DOE Defends 'Hot' Repository Design

Facing major criticism from a key scientific oversight panel, the Department of Energy came to a public meeting in mid-May armed with a slew of new studies to justify its design for the $60 billion Yucca Mountain nuclear waste repository in Nevada.

The new research, however, appeared so fundamentally different from previous work that members of the oversight panel--the Nuclear Waste Technical Review Board (NWTRB)--asked why DOE had not done these studies before. And they also questioned if the department could still meet its goal of completing a construction permit application for the underground repository by the end of the year in light of the new research.

"Yes" was the resounding answer from top DOE officials, including Margaret Chu, director of the Office of Civilian Radioactive Waste Management, who attended the entire two-day meeting.

The meeting's focus was corrosion of the waste casks, designed to hold some 70,000 metric tons of high-level radioactive waste inside the Nevada mountain, isolating the waste for at least 10,000 years, as required by regulation.

The importance of the casks' integrity to the overall barrier system has steadily grown as scientists learned more about the mountain's volcanic history as well as its climate and the porosity of its geology. Rainwater has been found to pass from the surface through the underground repository to groundwater 1,000 feet below the tunnels where the waste packages will be left.

The meeting was called by NWTRB, a congressionally established oversight board of geologists, engineers, and materials scientists. The board announced last year that it believed the casks could be penetrated because of localized corrosion.

The board issued a strongly worded letter and report to DOE in October 2003 and restated its concerns at the meeting, which was attended by some 100 people, mostly DOE science staff and consultants but also scientists from the Nuclear Regulatory Commission, electric utilities, and the state of Nevada, as well as a handful of residents.

OPENING THE MEETING, board member Ronald M. Latanision, a materials science and engineering professor at Massachusetts Institute of Technology, laid out the board's perspective in sharp terms.

He stressed that the board's views were based on data that DOE had presented to the board, which has no regulatory power but assesses the scientific validity of DOE's work.

The board is concerned that the temperature of the waste casks and the wet environment in the tunnels will lead to localized corrosion of the huge casks before 10,000 years and long before the radioactive waste would be safe, which will be hundreds of thousands of years.

Of particular interest is the first 1,000 years, when the thermally hot waste packages are left in the closed tunnels--the "thermal pulse" period when the temperatures of the casks will reach 160 to 180 °C.

After studying data that DOE provided last year, the board said that deliquescence-induced localized crevice corrosion of the casks is a likely result of above-boiling temperature and water seepage through the tunnels. Board members believe that sufficient levels of chloride exist in the water passing through the tunnels to bring about cask corrosion, eventually leading to perforation.

In its October report, the board cited DOE deliquescence experiments showing that brines from calcium chloride or magnesium chloride may be created at around 150 °C and even lower. The board fears these brines will lead to crevice corrosion even of an exotic nickel alloy DOE has selected for the casks.

Latanision added that the board had concluded that "all" the conditions necessary for localized corrosion are present, and he punctuated the importance by noting that the board's report was signed by all its members.

Latanision explained that during the thermal pulse period, localized corrosion will be particularly insidious and propagation rates can be extremely rapid and difficult to predict. Therefore, the board encouraged DOE to avoid these conditions altogether by modifying its design.

Unlike volcanic activity, water transport, and other natural conditions, where the only recourse is mitigation, localized cask corrosion can be eliminated entirely by shifting the design to a low-temperature environment, board members said.

They urged DOE to consider a "cold" repository design, in which wastes are maintained at a temperature below boiling during the thermal pulse period to reduce deliquescence and to limit the impact that high heat brings to an environment that is so difficult to characterize.

Members also urged DOE not to fall back on a "total system performance assessment" to justify its design. Latanision stressed that it was "absolutely necessary" to gain a fundamental understanding of the repository system for it to be licensable.

As he ended his presentation, Latanision said, "We want to open this meeting up and have as full a discussion as possible." Therefore, the board would allow questions from audience members as well as board members during presentations. Scientists with the Nuclear Regulatory Commission, the state of Nevada, and the Electric Power Research Institute would speak the first day, he said, and DOE would have the entire second day.

"DOE asked to have its presentation last, and frankly this is not the way I would have liked it," he said. Consequently, a full day of discussion took place without most parties knowing that DOE would introduce a host of new studies on day two.

DOE began its day at 8 AM and presented about 300 slides and testimony from more than a dozen scientists over 11 hours.

As DOE plowed through its material, its scientists said calcium chloride and magnesium chloride were extremely unlikely to be present in the repository, and if so, they will rapidly transform to nondeliquescent phases because of their instability at high temperatures. Also, any acid gases generated as a result of salt deliquescence will be dispersed and dissolved into water held in the rock.

Acid had become a concern as a result of a series of bench-scale tests that Nevada-funded scientists had recently run. Those data were presented on the first day of the conference by researchers, including Roger W. Staehle, an adjunct professor in chemical engineering and materials science at the University of Minnesota. They had found corrosion in cask material using water similar to that found in the mountain after subjecting the material to the range of temperatures anticipated for that environment.

BUT DOE COUNTERED that, in fact, the heat from the waste package will drive corrosive elements in the water-based environment from the casks themselves. The department also criticized the state studies.

If conditions for corrosion occur, the special cask material--termed alloy 22--will hold it in check, said Joe H. Payer, a Case Western Reserve University materials science and engineering professor who is under contract to DOE.

Payer acknowledged that some corrosion of alloy 22 was possible but not nearly enough to penetrate the cask. The casks have two shells: a 20-mm outer shell of alloy 22 and a 50-mm inner shell of stainless steel.

Payer said alloy 22 had the thickness of about 12 quarter coins, and at most, only the width of one quarter could be eaten through during the thermal pulse. He estimated that penetration would take 1,600 to 160,000 years.

As the board and audience quizzed DOE, board member Mark D. Abkowitz, Vanderbilt University professor of civil and environmental engineering, noted, "I think we have a situation in which corrosion has been a concern for more than 10 years, and last fall we put an exclamation point on that message.

"But what I heard today is that the environment under which tests have been conducted for many years and many millions of dollars have been spent is not an environment that is plausible. And that this is something that has just been learned over the past few months and is being presented to us today. So the board's concerns, in DOE's eyes, are nonstarters."

The new information and conclusions were a "rather sudden set of accomplishments," Abkowitz said. "In a project this complex, when the pace of learning is occurring at such a high rate, it tells me there is probably a whole lot more learning that needs to go on in this and other areas."

In response, DOE officials strongly disagreed with the need for more fundamental study. They said the board's conclusions in October were not supported by data that DOE had presented to the board in the past.

As the day came to a close, Latanision told reporters: "We have heard a lot of information that is absolutely new. Right now, I still think the solution could be a design change, and I have to question some of the slam-dunks presented in the transparencies. But I am only saying now that we need to digest them, and I can't make a judgment."

In particular, he singled out the corrosion data as entirely new: "Seems to me that [DOE] has done an awful lot of experiments that are not of any value [now.] If there is a silver cloud in all this, we have forced a more realistic assessment of what should be done."

Following the meeting, Chu stressed that DOE's design is for a hot repository. "We have not analyzed a cold repository. A cold repository is not our program," she told C&EN.

She stressed the value of the total system performance assessment approach, saying the board should consider the total system--not just the casks--to gauge the site's suitability.

Disagreeing with Chu was Judy Treichel, executive director of the Nevada Nuclear Waste Task Force, a nonprofit public interest group.

Treichel traced a history of DOE increasingly basing its design on a total systems approach. "We worry that growing uncertainties can be disguised as 'manageable weaknesses,' " and create a "false illusion of confidence and accuracy."

"In 1997, the acceptable thermal load was unknown, and we are still arguing about it," Treichel said, adding that, in the meantime, DOE had taken a "lemon and made lemonade," saying heat will now create a thermal barrier to drive off liquids.

"I just want to use plain words," added Staehle in comments at the end of the meeting. "It seems to me that we have a waste package emplaced in rocks that are full of chemicals, we've got a heated surface and an oxygen environment, and we've got water," Staehle said. "The issues have to do with how this is distributed. We do know alloy 22 can be perforated--rapidly, in some environments."

Staehle chided DOE for selecting a limited environmental model. "We need a set of answers, a set of different credible environments that we can identify and investigate. This is a complex system."

Latanision predicted that a report would be issued by the board in a matter of weeks.