ERUPTION TO DOSE: COUPLING A TEPHRA DISPERSAL MODEL WITHIN A PERFORMANCE ASSESSMENT FRAMEWORK

Sandia National Laboratories (SNL) continued evaluation of total system performance assessment (TSPA) computing systems for the previously considered Yucca Mountain Project (YMP). This was done to maintain the operational readiness of the computing infrastructure (computer hardware and software) and knowledge capability for total system performance assessment (TSPA) type analysis, as directed by the National Nuclear Security Administration (NNSA), DOE 2010. This work is a continuation of the ongoing readiness evaluation reported in Lee and Hadgu (2014), Hadgu et al. (2015) and Hadgu and Appel (2016), and Hadgu et al. (2017). The TSPA computing hardware (2014 server cluster -CL2014) and storage system described in Hadgu et al. (2015) were used for the current analysis. One floating license of Gold Sim with Versions 9.60.300, 10.5, 11.1 and 12.0 was installed on the cluster head node, and its distributed processing capability was mapped on the cluster processors. Other supporting software were tested and installed to support the TSPA-type analysis on the server cluster. The FY18 task included developing an inventory of software used for the Yucca Mountain Project process models and preliminary assessment of status of the software; enhancing security of the cluster and setting a backup system. The 2014 server cluster and supporting software systems are fully operational to support TSPA-LA type analysis. 3 ACKNOWLEDGEMENTS The authors would like to express their gratitude to Tito Bonano (8840) and Kevin McMahon (8842) for their technical interest and programmatic support. The authors would also like to thank David Sassani (8842) for technical review of the report. 4

The Engineered Barrier System Degradation, Flow, and Transport Process Model Report (EBS PMR) is one of nine PMRs supporting the Total System Performance Assessment (TSPA) being developed by the Yucca Mountain Project for the Site Recommendation Report (SRR). The EBS PMR summarizes the development and abstraction of models for processes that govern the evolution of conditions within the emplacement drifts of a potential high-level nuclear waste repository at Yucca Mountain, Nye County, Nevada. Details of these individual models are documented in 23 supporting Analysis/Model Reports (AMRs). Nineteen of these AMRs are for process models, and the remaining 4 describe the abstraction of results for application in TSPA. The process models themselves cluster around four major topics: ''Water Distribution and Removal Model, Physical and Chemical Environment Model, Radionuclide Transport Model, and Multiscale Thermohydrologic Model''. One AMR (Engineered Barrier System-Features, Events, and Processes/Degradation Modes Analysis) summarizes the formal screening analysis used to select the Features, Events, and Processes (FEPs) included in TSPA and those excluded from further consideration. Performance of a potential Yucca Mountain high-level radioactive waste repository depends on both the natural barrier system (NBS) and the engineered barrier system (EBS) and on their interactions. Although the waste packages are generally considered as components of the EBS, the EBS as defined in the EBS PMR includes all engineered components outside the waste packages. The principal function of the EBS is to complement the geologic system in limiting the amount of water contacting nuclear waste. A number of alternatives were considered by the Project for different EBS designs that could provide better performance than the design analyzed for the Viability Assessment. The design concept selected was Enhanced Design Alternative II (EDA II).

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.

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).

Performance assessments of geologic repositories for high-level nuclear waste will be used to determine regulatory compliance. The determination, that with a “reasonable expectation” regulatory limits are met, is based on the presumption that all of the relevant physical, chemical and biological processes have been modeled with enough accuracy to insure that a confident judgment of safety may be made. For the geologic disposal of high-level nuclear waste, this generally means that models must be capable of calculating radiation exposures to a specified population at distances of tens of kilometers for periods of tens to hundreds of thousands of years. A total system performance assessment will consist of a series of cascading models that are meantin tototo capture repository performance. There are numerous sources of uncertainty in these models: scenario uncertainty, conceptual model uncertainty and data uncertainty. These uncertainties will propagate through the analysis, and the uncertainty in the total system analysis must necessarily increase with time. For the highly-coupled, non-linear systems that are characteristic of many of the physical and chemical processes, one may anticipate emergent properties that cannot, in fact, be predicted. For all of these reasons, a performance assessment is not in and of itself a sufficient basis for determining the safety of a repository, but it remains a necessary part of the effort to develop a substantive understanding of a repository site.

A vitrification technology utilizing a lanthanide borosilicate (LaBS) glass appears to be a viable option for dispositioning excess weapons-useable plutonium that is not suitable for processing into mixed oxide (MOX) fuel. A significant effort to develop a glass formulation and vitrification process to immobilize plutonium was completed in the mid-1990s to support the Plutonium Immobilization Program (PIP). Further refinement of the vitrification process was accomplished as part of the Am/Cm solution vitrification project. The LaBS glass formulation was found to be capable of immobilizing in excess of 10 wt% Pu and to be very tolerant of the impurities accompanying the plutonium material streams. Thus, this waste form would be suitable for dispositioning plutonium owned by the Department of Energy-Office of Environmental Management (DOE-EM) that may not be well characterized and may contain high levels of impurities. The can-in-canister technology demonstrated in the PIP could be utilized to dispose of the vitrified plutonium in the federal radioactive waste repository. The can-in-canister technology involves placing small cans of the immobilized Pu form into a high level waste (HLW) glass canister fitted with a rack to hold the cans and then filling the canister with HLW glass. Testing was completed to demonstrate that this technology could be successfully employed with little or no impact to current Defense Waste Processing Facility (DWPF) operation and that the resulting canisters were essentially equivalent to the present HLW glass canisters to be dispositioned in the federal repository. The performance of wastes in the repository and, moreover, the performance of the entire repository system is being evaluated by the Department of Energy-Office of Civilian Radioactive Waste Management (DOE-RW) using a Total System Performance Assessment (TSPA) methodology. Technical bases documents (e.g., Analysis/Modeling Reports (AMR)) that address specific issues regarding waste form performance are being used to develop process models as input to the TSPA analyses. In this report, models developed in five AMRs for waste forms currently slated for disposition in the repository are evaluated for their applicability to waste forms with plutonium immobilized in LaBS glass using the can-in-canister technology. Those AMRs address: high-level waste glass degradation; radionuclide inventory; in-package chemistry; dissolved concentration limits of radioactive elements; and colloid-associated radionuclide concentrations. Based on evaluation of how the models treated HLW glass and similarities in the corrosion behaviors of borosilicate HLW glasses and LaBS glass, the models in the AMRs were deemed to be directly applicable to the disposition of excess weapons-useable plutonium. The evaluations are summarized.

''Total System Performance Assessment-License Application (TSPA-LA) Methods and Approach'' provides the top-level method and approach for conducting the TSPA-LA model development and analyses. The method and approach is responsive to the criteria set forth in Total System Performance Assessment Integration (TSPAI) Key Technical Issues (KTIs) identified in agreements with the U.S. Nuclear Regulatory Commission, the ''Yucca Mountain Review Plan'' (YMRP), ''Final Report'' (NRC 2003 [163274]), and the NRC final rule 10 CFR Part 63 (NRC 2002 [156605]). This introductory section provides an overview of the TSPA-LA, the projected TSPA-LA documentation structure, and the goals of the document. It also provides a brief discussion of the regulatory framework, the approach to risk management of the development and analysis of the model, and the overall organization of the document. The section closes with some important conventions that are used in this document.

Total System Performance Assessments are an important component in the evaluation of the suitability of Yucca Mountain, Nevada as a potential site for a mined geologic repository for the permanent disposal of high-level radioactive wastes in the United States. The Total System Performance Assessments are conducted iteratively during the site characterization to identify issues which should be addressed by the characterization and design activities as well as providing input to regulatory/licensing and programmatic decisions. During fiscal years 1991 and 1992, the first iteration of Total System Performance Assessment (hereafter referred to as TSPA 1991) was completed by Sandia National Laboratories and Pacific Northwest Laboratory. Beginning in fiscal year 1993, the Civilian Radioactive Waste Management System Management and Operating Contractor was assigned the responsibility to plan, coordinate, and contribute to the second iteration of Total System Performance Assessment (hereafter referred to as TSPA 1993). This document presents the objectives, approach, assumptions, input, results, conclusions, and recommendations associated with the Management and Operating Contractor contribution to TSPA 1993. A parallel effort was conducted by Sandia National Laboratories and is reported in Wilson et al. (1994, in press).

The Yucca Mountain Project (YMP) is currently designing a geologic repository for high level nuclear waste. The design encompasses two distinct phases, the pre-closure period where temperatures within the repository will be controlled by active ventilation, and the post-closure period where the repository will be sealed. A prerequisite for designing the repository is the ability to both understand and control the heat generated from the decay of the nuclear waste. This decay heat affects the performance of both the waste packages and the emplacement drift. The ability to accurately model the complex heat transfer within the repository is critical to the understanding of the repository performance. Currently, computational fluid dynamics codes are being used to model the post-closure performance of the repository. Prior to using the codes on the project they were required to be thoroughly validated. Eight pilot-scale tests were performed at the Department of Energy North Las Vegas Atlas Facility to evaluate the processes that govern thermal transport in an environment that scales to the proposed repository environment during the post closure period. The tests were conducted at two geometric scales (25 and 44% of full scale), with and without drip shields, and under both uniform and distributed heat loads. The tests provided YMP specific data for model validation. A separate CFD model was developed for each of the four test configurations. The models included the major components of the experiment, including the waste packages (heated steel canisters), invert floor, and emplacement drift (insulated concrete pipe). The calculated model temperatures of the surfaces and fluids, and velocities, are compared with experimental data.

The purpose of this report is to document the abstraction model being used in total system performance assessment (TSPA) model calculations for radionuclide transport in the unsaturated zone (UZ). The UZ transport abstraction model uses the particle-tracking method that is incorporated into the finite element heat and mass model (FEHM) computer code (Zyvoloski et al. 1997 [DIRS 100615]) to simulate radionuclide transport in the UZ. This report outlines the assumptions, design, and testing of a model for calculating radionuclide transport in the UZ at Yucca Mountain. In addition, methods for determining and inputting transport parameters are outlined for use in the TSPA for license application (LA) analyses. Process-level transport model calculations are documented in another report for the UZ (BSC 2004 [DIRS 164500]). Three-dimensional, dual-permeability flow fields generated to characterize UZ flow (documented by BSC 2004 [DIRS 169861]; DTN: LB03023DSSCP9I.001 [DIRS 163044]) are converted to make them compatible with the FEHM code for use in this abstraction model. This report establishes the numerical method and demonstrates the use of the model that is intended to represent UZ transport in the TSPA-LA. Capability of the UZ barrier for retarding the transport is demonstrated in this report, and by the underlying process model (BSC 2004 [DIRS 164500]). The technical scope, content, and management of this report are described in the planning document ''Technical Work Plan for: Unsaturated Zone Transport Model Report Integration'' (BSC 2004 [DIRS 171282]). Deviations from the technical work plan (TWP) are noted within the text of this report, as appropriate. The latest version of this document is being prepared principally to correct parameter values found to be in error due to transcription errors, changes in source data that were not captured in the report, calculation errors, and errors in interpretation of source data.

ABSTRACTShort-term static tests were conducted with a surrogate high-level waste glass to measure the effects of pH and dissolved iron on the glass dissolution rate. The tests were conducted to determine if a term to account for the effects of dissolved iron is needed in the glass degradation model developed for Total System Performance Assessment (TSPA) calculations for the Yucca Mountain disposal system license application. The glass degradation model includes terms for dependencies on temperature, pH, and chemical affinity. A series of tests was conducted at 90 °C in various pH solutions without iron and with added FeCl3, Fe2O3, Fe3O4, and FeOOH. Tests were conducted at glass surface area/solution volume (S/V) ratios about 2 and 10 m-1for between 2 and 21 days. Solution concentrations of boron were used to measure the extent of reaction and calculate the glass dissolution rates. Similar rates were measured in tests conducted with and without iron at each pH. Both the results of the tests with and without iron showed V-shaped pH dependence curves with minima at near-neutral pH values. The pH dependencies (η) are about 0.44 in basic solutions and –0.49 in acidic solutions, based on the combined results of tests with and without iron. These are within the range of values for the pH dependence in the TSPA model for site recommendation.

In recent years, there has been a growing interest in ensemble approaches for modelling volcanic plumes. The development of such techniques enables the exploration of novel methods for incorporating real observations into tephra dispersal models. However, traditional data assimilation algorithms, including ensemble Kalman filter methods, can yield suboptimal state estimates for positive-definite variables such as volcanic aerosols and tephra deposits. This study proposes two new ensemble-based data assimilation techniques for semi-positive-definite variables with highly skewed uncertainty distributions, including aerosol concentrations and tephra deposit mass loading. The proposed methods are applied to reconstruct the tephra fallout deposit resulting from the 2015 Calbuco eruption using an ensemble of 256 runs performed with the FALL3D dispersal model. Two datasets of deposit thickness measurements are considered: an assimilation dataset including 161 observations, and a validation dataset for an independent assessment of the methods. Results show that the assimilation leads to a significant improvement over the first-guess results, obtained from the simple ensemble forecast. The spatial distribution of the tephra fallout deposit thickness and the ashfall volume according to the analyses are in good agreement with estimations based on field measurements and isopach maps reported in previous studies. Both assimilation methods show a similar performance in terms of evaluation metrics and spatial distribution of the deposit. Finally, the potential application of the methodologies for the improvement of ash-cloud forecasts produced for operational models is also discussed.

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 objective and scope of this calculation is to create the appropriate parameter input for MING 1.0 (CSCI 30018 V1.0, CRWMS M&O 1998b) that will allow the testing of the results from the MING software code with both scientific measurements of microbial populations at the site and laboratory and with natural analogs to the site. This set of calculations provides results that will be used in model validation for the ''In-Drift Microbial Communities'' model (CRWMS M&O 2000) which is part of the Engineered Barrier System Department (EBS) process modeling effort that eventually will feed future Total System Performance Assessment (TSPA) models. This calculation is being produced to replace MING model validation output that is effected by the supersession of DTN MO9909SPAMING1.003 using its replacement DTN MO0106SPAIDM01.034 so that the calculations currently found in the ''In-Drift Microbial Communities'' AMR (CRWMS M&O 2000) will be brought up to date. This set of calculations replaces the calculations contained in sections 6.7.2, 6.7.3 and Attachment I of CRWMS M&O (2000) As all of these calculations are created explicitly for model validation, the data qualification status of all inputs can be considered corroborative in accordance with AP-3.15Q. This work activity has been evaluated in accordance with the AP-2.21 procedure, ''Quality Determinations and Planning for Scientific, Engineering, and Regulatory Compliance Activities'', and is subject to QA controls (BSC 2001). The calculation is developed in accordance with the AP-3.12 procedure, Calculations, and prepared in accordance with the ''Technical Work Plan For EBS Department Modeling FY 01 Work Activities'' (BSC 2001) which includes controls for the management of electronic data.

This report synthesizes the results of volcanism studies conducted by scientists at the Los Alamos National Laboratory and collaborating institutions on behalf of the Department of Energy's Yucca Mountain Project. Chapter 1 introduces the volcanism issue for the Yucca Mountain site and provides the reader with an overview of the organization, content, and significant conclusions of this report. The hazard of future basaltic volcanism is the primary topic of concern including both events that intersect a potential repository and events that occur near or within the waste isolation system of a repository. Future volcanic events cannot be predicted with certainty but instead are estimated using formal methods of probabilistic volcanic hazard assessment (PVHA). Chapter 2 describes the volcanic history of the Yucca Mountain region (YMR) and emphasizes the Pliocene and Quaternary volcanic record, the interval of primary concern for volcanic risk assessment. The distribution, eruptive history, and geochronology of Plio-Quaternary basalt centers are described by individual center emphasizing the younger postcaldera basalt (<5 Ma). The Lathrop Wells volcanic center is described in detail because it is the youngest basalt center in the YMR. The age of the Lathrop Wells center is now confidently determined to be about 75 thousand years old. Chapter 3 describes the tectonic setting of the YMR and presents and assesses the significance of multiple alternative tectonic models. The distribution of Pliocene and Quaternary basaltic volcanic centers is evaluated with respect to tectonic models for detachment, caldera, regional and local rifting, and the Walker Lane structural zone. Geophysical data are described for the YMR and are used as an aid to understand the distribution of past basaltic volcanic centers and possible future magmatic processes. Chapter 4 discusses the petrologic and geochemical features of basaltic volcanism in the YMR, the southern Great Basin and the Basin and Range province. Geochemical and isotopic data are presented for post-Miocene basalts of the Yucca Mountain region. Alternative petrogenetic models are assessed for the formation of the Lathrop Wells volcanic center. Based on geochemical data, basaltic ash in fault trenches near Yucca Mountain is shown to have originated from the Lathrop Wells center. Chapter 5 synthesizes eruptive and subsurface effects of basaltic volcanism on a potential repository and summarizes current concepts of the segregation, ascent, and eruption of basalt magma. Chapter 6 synthesizes current knowledge of the probability of disruption of a potential repository at Yucca Mountain. In 1996, an Expert Elicitation panel was convened by DOE that independently conducted PVHA for the Yucca Mountain site. Chapter 6 does not attempt to revise this PVHA; instead, it further examines the sensitivity of variables in PVHA. The approaches and results of PVHA by the expert judgment panel are evaluated and incorporated throughout this chapter. The disruption ratio (E2) is completely re-evaluated using simulation modeling that describes volcanic events based on the geometry of basaltic feeder dikes. New estimates of probability bounds are developed. These comparisons show that it is physically implausible for the probability of magmatic disruption of the Yucca Mountain site to be greater than 10{sup -7} events per year. Bounding probability estimates are used to assess possible implications of not drilling aeromagnetic anomalies in the Arnargosa Valley and Crater Flat. The results of simulation modeling are used to assess the sensitivity of the disruption probability for the location of northeast boundaries of volcanic zones near the Yucca Mountain site. A new section on modeling of radiological releases associated with surface and subsurface magmatic activity has been added to chapter 6. The modeling results are consistent with past total system performance assessments that show future volcanic and magmatic events are not significant components of repository performance and volcanism is not a priority issue for performance assessment studies.