short-lived, low and intermediate level waste, SFR, in operation since 1988. In Sweden, the preferred method for final disposal of spent fuel is to encapsulate it in copper canisters and dispose them in a deep geological repository in crystalline host rock. SKB is planning to build an encapsulation plant adjacent to the central storage for spent fuel, CLAB. The siting for the deep repository has not yet been selected. A siting program with feasibility studies was completed in 2001. Early 2002 SKB received the necessary permits to start the site investigation at two potential sites for siting of the deep repository in Sweden. The site investigation at these sites started early 2002 and will be completed during 2007. Over the years, a number of generic studies of the layout of the operational area(s) above ground and underground facilities have been performed. During the site investigation phase the deep repository will be developed to conceptual design status and a number of design studies will be performed. These design studies are called Design Justification Statements (DJS). One important DJS is the selection of access routes from the ground level to the disposal level at tentatively 500 m depth and that study will be completed shortly. The repository design and layout of the disposal areas will be based on site specific conditions and results from demonstration of handling and equipment for canisters, buffer and backfilling. Some of these demonstrations have already been performed at Äspö HRL but additional development and testing will be needed. The design and repository engineering work during the site investigation phase will be presented in this paper. The long-term safety of the repository will also be determined based on the arrangement of the underground area of the repository and the site-specific geological conditions. The deep repository facility description and the long-term safety assessment are two important reports that will be included in the licence application for siting and construction of the deep repository. The time needed for receiving the necessary permits is estimated to about two years. The construction time including installation and final commissioning of equipment is estimated at about 6 years. Deposition of copper canisters with spent fuel is planned to start late 2015 and end about 2050.
Transcript
WM’03 Conference, February 23-27, 2003, Tucson, AZ
DEVELOPMENT OF THE SWEDISH DEEP REPOSITORY FOR SPENT NUCLEAR
FUEL IN CRYSTALLINE HOST ROCK
Stig Pettersson, Eva Widing Swedish Nuclear Fuel and Waste Management Co (SKB)
The Swedish Nuclear Fuel and Waste Management Company, SKB, has developed a system that ensures the safe handling of all kinds of radioactive waste from the Swedish nuclear power plants for a long time period ahead. The keystones of this system are:
• A transport system with the ship M/S Sigyn which has been in operation since 1983.
• A central interim storage facility for spent nuclear fuel, CLAB, in operation since 1985.
• A final repository for short-lived, low and intermediate level waste, SFR, in operation since 1988.
In Sweden, the preferred method for final disposal of spent fuel is to encapsulate it in copper canisters and dispose them in a deep geological repository in crystalline host rock. SKB is planning to build an encapsulation plant adjacent to the central storage for spent fuel, CLAB. The siting for the deep repository has not yet been selected. A siting program with feasibility studies was completed in 2001. Early 2002 SKB received the necessary permits to start the site investigation at two potential sites for siting of the deep repository in Sweden. The site investigation at these sites started early 2002 and will be completed during 2007. Over the years, a number of generic studies of the layout of the operational area(s) above ground and underground facilities have been performed. During the site investigation phase the deep repository will be developed to conceptual design status and a number of design studies will be performed. These design studies are called Design Justification Statements (DJS). One important DJS is the selection of access routes from the ground level to the disposal level at tentatively 500 m depth and that study will be completed shortly. The repository design and layout of the disposal areas will be based on site specific conditions and results from demonstration of handling and equipment for canisters, buffer and backfilling. Some of these demonstrations have already been performed at Äspö HRL but additional development and testing will be needed. The design and repository engineering work during the site investigation phase will be presented in this paper. The long-term safety of the repository will also be determined based on the arrangement of the underground area of the repository and the site-specific geological conditions. The deep repository facility description and the long-term safety assessment are two important reports that will be included in the licence application for siting and construction of the deep repository. The time needed for receiving the necessary permits is estimated to about two years. The construction time including installation and final commissioning of equipment is estimated at about 6 years. Deposition of copper canisters with spent fuel is planned to start late 2015 and end about 2050.
WM’03 Conference, February 23-27, 2003, Tucson, AZ
INTRODUCTION
The responsibility for the management of the spent nuclear fuel, as well as for other types of radio-active waste from nuclear power production, lies with the operators of the nuclear power plants, i.e. the four nuclear utilities. The utilities have jointly formed SKB, the Swedish Nuclear Fuel and Waste Management Company, to safely manage the spent fuel and radioactive waste from the reactors to final disposal. The task of SKB is thus to plan, construct, own and operate the systems and facilities necessary for transportation, interim storage and final disposal.
SKB has developed a system that ensures the safe handling of all kinds of radioactive waste from the Swedish nuclear power plants for a long time period ahead. The keystones of this system are:
• A transport system with the ship M/S Sigyn which has been in operation since 1983.
• A central interim storage facility for spent nuclear fuel, CLAB, in operation since 1985.
• A final repository for short-lived, low and intermediate level waste, SFR, in operation since 1988.
However, new facilities will be required for safe management of all type of waste from operation and decommissioning of the nuclear plants including CLAB. The new facilities that are foreseen today are listed below: • An encapsulation plant for encapsulation of the spent fuel in copper canisters with a cast iron
insert. In SKB working plans the encapsulation plant will be located adjacent to CLAB. The application for the siting and construction of the encapsulation plant is scheduled for end 2005 and start of test operation with spent nuclear fuel is scheduled end 2015.
• For the supply of canisters with inserts to the encapsulation plant, a canister factory will be needed. The siting of the canister factory is not decided.
• A deep repository for encapsulated spent nuclear fuel in copper canisters will be needed. The siting of the deep repository is not decided but site investigation is ongoing at two sites in Sweden. This paper will describe the engineering and design work for the repository during the site investigation phase.
• The existing repository for low level operational waste (SFR at Forsmark) need to be expanded due to plans for extended operation time of the reactors.
• A final repository for decommissioning waste from the nuclear power plants, CLAB and the encapsulation plant will also be needed. The preliminary siting of this repository is at SFR at Forsmark.
• For the safe disposal of the low and intermediate level waste from the decommissioning of reactor vessel internals, so called core components, a repository will also be needed in the future. However, the intention is to store the core components in steel containers in rock vaults and the conditioning and disposal of this type of waste would take place some 30 years from now. The siting of this repository is not decided but it could be co-located at the deep repository or at SFR at Forsmark.
• A transport system with terminal vehicles, transport casks for spent fuel and canisters with encapsulated fuel in copper canisters will be needed. As new transport vessel replacing the existing vessel, M/S Sigyn, is also foreseen in the future.
PLANNING FOR DEVELOPMENT OF THE DEEP REPOSITORY
The site investigation at two potential sites for the deep repository in Sweden started during 2002. The site investigation will be completed during 2006 according to the main time schedule presented in
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SKB´s R&D program 2001. The application for necessary licenses and permits for the construction and operation of the repository at the selected site will be submitted to the authorities 2007. The required permits to start construction work are expected to be received early 2009. However the time schedule for the development of the deep repository is for obvious reasons uncertain due to the nature of the project. An extract of the main time schedule for the deep repository from the R&D program 2001 is shown in Figure 1 below.
Fig. 1. Time schedule for the development of the deep repository. During the site investigation phase that also include the license application period, the design of the deep repository will be developed from feasibility design to conceptual design. For parts of the repository, for instance ramp and shaft access area, also detailed design will be performed. During this period all necessary equipment for transport, handling of buffer material and canisters must be developed and tested. The testing of equipment has already started at our Äspö Hard Rock Laboratory. The excavation of the shaft and ramp down to repository level will take three to four years and during that time the detailed design of the remaining part of the repository can be performed.
The present capacity of the central interim storage CLAB is 5000 tonnes of uranium. An extension up to 8000 tonnes is under way. The Government gave permission for this expansion in August 1998. Construction of a second storage cavern has started and the work shall be finished mid 2004. The program for the extended capacity is co-ordinated with the time schedule for the encapsulation plant and the final repository. The capacity in CLAB will be sufficient for storing all nuclear fuel if the encapsulation and disposal will come into regular operation not later then 2023. If the regular operation would not start before that date new storage capacity would be required at CLAB. However, according to the present planning the regular operation could start about 2020.
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DESIGN PREMISES
General
The design premises for the repository for spent nuclear fuel have recently been compiled into one document. Previously the design premises were documented in many reports written at different occasions, by different experts and for different purposes. When the site investigations were initiated there was a need to compile the design premises for the different parts of the repository into one document and to structure the information. Furthermore the authorities required that SKB should account for how the requirements in laws and regulations are reflected in system design, and how results from performance and safety analyses are used in the design work. Another purpose of compiling the design premises into one document was to get a common basis for the design of the different parts of the repository. The Swedish reference alternative to handle spent nuclear fuel, and the alternative SKB intends to implement, is geological disposal according to the KBS-3-method. The main function of a KBS-3-repository is to isolate the spent fuel from man and environment. Secondly, if the isolation is broken, the repository shall retard the release of radionuclides. The spent fuel is after a period of interim storage encapsulated in watertight canisters and transported to the geological repository. The canisters are deposited at about 500 meters depth in crystalline bedrock and surrounded by a clay buffer containing swelling minerals. The different parts of a KBS-3-repository are shown in Figure 2.
Fig. 2. General principles of the Swedish deep repository with barrier systems.
Society’s requirements on safety and protection of man and environment as they are expressed in laws, regulations and international treaties constitute the basis for the design premises. The design requirements presume that geological disposal according to the KBS-3-method fulfils society’s requirements on a strategy to handle spent nuclear fuel. The design premises comprise:
− copper canister − bentonite buffer − backfill − hard rock facility including:
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− deposition tunnels and holes − other rock cavities (ramp, shafts, drifts, halls and boreholes) − plugs
The report with design premises for the KBS-3-repository was completed at the end of 2002. During 2003 the design premises will be put into a database and simultaneously reviewed. This work is carried out in co-operation with Posiva (Finland) who also are planning a KBS-3-repository.
Requirement Compilation The design premises can be divided into requirements and constraints. On a general level the requirements can be divided into legal requirements and owners requirements. The legal requirements are derived from texts in laws, regulations and treaties. The owner requirements express desired technical feasibility and efficiency. From legal and owner requirements more specific requirements expressing desired function, capability or characteristic of the repository and its different parts can be derived. The latter are referred to as system requirements. The constraints comprise the characteristics of the spent nuclear fuel and the repository site and of concept dependent matters such as interfaces between the different parts of the repository, situations and processes occurring during construction and operation and post closure processes that may impact system performance. Based on the system requirements and the constraints design requirements are formulated. The design requirements express quantifiable factors such as loads, dimensions and qualities that govern the design of the different parts of the repository. Each design requirement must be linked to a system, legal or owner requirement and most of them are also associated to a constraint. The main system requirements of the KBS-3-repository are to isolate the spent fuel from man and environment and to retard the release of radionuclides to the environment. These requirements are derived from the legal requirement of a multi-barrier system with several barriers that complement each other to achieve safety. The corresponding system requirement for the canister is to enclose the spent fuel and prevent dispersion of radionuclides to the surroundings. The buffer shall contribute to isolation and retardation by only permitting transport of solutes via diffusion. The hard rock facility shall be constructed, operated and sealed in such a way that the bedrock at the repository site after closure of the facility provides a suitable and long term stable environment for the buffer and canister. For each part of the repository, design requirements are formulated based on the system requirements and a systematic examination of: − couplings to other parts of the repository, − situations and processes occurring during construction and operation and − post-closure processes that impact the long-term safety.
Use of Design Premises in Optimisation Studies The design premises comprise a compilation of relevant laws, regulations and treaties followed up by more specific system requirements and design requirements. When evaluating different possible system designs the fulfilment the design requirements can be used as basis for the comparison. Using the same basis for different optimisation studies will hopefully facilitate both the realisation and review. In the optimisation of the different parts of the repository and in the Design Justification Statements the requirements associated to the following issues are considered: − Long term safety after closure. − Safety during construction. − Safety during operation. − Environmental impact. − Technology and feasibility. − Costs and time schedule. − Retrieveability of canisters.
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Requirement Compilation
The requirements on a final repository can be listed according to their origin. • Institutional Requirements • Owner Requirements • System Requirements
Institutional Requirements
This consists of rules, prescribed in laws, regulations and governmental decisions. Specific for radioactive matters are the requirements from the Nuclear Safety Inspectorate and rules for radiation protection.
Other rules of the same character are those concerning all industrial enterprises, such as occupational safety and health and environmental protection rules.
Common to these requirements is that they are in general difficult to change and non-negotiable.
Owner Requirements
This group implies guidelines, based on governmental decision and legislation, the owners’ requirements for disposal and its implementation. It can also involve international agreements, outside the institutional ones.
These requirements can be modified and changed according to negotiations System Requirements
In this group SKB´s own requirements will be found, derived and justified from institutional and owners´ requirements for the selected concept in geological environment (crystalline rock). One of the basic rules is that the design shall be based on the KBS-3 concept with vertical deposition of the canister and buffer in deposition holes in the floor of the deposition tunnel, the KBS-3V concept.
The rules are often discussed, and they are subject to optimisation. They can be defined as system functions: e g what is the purpose of the system, how and when should it be executed.
For the optimisation of the repository and the preparation of Design Justification Statements for documentation of the design studies the following will be considered:
− Long term safety after closure. − Safety during construction. − Safety during operation. − Environmental issues. − Technology and feasibility. − Costs and time schedule. − Possibility to retrieve the canisters with spent fuel.
Requirements Directed to Different Parts of the System
Derived from the overall requirements discrete demands can be specified for the different parts of the disposal system. The system consists of the following parts: • Copper canister. • Bentonite buffer. • Tunnel backfilling. • Bedrock
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One additional important element in the system is the depth of the repository, which can be added as a part of the requirements on the bedrock.
Copper canister
The copper canister which provides the initial isolation of the spent fuel shall be gas and water tight. It shall be resistant against corrosion in the geological environment for a period in the order of 100 000 years.
The canister shall also be mechanically strong in order to withstand handling during disposal operations and forces in the repository, such as handling and deformation forces, internal pressure, hydrostatic pressure, swelling pressure of the surrounding buffer, loading of possible ice sheet, small bedrock movements.
Further requirements on the canister are that the sealing method must not weaken the long-term properties of the copper. The canister must be designed to prevent criticality, to give sufficient protection for radiation and a good thermal conductivity.
The canister should be free of surface contamination before being placed in the transport cask for transfer to the repository. In the repository only gamma and neutron radiation must be accounted for. However, in the operation procedure for the deposition of the canister, surface contamination control will be carried out on equipment that has been in contact with the canister, e g the internals of the transport casks, the shielded tube for the deposition machine and the handling tools.
Bentonite buffer
The function of the buffer is to separate the canister from the bedrock and prevailing processes. It shall adjust its form according to small movements of bedrock. The bentonite shall limit mass transfer around the canister so that diffusion is the only transport mechanism. It has to be chemically and mechanically stable and it must not imply any harmful effects on other barriers.
In order to fulfil these functions the buffer shall have a suitable bearing capacity and give a sufficient swelling pressure when absorbing water. The hydraulic conductivity should be low and the thermal conductivity shall be sufficiently high.
Bedrock movements shall be absorbed through a suitable plasticity in order to protect the canister.
The diffusion function shall be obtained by the ability of the buffer to act as a filter and sorption medium for particles, corrosion products, radionuclides and colloids and a sufficient chemical buffering capacity is needed. The chemistry has to limit the growth of micro-organisms. A suitable gas permeability is necessary.
Bedrock
The function of the bedrock is to isolate the repository from the biosphere and to provide protection against surface and near surface processes.
It shall provide favourable and predictable rock mechanical, chemical and hydro-geological conditions for the engineered barriers to retain their good isolation properties.
These requirements are obtained in the process of the selection of the repository site and in particular the bedrock block or blocks in which the disposal will take place. The studies of the proposed sites will focus on important bedrock properties. Among these properties can be mentioned deformation zones, the distribution of fractures and their characteristics, the chemistry and hydrology.
Tunnel backfilling
The function of the backfilling is to keep the buffer in position in the deposition hole and to support tunnel walls and ceiling.
The requirement on the backfilling is to enable the conditions to return as close as possible to those prevailing in the rock before construction. The backfilling must not imply harmful effects on other barriers. It must be chemically and mechanically stable, shall have sufficient density and low compressibility, low hydraulic conductivity and an ability to retain radionuclides (sorption capacity).
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These functions should be obtained by a careful selection of the materials and compaction methods, that provides sufficient swelling pressure.
Another demand on the backfilling is that it should make inadvertent intrusion to the repository difficult. RELATION BETWEEN SITE INVESTIGATIONS, REPOSITORY ENGINEERING AND SAFETY ANALYSIS.
Current Status of Site Selection
SKB is currently pursuing site investigations at two candidate sites for a deep repository for spent nuclear fuel. The sites are Forsmark about 170 km north of Stockholm and Oskarshamn in the south part of Sweden about 400 km from Stockholm. The investigations will be carried out in two stages, an initial investigation followed by a complete investigation, should the results after the initial stage be favourable. A preliminary safety evaluation is carried out for each site after the initial stage, based on available field data and preliminary layouts for the deep repository in this stage. The main objectives of the evaluation are:
• to determine whether it is recommendable to pursue a complete site investigation at the candidate site, based on the prospects of fulfilling long-term safety criteria,
• to provide feed-back to continued site investigations and site specific repository design, should it be decided to pursue the investigation and
• to identify site specific scenarios and geoscientific issues for further analyses.
A site description with a site model will be issued from the site investigation data. The description forms the base for a proposed repository layout. An important task for this description is to show the feasibility of constructing the repository at the suggested site and to describe any critical uncertainties in the rock properties which need to be further investigated.
The site model, the site understanding and the repository layout form important parts of the basis for preliminary safety evaluations and, at later stages, comprehensive safety assessments. Several safety-related analyses will be made during the investigation and the subsequent data treatment. This includes an analysis and implementation of earthquake respect distances and thermal calculations yielding minimum canister distances.
As more data emerge from field investigations, the site model, repository layout and safety evaluations are revised and refined. Information from safety evaluations is fed back to both field investigations and repository engineering throughout the site investigation phase. The preliminary safety evaluation is an important occasion for more detailed and formalised feedback.
The methods for pre-investigation of candidate sites have been tested at the Hard Rock Laboratory at the island of Äspö situated close to the Oskarshamn Nuclear Power Plant. The Äspö HRL is also used for development of detailed investigation methodology and tests of models for description of the barrier function of the host rock. Tests of construction methods as well as demonstrations of technology and function of important parts of the repository system are also conducted at Äspö HRL.
Site Selection Factors
In the selection of sites consideration has to be given both to the underground characteristics and to the conditions on the surface.
The factors of importance on the site ground surface are for instance topography and soil, communications and infrastructure. The land usage is the first point to be considered, e g environmentally protected areas which must not be disturbed.
The accessibility to the underground is a factor that concerns both the surface conditions and the bedrock characteristics. A ramp will require a certain width of the land, depending on how it is drawn and how it is adapted to the infrastructure.
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The bedrock must be studied regarding many aspects as mentioned in the requirement chapter. Investigations cover several geo-scientific areas, thermal properties, water transportation characteristics. The extent of acceptable rock must also be big enough to comprise the whole repository.
DESCRIPTION OF THE DEEP REPOSITORY
General
The purpose of this chapter is to give a description the design and construction of the deep repository. The capacity of the deep repository shall be large enough to accommodate all spent nuclear reactor fuel from the Swedish nuclear power program and is estimated at about 9 000 ton tonnes. That corresponds to about 4 500 copper canisters of which 200 to 400 is planned to be disposed during the initial operation phase and the rest during the regular operation.
The annual disposal rate is planned to be up to 200 canisters and it shall be possible to dispose one canister per working day.
The investment cost for the repository up to start initial operation is in the order of 4.5 billion SEK or 0.5 billion USD. The cost for the repository from start initial operation to the final backfilling and closure of the underground facilities and decommissioning of surplus facilities above ground is in the order of 9 billion SEK or 1 billion USD.
During the initial operation phase about 200 people will be involved. That number will increase to about 240 during regular operation as excavation of new deposition tunnels will be part of the operation of the repository.
Design and Construction of the Deep Repository
Over the years, a number of generic studies of the layout of the operational area(s) above ground and underground facilities of the repository have been performed.
The underground facility of the repository consists of a central service area, an area for deposition of canisters with spent fuel during the initial operation and a deposition area for regular operation.
The deposition tunnels needed for the initial operation will be excavated before start of deposition. Most of the deposition holes will also be bored. However, the excavation of tunnels for the regular operation will be made progressively. When the regular operation starts, some 10 tunnels have been excavated and prepared in one branch of the deposition area. About five tunnels will be filled in a year and in the meantime another five tunnels are excavated in the parallel branch. The works can then be shifted at intervals of approximately one year with stepwise excavation of some five tunnels until all the spent fuel has been deposited in some 100 tunnels.
Different access routes from the ground level to the repository level at 500 m below ground have also been investigated. The access routes studied are mainly by shafts only or a ramp for the heavy and bulky transports in combination with different service shafts. Further, the ramp alternative could be arranged as a spiral or as a straight ramp in combination with service shafts. For the straight ramp arrangement a secondary operational area would be located just above the central service area of the deep repository. However for the access with a spiral ramp only one operational area located perpendicular above the central service area. This alternative is illustrated in Figure 3.
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Fig. 3. Illustration of the deep repository with spiral ramp access
In the evaluation process of the access routes, the cost and time schedule for shaft sinking to repository level are being studied. The time needed for the access down to the repository level could be of great importance as well as the possibility to carry out rock characterisation work during the excavation of the shaft or the ramp. Other factors that may influence the selection between access routes are constructability, operational safety and long term safety.
SKB’s reference design entails that:
• The tunnels are excavated using conventional drill and blast technology.
• The canisters are deposited one by one in holes bored in the floors of tunnels.
• The canister will have full radiation shielding during all handling steps during the deposition process.
• The buffer consists of highly compacted pure bentonite.
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• Backfilling after completed deposition of canisters in a tunnel is made with a mixture of bentonite and crushed rock.
The drilling of deposition hole, emplacement of buffer material, emplacement of canister and backfilling of deposition tunnel is shown in Figure 4. The operational area comprises a terminal building for receiving transport casks containing encapsulated fuel, a production building for preparation of buffer and backfilling material, a supply building for electric power, offices, workshops and information building and a garage. The operational area also comprises an elevator and ventilation building over the elevator and ventilation shafts. About 0.5 km2 is needed for the operational area. Space for storage of excavated rock occupies a large portion of this area. Local conditions may influence the size and arrangement of the operational area.
Access and Escape Routes
Access to the underground facility is, in the reference case, achieved via two ordinary routes, the ramp and a shaft with one or two elevators. They also serve as escape routes and access ways for the fire brigade. An additional shafts used for exhaust air is placed in the far end of the deposition area for regular operation. In the deposition area for regular operation the investigation tunnels will be connected at the far end before start of operation. This means that there is always one escape route available
Deposition Tunnels
The deposition tunnels are linked to the service area by tunnels for transport, communication, ventilation and utility lines. The deposition holes are approximately 8 m deep and have a diameter of 1.75 m. The canisters are embedded in a buffer of highly compacted bentonite, delivered from the production building through the ramp or through a shaft.
The total excavated solid rock volume is in the order of 1,850,000 m3 and the excavated volume of the deposition tunnels is about 50 % of the total excavated volume.
In the reference case, the deposition tunnels are separated by 40 m and the spacing between the deposition holes is 6 m. The distance is determined by the need to limit the temperature on the canister surface and the bentonite buffer.
The deposition area for regular operation is designed for the disposal of about 4 100 canisters. This disposal capacity necessitates excavation of somewhat more than 100 tunnels. Each deposition tunnel will contain about 40 deposition holes. The tunnels are assumed to be straight and parallel. They are connected by two main communication tunnels, which are forming a loop with the rest of the tunnel system. The length of the deposition tunnels has been set to 265 m during the feasibility design. The actual length will adjusted based of actual geological condition before the start of the excavation of the individual deposition tunnel. Due to the geological rock conditions the length of the deposition tunnel may be shorter but also longer.
In the real case the deposition area and the length of the deposition tunnels have to be adapted to the rock quality and properties, such as fracture zones, which can result in a division into smaller parts. Additional transport tunnels will then connect the parts.
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Fig. 4. Drilling of deposition hole, emplacement of bentonite buffer, emplacement of copper canister and backfilling of deposition tunnel
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Central Service Area
The central service area consists of seven halls and a tunnel-shaft arrangement for rock masses and backfill material. The area covers about 275 m*110 m. All halls are connected at each end to two parallel transport tunnels.
The halls are in order:
1 One hall for reloading the canisters from the transport cask to the radiation shielding tube.
2 One hall containing a workshop and a storage.
3 One elevator hall connected via a shaft to the elevator and ventilation building on ground level.
4 One ventilation hall, to which the fresh air is conducted from the ventilation building on ground level. The air is distributed to the various halls, to the ramp and to the deposition areas.
5 One electric power hall where the power from ground is transformed and distributed to various needs in all parts of the underground area.
6 One vehicle hall for parking vehicles when not in operation.
7 One rock drainage hall with two basins for collecting drainage water from all the underground area.
Access Ramp
During the regular operation period the ramp shall be used for all transports between the ground level and the deposition level. An exception is the transport of personnel that is mainly made via the elevator in the elevator shaft to the elevator hall.
Shafts
There are four shafts connecting the surface level with the deposition level: one elevator shaft and two ventilation shafts to the central service area and the one exhaust air shaft at the far end of the repository area. All ventilation shafts would be excavated by the raiseboring technique. The elevator shaft with the larger diameter will be excavated with raiseboring technique and stoping.
A fifth shaft may be excavated as a blind shaft and the excavation work would start at the same time as for the ramp. A skip will be installed in this shaft and the shaft would be used for transport of excavated rock, instead of using electrical or diesel driven truck in the ramp. The skip shaft would add additional cost for the repository but it will also shorten the construction time with 18 to 24 months.
SUMMARY OF THE REPOSITORY ENGINEERING WORK DURING THE SITE INVESTIGATION PHASE
The site investigation will be done in two steps, initial site investigation during 2002 to 2004 and what is called “complete site investigation” during 2005 to 2007. The results from the site investigation will be presented to the repository engineering group as different models for rock structure, hydrogeological model, etc, and all information will be included in a site description. Stress levels and direction of stresses in the rock mass will be presented as well as the thermal properties of the rock mass.
The adaptation of the above ground facilities and the deposition areas of the deep repository to the specific site condition will require long and hard work. The engineering aspect of the repository as an industrial facility is one aspect but the local people near the site and the local decision-makers in the community must also accept the repository.
The surface facility must be developed with the following guiding constrains:
− Detailed site plan for the use of the area in question.
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− Topography and ground conditions in the area.
− Transport and communication to the site (type of roads, suitable harbours for transport of bentonite and canisters with spent fuel, etc.).
− How can the supply of electrical power, water and sewage system, district heating, etc, be arranged.
− Must the facility be able to provide all kinds of service and maintenance or can local contractors be used.
The underground facility must be developed with the following guiding constrains:
− The arrangement of the above ground facility.
− The selected access route down to the disposal areas. What is the best option, transport in ramp for the heavy and bulky material or will it be only shaft access to the deposition areas.
− Site specific geological conditions:
• Geology with fracture zones.
• Thermal properties of the host rock that will be used for disposal of canisters.
• Geo-hydrological conditions.
• Rock mechanical conditions.
• Water chemistry at deposition level.
The geology of the host rock will determine the arrangement of the disposal areas due to the location of deformation zones and water carrying structures. The location of fractures and rock properties will determine the preferred arrangement and tunnel orientation taking into account of the tunnel system and the requirements and selection of rock support. The rock support could be bolting, shotcrete, steel or concrete lining, etc.
The thermal properties of the host rock will determine the size of the disposal areas in order not to exceed maximum canister surface temperature. The stress levels due to the temperature increase of the host bedrock must be calculated. The temperature of the host rock will also increase with the depth for the location of the deposition areas.
The geo-hydrological conditions in the host rock will determine the transport mechanism of the water and also the possibility to seal water in-leakage by grouting. The size and properties of the fractures will determine how easy or difficult it will be to seal the fractures to get acceptable working conditions during construction and operation of the repository. There will also be stringent requirements not to lower the ground water level in the vicinity of the repository.
The ground water pressure will affect the selection of design and construction of rock support and sealing plugs, etc.
The rock mechanical conditions and in-situ stresses will have an impact on the selected tunnel orientation with regard to the direction of dominant horizontal tensile stresses. The stress levels will increase with deeper location of the disposal areas and increase the risk for spalling.
The water chemistry at the disposal level will have an influence on the design and construction of the disposal areas. The salt content in the ground water will increase with depth and will have an impact on the buffer and backfilling material. To get the same swelling pressure in the backfilling, the bentonite content has to be increased. With higher salt content in the ground water, the corrosion of installations and equipment will be troublesome.
Other aspects, which are being treated, are environmental issues, technique, economy and possibility to retrieve the canisters with spent nuclear fuel.
A preliminary deep repository description for the conceptual phase (Layout D1) is scheduled for completion in the end of 2004 based upon the information from the initial site investigation. The site
WM’03 Conference, February 23-27, 2003, Tucson, AZ
description and the layout of the underground areas will be the base for a preliminary safety evaluation to confirm if the site would still be suitable and that the site investigation should continue or if the site has to be abandoned.
The layout of the facility will also be based on development and demonstration work carried out at SKB’s Äspö hard Rock Laboratory. The situation at Äspö is presented in a separate paper in this conference.
The deep repository description for the conceptual phase (Layout D2) is scheduled for completion early 2007 based on the result of the complete site investigation. The new site description and the revised layout of the underground areas for disposal of the encapsulated spent fuel will be the base for the long-term safety evaluation report which will be included in the licence application. The environmental impact report will also be part of the licence application as well as the repository safety operation report.
The work with the repository engineering will be intensive during the coming years. It will also be very challenging and will require that a number of options and alternative have to be investigated during the design selection process.
References:
1. SKB R&D report 2001, September 2001.
2. SKB report TR-01-03, December 2001 ”Integrated account of method, site selection and program prior to the site selection phase”.
3. SKB report R-01-57, December 2001, “Deep repository facility description – Access via straight ramp with two operational areas” (In Swedish).
4. SKB report R-02-19, April 2002, “Deep repository facility description – Access via shafts only and one operational area ” (In Swedish).
5. SKB report R-02-23, April 2002, “Deep repository facility description – Access via spiral ramp and one operational area” (In English).
6. Äspö Hard Rock Laboratory, Status Report 2002. WM´03 conference, secession 64, Underground Research Facility Update, Christer Svemar, Tommy Hedman, Stig Pettersson SKB (2003).
7. The Swedish program has entered the site selection phase, WM´03 conference, secession 22, Global Perspectives II, Peter Nygårds, Tommy Hedman and Torsten Eng SKB (2003).
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