Nuclear Waste Storage Undergoes Simple Shift

By Keith Rogers

By KEITH ROGERS

REVIEW-JOURNAL

In the world of nuclear waste, simpler is better.

That’s the case with the so-called “new path” that government scientists embarked on this week to improve the design and safety of the planned nuclear waste repository at Yucca Mountain, 100 miles northwest of Las Vegas.

In another respect, it’s also the course spelled out by independent experts in the Oct. 21 issue of the prestigious journal Science.

For different reasons, the simple-is-better philosophy appears to be gaining momentum among scientists for tackling what many of them consider to be a huge environmental problem: how to safely dispose of the most lethal radioactive materials on Earth, the stuff left over from splitting atoms to generate nuclear power.

Thomas H. Pigford, professor emeritus of nuclear engineering at the University of California, Berkeley, says the best way to attempt to protect generations for hundreds of thousands of years from dissolved radioactivity escaping into groundwater from 77,000 tons of nuclear waste entombed in the mountain is to surround the metal disposal containers with sand and gravel.

Ten percent, or 7,700 tons, would be in the form of solidified, heavy metal from highly radioactive defense waste turned into glass logs. The rest would be solid spent fuel from commercial power reactors.

In the Science article, Pigford and co-author Luther J. Carter, an independent science writer, suggest that a layer of dry, coarse gravel topped with a layer of fine sand or finely ground volcanic tuff – the same stuff the mountain is made of – would create a capillary action system. The layers would slowly draw away water that seeps through the mountain’s cracks and drips down from the ceilings of tunnels where metal waste canisters are sealed.

“All the waste containers beneath the gravel will corrode over time from the water vapor and oxygen present,” Pigford and Carter wrote. “Eventually radioactive elements dissolved in water will emerge from the failed containers, diffuse along gravel particle surface and … remain trapped there for hundreds of thousands of years.”

Their calculations, based on a 1995 performance assessment by a Yucca Mountain Project contractor, show that “the radiation dose to future people from a repository using a capillary barrier would be lower at all times by a factor of 1 million than the one envisioned by the Yucca Mountain Project today,” the article states.

Instead of sand and gravel, the current design calls for installing expensive drip shields made of titanium to divert water migrating through the mountain.

However, Pigford and Carter say neither the drip shield nor nickel-based Alloy 22, which forms the outer shell of the waste- disposal canisters, is needed. The cost savings would be considerable, considering the price tag for each of the 14,700 canisters with drip shields would be $900,000.

In an e-mail, a spokesman for the Energy Department’s Office of Repository Development in Las Vegas said the idea of using backfill and even depleted uranium as a barrier system previously has been reviewed.

“Currently, we are undertaking an exploratory study related to the potential benefits of backfilling,” wrote Allen Benson, the DOE spokesman. “The safety case at Yucca Mountain will be outlined in the license application when it is submitted to the” Nuclear Regulatory Commission.

Nevertheless, Benson said the drip shield design is still under consideration by project scientists. In addition, he said the waste package, or disposal canister, for spent nuclear fuel from commercial power reactors hasn’t changed.

“The waste package still consists of an outer corrosion barrier of nickel-based Alloy 22, approximately 1 inch thick, and an inner vessel of stainless steel, approximately 2 inches thick,” Benson wrote in an e-mail Wednesday.

His message came in response to a new design approach for using standardized canisters to deliver spent nuclear fuel to Yucca Mountain.

DOE’s announcement Tuesday said this “new path” design differs from the previous design in that solid, spent fuel pellets inside metal cladding assemblies would be sealed in these canisters at reactor sites, then hauled to Yucca Mountain on rail cars or trucks.

Once at the mountain, they would remain sealed with the transport canister being put inside a double-layered, disposal sheath canister for being moved by rail into permanent storage tunnels, or drifts.

Paul Golan, DOE’s acting director for Civilian Radioactive Waste Management, stated that with this “new path forward … we are confident that the simpler we make the design, the more reliable the project will be.”

He said this eliminates the need for huge, multibillion-dollar surface facilities for handling and repackaging spent fuel assemblies to be put into disposal canisters.

“It also reduces the potential hazards caused by the oxidation of bare spent nuclear fuel during handling,” DOE officials stated in their announcement.

In an interview Tuesday, Golan said there still will need to be a special, inert facility at Yucca Mountain for repackaging “off- normal” spent fuel that arrives in damaged cladding.

A previous study found that thousands of fuel assemblies would arrive damaged including some with undetected leaks and cracks from which fuel could oxidize and result in powderlike contamination.

After Pigford heard about DOE’s “new path” design, he said, “It’s certainly a change … that brings so many potential changes that it’s going to require a lot of study.”

He was referring to a claim in a trade publication, “The International Radioactive Exchange,” that the new design eventually would involve reprocessing of spent fuel at reactor sites. While reprocessing is a way of recycling some unused fuel back for commercial power generation, the waste left for disposal at Yucca Mountain would have to be solidified, such as the 10 percent received as highly radioactive defense waste.

“These words about reprocessing are major,” Pigford said in a telephone interview Wednesday. “Instead of spent fuel elements, they would be borosilicate glass containing fission products and containing some radioactive, transuranic elements.”

If this course is pursued, the whole aspect of the Yucca Mountain Project would change in light of new data required for heat loading on the repository and many other factors, Pigford said.

“That’s not just storing spent fuel from reactors, that would be storing a new material,” he said. “Sure the military waste at Savannah River (S.C.) and Hanford (Wash.) looks like reprocessing waste. But it’s different from commercial spent fuel. There’s an enormous difference that has a big effect upon the design of Yucca Mountain. … It puts us in a whole new era.”