-TP ý_ 114V_ PNL-8444 D/V UC-814 Preliminary Total-System Analysis* of a Potential High-Level Nudear Waste Repository at-Yucca Mountain P. W. Eslinger W. E. Nichols L A. Doremus M. D. White D. W. Engel D. W. Langford T. B. Miley S. 1. Ouderkirk Me T. Murphy January 1993.. Prepared for the US. Department of Energy under Contract DE-AC06-76RLO 1830 Pacific Northwest Laboratory Operated for the US. Department of Energy by Battelle Memorial Institute OBaltelle 11:103 1 °301120191 930131 FR WASTE fIW W 11 PDR ,6W 4%4:$ki DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. 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PACIFIC NORTHWEST LABORATORY operated by BATTELLE MEMORIAL INSTITUTE. forthe UNITED STATES DEPARTMENT OF ENERGY under ContractDE-ACO6-76RLO 1830 Printed'in the United States of America Available to DOE and DOE contractors from the Office of Scientific and Technical Information, P.O. Box 62, Oak Ridge, TN 37831; prices available from (615) 5764401. FTS 626"401. Available to the public from the National Techdical Information Service, U.s. Department of Commerce, 5233 Port Royal Rd., Springfied, VA 22161. a PNL-8444 UC-814 Preliminary Total-System Analysis of a Potential High-Level Nuclear Waste Repository at Yucca Mountain P. W. Eslinger W. E. Nichols L. A. Doremus M. D. White D. W. Engel D. W. Langford(&) T. B. Miley S. J. Ouderkirk(a) M. T. Murphy January 1993 Prepared for U.S. Department of Energy under Contract DE-AC06-76RLO 1830 Pacific Northwest Laboratory Richland, Washington 99352 (a) Wesinghouse Hanford Company Richland, Washington 99352 ,I '. I Summary The placement of high-level radioactive wastes in mined repositories deep underground is considered a disposal method that would effectively isolate these wastes from the environment for long periods of time. In the United States, the U.S. Department of Energy (DOE) is responsible, and, within certain limitations, has the authority to implement the provisions of the Nuclear Waste Policy Act of 1982, as amended in 1987, that provides the regulatory framework for development of a mined geologic repository. However, before a repository can be used for the disposal of nuclear waste it must meet standards established by the U.S. Nuclear Regulatory Commission (NRC 1985), the Environmental Protection Agency (EPA 1985), and must be licensed by the NRC. The Performance Assessment Scientific Support (PASS) program at Pacific Northwest Laboratory (PNL) provides modeling capabilities to the DOE to assist in assessing the performance of any potential repositories. This report describes modeling performed at PNL between May and November 1991 addressing the performance of the entire repository system related to regulatory criteria established by the EPA in 40 CFR Part 191. The geologic straigraphy and materal properties used in this study were chosen in cooperation with performance assessment modelers at Sandia National Laboratories (SNL). Sandia modeled a similar problem using diffrent computer codes and a different modeling philosophy. Pacific Northwest Laboratory performed a few model runs with very complex models, and SNL performed many runs with much simpler (abstracted) models. The modeling presented here is substantially different from previous modeling attempts. Models have been added for gas-phase movement of I4C, human intrusion, volcanic intrusion, glass waste forms, and tectonic induced variations. When compared to earlier calculations, a much broader set of conditions has been addressed. The new models consider thermally driven gas phase movement of radionuclides, and movement of radionuclides in the saturated zone. In addition to model development, new information on radionuclide sorption has been obtained, and new geochemical calculations have been performed to estimate the waste form solubility. The modeling approach described in this report incorporated the latest known data and conceptual model assumptions. However, definitive data from the site are not yet available for many parameters. Consequently, many values were chosen for this exercise based on expert judgment or modeling convenience. Unfortunately, this work did not receive the widespread review within DOE required to ensure that the latest available data were used. For these reasons, the results presented here are not sufficiently advanced to be useful in formulating prudent judgments about site suitability or the expected risk of the potential repository. As a result, the work reported here does not provide a sufficient basis to establish a "baseline" performance assessment. In addition to the cumulative release modeling, doses were estimated for several of the release scenarios. Working Draft 4 of 40 CFR Part 191 (February 3, 1992) includes the possibility that both individual and population dose limits may be established as repository performance criteria. The dose results presented here indicate that the potential repository may be able to meet both individual and population dose criteria. Substantial work is still required to define the exposure scenarios and environmental radionuclide concentrations needed to complete a definitive analysis of the doses that will be caused by postclosure repository operation. iii that will have to be The scenarios modeled are not exhaustive of all possible scenarios they were 'selected because they are believed to contribute the considered for licensing. However, for this of any scenarios to be considered. In summary, the analyses accomplished largest releases potential site for a high support the continued site characterization of Yucca Mountain as a report by the EPA relative to level nuclear waste repository and indicate that the limits established cumulative release of radionuclides are not exceeded. iv Contents Summary ............................................... ......... 1.0 Introduction ......................................... ... ................ ....... .. 1............ 1.1 Background ............................................................................ 1.. 1.2 History ..... ........ "" 1................... "1.3 Objectives ....................... ............... 1.2 1.4 Link with Other OCRWM Tasks .................. .. ................................... 1.3 1.5 Differences from Previous Calculations .............................................. 1.3 1.6 Caveats .................... ............................ 1.3 2.0 Overview of Modeling Approach ............................................................. 2.1 2.1 Site Location ............................................................................. 2.1 2.2 Radionuclide Release Scenarios ......................... 2.1 2.3 Choice of Radionuclides ............................ 2.3 -2.4 Models .................................. 2.5 2.4.1 Radionuclide Source Tcrzm ................................................... 2.5 2.4.2 Unsaturated Zone Hydrology and Specie Mass Tmnsport ........... 2.5 2.4.3 Saturated Zone Hydrology and Specie Mass Transport ................... 2.6 2.4.4 Human Intrusion .................... .. ..... 2.6 2.4.5.- Tectorism .................. *.................................................... 2.6 2.4.6 Basaltic Volcanrsm ............................................................. 2.6 2.4.7 Dose ............... of....... *** ........................ 2.6 3.0 Source-Term Analysis ........................................................................ 3.1 3.1 Description of Release Models ........................................................ 3.1 3.1.1 -Unsaturated Zone ............................................................... 3.1 3.1.2 Saturated Zone ................................................... ............ 3.2 V 3.4 3.1.3 Glass Dissolution Model -................. ..... .. 34 3.2 Source-Term Analysis Description ............................... 3.3 Source-Term Data DescriPtion ......................................................... 3.6 3.9 3.4 Base-Scenario Source-Term Results .......................................... 3.15 3.5 Human-Intrusion Scenario Source-Term Results .. ............ 4.1 4.0 Unsaturated Zone Hydrology and Transport ............. ............................ 4.1 Unsaturated Zone Description .- ........ *............... 4.1 4.1 4.1.1 Hydrologic and Transport Properties .................. ......... •...... 4.1.2 FracturtiNatrix Equivalent-Continuum Model ............... ...... 4.3 .0............................4.5 4.2 liquid Phase .............o....... ... 00 . 4.2.1 Method and Assumptions ..................................................... 4.9 4.2.2 Conceptual and Numerical Models ........... ,.......... .............. 4.9 4.2.3 Simulation Results ............. ............... **............