Paving the way for a decision later this year on whether to recommend construction of a permanent repository at Yucca Mountain in Nevada, the US Department of Energy (DoE) issued its long-awaited “Yucca Mountain Science and Engineering Report”.
The 1000-page report carefully avoids any position on the Yucca Mountain site’s suitability, and it contains no startling new revelations. Rather, it summarises technical information from site characterisation studies conducted since 1987 and more than 20 years of scientific research regarding potential problems that could affect the repository’s performance. These range from hydrologic, geochemical, thermal and mechanical processes to seismic events, volcanic eruptions, and the possibility of future human intrusion.
The DoE report, however, does provide a look at new repository design concepts the agency has been evaluating since it issued the Yucca Mountain viability assessment report in 1998.
Among new concepts outlined in the report are:
•A flexible design that would allow the repository to operate in a range of thermal modes, either above or below the boiling point of water. While DoE has favoured a higher operating temperature as a way to keep the waste packages dry longer by evaporating moisture in the rock, the Nuclear Waste Technical Review Board (NWTRB) has urged evaluation of a cooler-temperature design that would reduce uncertainties in repository performance assessment modelling.
•Installation of titanium drip shields over the waste packages prior to repository closure to divert any moisture that might drip from the drift walls for thousands of years.
•Providing future generations with the option to monitor the repository for up to 300 years before a decision is made whether to decommission and close it.
The report acknowledges that DoE has not yet completed an evaluation of all uncertainties in its performance assessment models, but says this will be done before energy secretary Spencer Abraham decides whether or not to recommend the Yucca Mountain site to the White House. DoE estimates the total cost of the programme at $58 billion.
If Abraham recommends the Yucca Mountain site, and President Bush and Congress endorse his decision over the objections of Nevada officials, DoE would apply to the Nuclear Regulatory Commission for a licence to build the repository in 2002.
The report anticipates that the repository would dispose of about 63,000t of commercial spent fuel, about 2333t of DoE spent fuel, and about 4667t of DoE high-level waste.
Under the Nuclear Waste Policy Act, the amount that can be stored in the first US geologic repository is limited to 70,000t of heavy metal (THM), until a second repository is in operation.
The DoE data suggest that, if many US nuclear plants receive 20-year licence extensions and there is a revival of nuclear plant ordering in the United States, the agency may need to begin preparing for a second repository before long. By 2035, the amount of commercial spent fuel in storage at dispersed sites — currently more than 40,000 THM — will double if all operating US plants complete their initial 40-year licence period, the report says.
The report was originally scheduled for release in December 2000. It was delayed for several months, however, while the DoE Inspector General (IG) investigated charges that it was a biased effort — intended to support a favourable site recommendation.
The IG undertook the investigation after DoE’s critics seized upon a contractor’s internal memorandum suggesting the science and engineering report could be used to persuade Congress to back the Yucca Mountain site.
The IG subsequently concluded that the memo was “inappropriate”, but that the waste programme’s integrity had not been compromised.
DoE plans to hold public hearings on the science and engineering report in late summer or autumn.
President Bush’s national energy strategy says the government should stay on course to evaluate nuclear waste burial, and also examine longer-term technologies that could reduce the quantity and toxicity of radioactive material that would be stored.
It does not make detailed pronouncements about Yucca Mountain but repeats a Bush campaign pledge that decisions about a Nevada repository will be based on “best science”.
“The administration will continue to study the science to determine whether to proceed,” says an excerpt from the report.
Decisions on whether to recommend the site for waste burial are expected late this year.
An administration spokesman said recommendations on nuclear waste disposal are “all consistent with campaign promises” made by Bush and vice president Dick Cheney last year in Nevada. The campaign had said it would oppose efforts to establish a temporary repository in the state.
The strategy says a repository would be needed even if waste alternatives pan out. It states: “There is growing interest in new technology known as accelerator transmutation, which could be used in combination with reprocessing to reduce the quantity and toxicity of nuclear waste. While this approach does not obviate the need for geologic disposal of nuclear waste, it could significantly optimise the use of a geologic repository.”