[cdn-nucl-l] Sandia research with new reactor could mean significant savings in the eventual safe transport, storage, disposal of nuclear waste

["Adam McLean" <adam.mclean@utoronto.ca>]

Posted on the Sandia news service on November 5, 2002 and at:
http://www.sandia.gov/news-center/news-releases/2002/nuclear-power/burnu
p.html

Adam

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FOR IMMEDIATE RELEASE
November 5, 2002 
 
Sandia research with new reactor could mean significant savings in the
eventual safe transport, storage, disposal of nuclear waste  

NEW REACTOR: Sandia researcher Gary Harms conducts experiments with a
new Sandia-built reactor that are paving the way toward possible changes
in regulations on transport and storage of nuclear waste. (Photo by
Randy Montoya)
Download 300dpi JPEG image, 'reactchamber.jpg', 832K (Media are welcome
to download/publish this image with related news stories.)
 
ALBUQUERQUE, N.M. - Recent experiments by Sandia National Laboratories
researcher Gary Harms and his team are using a new Labs-built reactor to
provide benchmarks showing that spent nuclear fuel - uranium that has
been used as fuel at a nuclear power plant - is considerably less
reactive than the original fresh fuel. This could mean significant
savings in the eventual safe transport, storage, and disposal of nuclear
waste.

"The conservative view has always been to treat spent fuel like it just
came out of the factory with its full reactivity," Harms, project lead,
says. "This results in the numbers of canisters required in the handling
of spent nuclear fuel to be conservatively high, driving up shipping and
storage costs."

The more realistic view is that as nuclear fuel is burned, the
reactivity of the fuel decreases due to the consumption of some of the
uranium and to the accumulation of fission products, the "ash" left from
burning the nuclear fuel. Accounting for this reactivity decrease,
called burnup credit, would allow for the spent nuclear fuel to be
safely packed in more dense arrays for transportation, storage, and
disposal than would be possible if the composition changes were ignored.

"Allowing such burnup credit would result in significant cost savings in
the handling of spent nuclear fuel," Harms adds.

This seems obvious on the surface, but in the ultraconservative world of
nuclear criticality safety, an effect must be proven before it is
accepted. Thus, prior to the Nuclear Regulatory Commission ever agreeing
to the more realistic view, it would have to be proven in actual
experiments and compared to computer models showing the same effects.

In 1999 Harms obtained a three-year grant from the Department of
Energy's (DOE) Nuclear Energy Research Initiative to make benchmark
measurements of the reactivity effects that fission products have on a
nuclear reactor. The project was called the Burnup Credit Critical
Experiment (BUCCX). Rhodium, an important fission product absorber, was
chosen for the first measurements.

To do this the BUCCX team first designed and built a small reactor,
technically called a critical assembly, which uses low-enriched fuel.
The control system and some of the assembly hardware for the reactor
came from the 1980s-era Space Nuclear Thermal Propulsion (SNTP) Critical
Experiment project, designed to simulate the behavior of a nuclear
rocket reactor.

The reactor, which operated during the experiments at a lower power than
a household light bulb, was subjected to several layers of safety
reviews. During the experiments, it performed safely exactly as
predicted.

"It took us most of the three years to build the reactor and get
authorization to use it. Only in the last few months have we begun
actual experiments," Harms says. "Much of the time was involved in
getting approvals from Sandia and DOE and to make sure it meets all ES&H
concerns."

The core of the BUCCX consists of a few hundred rods full of pellets of
clean uranium that originally came from the nuclear powered ship NS
Savannah. Thirty-six of the rods can be opened to insert experiment
materials between the fuel pellets. Prior to conducting experiments with
the rhodium, the researchers loaded the reactor to critical with only
the uranium fuel. This provided a baseline point of where uranium goes
critical - information that could be compared to later experiments.

Then, the BUCCX team added about 1,200 circular rhodium foils between
the uranium pellets in the 36 rods. The intent was to measure the extent
to which the rhodium reduced the reactivity of the uranium.

"We then compared the critical loading of the assembly with the rhodium
foils to the critical loading without rhodium," Harms says.

And, not to anyone's surprise, it took significantly more fuel to reach
critical with the rhodium-doped rods than without them.

Months before running the physical experiments on the reactor, Harms was
modeling on Sandia's sophisticated computers to determine where the
uranium doped with rhodium would go critical.

"I was curious," Harms says, "I did calculations ahead of time so I
could lay out the experiment and get a peek at what the experiments
would say. In the end, I was fairly impressed with how accurate the
calculations were compared to the actual physical experiments."

Of course, the computer codes weren't perfect, and had a small bias when
compared to other criticality safety benchmarks. And in analyzing the
actual experiments in the reactor, Harms took that bias into account.

Harms says two other fission products absorb neutrons better than
rhodium. However, he selected rhodium to run the experiments because it
is one of the few byproducts of fission that has a single stable
isotope, which means the experiment would not be contaminated by the
effects of other isotopes. Also, no one else has done any experiments
with rhodium in a critical assembly. Subsequent experiments could
address the dozen or so other fission products that are important to
burnup credit.

Also, to his knowledge, no other lab in the US is doing actual burnup
credit experiments. Oak Ridge National Laboratory is running codes to
determine how much the reactivity of spent fuel is reduced by fission
products, but not doing actual experiments.

At the end of the three-year funding period, Harms says the Sandia
program has come a long way in proving that the reactivity of spent fuel
is considerably less than that of fresh fuel.

"In essence Sandia is helping pave the way for the Nuclear Regulatory
Commission to address the safe and cost-efficient transport and storage
of nuclear waste," Harms says.

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Sandia is a multiprogram laboratory operated by Sandia Corporation, a
Lockheed Martin Company, for the United States Department of Energy's
National Nuclear Security Administration under contract
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responsibilities in national security, energy and environmental
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