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[cdn-nucl-l] Super-ceramics for nuclear waste
Posted in the International Herald Tribune on July 18, 2002 and at:
Super-ceramics for nuclear waste
Matthew L. Wald The New York Times Thursday, July 18, 2002
NEW BRUNSWICK, New Jersey While lawyers, senators and even an occasional
geologist argue over whether Yucca Mountain in Nevada is a suitable
place to store nuclear waste, scientific entrepreneurs are focusing on a
finer detail: how the waste can be packaged to isolate it for 10,000
The Senate's approval of Yucca on July 9 makes the question urgent.
So far, the U.S. Energy Department, which is in charge of Yucca, has
chosen conventional materials; its current plan is for a cask made of
steel alloyed with chrome, molybdenum and nickel, called alloy 22, and a
titanium tent above that. The department says it needs 10,000 packages,
and plans to spend about $500,000 a package, so the financial incentives
are significant. But some scientists doubt that anything metal can sit
for thousands of years without rusting in rock that has rainwater
percolating through it. A few are offering alternatives, including
recently developed ceramics and polymers. In a laboratory here at
Rutgers University, a startup company, Nucon, is showing off a scale
model made of an odd new ceramic. Ceramics are not known for strength,
but this has the same compression strength as steel, Nucon says.
Ceramics can be used as thermal insulators, but this one is cold to the
touch, a sign that it conducts heat readily. This is desirable in a
material that must isolate heat-generating waste that cannot be allowed
to heat itself to the melting point. This ceramic has these unusual
traits because it has exceptional density and has been baked in a
process called cintering.
Cintering gives it another odd quality. Tapped with a metal pen, the
model rings like a bell. The sound brings a smile to the face of the
company's chief engineer, Adam Khizh, who came to this country from
Russia nine years ago. "Perfect cintering," Khizh said. "The sound is
Nucon's material is a spinel, or magnesium-aluminum oxide. Oxidation
(or, in plain English, rust) is the big worry at Yucca. But oxides do
not rust; they have already oxidized.
So far, no one has cast ceramic containers large enough to hold bundles
of spent nuclear fuel rods. Engineers at Nucon hope to do this, although
its model is about the size of half a large watermelon.
Jared Cohon, chairman of the Nuclear Waste Technical Review Board, a
panel established by Congress, said that early plans for Yucca had
included a ceramic covering over the metal, and that this was dropped
because engineers doubted that the covering could withstand rough
handling. But a ceramic that could would be appealing, he said, because
it would resist corrosion far better than metal.
Some ceramics experts are dubious. Delbert Day, a professor of ceramic
engineering at the University of Missouri at Rolla, and a former
president of the American Ceramic Society, said it might be simpler to
protect the steel by encasing it in concrete.
Paige Russell, the Yucca project's technical lead for waste package
design and testing, said that no final packaging decisions had been made
and that the design of the containers so far was "conceptual." But for
now, she said, the Energy Department has come down in favor of a
material it knows better: metal. She said the project would probably
choose proven rather than experimental materials.
"We have to understand the performance of the material over time," she
said. "We have to understand the performance of the material in
different environments, how to manufacture and fabricate the material.
There are a lot of positives to using materials that are already known
and have been used in industry."
Ceramics are used to stabilize high-level nuclear waste, but only as a
matrix material, not as a wrapper.
For low-level wastes - as opposed to the high-level spent fuel that the
department wants to bury at Yucca - the Energy Department is trying a
new material, a polymer foam to bind up radioactive materials
But the big challenge is spent reactor fuel, which will remain intensely
radioactive for centuries and has many components that policy-makers
want to keep out of underground water for millennia.
The Energy Department's early plan was for ordinary steel, but it moved
up to alloy 22 for better corrosion resistance. But alloy 22 is harder
to weld than ordinary carbon steel, and welds, experts say, are where
failure most often occurs.
The department's plan is for a "drip shield" of titanium over each
container. The containers would be 6 to 7 feet in diameter, and about 16
feet long, to be parked in a line, filling the tunnel like subway cars.
Nucon sees instead giant, elongated watermelons made of ceramic, 18 feet
long, with a wall 3 inches thick and an inner diameter of 5.5 feet. The
ovoid shape is a way to reduce the risk of cracking the ceramic if it
bangs into something; with rounded ends, the force of impact would be
better distributed around the container.
Nucon believes it has made an important advance in being able to cast
thick ceramics. Making a thick ceramic is a challenge because cintering
requires even heating and cooling, increasingly difficult in thick
structures. Nucon's solution is a combination of conventional thermal
heating, plus microwave energy, which heats evenly.