The Nuclear Fuel Cycle

Transcription

1 Advanced Summer School of Radioactive Waste Disposal with Social-Scientific Literacy UC Berkeley, Berkeley, California, August 3, 2008 Geological l Disposal of Radioactive Waste: Concepts, Technical Bases and Safety Assessment Dr. Mick Apted Monitor Scientific LLC Denver, Colorado mapted@monitorsci.com Monitor Scientific The Nuclear Fuel Cycle

2 The Nuclear Fuel Cycle is Not Just for Power Production Presentation Outline Part 1: Radioactive wastes for geological disposal Part 2: Strategies and design bases for disposal of radioactive wastes in geological repositories Part 3: Safety assessment (SA) of long- term waste isolation and safety of a geological repository Short coffee break Part 4: Real-time Repository Design and SA by students t

3 1. Radioactive Wastes for Geological Disposal Basic radiation concepts Classification of nuclear waste Nuclear wastes for geological disposal Spent nuclear fuel (SNF) High-level waste (HLW) from reprocessing SNF (e.g., borosilicate glass) Intermediate-level waste (ILW), also from reprocessing SNF Basic Concepts (1) Isotopes (Radionuclides) Atomic number (Z) =Number of protons in nucleus Number of neutrons in nucleus (N) Mass number (A = Z + N) He-4 (Z=2, N=2, A=4) U-238 (Z=92, N=146, A=238) U-235 (Z=92, N=143, A=235) Radioelement = all radionuclides with same Z Plutonium (Pu) is a radioelement, Pu-239 is a radionuclide

4 Basic Concepts (2) Becquerel (Bq) unit of activity defined as 1 nuclear disintegration per second (1 Bq = 2.7E 11 curies). Heat output calculated by knowing the heat produced per disintegration (watts/bq). Alpha (α) particles Helium nucleus (He-4). Beta (β) particles Beta (+) anti-electron (Z 1 ) Beta ( ) electron (Z +1 ) Gamma (γ) ray Changes in atomic nucleus (e.g. alpha, beta emission) (X-ray: change in orbital electrons). Neutron (n) Mostly released by nuclear fission. Basic Concepts (3)

5 Basic Concepts (4) Half-life Specific activity (SA) measurement of radioactivity yp per unit mass, inversely proportional to isotope half-life (t 1/2 ) and mass number (A). Basic Concepts: Decay Series & Daughters (5) Neptunium Series (simplified) Uranium Series (simplified) Actinium Series (simplified) Thorium Series (simplified) 242 Am (432 a) 242 Pu (3.76E5 a) 243 Am (7.38E3 a) 240 Pu (6569 a) 237 Np (2.14E6 a) 238 U (4.47E9 a) 239 Pu (2.41E4 a) 236 U (2.34E7 a) 233 U(1 (1.59E5 a) 234 U(2 (2.45E5 a) 235 U(7 (7.04E8 a) 232 Th (1.41E10 a) 229 Th (7.34E3 a) 230 Th (7.7E4 a) 231 Th (25.5 h) 228 Ra (5.75 a) 225 Ra (14.8 d) 226 Ra (1600 a) 227 Ac (21.8 a) 228 Th (1.91 a) 205 Tl (stable) 206 Pb (stable) 207 Pb (stable) 208 Pb (stable) Source: Roddy et al., 1986, Physical and Chemical Characteristics of Commerical LWR Spent Fuel, ORNL/ TM-9591/ V1&R1, Oak Ridge National Laboratory, Oak Ridge, TN.

6 Relating Radiation Energy to Health Effects Absorbed dose, D (J/kg) or (Gy) Energy imparted by ionizing radiation per unit mass In radiation protection it refers to the average dose absorbed b by a tissue, an organ or the whole body Does not account for the differences in effects according to the type and the energy of the radiation or the differences in radio-sensitivity of the living tissues exposed. For radiation protection the following factors are used to achieve this: Radiation weighting factor Tissue weighting factor Equivalent Dose, H T Equivalent Dose, H T, is expressed in sievert (Sv) and is represented by a summation that includes all the components of the radiation field: H T where = R w R D TR D T,R is the absorbed dose in tissue or organ T from radiation R (J/kg) w R is the radiation weighting factor (unitless) 1 Sievert = 100 rem)

7 Potential Dose-Risk Response Relationships Estimated and Real Radiation Doses Individual annual radiation doses in millisieverts Typical calculated impacts of a HLW repository Below Concern ICRP Band 3 Low Concern - Repository constraint? Natural Background Global Average Typical Range Would we intervene? Ramsar Flight to Japan April 2002 Unlikely to be justified May be necessary Almost always justified

8 Definition of Radioactive Waste Radioactive waste is defined by the IAEA as being: "Any material that contains or is contaminated by radionuclides at concentrations or radioactivity levels greater than the exempted quantities established by the competent authorities, and for which no use is foreseen". It is normally national policy that t determines what is considered waste, e.g. spent fuel is a resource in France, Japan, Russia and the United Kingdom, but a waste in the Finland, Sweden, Canada and the United States. Note that because of radioactive decay, radioactive wastes will trend toward becoming less hazardous wastes over time. Classification of Radioactive Wastes Objective of waste classification by the type and abundance of radionuclides is to provide a system that: Makes it easier to determine how to safely handle the wastes Indicates the length of time wastes will require control/handling/isolation Informs suitable disposal options

9 IAEA Classification of Radioactive Wastes Waste Class Typical Characteristics Possible Disposal Options Exempt Waste (EW) Low and Intermediate Level Waste (LILW) - Short Lived (LILW-SL) activity levels at or below clearance levels activity levels above clearance levels thermal power below about 2kW/m 3 restricted long-lived radionuclide concentrations, e.g. long lived α-emitters average < 400 Bq/g or 4000 Bq/g maximum per package no radiological restrictions - normal land fill near surface facility near surface or geological p p g repository - Long Lived (LILW-LL) long-lived radionuclide concentrations exceeding limitations for short-lived wastes geological repository High Level Waste (HLW) spent nuclear fuel (SNF) reprocessed SNF [e.g., borosilicate glass (BSG)] thermal power greater than about 2kW/m 3 long lived radionuclide concentrations exceeding limits for short-lived wastes geological repository Inventories of Radioactive Waste (1/2) Key radionuclides- within the fuel cycle, radionuclides arise in three groups: Activation Products: these arise from the interactions of neutrons with reactor materials. Some important activation products are: C-14, Cl-36, Co-60, Ni-59, Ni-63. Fission Products: these arise when a nuclide undergoes fission. Some important fission products are: Sr/Y-90, Tc-99, I-129, Cs-137/Ba-137m.

10 Inventories of Radioactive Waste (2/2) Actinides: these radionuclides are produced when fertile isotopes, like U-238, absorb neutrons that lead to the production of trans-uranic actinides, e.g. Np, Pu, Am, Cm. Spent Fuel as a Waste Form Multiple Sources Stoichiometric UO 2.00 matrix Irradiated zircaloy cladding Volatile fission products migrated to voids, cracks, gap, and grain boundaries ( instant release fraction ) 5-metal phase Crud (from irradiation of coolant water) Radiation field Initially high α, β and γ. After 1000 years, mostly α. Possible radiolysis of contacting air or groundwater.

11 Heat-Producing Radionuclides Energy of radiation from radioactive-decay produces heat Decay heat: dominated by a few nuclides Co-60 ~ W/Bq; t 1/2 = 5.27 years Sr-90/Y-90 ~ W/Bq; t 1/2 = 29.1 years Cs-137/Ba-137m ~ W/Bq; t 1/2 = 30 years Actinides (in Spent Fuel): 241 Am, 240 Pu, 239 Pu ~ W/Bq Computed from radionuclide inventory using Spreadsheet (good estimate) Decay codes (ex. ORIGEN) that solve Bateman equations for decay chains. Characteristics of Discharged Spent Fuel Activity Per MTHM PWR 33 GWd Burnup 10-fold decrease in heat generation in 300 years Source: Roddy et al., 1986, Physical and Chemical Characteristics of Commerical LWR Spent Fuel, ORNL/ TM-9591/ V1&R1, Oak Ridge National Laboratory, Oak Ridge, TN.

12 Vitrified HLW (e.g., Borosilicate Glass) Amorphous structure of glass accepts wide range of cationic species. Many different glass formulations. Waste loadings of radionuclides wt.%. Lower thermal loading per unit mass compared dto SNFbecause of removal of some heatproducing nuclides. Heat output depends on aging before reprocessing, storage before disposal, inventory, total mass. ~5000 years ~200,000 years

13 200,000 9, ,000 8, ,000 7,000 6,000 tivity relative act 140, , ,000 80,000 relative activity 5,000 4, ,000 2,000 1,000 60, ,000 years 20, ,000 40,000 60,000 80, ,000 years Common Sense Representation of Hazard 200, , rela ative activity 160, , , ,000 80,000 60,000 40,000 20,000 Archaeological analogues show that we can deal with this period well Radiological impacts of deep uranium ore bodies suggest limited concern about this period We should not expect to do better than Nature at long times , , , , , years

14 Wastes Generated from Reprocessing 100 MTU (45 m 3 ) of Spent Fuel Glasses Metals Minerals Grout/ Cements also Spent MOX Waste Stream Waste Composition Category Volume, m 3 FPs + minor TRU Borosilicate glass HLW Uranium (excess) U 3 O 8 powder Storage/ disposal 18 Cesium/strontium Cs/Sr alumino-silicate Storage 1.1 Hulls + Tc, sludge Zr-Fe based alloy HLW 0.6 Compacted hulls Non-TRU Zr HLW 6.1 Iodine-129 Potassium Iodate HLW Krypton Zeolite/aluminosilicate HLW Tritium Grout HLW <0.01 Lanthanide FPs LABS glass HLW 0.31 Carbon-14 Sodium carbonate HLW Solvents, resins, etc. Grout ILW ~130 Sources: J. Laidler, ÒGNEPSpent Fuel Reprocessing Waste Streams and Disposition OptionsÓ, May 15, 2007, US Nuclear Waste Technical Review Board and D. Davidson (AREVA), ÒBriefing to the NRC Advisory Committee on Nuclear WasteÓ, May 16, Intermediate Level Wastes + Exotics Metals/ Cladding Lot s ofit! Relatively high activity (inventory) of soluble, long-lived, dose-contributing nuclides!! Where to put it? Not in trenches! Grout/ Cement

15 2. Strategies for Geological Disposal Why geological disposal? What are the basic strategies used to assure long-term, safe isolation in geological repositories? What are some examples of repository design concepts now being implemented internationally? Where geological disposal began US N ti l A d f US National Academy of Sciences, 1957

16 Objectives of Geological Disposal PREVENT any releases reaching people and the environment in harmful concentrations ISOLATE radioactivity from people by deep burial in rock typically, m CONTAIN waste for >300 years to allow decay of inventory and heat production MAINTAIN a stable geological cocoon for engineered barrier system for hundreds of thousands of years Image: SKB, Sweden Approaches for Safe, Long-term Geological Disposal of Radioactive Wastes Multiple Barriers Natural Barrier System (NBS) of host rock and groundwater Engineered Barrier System (EBS) of waste form, canister for emplacement, possibly other barriers to assure or enhance isolation (NOT to compensate for a bad site ) Multiple Isolation Strategies Delay and Decay Constrain Concentration

17 The Multiple Barrier Concept each barrier acting in concert with the others to isolate, contain and reduce impacts Engineered barriers solid waste-form metal container metal overpack buffer/backfill geological environment dilution Natural barriers Safety Strategy #1: Delay and Decay Containment If containment time (t c ) is >10- times greater than t 1/2 for a given nuclide, that nuclide inventory will effectively decay to insignificance: reduction factor = 2 tc/t1/2 = 2 10 = 1024 Containment for years is an effective approach to eliminating heat producing nuclides (Co-60, Sr-90, Cs-137) and, for HLW, allowing repository to approach ambient temperature before waste contact by groundwater Less effective for long-lived and non-sorbing nuclides Transport If transit time (t) of released nuclides to travel across EBS and/or NBS is >10-times greater than t 1/2 for a given nuclide, that nuclide will effectively decay to insignificance during transport: reduction factor = 2 t/t1/2 = 2 10 = 1024 Short-lived Sr-90, Cs-137 eliminated by transport and containment (latent safety functions). Effectiveness: diffusive transport >> advective transport longer path length >> shorter path length

18 Decay During Containment 1.E+07 Eliminated Nuclides 1.E+06 1.E+05 1.E+04 t = 0.1t c C-14 Tc-99 Cs-137 Am-241 Pu-239 Np E+03 1.E+02 t = t c = 1000 years 1.E+01 1E 1.E+01 1E 1.E+02 1E 1.E+03 1E 1.E+04 1E 1.E+05 1E 1.E+06 1E 1.E+07 Half-Life (a) Decay During Transport Transp port Tim me (a) 1.E+07 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 Eliminated Nuclides Sm-151 Cs-137 t = t 1/2 Cm-245 Am-243 Zr-93 Pu-239 Cm-246 Am-241 Pu-240 Pu-242 Sn-126 Th-229 Th-230 U-234 U-233 Np-237 t = 10t 1/2 Ra-226 Tc-99 Cs-135 Pd-107 Se-79 Buffer thickness = 0.7 m 1.E+01 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Half-Life (a)

19 Example: H12 Defense-in-Depth by Decay h Through me Field er (a) (a) sport Time Ti T gh nd Far-F Buffe Tr ranspor Trans Buffer Throug an 1.E+07 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 Eliminated Nuclides Sm-151 Cs-137 t = 10t 1/2 t = t 1/2 Cm-245 Am-243 Zr-93 Pu-239 Cm-246 Am-241 Pu- 240 Pu-242 Sn-126 Th-229 Th-230 U-234 U-233 Np-237 Ra-226 t = t c =1000 years c Flow rate = 1 m/a Far-field path = 100m Tc-99 Cs-135 Pd-107 Se-79 Buffer thickness = 0.7 m 1.E+01 1.E+01 1.E+02 1.E+03 1.E+04 1.E+05 1.E+06 1.E+07 Half-Life (a) Safety Strategy #2: Constrain Concentration Defense-in-Depth: Depth: Factors that Constrain Concentration Regulatory Target (e.g., dose, drinking water) Quantifiable non-decay Initial Concentration factors that reduce or in Waste Form limit it the concentration ti of nuclides. Repository Layout Factors constraining concentration can occur over time and space. Distribution in Container Failure (x,t) Factors constraining concentration can occur Shared Solubility Limits naturally and can also be engineered. COMPLIANCE Factors constraining Dispersion during concentration can Transport reduce (by many orders of magnitude!) Aquifer Mixing i radionuclide releases.

20 First Step: Site Characterization and Selection Selecting sites to be characterized is constrained by Public acceptance, on both local and national scale Available geological diversity (compare USA vs. Netherlands) Tectonic stability (Saturday field trip) Competing land use (e.g., national parks, oil & gas production) Transportation requirements Hydrological factors Dry site: salt formations No-flow site: low-permeability rocks (e.g., clay) Low-flow o site: fractured rocks (e.g., granites) Geochemical factors Low oxygen/ low redox potential (Eh) Near-neutral ph Low concentrations of complexing anions and organics EBS Materials Selection: Canister Types of canister materials based on corrosion mode Corrosion-allowance metals having moderate rates of general corrosion in water (e.g., mild steel) Corrosion-resistant metals have inert oxide surfacelayer, with extremely low rates of general corrosion in water but may eventually fail by localized corrosion (e.g., stainless steels, titanium) Metals whose corrosion rate is not controlled by corrosion kinetics but by diffusion-limited transport of reactants to the metal s surface (e.g., copper by dissolved d sulfide, oxygen) Advanced materials, including amorphous metals and ceramics Compatibility with repository environment

21 Canister + Waste Form = Waste Package SNF Waste Package HLW Waste Package EBS Materials Selection: Buffer (1) Diffusion barrier ( buffer ) placed between waste package and host rock Assure slow rate of diffusion and high sorption by buffer material, leading to long transit time of nuclides, hence, significant ifi decay during transport t out of the EBS Decouple release-rate behavior of WP from possible future changes in site hydrology due to climate change Filter radionuclide-bearing colloids during transport across buffer Constrain the corrosion rate of possible copper canister to the slow rate of diffusive transport t of reactants ts (no reliance on corrosion os o kinetics) Minimize potential impacts on WP from any geomechanical processes from surrounding rock (e.g., faulting, rock creep, spalling, rockfall) Operational factors to be considered Ease of emplacement and impact on WP retrieval QA of as-emplaced buffer Impact on thermal management (delayed emplacement with ventilation)

22 EBS Materials Selection: Buffer (2) re-saturation: smectite absorbs water and swells impermeable: diffusive transport of (1) reactants to canister, and (2) nuclides from failed canister retardation of nuclides via ion-exchange + surface adsorption filters colloids/ particulates permeable to gas? bentonite is smectite-rich rock type fabrication, emplacement and durability issues FEBEX ~2 m EBS Materials Selection: Buffer (3) Buffer for Unsaturated Sites Capillary-breaking process: Sand-over-Gravel Layers Laboratory data and 3000-year old burial mounds in Asia confirm robust performance against earthquakes and climate change I meter of gravel = 90 meters of clay! Water Sand Gravel UC Berkeley NE-170 (2007)

23 EBS Materials Selection: Prefabricated EBS Modules (PEMs) IWP (Integrated Waste Package) MCM (Multi-Component Module) Need for Thermal Management for HLW/ SNF EBS and NBS materials have thermal limits affecting ~10-fold decrease in properties and performance. heat generation rate in first 300 years; Thermal constraints on potential thermal repository operations (hence, management by impact on repository design storage and/or and layout). containment in Many repository processes a canister are thermally activated or are temperature-sensitive Waste-form dissolution Diffusion Radioelement solubility Corrosion Variation in heat production of vitrified ifi HLW with time after emplacement (assuming 54 years out-of-reactor ) From: JNC (2000c) H12 SR2

24 Safety Functions: What is each barrier expected do? Bedrock isolate EBS from biosphere remain geomechanically stable favorable geochemistry for EBS limit groundwater flow Tunnel Backfill keep EBS in place prevent fast pathways for release keep tunnels stable path a s for release Canister Buffer mass-transport is diffusion limited isolate canister from rock conduct heat filter colloids aid waste emplacement isolate waste from water withstand mechanical loads conduct heat Repository Designs: KBS-3 (Sweden, Finland) Backfill layout for under the sea-bed inclined layout Spent fuel 500-m depth in granite 4,000 Cu-canisters 0.3-m bentonite buffer Bentonite + sand backfill Vertical emplacement Reducing, neutral ph, saline groundwater Safety Case includes year containment, low UO 2 dissolution rate, low radioelement solubilities, 50-m far-field fi transport. t

25 Repository Designs: H12 (Japan), K-1 (Switzerland) HLW glass (MOX?) > 300-m depth in hard or soft rock (volunteer site) 40,000 C-steel packages 0.7-m clay + sand buffer Emplace vertical or horizontal Reducing, neutral ph, saline/dilute water Safety Case includes year containment, low radioelement solubilities, 100-m far-field transport. Repository Designs: Yucca Mountain (USA) Spent fuel & DHLW 300-m depth in tuff, 300-m above the water table 40,000 C22 packages + Ti shield Horizontal emplacement in open drift (currently no backfill) High heat load leads to initial dry-out of tuff, delaying onset of corrosion and preventing aqueous releases of nuclides Unsaturated, oxidizing, neutral, g dilute water Safety Case includes >10 4 -year containment, distributed failure, limited water, slow UZ transport, matrix diffusion.

26 Repository Designs: WIPP/ TRU (USA) ¾ Operational repository in bedded salt formations ¾ Disposal of non-heat producing p g defense transuranic wastes (TRU) ¾ Minimal EBS because of no flowing water, sealing of wastes by creeping salt ( dry site ) French HLW Disposal p in Clay: y No-flow Site Image: ANDRA

27 CARE Disposal: Concept emphasising staging (underground storage) and ease of reversal increased acceptance? Extended open period (up to 300 years) - easy reversal but safeguards / security concerns? Disposal in Deep Borehole Options Low T Option: engineered barriers Could be extremely safe if site can be sufficiently characterized, emplacement is assured, and no retrieval option High T Option: rock melting

28 Flexibility Provided by Range of Viable Options Barrier Concepts for LLW Near-Surface Disposal Resistive-layer barrier: lower permeability material (e.g., clay) that resist flow-through of infiltrating water Conductive-layer barrier: high permeability captures and diverts flow of infiltrating water (Richards barrier or capillary-breaking layer) Bioengineered (vegetation) barrier: promotes evapo- transpiration of infiltrating water Waste encapsulated in low permeability cement/ grout waste matrix.

29 Guiding Principles for Repository Design Characterize repository environment (NBS) Dry or No-flow Site: minimal i EBS Geochemical conditions can strongly affect radioelement concentrations and the stability of EBS materials Require robustness of barriers and isolation process: Effectiveness: how much does a barrier or process contribute to reducing peak radionuclide release rates? Reliability: are there multiple-lines of evidence supporting expected performance over repository time scales? Include operational and emplacement requirements Examine design trade-offs, cost impacts, and potential for design optimization after assuring safety 3. Assessing Long-term Safety Perspectives on Safety Assessment Calculating future radiological doses What if? scenarios Multiple lines of evidence to support safety assessments

30 Who Needs to Know It Is Safe? Implementer: to have confidence to present and defend a license application Regulator: to be able to license the repository Elected Government Representatives: to be justified in taking a decision to proceed Public: to have confidence in and accept the whole process The relative roles and participation of these stakeholders varies from country to country Different Stakeholders Want Different Things from Safety Assessment Regulator an independent assessment of quantitative dose or risk compared to established safety standards many regulations require treatment of uncertainty, supporting arguments, and multiple lines of evidence' Funder (e.g. Utilities) needs confidence to invest the money in disposal rather than continued storage Elected Government Representatives wants the PUBLIC to be happy and nothing to go wrong

31 .the PUBLIC wants to be sure that: the repository will not affect their health or that of their children and grandchildren the arguments they hear are based upon something that they can understand and relate to their experience they can see the relevant information and that nothing is being hidden from them the regulator is convinced about the safety that any independent experts who they trust are also happy with the safety..and (probably) that there are no long-term (>>1000 years) environmental impacts The public worries about different things than the experts worry about...

32 An Independent, Competent Regulator is Key Assure that post-closure safety meets internationally accepted standards for members of the public Confirm that uncertainty has been properly considered: Are system models complete and valid? Are appropriate parameter values and ranges identified? Are risk-informed analyses conducted to identify which processes and parameters are most important to safety? Are what if? scenarios analyzed? Proof in the scientific sense is not possible! Conduct independent review and assessment of safety Require Implementer to present multiple lines of evidence Disposition of concentration and fluxes of natural and repository- Sub-system derived radioactivity Models and radiotoxicity for Safety over long Assessment time periods 1. Repository Environment In surrounding rock formations and groundwaters 5. Biosphere Dose In surface waters: rivers, lakes Container Waste Buffer or backfill In different regions of 3. the Radionuclide engineered barrier system Release From EBS Decayed situ 2. Initial Containment 4. In Geosphere the repository system Transport

33 Calculational Approach Application of well-established mass-transfer theory Key Processes and Parameters Advection (Darcy s law) Diffusion (Fick s law) Radioactive decay and in-growth Waste-form dissolution rate Radioelement solubilities Radioelement sorption on different barriers Matrix diffusion Colloidal transport (if relevant) Aquifer mixing and dilution Isotopic exchange Waste Form + Water Catalyzes reactions (waste forms, even glass, inert in absence of water) Behaves as a reactant (waste form dissolution) Limited carrying capacity (radioelement solubility) (radioelement solubility) Pathway for Radionuclide Migration Back to the Surface

34 Controls on Radioelement Release by Water Waste Form C i (t) = time-dependent concentration of radionuclide i in water contacting the waste form M = dissolution rate radioelement i released Ci C (t) Q = transport rate Case 1: F > 1 Ci () = CSAT (generally applicable for all radioelements) Flux Ratio = F = M/ Q Case 2: F << 1 Ci () = [M/ Q] CSAT Ci () = F CSAT (applicable only under limited circumstances) Fate of Radioelements, Not Fate of Waste Forms Co oncent tration Dissolution rate (glass) Leach rate (cement) Corrosion rate (metals) nradioelement Solubility Laboratory WAC/ PCT Tests Repository Performance (days to to few years) (>10 s years) Time

35 Fate of Radionuclides throughout Repository System (e.g., Swiss clay site) >99.9% of inventory remains within EBS Assumed 10 4 year containment time Nagra EN Fig Safety Assessment: Calculation of Compliance with Dose Standard

36 Treatment of Credible Scenarios: What if? Tectonic stability? Climate change? Susceptible to drastic changes in ambient hydrology and geochemistry? Thermal alteration of EBS? Potential formation of gas? Formation of potential fast pathways from repository to surface? Faulting of EBS? Climate Change: Europe 18,000 years ago Thick ice sheets Extensive permafrost Sea level as low as -165 m Followed by very rapid deglaciation Lik l t i ( l Likely to occur again (several times over next 1 Ma)

37 Neodani fault at at Midori, i October 1891, M8 event 6m 6m vertical, 3m 3m horizontal displacement Photo: Photo: B Koto Koto Need for Multiple Lines of Evidence Uncertainties in extrapolating today s conditions into the future Increased confidence from defense -in-depth technical arguments Matching of time scales between repository assessment and natural/ archaeological analogues Calculated l doses can have relatively l large uncertainties ti Verification/ validation of safety assessment codes (e.g., analyze an analogue using the same component models) Establish the degree of conservatism in safety assessments Different Stakeholders find different arguments compelling level of understanding varies among Stakeholders level of belief varies among Stakeholders Analogues more easily understood than complex computer codes

38 Natural Analogue (in general) stability of materials metals (Cu, Fe) clays cement/concrete UO 2 stability of the deep environment slow transport geochemical buffering radiological behavior studies of high natural background areas fluxes of elemental analogues Natural Analogue: Cigar Lake Uranium Deposit, Canada ~100,000 tons uranium >1 billion years old no surface radiological signature

39 Roman Cement: 1700 years Wood: 1,500,000 years Roman Iron: 1900 years Analogues exist for many materials, including copper, iron, cement, steel, organic materials... Archaeological Analogues of Materials: Glass: Years Old What is the point? Some safety assessments assume that vitrified HLW dissolves completely in 1000 years. illustrates conservatism

40 Analogue: Human vs. Hazard Timescales 300-year Open Period 10,000 Main YM Performance Period Classical Period (Greece, Rome) Iron Age Bronze Age First Textiles (Catal Huyuk) First Cu Smelting (Catal Huyuk) end of last glacial period in N America First Pottery (Japan) Relative Activity of HLW 5,000 First Glass Chalcolithic Period ( Copper Age ) First Cities (Sumeria) First Writing and use of the Wheel Neolithic Period ( New Stone Age ) ka BP 0 Limits to Safety Assessment of Geological Disposal Elements to be represented EBS & host rock Hydrogeological system Surface environment processes Radiological exposure modes Changes acting on these elements Geological change Climatic change Ecological change Human activities human intrusion Individual habits years Predictability of changes into the future?