Southwest Research Institute 1995 Annual Report

The Center for Nuclear Waste Regulatory Analyses (CNWRA) provides timely and sustained technical assistance and research in support of the U.S. Nuclear Regulatory Commission's (NRC) high-level nuclear waste management regulatory program. To aid other nations in their planning for high-level waste storage, the NRC has permitted the CNWRA to cooperate with other nations in their activities toward the same end. Thus far, collaborative programs have been conducted with the Swedish Radiation Protection Institute as well as with groups in the United Kingdom and Canada.

The CNWRA aids the NRC in developing criteria with which to assess the adequacy of the license application to be submitted by the U.S. Department of Energy (DOE) for designing, constructing, operating, and ultimately closing an underground repository for spent nuclear reactor fuel and high-level nuclear wastes. Current emphasis is on a repository at Yucca Mountain, Nevada. Through a variety of research, inspection, and analysis activities, the CNWRA assists the NRC in formulating and providing guidance to the DOE, evaluating repository site characterization and design activities, and efficiently implementing the repository licensing process. These efforts will help determine whether the planned repository can adequately protect the radiological health and safety of the public in future millennia.

The CNWRA completed a multiyear laboratory and analytical investigation of cation exchange, the reaction kinetics and solubility of selected minerals, and conceptual and numerical geochemical modeling of repository systems. The investigation demonstrated that a simple thermodynamics formulation, the Margules equation, was adequate to predict ion exchange between aqueous solutions and a zeolite mineral that is commonly found in the Yucca Mountain vicinity.

Experimental and modeling studies continued on the chemical behavior of mineral phases important to radionuclide sorption. Scientists compared three mathematical models with laboratory data to assess the models' abilities to predict the effects of changing system chemistry on sorption rates. The models were found to match the experimental data well. Because the models are based on accepted geochemical principles, it is believed they will be effective in predicting most geochemical conditions of interest at Yucca Mountain.

Scale-model experiments, dimensional analyses, and numerical modeling were completed for a thermohydrology project. Results indicated that, for nonlinear thermohydrologic processes, dimensional analysis was not generally applicable. It was concluded that thermohydrologic experiments would have to be conducted at different scales to develop appropriate rules for scaling.

As part of an effort to understand the complex coupling of thermal, hydrological, chemical, and mechanical processes, a study of mechanical-hydrological coupling was initiated under the aegis of an international cooperative study on the development and validation of coupled models. Preliminary results indicate significant channeling of flow within rock joints and relatively large changes in permeability when the joints are subjected to normal and shear displacements.

To assess certain aspects of underground facility design, CNWRA research has focused on the effects of repeated seismic loads on the response of jointed rock masses. Reports published this year addressed the characterization of rock joints using a novel fractal approach, displacements near underground openings and changes in groundwater level caused by nearby mining-induced seismic events, and laboratory studies of single rock joints and scale-model underground openings.

Scientists completed a three-dimensional geologic framework model using EarthVision" software. This preliminary model includes eight stratigraphic units at Yucca Mountain, three potentially important faults, and the water table. Appropriate data are also being incorporated into an extensive geographic information system database to use during review of critical geochemical, hydrological, seismic, tectonic, and volcanological features of the proposed repository site.

The behavior of faults in the Yucca Mountain area will be important to repository performance. Studies this year examined factors influencing deformation in curved fault systems. In addition, software was developed to evaluate the tendency of faults and fractures to dilate or slip, depending on the orientation and magnitude of in situ stresses. Efforts to integrate this software into a three-dimensional dynamic simulation code are under way.

Because of its potentially significant consequences, the probability of volcanism in the vicinity of a repository continues to be a focus of investigation. The spatial distribution and timing of basaltic volcanism near the Yucca Mountain site were modeled using three nonhomogeneous methods to obtain nonparametric estimates of spatial and spatio-temporal recurrence rates. Active volcanoes are also being studied to evaluate the range of consequences of volcanic eruptions.

Engineers completed an experimental investigation of localized corrosion, stress corrosion cracking, materials thermal stability, and hydrogen embrittlement. The primary objective was to identify a critical parameter for predicting corrosion rates in waste container materials. It was found that the electrical potential below which corrosion stops can provide bounding values for long-term pitting, crevice corrosion, and stress corrosion cracking in nickel-based alloys. In addition, the CNWRA began to assess microbial effects on corrosion.

The CNWRA developed a computer code to demonstrate the ability to evaluate waste package containment in the context of regulatory requirements. In addition, a relatively simple repository geometry was used to simulate the evolution of potential waste-package environments. A two-phase flow model was used to calculate relative humidity in the vicinity of waste packages and to calculate the degree of saturation above and below the waste-emplacement horizon.

Studies were initiated under the third phase of the joint CNWRA-NRC iterative performance assessment effort. Investigations were selected based on previous studies that indicated either large uncertainty or high impact on repository performance. These include infiltration of water from natural sources, identification of key radionuclides, and assessment of exposure to future generations.

Copyright© 1995 by Southwest Research Institute. All rights reserved under U.S. Copyright Law and International Conventions. No part of this publication may be reproduced in any form or by any means, electronic or mechanical, including photocopying, without permission in writing from the publisher. All inquiries should be addressed to Communications Department, Southwest Research Institute, P.O. Drawer 28510, San Antonio, Texas 78228-0510.