ONDRAF/NIRAS Repository Concept for Category C Wastes First full draft report

Belgian agency for radioactive waste and enriched fissile materials Evolution of the Near-Field of the ONDRAF/NIRAS Repository Concept for Category C Wastes First full draft report Editor and principal author: Stephen Wickham, Galson Sciences Ltd NIROND-TR 2007-07E April 2008

ONDRAF/NIRAS NIROND-TR report 2007-07E GEOLOGICAL DISPOSAL PROGRAMME Evolution of the Near-Field of the ONDRAF/NIRAS Repository Concept for category B and C wastes First full draft report Editor and principal author: Stephen Wickham, Galson Sciences Ltd April 2008

This report was written on behalf of ONDRAF/NIRAS by: Editor and principal author: Stephen Wickham, Galson Sciences Ltd. It was reviewed by: Bennett, D., Galson, D., Hooker, P. (Galson Sciences Ltd) It was approved by: Dierckx A. (ONDRAF/NIRAS) Contact person and address: Stephen Wickham, Galson Sciences Ltd., smw@galson.sciences.co.uk Contact person at NIRAS/ONDRAF : r.gens@nirond.be Credits : The content of this report has been discussed with many ONDRAF/NIRAS and SCK CEN experts, who have been asked to verify whether the content properly reflects their original contribution, and to confirm, in each field of expertise, that the complete set of relevant studies has been properly taken into account. The following specialists have been involved in this exercise: Eef Weetjens, Xavier Sillen and Ann Dierckx: end-user perspective Eef Weetjens, Xavier Sillen and Xiangling Li: THM behaviour Maarten Van Geet: chemical perturbations in the EDZ Lian Wang: chemical processes in concrete Frank Druyts and Bruno Kursten: corrosion aspects Hughes Van Humbeeck: general design issues Robert Gens: overall consistency. Robert Gens is also an editor of the report. The storyboard diagrams in Section 3 were drawn by Liz Harvey (Galson Sciences Ltd) ONDRAF/NIRAS Avenue des Arts 14 1210 BRUSSELS BELGIUM www.nirond.be The data, results, conclusions and recommendations contained in this report are the property of ONDRAF/NIRAS. The present report may be quoted provided acknowledgement of the source. It is made available on the basis that it will not be used for commercial purposes. All commercial uses, including copying and re-publication, require prior written authorization of ONDRAF/NIRAS.

ONDRAF/NIRAS Document data sheet Document type Technical Report Publication date April 2008 Status of the document PD + IM IM MM First full draft report Series GEOLOGICAL DISPOSAL PROGRAMME Title Evolution of the Near-Field of the ONDRAF/NIRAS Repository Concept for Category C Wastes Author(s) of the primary document Stephan Wickham, Galson Sciences Ltd. Reference number N/A ONDRAF/NIRAS number NIROND TR-2007-07 E Author(s) of the integration module Order number 2007008527 Contract title IM identification number IM total no. of pages 0542d-1 N/A To be filled in for the management module only Author Distribution list Subcontractor identification GALSON N/A Date N/A MM identification number N/A MM total no. of pages N/A Analytical code 253-B61 FIDES code GCE11

Executive Summary This report provides a synthesis of the expected evolution of the engineered barrier system (EBS) in the ONDRAF/NIRAS concept for deep disposal of Category C waste (i.e., high-level radioactive waste (HLW) and spent fuel). It presents an up-to-date scientific view of the key processes involved and serves the following functions: It provides the scientific basis on which a reference model for the evaluation of repository safety can be constructed. It provides the necessary basis on which to construct scenarios for evaluation and demonstration of the safety case for deep disposal. It will be used in a formal exercise for checking completeness against a Features, Events and Processes list (FEP-list) specific to the Belgian disposal system. The reference model, development of scenarios for safety calculations and completeness checking will be reported in separate future projects. The report summarises the detailed phenomenological studies (currently available Level 5 Reports) within the ONDRAF/NIRAS Safety and Feasibility Case 1 (SFC-1) programme that are relevant to the EBS, and discusses the thermal, hydrological, mechanical, chemical, biological and radiation processes that may occur within the near-field. The report describes the key processes that may occur during repository construction and operation, and through the post-closure period. The report focuses on the disposal concept for HLW and spent fuel only. The expected evolution of the EBS concept for Long-Lived Low- and Intermediate-Level Waste (LILW-LL - Category B waste), and of the repository sealing system, will be presented in separate, additional reports. The report does not address spent fuel and HLW glass dissolution, radionuclide release and transport, or processes occurring within the damaged or disturbed zone of the Boom Clay, such as alkaline plume migration, because these processes will be reported in other SFC-1 documents. The report will be fully updated in future as further Level 5 reports and other literature become available. ONDRAF/NIRAS is considering the feasibility of disposing of LILW-LL, HLW and spent fuel in a deep geological repository excavated in the Boom Clay formation. The disposal concept for HLW and spent fuel is based on ensuring complete containment of the radioactivity during the thermal phase when temperatures in the repository will be significantly higher than those of the surrounding host rocks because of radioactive decay of the waste. The thermal phase corresponds to the period over which the performance of safety-critical disposal system components, primarily the retardation capability of the Boom Clay, could be compromised by the thermal transient, and will last for at least hundreds of years after emplacement of vitrified HLW, and possibly up to a few thousand years for spent fuel. The concept also seeks to avoid dissolution of spent fuel under radiolytically-induced oxidising conditions. A key component of the EBS design for HLW and spent fuel is the supercontainer. The supercontainer has been designed on the basis of the Contained Environment Concept, the intention of which is to establish and maintain a chemical environment around the overpack that is favourable to achieving the desired performance (containment of the radioactivity NIROND-TR 2007-07 E, April 2008 i

during the thermal phase). In the supercontainer, containment is achieved by placing the waste in a carbon steel overpack and surrounding the overpack with a Portland Cement (PC) concrete buffer. An outer stainless steel envelope may also be placed around the buffer and, if present, the envelope may be sealed or perforated to allow free entry and exit of water and gas. The sealed overpack encloses the canisters of HLW or spent fuel assemblies, and must contain and prevent the release of the radioactive waste for the duration of the thermal phase. Once fabricated, the supercontainer will be emplaced horizontally in tunnels excavated in the Boom Clay. The tunnels will be lined and supported with concrete wedge blocks (the tunnel liner). The void space between the supercontainer and the tunnel liner will be backfilled with a cementitious material (the backfill) before the tunnels are sealed with concrete or clay plugs. Immediately adjacent to the tunnel liner lies an excavation damaged zone (EDZ) of repository host rock that may have become damaged by excavation of the tunnels, and within which the changes to some of the host-rock parameter values are significant enough potentially to affect the performance of the disposal system. Beyond this lies an excavation disturbed zone (EdZ), where there are changes to some of the host-rock parameters relative to their nominal values but no significant impacts on long-term safety or repository performance. During the thermal phase, radiogenic heating will raise temperatures significantly throughout the disposal system near-field. Preliminary estimates of the thermal evolution of the system have been made based on generic parameters. Assuming a 60-year cooling time before supercontainer assembly, after 5 years most of the interior of the supercontainer is likely to be above 60 C, whether the waste is vitrified HLW or spent fuel. The peak temperature within the supercontainer, backfill and tunnel liner will be experienced within 5 years of supercontainer assembly and emplacement for vitrified HLW, and within 20 years for spent fuel. For vitrified HLW, the temperature close to the overpack will be >~80 C for a period of about 15 years, For spent fuel, the temperature close to the overpack will be >~90 C for a period of about 30 years. The supercontainer design and waste cooling time have been chosen so that the temperature at the outer surface of the overpack will not exceed 100 C for either waste type. The temperature at the outside edge of the buffer will rise to a maximum of ~65 C (after 5 years for HLW) and ~80 C (after 15 years for spent fuel). The temperature close to the backfill/liner/boom Clay interfaces will rise to a maximum of ~55 C (after 10 years for HLW) and ~70 C (after 15 years for spent fuel). These modelling results may be revised in future as a more complete set of input parameters becomes available. After emplacement of the backfill and sealing of the tunnels, the backfill is likely to exert a large suction potential and its saturation state will therefore increase rapidly. However, this will initially occur at the expense of the saturation of the Boom Clay close to the tunnel lining. Preliminary calculations indicate that about 1 year after backfilling, the concrete backfill materials will be ~99% saturated, but the degree of hydraulic saturation in the first decimetres of the surrounding Boom Clay will be lower than this. After about 2 years, however, both are likely to become completely saturated. In the scoping calculations the timescale is mainly dependent on the unsaturated behaviour of the EBS materials and Boom Clay, the porosity of the backfill, the hydraulic conductivity of the Boom Clay and the hydraulic gradient. However, the complexity of the resaturation process increases further when the interaction of the EBS NIROND-TR 2007-07 E, April 2008 ii

with the EDZ and host rock is taken into account, as demonstrated by the BACCHUS2 and RESEAL in-situ experiments. Calculations based on assumed material properties and the likely volume of unsaturated buffer materials, and assuming no stainless steel envelope, suggest that after hydrostatic pressures are attained in the backfill, the time required for the buffer to become saturated may be approximately 2 years. As for the backfill, a buffer material exerting a large suction can lead locally to temporary de-saturation of the Boom Clay close to the repository tunnels. The heat emanating from the waste may influence the distribution of water in the near-field. Close to the overpack, free water initially contained within the concrete buffer will be heated through contact with the overpack assembly. However, modelling suggests that the temperature increase in the buffer will induce little dehydration of hydrous cementitious phases, cause little porosity increase, generate little vapour, and is not likely to generate a dry zone adjacent to the overpack. Radiogenic heating and associated processes may potentially lead to various mechanical effects within the near-field, including expansion and contraction, solid volume changes, porosity changes, and cracking of the buffer. Scoping studies indicate that the tensile stresses expected as a result of the heat of concrete buffer hydration will be well below the tensile strength of the concrete. However, the main importance of the buffer is to condition the chemical composition of the supercontainer pore fluid, and this primary function will continue to be fulfilled regardless of any mechanical effects. After hydration, the solid phase assemblage within the buffer concrete will mostly comprise portlandite (Ca(OH) 2 ) and Calcium-Silicate-Hydrate gel. Initial buffer pore fluid ph values at 25 C may be well in excess of 12.4 due to the presence of alkali metal hydroxides. The initial redox conditions within the buffer concrete will be oxidising, but poorly poised. Similar initial conditions are likely within the backfill. Subsequently, a wide range of chemical effects may occur to alter the initial chemical conditions. The initially very high ph of the pore water in the cement barriers may decrease to slightly lower values (e.g., ph ~ 12.4 at 25 C, ph ~11 at 100 C) if the readily soluble alkali metal hydroxides are leached or diffuse away, and heating of the concrete buffer close to the overpack leads to the formation of high-temperature cement solid phases such as afwillite. However, the large mass of portlandite in the buffer will continue to buffer ph at levels >11 throughout the thermal phase and for a long time thereafter. At the irradiation rates expected within the buffer, radiolysis is not expected to have a significant impact on corrosion, but it may serve to prolong the duration of the aerobic phase. An experimental programme to investigate the possible impact of gamma radiolysis on corrosion of the carbon steel overpack at the expected irradiation rate within the supercontainer is currently underway. After the effects of radiolysis have ceased, anaerobic corrosion processes are likely to prevail at the overpack surface. Gas generation calculations indicate that hydrogen production due to gamma radiolysis of water will be small compared with hydrogen production due to anaerobic corrosion of steel components. Scoping calculations suggest that following buffer saturation, corrosion within the supercontainer will generate gas at a rate that is too fast for it to be removed by molecular NIROND-TR 2007-07 E, April 2008 iii

diffusion of dissolved gas, implying that a free gas phase will develop. Such gas would tend to migrate out of the supercontainer. An experimental programme to investigate further the generation of gas by corrosion of the overpack, and its potential consequences, is currently underway. If present, the external surface of the envelope may be exposed to various solutes, including chloride, carbonate, bicarbonate and various sulphur species in Boom Clay pore waters. Reaction with these solutes could lead to perforation of an initially intact envelope within a few years if aggressive ions (e.g., Cl -, S 2 O 2-3 ) reach the stainless steel/backfill interface before the oxygen in this region is consumed. However, it is likely that conditions at the surface of the envelope will rapidly become anaerobic, such that the corrosion potential falls below the pitting potential and, therefore, that perforation of an initially intact envelope will take much longer (> 100 years). Dark, hot, high-ph conditions, combined with low porosity will tend to suppress microbial activity, but the presence and possible persistence of microbes within the near-field cannot be fully discounted. Microbes could reduce oxidised sulphur present in the repository and the resulting reduced sulphur is capable of promoting pitting corrosion. Therefore, the potential for microbial activity within the buffer and the potential impact of reduced sulphur species on corrosion under high-ph conditions are being investigated further. The range of processes that will affect the chemistry of the near-field is extremely complex, and further more detailed modelling and experimental work is required in order to constrain the magnitude of the uncertainty associated with the various processes. This applies, in particular, to the likely concentration of aggressive species at the surface of the overpack. During the thermal phase, the interplay of processes within the supercontainer will be particularly complicated when the envelope is perforated or absent, with different fluids (water and gas) and different chemical species moving in opposite directions in response to hydraulic, temperature and chemical potential gradients. NIROND-TR 2007-07 E, April 2008 iv

Executive Summary i. Table of contents v. 1 Introduction 1 1.1 Background 1 1.2 Safety Functions and Functional Requirements of the EBS Design Concept 4 1.3 Scope of the Report 4 1.4 Structure of the Report 5 2 System Description 7 2.1 Category C Disposal System Overview 8 2.2 The Waste Form 13 2.2.1 Vitrified HLW 13 2.2.2 Spent Fuel 14 2.3 The Supercontainer 14 2.3.1 Overpack 15 2.3.2 Buffer 16 2.3.3 Envelope 20 2.4 The Backfill 21 2.5 The Tunnel Liner 21 2.6 The Boom Clay 22 2.6.1 The Excavation Damaged Zone (EDZ) and Excavation disturbed Zone (EdZ) 24 3 Overview of Expected Evolution 27 3.1 Summary of Expected Evolution 29 3.1.1 Expected Evolution with Envelope Absent or Initially Perforated 31 3.1.1 Expected Evolution with Envelope Absent or Initially Perforated 32 3.1.2 Expected Evolution with an Initially Intact Envelope Present 40 3.2 Expected Evolution at Key Near-Field Locations 47 3.2.1 Thermal Effects 48 3.2.2 Hydraulic and Hydrothermal Effects 49 NIROND-TR 2007-07 E, April 2008 v

3.2.3 Mechanical Effects 50 3.2.4 Chemical Effects 50 3.2.5 Biological Effects 52 3.2.6 Radiation Effects 53 4 Thermal Evolution 55 4.1 Thermal Processes in the EBS 55 4.2 Expected Thermal Evolution 56 4.2.1 Vitrified HLW 57 4.2.2 Spent Fuel 59 4.3 Main Uncertainties 60 5 Hydraulic Evolution 63 5.1 Hydraulic Processes Within the EBS 63 5.2 Expected Hydraulic Evolution 64 5.2.1 Saturation of the Backfill and Buffer 64 5.2.2 Thermal Dehydration Effects 68 5.2.3 Gas Effects 71 5.3 Main Uncertainties 73 6 Mechanical Evolution 75 6.1 Mechanical Processes Within the EBS 75 6.2 Expected Mechanical Evolution 76 6.2.1 Thermal Stresses 76 6.2.2 Corrosion Effects 77 6.3 Main Uncertainties 77 7 Chemical Evolution 79 7.1 Chemical Processes in the EBS 79 7.2 Initial Chemical Conditions 80 7.2.1 Initial Chemical Conditions in the Boom Clay 80 7.2.2 Initial Chemical Conditions in the Buffer 81 7.3 Expected Evolution During Repository Construction and Operation 83 7.3.1 Oxidation Effects 83 7.3.2 Introduction of Microbes 86 NIROND-TR 2007-07 E, April 2008 vi

7.3.3 Effect of Radiolysis 86 7.3.4 Effect of Elevated Temperatures 88 7.4 Expected Chemical Evolution and Effects after Repository Closure 89 7.4.1 The Return to Anaerobic Chemical Conditions 89 7.4.2 Effect of Elevated Temperatures 90 7.4.3 Interactions with Envelope Absent or after Envelope Perforation 91 7.5 Main Uncertainties 93 8 Electrochemical Evolution of Metallic Barriers 95 8.1 Electrochemical Processes Within the EBS 95 8.2 Corrosion of the Envelope 96 8.3 Corrosion of the Overpack 97 8.4 Effect of Radiation on Anaerobic Corrosion of the Overpack 101 8.5 Main Uncertainties 102 9 Conclusions 105 10 References 110 Input Data and Boundary Conditions to Support Modelling Studies of the Belgian EBS Design for HLW Disposal 117 A1 Introduction 117 A1.1 Background 117 A1.2 Structure of Appendix A 117 A2 Supercontainer Dimensions 117 A3 HLW Inventory and Radiolysis Calculations 121 A4 Concrete Buffer, Backfill and Wedge Blocks 125 A4.1 Cement 125 A4.2 Aggregate 125 A4.3 Concrete Proportioning 125 A4.4 Assumed Concrete Mix 125 A4.5 Superplasticiser 126 A4.6 Nominal physical properties of concrete buffer 126 A4.7 Water content at 60 C 127 A4.8 Physical properties of filler 128 NIROND-TR 2007-07 E, April 2008 vii

A4.9 Backfill 129 A4.10 Tunnel Liner 129 A5 Boundary Conditions for T-H Modelling 130 A5.1 Thermal conductivity of surrounding media 130 A5.2 Thermal power of vitrifed HLW 130 A6 Carbon Steel Overpack and Stainless Steel Envelope 132 A7 Boom Clay 135 A8 References 139 NIROND-TR 2007-07 E, April 2008 viii

1 Introduction 1.1 Background The Belgian radioactive waste management organisation, ONDRAF/NIRAS, is responsible for developing a deep disposal facility for Category B waste (i.e., low-level and intermediate-level long-lived radioactive waste (LILW-LL)) and Category C waste (i.e., vitrified high-level radioactive waste (HLW) and spent fuel). A primary aim of the ONDRAF/NIRAS programme for Category B and C wastes is to establish the feasibility of a deep disposal facility, without any presumption about precise repository location. Boom Clay, a poorly indurated argillaceous formation, is the reference medium for hosting such a disposal facility, but no official siting decision has yet been taken by the Belgian authorities. Most of the information related to the Boom Clay comes from the underground research laboratory (HADES) located beneath the Mol-Dessel region in north-east Belgium. This region also serves as a reference site for methodological research associated with the Category B and C waste disposal programme. There is an international consensus that the principal objective of any facility for the disposal of radioactive waste is to provide long-term safety by protecting humans and the environment from harm. This objective was stated by the International Commission on Radiological Protection (ICRP) as follows [35]: The principal objective of disposal of solid radioactive waste is the protection of current and future generations from the radiological consequences of waste produced by the current generation. The same objective was stated by the International Atomic Energy Agency [34] as follows: The objective of radioactive waste management is to deal with radioactive waste in a manner that protects human health and the environment, now and in the future, without imposing undue burdens on future generations. The commonly adopted management strategy to achieve this objective is to concentrate and contain the waste and to isolate it from the biosphere, and this strategy forms the starting point for the design of the Belgian deep disposal concept. ONDRAF/NIRAS has adopted a step-wise approach to disposal facility design. Various types of information and information sources, as identified in the ONDRAF/NIRAS safety strategy [54], inform the requirements of the disposal facility design, including: International principles of radioactive waste management and international recommendations and guidance. Boundary conditions, both external, as specified in national laws and regulations, and internal, as specified by ONDRAF/NIRAS early in the programme and unlikely to change. Disposal system development is then guided by various strategic choices, consistent with the principles and boundary conditions, and is carried out on two levels: NIROND-TR 2007-07 E, April 2008 1

At a higher, more general level, the reference disposal concept is developed. The concept is developed to be robust to any reasonably foreseeable changes in principles, boundary conditions or knowledge. At a lower, more detailed level, specific design choices and decisions regarding repository implementation are made. The detailed design may be modified as the programme progresses through successive stages in order to adapt the repository to meet relevant requirements more successfully, take advantage of advances in science and engineering, and enable more robust safety and feasibility statements to be made. The general safety functions in the Belgian radioactive waste disposal concept are given in Table 1.1 [53]. Isolation (I-function) The objective of the I-function is to isolate the waste from man and the surface environment, in order to prevent direct access to the waste and to protect the disposal facility from the effects of surface processes. The I-function is divided into two sub-functions: I1 Reduce the likelihood and possible consequences of inadvertent human intrusion. I2 Create stable conditions for the waste and disposal system components in order to shield the repository from changes and disturbances at the surface or in the sub-surface. Engineered Containment (C-function) The objective of the C-function is to prevent for as long as possible any dispersion of contaminants outside the waste form and its primary containment. Delay and Attenuate Releases (R-function) The objective of the R-function is to retard and limit the eventual release of contaminants from the disposal system, after such time as the C-function is no longer present. Three sub-functions have been defined that contribute to both the delay and the attenuation of releases: R1 Limit the release from the waste form. Various physical and chemical mechanisms contribute to the resistance to leaching, such as slow dissolution and low solubility limits. R2 Limit water flow through the system. This function results in limited quantities of water infiltrating or moving through the system due to the presence of low permeability barriers (the Boom Clay and the tunnel and shaft seals). R3 Retard contaminant migration. Processes such as contaminant precipitation and sorption contribute to this sub-function, whereby contaminants released from the waste are mostly retained for an extended period. 2 NIROND-TR 2007-07 E, April 2008

Table 1.1 - General safety functions of the Belgian radioactive waste disposal concept, the barriers or components of the disposal system likely to provide or contribute to these functions, and the time frame over which they are expected to operate [53]. Safety function Sub-function Likely contributing component Time frame (y) Isolation (I) Reduce likelihood and consequences Institutional controls 10 2 of human intrusion (I1) Geological barrier 10 6 Create stable conditions for the disposal system (I2) Stable geological setting 10 6 Engineered containment (C) Prevent releases for as long as possible Engineered barriers 10 3-10 4 Delay and attenuate the releases (R) Limit release from waste form (R1) Limit water flow through system (R2) Conditioned waste form Geological barrier, seals 10 4 (vitrified waste) - 10 5 (spent fuel) 10 6 Retard contaminant migration (R3) Geological barrier 10 6 Engineered containment must be provided for the duration of the thermal phase and the precise time frame may be modified based on future studies. The thermal phase corresponds to the period over which the performance of safety-critical disposal system components, primarily the retardation capability of the geological barrier, could be compromised by the thermal transient. Table 1.1 demonstrates that the Engineered Barrier System (EBS) is primarily responsible for providing one function, the engineered containment function (C). The main contributor to the R2 function will be the geological barrier, the Boom Clay, although the tunnel and shaft seals will probably also contribute to this function. ONDRAF/NIRAS is preparing safety cases to demonstrate the long-term safety of its deep disposal concept for Category B and C wastes. The safety cases will include the assessment basis, an overall description of the disposal system, and a description of the scientific and technical data and understanding relevant to the assessment of system safety and feasibility. This report supports the safety case for disposal of Category C wastes by describing the expected evolution of the near-field of the Category C disposal system, focusing in particular on the EBS. The EBS evolution report is not yet complete and will be further developed over the coming months and years as further information (e.g., from research studies) becomes available. A companion report focuses on the expected evolution of the EBS for Category B waste. NIROND-TR 2007-07 E, April 2008 3

1.2 Safety Functions and Functional Requirements of the EBS Design Concept Based on the disposal system concept, a set of functional requirements has been established for each of the various sub-systems and components. The design of the EBS has been elaborated based on these functional requirements [30]. Feedback from focused research studies, scoping calculations and safety assessments is being used to further refine and improve the EBS design. The assignment of specific safety functions to the various sub-systems and components of the Category C disposal concept is a stepwise process as described in the ONDRAF/NIRAS safety strategy [54] [19]: Based on the strategic choices driving the disposal concept, the safety functions that are expected to be fulfilled by the system and its sub-systems and components are defined. Within the SFC-1 programme this was done in the period 2005-2006. The assignment of safety functions is justified in the light of ongoing phenomenological assessment of the disposal system, sub-systems and components. Within the SFC-1 programme this will be carried out in the period 2006 to 2010. Finally, the assignment of safety functions will be confirmed based on confidence in the phenomenological understanding of the disposal system. 1.3 Scope of the Report This report presents an up-to-date scientific view of the processes involved in the expected evolution of the EBS design for Category C wastes, and serves the following functions: It provides the scientific basis on which a reference model for the evaluation of repository safety can be constructed. It provides the necessary basis on which to construct scenarios for evaluation and demonstration of the safety case for deep disposal. It will be used in a formal exercise for checking completeness against a FEP-list specific to the Belgian disposal system. The reference model, development of scenarios for safety calculations and completeness checking will be reported in separate future projects. The report focuses on the expected evolution of the EBS in the disposal concept for Category C waste involving the current reference design, namely a BSC-1 supercontainer comprising a carbon steel overpack and a concrete buffer, with or without an outer stainless steel envelope. The Boom Clay is the reference host formation for the disposal concept. The following subject areas are excluded: The report does not address the expected evolution of the EBS for Category B waste, and the expected evolution of the tunnel and shaft seals. These are described in companion reports. The report does not address spent fuel dissolution or HLW glass dissolution because these issues are addressed elsewhere in ONDRAF/NIRAS s safety case documentation 4 NIROND-TR 2007-07 E, April 2008

programme. However, other processes operating within the overpack, such as corrosion of primary waste containers, are included. The report does not address radionuclide release and transport processes. The report does not address processes mainly operating within the Boom Clay surrounding the repository, such as alkaline plume migration. The report refers to various scoping calculations that explore the behaviour of the supercontainer EBS design. Scoping calculations are a form of quantitative analysis, based on simplified assumptions, that is intended to capture quickly a general understanding of the behaviour of an engineered or natural system. These calculations provide a first-order examination of the sensitivity of the system to critical parameters and may involve numerical modelling or simple mathematical or algebraic reasoning. It is important to note that many of these calculations were performed early in the development of the disposal concept. Although the results of these calculations are indicative about the timeframe and magnitude of certain processes, the reader should be aware that they do not represent a systematic phenomenological analysis. 1.4 Structure of the Report The report is structured as follows: Section 2 provides a component-by-component description of the near-field and EBS in the ONDRAF/NIRAS disposal concept for Category C wastes. Section 3 provides an overview of the expected evolution of the near-field at the system level and for key locations within the near-field. Section 4 summarises the expected thermal evolution of the near-field and identifies the main uncertainties. Section 5 summarises the expected hydraulic evolution of the near-field and identifies the main uncertainties. Section 6 summarises the expected mechanical evolution of the near-field and identifies the main uncertainties. Section 7 summarises the expected chemical evolution of the near-field, including the evolution with and without an envelope, and identifies the main uncertainties. Radiological and biological EBS processes are also summarised in this section. Section 8 describes the electrochemical processes that are expected to affect the various metallic components within the near-field, and identifies the main uncertainties. Section 9 presents conclusions and summarises the principal remaining uncertainties. Appendix A provides a compilation of input data and boundary conditions relevant to the disposal concept for Category C waste. NIROND-TR 2007-07 E, April 2008 5

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