Keywords: EPRI TR-100384 High-level radioactive wastes Pr6ject 3055-2 Radioactive waste disposal Interim Report Electric Power Risk assessment May 1992 Research Institute Radionuclide migration Models Demonstration of a Risk-Based Approach to High-Level Waste Repository Evaluation: Phase 2 Prepared by Risk Engineering, Inc. Golden, Colorado 9410180384 941004 PDR WASTE VM-- I PDR REPORT S U M M A R Y Demonstration of a Risk-Based Approach to High-Level Waste Repository Evaluation: Phase 2 An EPRI probability-based methodology to assess the performance of high-level nuclear waste repositories has been upgraded using realistic, credible scientific and engineering inputs. In particular, this methodology considers aqueous and gaseous pathways for release of radionuclides as well as possible releases triggered by inadvertent human intrusion and natural occurrences. Utilities and DOE can apply this methodology to identify critical technical issues that require further investigation. BACKGROUND With cosponsorship from Edison Electric Institute (EEI/UWASTE), INTEREST CATEGORIES EPRI revised and upgraded its high-level waste performance assessment method ology. This methodology is designed to encourage DOE to complete integrated assessments characterizing Nevada's Yucca Mountain as a prospective site for the Radioactive waste nation's high-level waste repository. The previous phase 1 report on this project management (EPRI report NP-7057) demonstrated the feasibility of a risk-based methodology Risk analysis, and illustrated potential insights from its application. management, and assessment OBJECTIVES Waste and water management 0 To convert phase 1 results from a reasonable, illustrative model to a realistic, Waste disposal and use credible phase 2 model. - To include new scenarios in the model, including gaseous releases; time KEYWORDS dependent changes in climate, infiltration, and repository temperature profile; and human intrusion. High-level radioactive wastes APPROACH The project team comprised experts in climatology, surface water Radioactive waste disposal and groundwater hydrology, tectonics, volcanology, geochemistry, waste package Risk assessment design, rock mechanics, human factors, and nuclear engineering. They developed a probability-based logic tree framework for performance assessment and desig Radionuclide migration nated the inputs, models, and uncertainties using discrete distributions on specific Models input assumptions. This approach considered scientific and engineering uncer tainty on the state of nature or on models and parameters used to represent future occurrences. RESULTS The calculational methodology integrates information over all possible combinations of uncertain inputs and for each combination determines the quan tity of radioactive release for 13 radionuclides. The probability distribution of re leases reflects uncertainties in inputs designated by the expert in each respective field. The methodology's calculational techniques have been applied to precipitation and surface water flow, groundwater flow, gaseous release, liquid release from the engineered barrier system, and human intrusion. The previous phase 1 model was revised to represent climate conditions, earthquake and volcanic occurrences, effect on the host rock, temperature changes, effects on the water table as a result of earthquakes, and changes in hydrologics and engineered barrier systems. Power Research Institute EPRI TR-1100384sTR-100384s Electric Power Research Institute DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS REPORT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) NAMED BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS REPORT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (11)THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (111)THAT THIS REPORT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IFEPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS REPORT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED INTHIS REPORT. ORGANIZATION(S) THAT PREPARED THIS REPORT RISK ENGINEERING, INC. Electric Power Research Institute and EPRI are registered service marks of Electric Power Research Institute. Inc Copyright © 1992 Electric Power Research Institute. Inc, All rights reserved ORDERING INFORMATION Requests for copies of this report should be directed to the EPRI Distribution Center, 207 Coggins Drive, PO. Box 23205, Pleasant Hill, CA 94523, (510) 934-4212. There is no charge for reports requested by EPRI member utilities and affiliates. ABSTRACT This project develops and applies a probability-based methodology to assess the performance of high level nuclear waste repositories. The particular method is an extension of the methodology demonstrated previously under Phase 1 of this project. Under the current application, aqueous and gaseous pathways for release of radionuclides are considered, as are possible releases induced by inadvertent human intrusion and by volcanic occurrences. Individual experts in the relevant scientific and engineering fields designate the inputs (and their uncertainties) for the analysis. These inputs are aggregated using the logic tree format so that calculations of levels of release for thirteen radionuclides can be made for all possible combinations of assumed input values. The probability distribution of releases reflects the uncertainties in inputs designated by the expert in each field. The methodology is applied to the proposed repository at Yucca Mountain, Nevada. The application indicates that, for high levels of possible release, the largest quantities of nuclides will escape the 4 repository by aqueous pathways. Gaseous release of 1 C may occur but will be important relative to releases by aqueous pathways only at lower levels of total release. Volcanic disturbances, earthquakes, and inadvertent human intrusion do not lead to large releases. Sensitivity studies for aqueous pathways indicate that critical factors affecting large releases are amount of groundwater infiltration, solubilities of radioelements and dissolution rate of the waste matrix, lateral diversion of the groundwater flow around the repository, characteristics of the engineered barrier system, and coupling between fracture and matrix flow. iii CONTENTS Section 1 INTRODUCTION 1-1 2-1 2 A PROBABILISTIC CLIMATE AND RAINFALL MODEL 2-1 INTRODUCTION 2-1 APPROACH TO THE PROBLEM 2-2 PRESENT CLIMATE AT YUCCA MOUNTAIN CHARACTERIZATION OF GLACIAL CLIMATE 2-5 PERIODICITIES OF GLACIAL/INTERGLACIAL (PLUVIAL/INTERPLUVIAL) EPISODES 2-7 2-7 CHARACTERIZATION OF GREENHOUSE CLIMATE 2-8 THE PRECIPITATION MODEL 2-11 ADJUSTMENTS OF MODEL PARAMETERS 2-12 ESTIMATES OF NET INFILTRATION 2-12 RESULTS OF MODEL RUN PROBABILITIES OF THE VARIOUS CLIMATE SCENARIOS IN THE FUTURE 2-13 2-16 DISCUSSION AND FUTURE IMPROVEMENTS 2-17 REFERENCES 3-1 3 NET INFILTRATION: MODELS AND CALCULATIONS 3-1 INTRODUCTION Background 3-1 Objectives and Approach 3-2 PHYSICAL AND BIOLOGICAL PROCESSES AFFECTING NET INFILTRATION 3-5 Soil Water Flow Model 3-5 VARIABILITY OF CLIMATE 3-7 3-9 SPATIAL VARIABILITY OF NET INFILTRATION 3-11 MODEL CALCULATIONS AND RESULTS 3-11 INTEGRATION OF MODEL RESULTS 3-11 Soil/Hydrologic Units 3-14 Climate Characterization 3-15 SUMMARY AND CONCLUSIONS v REFERENCES 3-16 4 CLIMATE-RELATED CHANGES IN THE WATER TABLE 4-1 SCOPE OF EVALUATION 4-1 REVIEW OF EXISTING INFORMATION 4-1 DESCRIPTION OF THE NODE 4-2 REFERENCES 4-4 5 EARTHQUAKES AND TECTONICS 5-1 SCOPE OF EVALUATION 5-1 EVALUATION OF SECONDARY FAULTING POTENTIAL 5-2 Earthquake Source Model 5-2 Frequency of Fault-Displacement Induced Canister Failure 5-2 EVALUATION OF EARTHQUAKE INDUCED STRESS CHANGES 5-6 SUMMARY 5-7 REFERENCES 5-13 6 WATER TABLE CHANGE DUE TO A NORMAL FAULTING EARTHQUAKE 6-1 INTRODUCTION 6-1 STRESS CHANGE FOLLOWING BASIN AND RANGE EARTHQUAKES 6-3 LAYERED MODEL AND THE COMPRESSIBILITY OF EACH LAYER 6-4 PORE VOLUME CHANGE DUE TO STRESS CHANGE 6-6 WATER TABLE CHANGE 6-8 Uncertainties in Stress Drop, Unsaturated Pore Space, Material Properties 6-10 Results 6-11 REFERENCES 6-13 7 VOLCANO OCCURRENCES 7-1 SCOPE OF EVALUATION 7-1 TEMPORAL MODELS OF VOLCANISM 7-2 SPATIAL MODELS OF VOLCANISM 7-3 THE PROPOSED SPATIAL MODEL 7-5 Example Models 7-6 Computer Code 7-8 Results of Monte Carlo Simulation 7-10 EFFECT OF VOLCANISM 7-12 LOGIC TREE FOR VOLCANISM 7-14 vi bection SUMMARY 7-15 REFERENCES 7-17 8 TEMPERATURE CHANGES CAUSED BY WASTE EMPLACEMENT 8-1 INTRODUCTION 8-1 MECHANISMS OF HEAT TRANSFER 8-1 ANALYSES OF REPOSITORY TEMPERATURE 8-5 SCENARIOS FOR HEAT TRANSFER AT YUCCA MOUNTAIN 8-7 TEMPERATURE SCENARIOS 8-8 SCENARIO LIKELIHOOD 8-10 REFERENCES 8-13 9 TIME-DEPENDENT, THERMO-MECHANICAL DAMAGE IN THE NEAR-FIELD ENVIRONMENT 9-1 INTRODUCTION 9-1 STRESS AND TEMPERATURE BOUNDARY CONDITIONS 9-3 Thermal Stresses Due to Temperature Boundary Condition 9-3 Mechanical Stresses Due to Far-Field Loading 9-5 THERMO-MECHANICAL DAMAGE MODEL