Center for Nuclear Waste Regulatory Analyses

The Center for Nuclear Waste Regulatory Analyses (CNWRA) at SwRI applies diverse scientific and engineering skills to support the Nuclear Regulatory Commission (NRC) in ensuring that the proposed high-level radioactive waste repository at Yucca Mountain, Nevada, will protect public health and safety as well as the environment. Parallel to this search for an acceptable permanent disposal site, the CNWRA helps evaluate possible hazards resulting from human activity and natural processes at proposed interim storage sites. In addition, non-nuclear programs in the earth and natural sciences continued to grow in breadth and importance. The staff made significant contributions to petroleum exploration and distribution and to groundwater assessment.

In cathodically protected pipelines, areas shielded from cathodic protection are susceptible to corrosion and stress corrosion cracking. CNWRA staff designed and assembled a prototype disbonded coating coupon that simulates a realistic pipeline coating disbondment. The coupon, used with a computer model, can extrapolate the measurements of the coupon to various disbonded geometries found on the actual pipeline, thereby providing a rapid assessment of the risk from disbondments and the efficacy of mitigative treatment.

A team of CNWRA and NRC staff has worked to evaluate the Department of Energy (DOE) site characterization results and analyses, as well as the proposed form of the waste and disposal containers. In response to legislative direction, the team reported on the sufficiency of this information for inclusion in a potential license application. The comments are anticipated to accompany a possible DOE site recommendation to the U.S. president, if DOE decides to recommend the site for development of a repository. The CNWRA staff assisted the NRC in completing the regulation that forms the basis for evaluating the proposed Yucca Mountain repository and the detailed plan containing the methods and criteria for conducting that evaluation. Investigations were conducted in state-of-the-art laboratories, at selected field sites, and with sophisticated computer models, several of which were developed by CNWRA staff.

Federal law allows NRC only 36 months (with the option to extend one year) to evaluate the DOE application to construct and operate a repository for high-level radioactive waste at Yucca Mountain. Given this constraint, the NRC and CNWRA staffs will complete their review of the potential license application in 18 months. The site for the proposed repository, which covers several square miles, must be designed to limit radiation doses to nearby inhabitants during a 10,000-year regulatory period. To focus its review on those aspects of the repository that pose the greatest health and environmental risks, CNWRA staff will be guided by risk insights, such as those derived from calculations using the Total-system Performance Assessment (TPA) code and results from DOE calculations. Staff members are updating this complex software tool, developed in 1991, to incorporate new information about the geology, climatology, and hydrology of the Yucca Mountain site, as well as modifications to DOE's proposed design for waste disposal containers and tunnel geometry.

Independent product evaluations and failure analyses for clients in the energy, marine, automotive, aerospace, and oil and gas industries put SwRI technologies and staff to work predicting and monitoring localized corrosion and designing cathodic protection systems. In one example of independent product evaluation, SwRI assessed non-abrasive pads used to prevent coating damage from concrete mattresses used to stabilize and protect subsea pipelines from impacts by ships. Staff members conducted studies in simulated seawater and showed that the permeable pads were superior to impermeable pads in permitting cathodic protection to reach the pipe surface.

CNWRA staff members are conducting experimental investigations and mechanistic modeling to investigate the processes that contribute to long-term performance of high-level radioactive waste containers. The ability of these containers to limit the release of radioactive materials for thousands of years is integral to the DOE strategy for safe disposal of this waste. Staff members are investigating the effects of welding, post-welding heat-treatments, and thermal aging on metallurgical conditions, corrosion modes, and corrosion rates using specialized analytical tools. To monitor corrosion processes, scientists are developing long-lasting sensors that can operate in aggressive environments including wet or dry conditions, elevated temperatures, and ionizing radiation, which will be important for performance-confirmation related activities. The complex chemistry of the potential environments inside the containers and the rates at which spent nuclear fuel cladding and high-level waste glass are degraded by corrosion are also being studied.

CNWRA staff members are working with internationally recognized consultants to evaluate the range of potential effects that basaltic magma, rising from deep in the earth, could have on subsurface tunnels containing radioactive waste. Although the odds of such a scenario occurring at the proposed Yucca Mountain repository are low, NRC's federal regulations require a detailed evaluation of the potential consequences, if the probability is at least one chance in 10,000 in 10,000 years. The CNWRA approach to evaluating magma-drift interactions uses a one-dimensional model to calculate changes in pressure that would occur in a closed-end, impermeable tunnel during the first seconds of magma flow. Scientists are also conducting experiments in CNWRA laboratories with materials analogous to basaltic magma. These experiments will help verify numerical models and evaluate model sensitivities to different physical conditions.

Rain and groundwater enhance the dissolution of limestone in the Edwards Aquifer, the primary source of drinking water for the city of San Antonio. The dissolution process is particularly efficient where there is high fault and fracture density, producing open cavities such as the small cave localized along the fault shown. CNWRA staff members are applying structural geology concepts and techniques to evaluate the structural controls on the Edwards Aquifer recharge zone. The investigations evaluate the distributions and characteristics of fault block deformation features. The results are then used to develop a calibrated rock deformation model for future assessments of the aquifer, focused on productivity, vulnerability, and contaminant transport.

Over time, the network of faults and fractures at Yucca Mountain has created pathways for water, gases, and heat through volcanic strata that would host the proposed repository. The network also defines the location, size, and shape of small rock-blocks that must be considered in analyses of future stability of underground openings and pillars. Characterization activities by the DOE have concentrated on one fault block beneath Yucca Crest, however, alternative repository designs include other fault blocks that have not been characterized in detail. CNWRA geologists have developed and are applying new techniques to estimate the location and amount of deformation in fault blocks based on the geometry of faulted rock layers and displacement vectors on the bounding faults. The results have important implications for potential fracture density and connectivity in fault blocks at Yucca Mountain and for the related stability of underground excavations and flow. The probability and consequences of seismotectonic activity causing fault displacements on the block-bounding faults of sufficient timing and magnitude to impact, in turn, design or performance measures, remains to be determined.

CNWRA engineers and scientists participated in an international cooperative project called DECOVALEX (DEvelopment of COupled models and their VALidation against EXperiments). The project supports the development of mathematical models of coupled processes in the geosphere and in their applications and validation against experiments in the field of nuclear waste isolation. These studies increase the understanding of thermal-hydrological-mechanical processes that affect rock mass stability and radionuclide release and transport from a repository to the biosphere. The studies also assess how these processes can be described using mathematical models. A modeling test performed at Yucca Mountain helped build confidence in DOE models and CNWRA models developed on behalf of the NRC to independently evaluate the safety case for the proposed repository.

SwRI is participating in an interdisciplinary initiative to manage and protect the Edwards Aquifer, a highly fractured limestone formation in south central Texas that serves as the water supply for more than 1 million people in San Antonio and the surrounding communities. This initiative combines analytical tools and expertise developed over more than a dozen years by SwRI hydrologists, geologists, geochemists, and geophysicists. Three programs are active in the initiative: one funded jointly by the Edwards Aquifer Authority (EAA) and the U.S. Army Corps of Engineers, and two sponsored by SwRI's internal research and development program. The team of scientists from SwRI and city, state, and federal agencies is developing a basis for more accurately estimating how freely water moves through the aquifer's honeycombed limestone to better define subsurface geological features of the Edwards Aquifer. The 3DStress™ computer program, developed at SwRI, is used to evaluate how geologic structures affect recharge into and flow through the aquifer. The results of these studies, integrated with related research by the EAA, San Antonio Water System, the U.S. Geological Survey, Texas Water Development Board, and the University of Texas Bureau of Economic Geology, will help ensure that the EAA aquifer management model, once it is developed, will accurately represent groundwater flow through the aquifer.

Staff members develop various corrosion sensors to detect the onset of localized corrosion, corrosion precursor events under coating and paint films, and corrosion under alternating wet and dry conditions. One localized corrosion sensor array developed using internal research funds uses sensitive galvanic current flowing between each electrode and a combination of other electrodes in an array. SwRI also developed a software program to acquire and analyze data.

Natural gas pipeline operators and regulators strive to provide increasingly reliable and safe service. Third-party damage and corrosion remain the most frequent causes of interruptions. Internal corrosion was the root cause of about 12 percent of gas pipeline incidents during the year 2000. Existing techniques to detect pipeline corrosion cannot, in practice, be applied to all segments of the pipeline system. SwRI collaborated with the gas industry to develop a methodology that identifies locations most susceptible to internal corrosion damage in pipelines that carry nominally dry gas but may suffer from short-term influxes of wet gas or liquid water (or other electrolyte). The methodology uses rigorous modeling of fluid flow to determine locations along a pipeline where corrosive liquids are likely to accumulate. This innovative approach permits a focused assessment of internal pipeline corrosion at high-risk locations. If these locations are confirmed to be free of corrosion, then other locations with similar conditions and locations less likely to accumulate corrosive liquids are considered free from corrosion and do not require further examination.

Corrosion costs the United States almost $400 billion each year, not including the effects of increased wastes, environmental damage, loss of life and property, and increased imports of energy and metals. Localized corrosion accounts for a majority of the unanticipated corrosion failures in the United States and around the world. SwRI is supporting OLI Systems, Inc., to develop a mechanistically based localized corrosion software tool that will enable prediction of localized corrosion in complex chemical processing streams. In the first year of a three-year program funded by the National Institute of Standards and Technology, Advanced Technology Program, the SwRI staff has developed experimental data for the model, contributed to model development, and developed corrosion monitoring tools for in-plant model validation.

As part of its work on interim storage of spent nuclear fuel, the SwRI staff supported an NRC "safeguards" study of storage casks and evaluated the proposed independent spent fuel storage facility on the reservation of the Skull Valley Band of Goshute Indians in Utah. The staff is independently evaluating seismic soil-structure interactions of storage casks placed on concrete pads, as well as the risk associated with potential aircraft crashes at the proposed facility.

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