Southwest Research Institute 1998 Annual Report, Center for Nuclear Waste Regulatory Analyses

Providing support to the Nuclear Regulatory Commission (NRC) on the high-level waste (HLW) management regulatory program continues to be the largest focus area of the Center for Nuclear Waste Regulatory Analyses (CNWRA). As this vitally important national program moved forward, political and economic forces played an increasing role in additional projects undertaken by CNWRA staff. Utility industry concerns over the continued safe storage of spent nuclear fuel at reactor sites have stimulated both government- and industry-sponsored initiatives to develop interim storage sites. The NRC has tasked the CNWRA to support evaluations of their proposed designs and sites. In addition, strong worldwide demand for uranium has increased work related to licensing uranium mining operations throughout the western U.S. During 1998, the CNWRA also made significant contributions toward solving technical problems for the petroleum exploration and gas pipeline industries.

In late 1998, the Department of Energy (DOE) will meet the first of a series of congressionally mandated milestones related to the HLW repository program. The DOE will complete an analysis of the proposed repository at Yucca Mountain, Nevada. This analysis, known as the viability assessment, will determine the technical and financial viability of the project and form the basis for a decision on whether to proceed with developing the proposed repository. Based on their independent scientific work, CNWRA engineers and scientists have developed, along with the NRC, review procedures and acceptance criteria to be applied to this important appraisal. The CNWRA will continue its research and development activities to ensure the availability of appropriate tools, laboratory and field data, mathematical models, and computer codes for an effective review of the final license application beginning in 2002.

Over the past year, CNWRA staff completed and delivered several versions of the Total-system Performance Assessment (TPA) code used to conduct probabilistic safety evaluations for the proposed HLW repository site. This sophisticated, modular code estimates the expected total radiological dose to nearby inhabitants from radionuclides that could be released from the repository over the 10,000-year compliance period. The TPA code is used to create thousands of simulations of the evolution of the repository. Variables include climate change, seismic and volcanic disruptions, movement of groundwater through unsaturated and saturated zones, corrosion of waste containers, dissolution of radioactive wastes, and possible transport of radioactive elements through rocks and soils to water wells. The effects of any changes in model parameters on repository performance can be readily evaluated using this code.

A two-part approach is being taken to evaluate the effects of HLW container material selection on repository performance. Mechanistic numerical modeling has demonstrated the importance of container life on radiological dose and provided quantitative estimates of the effect of changing the container material from a nickel-chromium-iron alloy (825) to a more corrosion-resistant alloy (C-22) with higher percentages of both nickel and molybdenum. The CNWRA container life model compares the repassivation potential, which depends on both alloy composition and the chloride concentration of the environment contacting the container, with the corrosion potential, which is dependent on the oxidizing potential in the environment. Complex processes, such as galvanic coupling between carbon steel and alloy 825 overpacks and the effects of steel corrosion products on coupling efficiency were calculated and an abstracted model was integrated into the TPA computer code. Long-term corrosion experiments were conducted to evaluate the mechanism for stable growth of localized corrosion in carbon steel and the environmental conditions under which stable pit growth can occur.

Ground motion caused by earthquakes is one of several natural phenomena critical to the engineering design of surface and subsurface facilities at the proposed HLW repository. Newly devised finite element simulations conducted by CNWRA engineers indicate that patterns of ground motion amplitudes from normal-fault earthquakes are controlled by the concentration of seismic energy generated by fault slip in a few shear-wave pulses that propagate along a path perpendicular to the fault. An important finding of this research is that maximum ground motion occurs along the path of this pulse, giving rise to a ground motion pattern that is significantly different from the patterns predicted using conventional ground motion attenuation models.

To provide technical support for NRC oversight of the DOE tank waste remediation system program, CNWRA engineers conducted an extensive survey of high-level nuclear waste vitrification, other waste solidification, and commercial glass melting processes. The results of the survey provide useful input to safety assessments of the treatment of radioactive waste at the Hanford Reservation in Washington. To quantify radiological risk, the complex chemistry associated with retrieving, mixing, separating, evaporating, and vitrifying tank wastes was evaluated. CNWRA staff are reviewing the disposal of aluminum-clad research reactor fuels and the classification of wastes stored in underground tanks at the Hanford, Washington, and Savannah River, South Carolina, sites.

In a study sponsored by the Pipeline Research Committee International, factors affecting internal corrosion of natural gas pipelines were systematically examined using a combination of factorial corrosion experiments, Raman spectroscopy of corrosion products, and thermodynamic modeling. The model showed the conditions under which precipitation of iron sulfide and carbonate films, and calcium and magnesium scales on the pipeline surface, would be expected. Raman spectroscopy was used to confirm precipitation of films and scales on the metal surfaces. When combined with experimental observations, this thermodynamic modeling provides a powerful approach for predicting when corrosion inhibitors should be introduced in pipelines.

The chemical environment beneath disbonded steel-pipeline coatings is important in determining the degree of cathodic protection needed, as well as the occurrence of stress corrosion cracking on the outside surface of the pipeline. To address this problem, CNWRA scientists developed a comprehensive reactive transport model known as TECTRAN for the Gas Research Institute. The effects of external groundwater composition, level of cathodic protection, and disbonded coating geometry were examined and compared to results obtained from controlled laboratory experiments. Qualitative agreement between the experiments and the model was obtained in this first phase of the three-year study. When completed, TECTRAN will be applied to risk assessment and management of pipelines.

CNWRA support to NRC regulation of the uranium mining industry includes technical and regulatory evaluation of license renewals, preparation of environmental assessments, and the development of regulations and regulatory guidance documents. In addition, CNWRA staff conduct evaluations of groundwater protection and cleanup programs, reviews of mining site surface contamination cleanup and reclamation designs, and assessments of seismic stability. This work integrates diverse technical skills in groundwater and surface water hydrology, geochemistry, geotechnical engineering, radiation health physics, and regulatory analysis to support timely licensing actions.

Copyright© 1998 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, phone (210) 522-2257, fax (210) 522-3547.