[cdn-nucl-l] Proton Guns Set Their Sights on Taming Radioactive Wastes

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

Posted in Science Magazine Volume 302, Number 5644, Issue of 17 Oct 2003,
pp. 379-381 and at:
http://www.sciencemag.org/cgi/content/full/302/5644/379

Adam

----------------------

Proton Guns Set Their Sights on Taming Radioactive Wastes
Dennis Normile

Once mooted as energy sources, nuclear reactors that substitute particle
accelerators for chain reactions are taking long-range aim at a new mission
KUMATORI, JAPAN--On the grounds of Kyoto University's Research Reactor
Institute, workers have dug into a hillside to give a 30-year-old
experimental nuclear reactor an unusual companion: a proton synchrotron.
When it starts up in fall 2005, the synchrotron will fire protons into the
heart of the reactor, straight down the axis of a cylinder of heavy metal
wrapped in a core of nuclear fuel. Neutrons dislodged from the target will
hurtle into the fuel, shattering atoms as they go.
It may seem a roundabout way to generate a nuclear reaction, and it is. But
this type of accelerator-driven system (ADS), as it's called, isn't
primarily designed to generate power. Instead, its aim is to transform some
of the nastier ingredients of spent reactor fuel into less troublesome
elements. The technology "has a unique role to play in treating nuclear
wastes," says Stefano Monti, a nuclear physicist at the Italian National
Agency for New Technologies, Energy, and the Environment (ENEA) in Rome.

Kyoto University, with its $10 million Kumatori Accelerator-driven Reactor
Test Facility (KART), is not alone. By the end of this year, the Joint
Institute for Nuclear Research (JINR) in Dubna, Russia, expects to start
building a $1.75 million experiment chamber for nuclear reactions at an
existing proton accelerator. And ENEA, the French Atomic Energy Commission
(CEA), and Germany's Forschungszentrum Karlsruhe are joining forces for the
$22 million TRIGA Accelerator-Driven Experiment (TRADE), which will add a
proton accelerator to an experimental reactor at ENEA's Casaccia Research
Center in Rome. The three partners expect to commit to funding the project
within this year and hope to win additional funding from the European Union
next year. That would allow construction to start in 2005. Researchers from
Los Alamos National Laboratory in New Mexico have been participating in
design work and will likely join the project formally next year as well.

More projects are on the horizon. Scientists in Japan are lobbying for a
reactor chamber to be added later to the Japan Proton Accelerator Research
Complex project, now under construction in Tokai, northeast of Tokyo. And in
Europe, scientists are already starting to talk up an Experimental
Accelerator-Driven System to follow on from TRADE.

The projects are cheap in an age when big physics facilities run to hundreds
of millions of dollars. Support for ADS research has been small but
consistent, Monti says. "At least in Europe, nuclear waste is the main issue
for public acceptance of nuclear power, and that justifies the effort to
develop this technology," he says, although he cautions that it will take "a
mosaic of technologies" to solve the problem.

The basic processes at work in an ADS--splitting atoms to change one element
into another--have been understood for almost a century. Similar schemes
were briefly studied in the 1950s to turn thorium into uranium-235 to fuel
nuclear reactors. The idea was revived in the 1980s when scientists started
wrestling with the problem of waste from nuclear power plants. The most
troublesome components of nuclear waste are long-lived fission products and
actinides --elements such as americium and curium --which have half-lives of
thousands of years. Researchers working separately at Brookhaven National
Laboratory in Upton, New York, and at Los Alamos started looking at using
subcritical, or non-self-sustaining, nuclear reactions to burn up these
wastes. They envisioned using an accelerator to fire a beam of protons at a
target surrounded by spent nuclear fuel. In what is called a spallation
reaction, the protons break target nuclei, producing neutrons that trigger
reactions in the surrounding material (see figure). Some radioactive
elements are rendered nonradioactive. Others absorb a neutron, become
unstable, and then either fission or decay. Actinides, for example, are
transmuted into uranium, which decays into shorter-lived radionuclides that
can be disposed of as low-level nuclear waste. Because the reaction is
subcritical, if the stream of protons is shut off, the reaction stops.

Chain of fuels. In an ADS, protons slamming into a heavy-metal cylinder
knock out neutrons that alter radioactive material. 
CREDIT: ADAPTED FROM JAPAN ATOMIC ENERGY RESEARCH INSTITUTE

Later, researchers concluded that it would be more practical to reprocess
spent fuel, extract long-lived fission products, and recycle and burn them
in commercial nuclear power plants. But reprocessing raises nuclear
proliferation concerns because it separates out plutonium, which can be
fashioned into a fission bomb. And in any case, actinides cannot be recycled
because peculiarities in the way they fission make it hard to control
reactions based on actinide-rich fuel.

Labs in the United States, Europe, and Japan studied ADS on paper, but the
technology never generated widespread interest until Carlo Rubbia took it
under his wing. In 1993, Rubbia, a 1984 Nobel laureate in physics who was
then director-general of CERN, the European particle physics laboratory,
championed ADSs as a way to generate energy. Relying on a subcritical
reaction, Rubbia believed, would eliminate any chance of a Chornobyl-like
accident. Moreover, ADS power plants would generate much less high-level
waste than conventional nuclear power plants and no material that could be
processed into nuclear weapons. They would also be able to "burn" thorium,
an element three times as abundant as uranium and much easier to process
into nuclear fuel.

A preliminary study suggested that the system would produce about 30 times
more energy than it consumed. But Rubbia never showed that his "energy
amplifier" would be economically competitive, says Massimo Salvatores,
director of research at CEA's Cadarache Center, and he eventually shelved
the idea. Rubbia, who is now high commissioner of ENEA, has since switched
focus: When he proposed the TRADE experiment in 2000, he talked exclusively
about treating nuclear waste.

Salvatores credits Rubbia with building the respect for ADSs that led to
funding for experiments on the separate components, a first step toward a
full system. The most notable of these is the 8-year-old MUltiplication of
an External Source (MUSE) experiment at the Cadarache Center, which studies
the nuclear reactions triggered in a subcritical core by neutrons from a
deuterium- tritium reaction. MUSE's neutrons, however, achieve energies of
only 14 million electron volts--less than a tenth the energy of neutrons
from spallation sources. That shortfall limits the sorts of research MUSE
can pursue. Other experiments have studied spallation sources, but none has
brought an accelerator and a reactor together. "To confirm the feasibility
of an ADS, you need to use both spallation neutrons and a subcritical
[reactor] system," says Kaichiro Mishima, a nuclear engineer at Kyoto
University.

That's where Kyoto's KART comes in. It will come on line in fall 2005, and
JINR's Subcritical Assembly in Dubna (SAD) will join it a year later. Both
will concentrate on the basic physics of ADS. TRADE, the most comprehensive
of the new experiments, will go further. Whereas KART and SAD will be run at
extremely low power, TRADE will generate several hundred kilowatts, allowing
researchers to study how increasing temperatures in the core affect the
reaction. It will also tackle practical issues such as cooling the target
and monitoring and controlling the reaction through start-up, shutdown, and
steady-state operation. "We want to validate that we can control the system
under these various conditions and that we can react if there is any
problem," Salvatores says. Such steps will be necessary to address safety
and licensing issues for a large-scale demonstration waste-transmutation
project planned for around 2015 and likely to cost several hundred million
dollars.

Even if ADSs sail through the experiments with flying colors, the technique
could still face an uncertain future. "An ADS is one of a number of
different options" for handling high-level waste, says Mike Cappiello, a
nuclear physicist at Los Alamos who is national director for transmutation
engineering for the U.S. Department of Energy. For example, although the
United States is interested in joining the TRADE project, the country could
decide to rely solely on the geological depository for high-level waste it
is developing at Yucca Mountain, Nevada. One factor that might give ADS an
edge over other options, Cappiello notes, is economics. A full-scale ADS
system would produce a significant amount of thermal power. "And there is
every incentive to recover that power to either produce electricity or make
hydrogen," he says. Ironically, ADSs might end up doing double duty as
energy amplifiers after all.