Engineered Barrier System Degradation, Flow, and Transport Process Model Report

This Analysis/Model Report (AMR) documents the development of an expected-case model for unsaturated zone (UZ) flow and transport that will be described in terms of the representativeness of models of the natural system. The expected-case model will provide an evaluation of the effectiveness of the natural barriers, assess the impact of conservatism in the Total System Performance Assessment (TSPA), and support the development of further models and analyses for public confidence building. The present models used in ''Total System Performance Assessment for the Site Recommendation'' (Civilian Radioactive Waste Management System Management and Operating Contractor (CRWMS M&O) 2000 [1532461]) underestimate the natural-barrier performance because of conservative assumptions and parameters and do not adequately address uncertainty and alternative models. The development of an expected case model for the UZ natural barrier addresses issues regarding flow-pattern analysis and modeling that had previously been treated conservatively. This is in line with the Repository Safety Strategy (RSS) philosophy of treating conservatively those aspects of the UZ flow and transport system that are not important for achieving regulatory dose (CRWMS M&O 2000 [153246], Section 1.1.1). The development of an expected case model for the UZ also provides defense-in-depth in areas requiring further analysis of uncertainty and alternative models. In general, the value of the conservative case is to provide a more easily defensible TSPA for behavior of UZ flow and transport processes at Yucca Mountain. This AMR has been prepared in accordance with the ''Technical Work Plan for Unsaturated Zone (UZ) Flow and Transport Process Model Report'' (Bechtel SAIC Company (BSC) 2001 [155051], Section 1.3 - Work Package 4301213UMG). The work scope is to examine the data and current models of flow and transport in the Yucca Mountain UZ to identify models and analyses where conservatism may be reduced and uncertainty may be better characterized either through new analyses or development of new, alternative, models. This AMR supports Site Recommendation (SR) activities by documenting analyses and models reported in ''FY01 Supplemental Science and Performance Analyses, Volume 1: Scientific Bases and Analyses'' (BSC 2001 [155950]). However, except for the model results reported in Section 6.4.2, this AMR does not support TSPA as documented in either ''FY01 Supplemental Science and Performance Analyses, Volume 2: Performance Analyses'' (BSC 2001 [1546591]) or ''Total System Performance Assessment for the Site Recommendation'' (CRWMS M&O 2000 [153246]). Furthermore, this AMR represents scoping studies to aid in determining whether work beyond the scope of the models and analyses used to support TSPA-SR (CRWMS M&O 2000 [153246]) is warranted. If the decision is made to use any of these models and analyses for TSPA for License Application, further verification and validation will be required. The model results reported in Section 6.4.2 are used for TSPA sensitivity studies as documented in ''FY01 Supplemental Science and Performance Analyses, Volume 2: Performance Analyses'' (BSC 2001 [154659], Section 3.2.2.3).

The purpose of this Analysis/Model Report (AMR) is to evaluate (by means of 2-D semianalytical and 3-D numerical models) the transport of radioactive solutes and colloids in the unsaturated zone (UZ) under ambient conditions from the potential repository horizon to the water table at Yucca Mountain (YM), Nevada. This is in accordance with the ''AMR Development Plan U0060, Radionuclide Transport Models Under Ambient Conditions'' (CRWMS M and O 1999a). This AMR supports the UZ Flow and Transport Process Model Report (PMR). This AMR documents the UZ Radionuclide Transport Model (RTM). This model considers: the transport of radionuclides through fractured tuffs; the effects of changes in the intensity and configuration of fracturing from hydrogeologic unit to unit; colloid transport; physical and retardation processes and the effects of perched water. In this AMR they document the capabilities of the UZ RTM, which can describe flow (saturated and/or unsaturated) and transport, and accounts for (a) advection, (b) molecular diffusion, (c) hydrodynamic dispersion (with full 3-D tensorial representation), (d) kinetic or equilibrium physical and/or chemical sorption (linear, Langmuir, Freundlich or combined), (e) first-order linear chemical reaction, (f) radioactive decay and tracking of daughters, (g) colloid filtration (equilibrium, kinetic or combined), and (h) colloid-assisted solute transport. Simulations of transport of radioactive solutes and colloids (incorporating the processes described above) from the repository horizon to the water table are performed to support model development and support studies for Performance Assessment (PA). The input files for these simulations include transport parameters obtained from other AMRs (i.e., CRWMS M and O 1999d, e, f, g, h; 2000a, b, c, d). When not available, the parameter values used are obtained from the literature. The results of the simulations are used to evaluate the transport of radioactive solutes and colloids, and to determine the processes, mechanisms, and geologic features that have a significant effect on it. They evaluate the contributions of daughter products of radioactive decay to transport from the bottom of the potential repository to the water table. The effect of the various conceptual models of perched water bodies on transport is also evaluated. Note that a more thorough study of perched water bodies can be found in another AMR (CRWMS M and O 1999d, Sections 6.2 and 6.6). The primary caveat for using the modeling results documented here is that the input transport parameters were based on limited site data. For some input parameters, best estimates were used because no specific data were available. An additional caveat is that the RTM is based on the conceptual models and numerical approaches used for developing the flow fields and infiltration maps, and thus they share the same limitations.

As mandated in the Nuclear Waste Policy Act of 1982, the U.S. Department of Energy (DOE) has been investigating a candidate site at Yucca Mountain, Nevada, to determine whether it is suitable for development of the nation's first repository for permanent geologic disposal of spent nuclear fuel (SNF) and high-level radioactive waste (HLW). The Nuclear Waste Policy Amendments Act of 1987 directed that only Yucca Mountain be characterized to evaluate the site's suitability. Three main components of the DOE site characterization program are testing, design, and performance assessment. These program components consist of: Investigation of natural features and processes by analyzing data collected from field tests conducted above and below ground and from laboratory tests of rock, gas, and water samples Design of a repository and waste packages tailored to the site features, supported by laboratory testing of candidate materials for waste packages and design related testing in the underground tunnels where waste would be emplaced Quantitative estimates of the performance of the total repository system, over a range of possible conditions and for different repository configurations, by means of computer modeling techniques that are based on site and materials testing data and accepted principles of physics and chemistry. To date, DOE has completed and documented four major iterations of total system performance assessment (TSPA) for the Yucca Mountain site: TSPA-91 (Barnard et al. 1992), TSPA-93 (Wilson et al. 1994; CRWMS M and O 1994), TSPA-95 (CRWMS M and O 1995), and the Total System Performance Assessment-Viability Assessment (TSPA-VA) (DOE 1998a, Volume 3). Each successive TSPA iteration has advanced the technical understanding of the performance attributes of the natural features and processes and enhanced engineering designs. The next major iteration of TSPA is to be conducted in support of the next major programmatic milestone for the DOE, namely the Site Recommendation (SR). The Total System Performance Assessment-Site Recommendation (TSPA-SR) will present a compliance evaluation of overall system performance against the guidelines and requirements in the revision of the DOE siting guidelines to be promulgated at 10 CFR 963, U.S. Nuclear Regulatory Commission (NRC) regulation for HLW disposal at proposed 10 CFR 63 (the proposed rule has been published at 64 FR 8640), and U.S. Environmental Protection Agency (EPA) environmental radiation protection standard to be promulgated at 40 CFR 197. At present, the NRC has issued the proposed 10 CFR 63 (64 FR 8640) for public comment whereas the EPA standard and the revised DOE siting guidelines are currently being developed. EPA has announced the release of 40 CFR 197 proposed rule on its website (www.epa.gov/radiation/yucca/rule.qui.htm) and the Federal Register announcement (which initiates the 90 day public comment period) is expected by the end of August, 1999. The purpose of this document is to present the overall goals, objectives, scope, methods, approach, and assumptions to be used in the development of the TSPA-SR. This document will serve as a communication tool for coordinating the DOE TSPA activities and for keeping the NRC staff informed of the TSPA activities for the SR.

The purpose of this study was to evaluate the influence of the intake of stable isotopes of carbon and iodine on the committed doses due to the ingestion of {sup 14}C and {sup 129}I. This was accomplished through the application of two different computational approaches. The first was based on the assumption that ground (drinking) water was the only source of intake of both {sup 14}C and {sup 129}I and stable carbon and stable iodine. For purposes of the second approach, the intake of {sup 14}C and {sup 129}I was still assumed to be only that in the ground (drinking) water, but the intake of stable carbon and stable iodine was assumed to be that in the drinking water plus other components of the diet. The doses were estimated using either a conversion formula or the applicable dose coefficients in Federal Guidance Reports No. 11 and No. 13. Serving as input for the analyses was the estimated maximum concentration of {sup 14}C or {sup 129}I that would be present in the ground water due to potential releases from the proposed Yucca Mountain high-level radioactive waste repository during the first 10,000 years after closure. The estimated concentrations of stable carbon and iodine were based on analyses of ground water samples collected in the Amargosa Valley, NV. Based on the accompanying analyses, three conclusions were reached. First, no dose estimate, using a conversion formula in which the ratios of the stable to radioactive isotopes of an element serve as input, should ever be made without including the stable element intake contributions from all components of the diet. Second, the study suggests that the dose coefficients for {sup 129}I in Federal Guidance Reports No. 11 and No. 12 which, in turn, are based on publications of the ICRP, may not be appropriate for application in developed nations of the world, especially those in which relatively large amounts of seafood are consumed and the use of iodized salt is common. The estimated average daily intake of stable iodine by the adult U.S. population, for example, is 400 pg. This is twice the value listed by the ICRP for Reference Man. This leads to a dose estimate that is too high by a factor of two. Although the ICRP accounts for stable isotope contributions through the selection of a corresponding biological half-time for iodine, the selection in this case may need reevaluation especially with respect to assessments of potential {sup 129}I releases from the proposed Yucca Mountain high-level radioactive waste repository. The third conclusion, which confirms earlier studies, is that an increase in the intake of either {sup 14}C or {sup 129}I will not lead to an increase in the dose if there is a corresponding increase in the intake of stable carbon or iodine such that the ratio of {sup 14}C or {sup 129}I to stable carbon or iodine does not change.

Knowledge of unsaturated flow and transport in porous media is critical for understanding the movement of water and solute through the unsaturated zone. The suction potential of rock determines the imbibition of water and, therefore, the moisture retention in the matrix. That, in turn, affects the relative importance of matrix flow and fracture flow, and their interaction, because greater suction potential moves more water from fractures into the matrix and therefore retards fracture flow. The moisture content as a function of the suction potential is called a moisture retention curve or a characteristic curve. Moisture-retention data are important input for numerical models of water movement in unsaturated porous media. Also important are the effect of sample history on the moisture-retention curves and whether there is significant hysteresis between wetting and drying measurements. The Yucca Mountain Site Characterization Project (YMP) of the U.S. Department of Energy is studying the suitability of the tuffaceous rock at Yucca Mountain, Nevada, for a potential high-level nuclear waste repository. The potential repository horizon will be in the unsaturated zone of the Topopah Spring member (densely welded) of the Paintbrush Tuff unit at Yucca Mountain. This unit is highly fractured. Therefore, transport of water within the near field of the nuclear waste package in the repository is strongly influenced by the suction potential of the repository host rocks at elevated temperatures. In a high-level nuclear waste repository, the rock mass around the waste packages will become dry because of the thermal load of the waste but will then re-wet during the cool-down period as the thermal output of the waste packages declines. Much of this process will occur at temperatures above ambient temperature. The goal of our work is to determine the importance of temperature and the wetting-drying hysteresis on the measured moisture retention curves of the densely welded tuff. For Topopah Spring tuff the suction potential is assumed to be primary due to the matric potential.

Overview of total system model used for the 2008 performance assessment for the proposed high-level radioactive waste repository at Yucca Mountain, Nevada

Since 1985, the State of Nevada has sponsored academic/private sector research into various health, safety, and environmental issues identified with the Yucca Mountain site. This research has been documented in scientific peer-reviewed literature, conferences, and workshops, as well as numerous state-sponsored University thesis and dissertation programs. This document is a bibliography of the scientific articles, manuscripts, theses, dissertations, conference symposium abstracts, and meeting presentations produced as a result of state-sponsored research.

This report describes the selection of water compositions for the total system performance assessment (TSPA) model of results from the thermal-hydrological-chemical (THC) seepage model documented in ''Drift-Scale THC Seepage Model'' (BSC 2004 [DIRS 169856]). The selection has been conducted in accordance with ''Technical Work Plan for: Near-Field Environment and Transport: Coupled Processes (Mountain-Scale TH/THC/THM, Drift-Scale THC Seepage, and Post-Processing Analysis for THC Seepage) Report Integration'' (BSC 2004 [DIRS 171334]). This technical work plan (TWP) was prepared in accordance with AP-2.27Q, ''Planning for Science Activities''. Section 1.2.3 of the TWP describes planning information pertaining to the technical scope, content, and management of this report. The post-processing analysis for THC seepage (THC-PPA) documented in this report provides a methodology for evaluating the near-field compositions of water and gas around a typical waste emplacement drift as these relate to the chemistry of seepage, if any, into the drift. The THC-PPA inherits the conceptual basis of the THC seepage model, but is an independently developed process. The relationship between the post-processing analysis and other closely related models, together with their main functions in providing seepage chemistry information for the Total System Performance Assessment for the License Application (TSPA-LA), are illustrated in Figure 1-1. The THC-PPA provides a data selection concept and direct input to the physical and chemical environment (P&CE) report that supports the TSPA model. The purpose of the THC-PPA is further discussed in Section 1.2. The data selection methodology of the post-processing analysis (Section 6.2.1) was initially applied to results of the THC seepage model as presented in ''Drift-Scale THC Seepage Model'' (BSC 2004 [DIRS 169856]). Other outputs from the THC seepage model (DTN: LB0302DSCPTHCS.002 [DIRS 161976]) used in the P&CE (BSC 2004 [DIRS 169860], Section 6.6) were also subjected to the same initial selection. The present report serves as a full documentation of this selection and also provides additional analyses in support of the choice of waters selected for further evaluation in ''Engineered Barrier System: Physical and Chemical Environment'' (BSC 2004 [DIRS 169860], Section 6.6). The work scope for the studies presented in this report is described in the TWP (BSC 2004 [DIRS 171334]) and other documents cited above and can be used to estimate water and gas compositions near waste emplacement drifts. Results presented in this report were submitted to the Technical Data Management System (TDMS) under specific data tracking numbers (DTNs) as listed in Appendix A. The major change from previous selection of results from the THC seepage model is that the THC-PPA now considers data selection in space around the modeled waste emplacement drift, tracking the evolution of pore-water and gas-phase composition at the edge of the dryout zone around the drift. This post-processing analysis provides a scientific background for the selection of potential seepage water compositions.

The Topopah Spring Member of the Paintbrush Tuff is an ash-flow cooling unit that is the candidate host rock for a potential high-level nuclear waste repository at Yucca Mountain, Nevada. The repository workings will be mostly confined to the member`s rhyolitic portion, which is chemically homogenous but texturally variable. This report describes the status of work to develop a useful internal stratigraphy for the rhyolitic portion of the member; our approach is to use an image analysis technique to map textural variations within the member as a function of stratigraphic height. Fifteen petrographic thin sections of Topopah Spring rhyolitic tuff were studied in each of two drill holes (USW GU-3 and USW G-4). Digital color images were collected in transmitted light for two scenes 1 cm on a side for each thin section. Objects within a scene were classified by color, and measurements of area, elongation, and roughness were determined for each object. Summary statistics were compiled for all measurements for each color component within a scene, and each variable was statistically examined for correlations with stratigraphic position. Our initial studies using image analysis have not yet produced a useful method for determining stratigraphic position within the Topopah Spring Member. Simplifications made in this preliminary application of image analysis may be largely responsible for these negative results. The technique deserves further investigation, and more detailed analysis of existing data is recommended. 9 refs., 11 figs., 4 tabs.

The design of a radioactive waste repository typically involves a multi-barrier system, including the natural barrier system (NBS), i.e., the host rock and its surrounding subsurface environment, and an engineered barrier system (EBS). The EBS is to be constructed from the man-made, engineered materials placed within a repository. The repository includes the waste form, waste canisters, buffer materials, backfill, and seals. The most common buffer material for EBS is compacted bentonite, which features low permeability and high retardation of radionuclides. Extensive studies concerning the behavior of bentonite backfill in crystalline and argillite/shale geologic environments for nuclear waste disposal have been conducted by means of laboratory experiments, numerical modeling, and large-scale in situ tests in Underground Research Laboratories (URLs) in Switzerland, France, Belgium, and Japan. This report includes the results of LBNL’s research activities conducted according to the objectives and scope of the work packages “SF-20LB01030802 Engineered Barrier System R&D - LBNL” and “SF-20LB01030806 Engineered Barrier System International Collaborations - LBNL” of the Spent Fuel and Waste Science and Technology (SFWST) (formerly called Used Fuel Disposal) Campaign of the Department of Energy’s (DOE) Office of Nuclear Energy. LBNL research studies included laboratory scale tests and modeling of the evolution of the EBS bentonite and associated coupled processes, and impacts of high temperature on parameters and processes relevant to the performance of crystalline and argillite repositories, including the evaluation of the technical basis for applying the maximum allowable temperature. The results of these studies are being addressing the technical elements necessary to evaluate the EBS design concepts. Emphasis is on the study of thermal, mechanical, and chemical processes that influence the performance of EBS, and the development of modeling capability for reliable assessment of these processes, and ultimately supporting the development of the GDSA model with detailed coupled THMC process models.

This report presents results of the Unsaturated Zone Flow Model Expert Elicitation (UZFMEE) project at Yucca Mountain, Nevada. This project was sponsored by the US Department of Energy (DOE) and managed by Geomatrix Consultants, Inc. (Geomatrix), for TRW Environmental Safety Systems, Inc. The objective of this project was to identify and assess the uncertainties associated with certain key components of the unsaturated zone flow system at Yucca Mountain. This assessment reviewed the data inputs, modeling approaches, and results of the unsaturated zone flow model (termed the ''UZ site-scale model'') being developed by Lawrence Berkeley National Laboratory (LBNL) and the US Geological Survey (USGS). In addition to data input and modeling issues, the assessment focused on percolation flux (volumetric flow rate per unit cross-sectional area) at the potential repository horizon. An understanding of unsaturated zone processes is critical to evaluating the performance of the potential high-level nuclear waste repository at Yucca Mountain. A major goal of the project was to capture the uncertainties involved in assessing the unsaturated flow processes, including uncertainty in both the models used to represent physical controls on unsaturated zone flow and the parameter values used in the models. To ensure that the analysis included a wide range of perspectives, multiple individual judgments were elicited from members of an expert panel. The panel members, who were experts from within and outside the Yucca Mountain project, represented a range of experience and expertise. A deliberate process was followed in facilitating interactions among the experts, in training them to express their uncertainties, and in eliciting their interpretations. The resulting assessments and probability distributions, therefore, provide a reasonable aggregate representation of the knowledge and uncertainties about key issues regarding the unsaturated zone at the Yucca Mountain site.

The US Department of Energy is evaluating a site located at Yucca Mountain in Nye County, Nevada, as a potential high-level nuclear waste repository. The rock at the proposed repository horizon (above the water table) is densely welded, devitrified tuff, and the fluid environment in the repository is expected to be primarily air-steam. A more severe environment would be present in the unlikely case of intrusion of vadose groundwater into the repository site. For this repository location, austenitic stainless steels and copper-based materials are under consideration for waste container fabrication. This study focuses on the effects of gamma irradiation on the electrochemical mechanisms of corrosion for the prospective waste container materials. The radiolytic production of such species as hydrogen peroxide and nitric acid are shown to exert an influence on corrosion mechanisms and kinetics.

ABSTRACTA two-layer waste package (carbon steel outer barrier and Alloy 825 inner barrier) is specified to dispose of high-level nuclear waste at the potential repository at Yucca Mountain. A set of improvements and more realism have been added to a stochastic waste-package degradation model which was developed for a recent total system performance assessment of the potential repository [1]. The waste-package surface is divided into “patches” to better represent the general corrosion of the carbon-steel outer barrier. The “corrosion-time” concept is developed to represent the corrosion of the carbon-steel outer barrier in changing exposure conditions with time such as those expected in the potential repository. With the patches approach and the corrosion-time concept implemented into the waste-package degradation model, sensitivity of the waste package degradation (failure and pitting degradation) to different threshold spalling thicknesses of the corrosion products from the carbon-steel outer barrier is analyzed. The results show that the waste-package pitting degradation is sensitive to the corrosion-products spalling thickness of the carbon-steel outer barrier. A greater pitting degradation of the waste packages is predicted with a smaller spalling thickness. Further understanding of the corrosion-products spalling in different exposure conditions (i.e., water chemistry, water contact mode, etc.) and its effects on carbon steel corrosion is needed to enhance the confidence in the waste-package performance modeling in the potential repository.

Geological repositories for disposal of high-level nuclear waste generally rely on a multibarrier system to isolate radioactive waste from the biosphere. The multi-barrier system typically consists of the natural barrier system (NBS), which includes the repository host rock and its surrounding subsurface environment, and the engineered barrier system (EBS). The EBS represents the man-made, engineered materials placed within a repository, including the waste form, waste canisters, buffer materials, backfill, and seals (OECD, 2003). The EBS plays a significant role in the containment and long-term retardation of radionuclide release.

This Analysis/Model Report (AMR) documents an analysis that was performed to estimate climatic variables for the next 10,000 years by forecasting the timing and nature of climate change at Yucca Mountain (YM), Nevada (Figure l), the site of a potential repository for high-level radioactive waste. The future-climate estimates are based on an analysis of past-climate data from analog meteorological stations, and this AMR provides the rationale for the selection of these analog stations. The stations selected provide an upper and a lower climate bound for each future climate, and the data from those sites will provide input to the infiltration model (USGS 2000) and for the total system performance assessment for the Site Recommendation (TSPA-SR) at YM. Forecasting long-term future climates, especially for the next 10,000 years, is highly speculative and rarely attempted. A very limited literature exists concerning the subject, largely from the British radioactive waste disposal effort. The discussion presented here is one method, among many, of establishing upper and lower bounds for future climate estimates. The method used here involves selecting a particular past climate from many past climates, as an analog for future climate. Other studies might develop a different rationale or select other past climatesmore » resulting in a different future climate analog.« less