SANDIA REPORT SAND91-0928 • UC-721 Unlimited Release Printed December 1991 Background Information Presented to the Expert Panel on Inadvertent Human Intrusion into the Waste Isolation Pilot Plant R. V. Guzowski, M. M. Gruebel, Editors W. D. Weart, S. G. Bertram-Howery, R. V. Guzowski, K. F. Brinster, P. N. Swift, S. B. Pasztor, Authors Prepared by Sandia National Laboratories Albuquerque, New Mexico 87185 and Livermore, California 94550 for the United States Department of Energy under Contract DE·AC04·76DP00789 '·(.·: ; ' Issued by Sandia National Laboratories, operated for the United States Department of Energy by Sandia Corporation. NOTICE: This report was prepared as an account of work sponsored by an agency of the United States Government. 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Pasztor2, Authors Sandia National Laboratories Albuquerque, NM 87185 ABSTRACT The Waste Isolation Pilot Plant (WIPP) is planned as a mined geologic repository for the disposal of transuranic (TRU) radioactive wastes generated by defense programs of the United States Department of Energy. One of the criteria for evaluating the suitability of the WIPP for disposal of TRU wastes is compliance with the United States Environmental Protection Agency's (EPA) standards for such facilities. The Containment Requirements of those standards require calculating cumulative releases of radionuclides to the accessible environment by all combinations of events and processes (scenarios) that may affect the escape and transport of radionuclides from the repository for 10,000 years after decommissioning of the facility. Because the release limits established by the EPA are probabilistic, scenario probabilities are also required. A panel of experts was convened to estimate the probabilities of occurrence of the events used in scenario development and to identify additional human-intrusion events for inclusion in a safety assessment of the WIPP. This report documents the background presentations that were made to the panel about the WIPP program, regulatory guidelines, the performance­ assessment program, and site-specific and regional geologic and hydrologic characteristics that may affect the WIPP disposal system. 1 Science Applications International Corporation 2 Tech Reps, Inc. CONTENTS I. INTRODUCTION ............................................................................................................................... 1-1 References ................................................................................................................................. 1-3 II. HISTORY AND OVERVIEW OF THE WASTE ISOLATION PILOT PLANT................................. 11-1 Mission of the WIPP ................................................................................................................. 11-4 Framework of Major Regulations Governing Deep-Geologic Disposal in the U.S. .................................................................................... 11-5 Characteristics of the WIPP .................................................................................................... 11-7 Receipt Schedule for the Wastes ........................................................................................... 11-10 Characteristics of the Shipping Packages for the WIPP ............................................................................................................................ 11-11 Surface and Subsurface Facilities.......................................................................................... 11-13 Performance Assessment Issues to be Resolved ................................................................ 11-15 References................................................................................................................................. 11-28 Ill. APPLICATION OF 40 CFR PART 191, SUBPART B TO THE WASTE ISOLATION PILOT PLANT ........................................................................ 111-1 PREFACE: Synopsis of Considerations for Inadvertent Human Intrusion ................................................................................................ 111-1 40 CFR Part 191, The Standard {1985) .................................................................................. 111-4 Subpart A ......................................................................................................................... 111-4 Subpart B ......................................................................................................................... 111-6 Controlled Area .......................................................................................................... 111-8 Reasonable Expectation ............................................................................................ 111-10 Status of the Standard..................................................................................................... 111-12 Containment Requirements .................................................................................................... 111-12 Performance Assessment............................................................................................... 111-12 Performance-Assessment Methodology........................................................................ 111-15 Scenario Development .............................................................................................. 111-15 Prediction of Releases ............................................................................................... 111-17 Uncertainties .............................................................................................................. 111-18 Compliance-Assessment Procedure ......................................................................... 111-20 Modifying the Requirements........................................................................................... 111-21 Assurance Requirements ........................................................................................................ 111-23 Individual Protection Requirements ...................................................................................... 111-24 Ground Water Protection Requirements ............................................................................... 111-27 Special Sources of Ground Water.................................................................................. 111-27 Requirement Not Relevant to the WIPP ......................................................................... 111-27 References ................................................................................................................................. 111-29 IV. SCENARIO DEVELOPMENT AND MODELING FOR PERFORMANCE ASSESSMENT........................................................................................... IV-1 Introduction ............................................................................................................................... IV-1 Regulatory Basis for Scenarios .............................................................................................. IV-1 Requirement of a Scenario-Selection Procedure................................................................. IV-2 Scenario-Selection Procedure ................................................................................................ IV-2 Application of the Scenario-Development Procedure to the WIPP .......................................................................................................... IV-7 Preliminary Modeling of Scenarios and the Use of Panels' Judgment ...................................................................................................... IV-7 References................................................................................................................................. IV-13 iii V. A SUMMARY OF THE HYDROGEOLOGY AND GEOMORPHOLOGY