Evolution of Environments in a Repository in Yucca Mountain
Charles Bryan, Geochemist; Sandia National Laboratories

Overview of how the environment at Yucca Mountain would be expected to change over time, and the ways in which these changes would affect the waste disposed of at the site.

The two chemical environments that could be expected in the Yucca Mountain tunnels were deliquescence (to dissolve and become liquid by absorbing moisture from the air) and seepage.  While deliquescence could begin taking place as soon as the waste was deposited, seepage would be possible only after the drift wall temperature dropped below 100 degrees Celsius.  Deliquescent brines are able to form in small amounts, and only at elevated temperatures.  Deliquescence of salts from dust around a waste package could create an "aqueous environment" on the surface.

Chemistry of Water Contacting Engineered Barriers
Roberto T. Pabalan, Institute Scientist; Center for Nuclear Waste Regulatory Analyses (CNWRA)

Water chemistry will be among the factors that influence corrosion of engineered barriers at Yucca Mountain, which in turn will be affected by deliquescence of inorganic salts, and the evaporation of seepage water.  Corrosion could occur in environments in which there is either no seepage because of high temperatures, or in an environment where brines form due to the evaporation of seepage water on the waste package.

Studies conducted by the CNWRA attempt to determine the chemistry of the water that could potentially reach the waste packages held at Yucca Mountain, examine DOE's technical framework, and incorporate these into the Nuclear Regulatory Commission's Total-system Performance Assessment.

The work done by CNWRA indicated that the evaporation of seepage water that could reach the waste packages at Yucca Mountain could form brines that support localized corrosion of alloy-22.  In addition, salts could deliquesce at higher temperatures and form brines.  There needs to be additional research done on dust samples from Yucca Mountain to better understand corrosion risks.

Update of State of Nevada Research on Waste Package Environments in Yucca Mountain     
Don Shettel, Geochemist and Chairman; Geosciences Management Institute, Inc.

A model has been developed to illustrate rapid penetration of alloy-22.  The model showed a process in which rapid perforation of alloy-22  could occur in "concentrated mixed acids."  This scenario shows that this perforation could take place in less than ten years if these mixed acids accumulate.  Based on this information, Mr. Shettel concluded that it provides "a credible foundation for early perforation of the alloy-22."

New Alloy-22 Data and Their Relevance to High-Temperature Localized Corrosion
Raul Rebak, Senior Scientist; Lawrence Livermore National Laboratory

While it is recognized that alloy-22 is susceptible to crevice corrosion and chloride-containing aqueous solutions, this is affected by "chloride concentration, temperature, electrochemical potential and nitrate concentration."  

Autoclave experiments were conducted to test the hypothesis that a nitrate to chloride ratio higher than .5 would spark crevice corrosion, not to simulate conditions within the drift.  In all three autoclave experiments, testing different conditions, crevice corrosion occurred in both vapor and liquid phases.  However, after nine months, "little or no general corrosion" occurred when tested at 160 degrees Celsius and 220 degrees Celsius.  Following the autoclave tests, "there may still be a need to investigate, under physically attainable natural conditions, the effect of time on crevice corrosion initiation and propagation" in certain environments.  The tests that have been conducted "continue to support the localized corrosion model for degradation of the waste package."

Update of State of Nevada Research on Corrosion of Alloy-22
Maury Morgenstein, Geologist and President; Geosciences Management Institute, Inc.

Current research focuses on the dust that is currently present at Yucca Mountain, but needs to factor in what changes will take place once we begin depositing waste material at the site. 

Water reaching the tunnels where waste would be stored could form brines as the temperatures rise.  As temperatures continue to rise and water evaporates, these brines could become airborne dust, changing the nature of the repository dust and creating the possibility for corrosion.

Using the Eastern Desert in Central Egypt as an example, Mr. Morgenstein looked for clues as to what kind of occurrences might take place at Yucca Mountain.  The examination of conditions in the Egyptian desert showed that void spaces "are coated with salt precipitates and grains are cemented at contact points where there has been suction due to dry out and there are brine solutions in fractures and pores in the host rock/sediments feeding the drying surface," and that "salts are precipitated in ponded situations where brines have evaporated."  The concept of nitrate as an inhibitor does not work under vapor conditions.

Corrosion of Salt Environments at Elevated Temperatures
Lietai Yang, Senior Research Engineer; CNWRA, SwRI

Potential for corrosion of alloy-22 and at what rates at high temperatures in areas where salts are present was discussed.  The Center for Nuclear Waste Regulatory Analyses study indicated a higher potential for corrosion than was shown in a de-aerated system, and that general corrosion was "the major mode of attack."  Tests on "the characterization and stability of the salt mixture at elevated temperatures are being performed," as well as tests aimed at verifying general corrosion rates and the vulnerability to localized corrosion.

Crevice Corrosion Initiation and Propogation Tests
Xihua He, Research Scientist; CNWRA, SwRI

Results of another test conducted by the Center for Nuclear Waste Regulatory Analyses on the initiation of crevice corrosion was presented.  Temperature, pH, the ratio of chloride concentration to that of corrosion inhibitors, and fabrication processes affected the susceptibility of alloy-22 to corrosion, and that a "strong tendency toward stifling and re-passivation was observed.  Uncertainties exist surrounding the impact of dust deliquescence brines, the "tendency toward stifling and re-passivation in elevated temperatures and more aggressive chemical conditions," and the "applicability of data obtained from experiments in fully-immersed condition to limited-volume systems."

Localized Corrosion Data and Analyses from the Materials Performance Thrust of the OCRWM Science and Technology Program
Joe Payer, Professor; Case Western Reserve University
Rob Kelly, Professor; University of Virginia

Sought to explain some of the conditions that must be met for corrosion to be initiated and maintained.  For crevice corrosion to continue once it is started, a critical crevice chemistry must be maintained.  Examined "scenario-specific" cases in an attempt to determine whether physical conditions will support corrosion after it has begun under different scenarios.  In order to accurately gauge the risk of corrosion of waste packages at Yucca Mountain, conditions like those found at Yucca Mountain must be used in formulations.

Corrosion Models to Support Total System Performance Assessments
Osvaldo Pensado, Senior Research Scientist; CNWRA, SwRI

Localized corrosion should be included in the performance assessment if seepage water were to contact waste packages during the "thermal pulse."  There are uncertainties regarding stifling and re-passivation of localized corrosion, and the "extent of attack."  Models suggested that thermally aged and welded materials were more susceptible to localized corrosion, while nitrate did appear to be an inhibitor of corrosion.  Based on findings of the tests, localized corrosion should be considered in a performance assessment, but if a drip shield is effective, the probability of localized corrosion decreases considerably. 

Developments in Modeling Localized Corrosion of Alloy-22
Fraser King, Principal Investigator; Integrity Consulting Ltd. 

In examining localized corrosion in multi-salt deliquescent systems, the Electric Power Research Institute (EPRI) conducted an independent analysis of available data in an attempt to develop a "best-estimate of waste package corrosion behavior."  EPRI's "decision tree" approach lays out a set of six questions, all of which must be answered yes for there to be technical or safety risks associated with early container failure.  Based on this approach, EPRI concluded that the formation of high-temperature deliquescent brines is not a cause for concern regarding corrosion of alloy-22 packages at Yucca Mountain.  Based on experimental data, EPRI concluded that even if localized corrosion did initiate, it would be stopped through stifling and re-passivation.

Development and Implementation of the Localized Corrosion Model
Neil R. Brown, Manager; Bechtel SAIC Company, LLC (DOE Contractor)

The localized corrosion model, which weighs the potential for long-term corrosion with the potential for crevice re-passivation, was presented.  For localized corrosion to occur, the answer to each of the following five questions must be "yes."

• Can multiple-salt deliquescent brines form at elevated temperature?
• If brines form at elevated temperature, will the persist?
• If deliquescent brines persist, will they be corrosive?
• If deliquescent brines are potentially corrosive, will they initiate localized corrosion?
• Once initiated, will localized corrosion penetrate the waste package outer barrier?

While the answer to the first two questions are "yes" and "sometimes," the answers to the last three are all "no," leading to the conclusion that localized corrosion due to dust deliquescence can be dismissed as "low consequence."

Summary of NRC Work and Waste Package Corrosion Risk Insights
Tae Ahn, Senior Materials Engineer; Technical Review Directorate, Division of High-Level Waste Repository Safety, Office of Nuclear Material Safety and Safeguards, NRC
Yi-Ming Pan, Principal Engineer; CNWRA, SwRI (NRC Contractor)

The formation of brines from salt mixtures at higher temperatures will be more likely to result in general corrosion than localized corrosion, and the brines that form from evaporation of seepage water are largely benign to alloy-22, "but some compositions could initiate localized corrosion of the waste package material."  The crevice corrosion of alloy-22 is frequently controlled through stifling and re-passivation.  In light of findings, general corrosion appears to be more predominant than localized corrosion.  In the future, efforts will be made to improve data in:

• Effects of long-term chemical or structural changes in passive film stability;
• Elevated temperature effects on corrosion rate;
• Chemistry of water contacting engineered barrier;
• Crevice corrosion initiation and propagation tests;
• Corrosion model to support independent total system performance assessments; and
• Integrated confirmatory tests on corrosion and evolution of near-field chemistry.

September 25 Transcript from NWTRB Website

September 26 Transcript from NWTRB Website

September 25 and September 26 Presentations from NWTRB Website

Nuclear Waste Technical Review Board Meeting
September 27, 2006; Amargosa Valley, Nevada

OCRWM Program and Project Overviews
Edward Sproat, Director; Office of Civilian Radioactive Waste Management, DOE

Introduced himself as the new director of DOE’s Office of Civilian Radioactive Waste Management, and laid out the program’s goals and schedule for opening the Yucca Mountain repository.  Under the “best achievable” schedule, DOE plans to begin the Nevada rail line by October 2009, begin construction of the repository in September 2011, and begin receiving fuel in March 2017.

Sproat next outlined four key strategic objectives facing OCRWM, which included the submittal of a “docketable” license application to the NRC by June 30, 2008; creating the culture within OCRWM necessary to develop and operate the Yucca Mountain repository, reducing government liability associated with the ability to accept commercial spent nuclear fuel, and the development of a national spent fuel transportation plan.

NRC Perspective on the Safety Case for Geologic Repositories
Lawrence Kokajko. Division of High Level Waste Repository Safety, NRC

Compared the roles and responsibilities of DOE and the NRC.

The approach established in 10 CFR Part 63 is based on “well-defined, incremental decision points,” which allow for the continuous development of knowledge, and the building of confidence and ongoing review. 

The phased decisions in Part 63 include:

• Construction authorization;
• License to receive and possess material; and
• Amendment for permanent closure of the facility updated by performance confirmation program.

The safety approach from NRC’s perspective includes safety analyses, plans and procedures, and “continued safety oversight.”  The safety analyses include pre- and post-closure operations, and evaluate “potential radiological exposures.”  Pre-closure safety plans and procedures include training and certification of personnel, and emergency plans, while post-closure includes waste retrieval and performance confirmation.  Key components of “continued oversight” are land use control, placement of permanent markers, and maintenance of records and archives. 

In regulation of high-level waste repositories, NRC operates based on a “risk informed approach,” which “provides an ‘informed’ and focused approach” including:

• Identification of important parameters, models and assumptions;
• Identification of important uncertainties; and
• Review of technical support in key areas.

Overview of OCRWM’s Yucca Mountain Repository System Post-closure Safety Case
Dr. J. Russell Dyer, Director; DOE-Office of the Chief Scientist

The Safety Analysis Report (SAR) as one that includes:

• Hazards surrounding the repository and the system of barriers;
• Arguments and support of safety recommendations; and
• Discussion and evaluation of uncertainties.

The SAR and its updates will provide the foundation for the NRC’s authorization of the construction, operation, and permanent closure phases of the waste repository lifecycle.  The submittal of a SAR is taken to mean that DOE has “confidence in system safety over the entire repository lifecycle.” 

Barrier Capability and the Assessment of System Performance
Peter Swift; Sandia National Laboratories

Described the technical components and predicted performance of barriers at the waste repository.  Confidence in the system is based on:

• Understanding components and their capabilities;
• Understanding system performance;
• A clear display of uncertainty; and
• Following a process that demonstrates completeness.

Various forms of information (e.g. models, studies, observational data, etc.) support confidence in the system performance.

The Role of Analogues
Dr. Patrick V. Brady, Senior Scientist; Sandia National Laboratories

Natural analogues have the potential to provide insights into the way the repository and waste packages perform under conditions similar to those at Yucca Mountain.  Evidence from Pena Blanca, Mexico and the Inchtuthil Legionary Fortress offer examples of what has actually taken place in the past beyond scientific models and calculations.

The Science and Technology Program
Dr. Mark T. Peters, Deputy to the Associate Laboratory Director; Argonne National Laboratory

The mission of the Science and Technology program is to “provide advanced science and technology to continually enhance our understanding of the repository system and to reduce the cost and schedule for the OCRWM mission,” with research areas of “materials performance,” “source term,” and “natural barriers.”

Materials performance research is aimed at advancing knowledge surrounding the corrosion of materials and improving performance.  Three major subjects within the issue are:

• Corrosion processes on metal surfaces covered with particulates and deposits;
• Evolution of corrosion damage by localized corrosion; and
• Evolution of the environment on metal surfaces.

Source term research is intended to improve “the understanding of the release mechanisms of key radionuclides from SNF and explore technical enhancements.”  Research on natural barriers improves understanding of both natural and engineered barriers in order to effectively isolate waste in the repository.

Information generated through the Science and Technology program will improve the confidence in the performance of the repository system over its entire lifecycle.

Performance Confirmation and Long-Term Science and Monitoring
Frank Hansen; Sandia National Laboratories

Described the importance and components of post-closure monitoring of a waste repository.  This information will eventually provide the basis for closure licensing.  Long-term testing is also expected to improve public confidence and allow adjustments based on improved knowledge.  Current program activities include:

• Develop performance test plans;
• Continue monitoring, testing, and data collection;
• Continue the total system performance assessment and modeling of underlying processes; and
• Develop data ranges and limits to performance confirmation parameters.

September 27 Transcript from NWTRB Website

September 27 Presentations from NWTRB Website