From: Michael M. Butler (mmb@spies.com)
Date: Fri Jan 31 2003 - 18:28:16 MST
HAIL, CAESAR
A clever new design could lead to a kinder, gentler form of nuclear power
http://www.economist.com/science/displayStory.cfm?story_id=1559861
The Economist print edition
IT SOUNDS impossible: a nuclear reactor that generates electricity from
nuclear waste. Yet that is what Claudio Filippone, a nuclear scientist and
director of the Centre for Advanced Energy Concepts at the University of
Maryland, proposed a few years ago. He has now devised an improved design,
called CAESAR (³clean and environmentally safe advanced reactor²) that is
even more counterintuitive. As well as being environmentally friendly‹it can
produce electricity without causing any extra pollution‹his new design could
also help prevent nuclear proliferation.
Conventional nuclear reactors run on fuel rods made largely of uranium-238,
enriched with about 4% uranium-235. If you hit a uranium-235 atom with a
neutron, it releases a tiny amount of energy and breaks up into smaller
nuclei and more neutrons, which can in turn hit other atoms. Pack atoms
close enough together, and a self-sustaining reaction occurs, producing a
vast amount of energy in the form of heat. Several things control the
reaction rate, including a ³moderator² that is inserted between the fuel
rods to slow down some of the neutrons so that they move slowly enough to
split atoms. After two or three years, there are so few uranium-235 atoms
left that the reaction cannot sustain itself. At this point, fresh fuel rods
are needed.
Nuclear Energy
Popular Mechanics posts an article about Dr Filippone's earlier work. See
also the Nuclear Research Institute of Rez.
In contrast, Dr Filippone's new design works by splitting the far more
numerous uranium-238 atoms inside fuel rods. Such atoms are very picky. To
get them to split, you have to hit them with a neutron going at exactly the
right speed. (Uranium-235 and plutonium atoms are far less choosy, so
getting a self-sustaining reaction going is relatively easy‹hence their use
in weapons.) The key to the new design is the unusual choice of moderator:
steam. Steam's density can be controlled very finely, so it can be used to
slow passing neutrons to ensure that they are moving at exactly the right
speed to split a uranium-238 atom.
The result is a tiny burst of energy, more neutrons and smaller nuclei, as
usual. But CAESAR exploits the fact that these smaller nuclei also decay to
produce additional, slow-moving neutrons, known as ³delayed neutrons². In a
conventional reactor, the moderator slows these neutrons down so much that
they cannot contribute to the reaction. But when steam is used as the
moderator, the delayed neutrons keep going until they hit another
uranium-238 atom. It should then be possible to maintain a self-sustaining
reaction in ³spent² fuel rods of pure uranium-238 for several decades. So
material now treated as waste could be used as fuel. The problem of
disposing of spent fuel will remain, but CAESAR is, in effect, a form of
waste storage that produces electricity.
The design also has implications for preventing nuclear proliferation. The
cores of existing reactors have to be accessible so that fuel rods can be
moved in and out. By adjusting the configuration of the core in the right
way‹by judicious positioning of graphite, for example‹almost any civilian
reactor can be made to produce plutonium, and thus to make weapons. Access
to the core is not necessary with CAESAR, as it could run for decades
without any need for refuelling. Thus it could be sealed. Countries could
then adopt the design to show that their nuclear intentions were entirely
peaceful.
Dr Filippone has tested some aspects of his design using the experimental
TRIGA reactor at the University of Maryland, and in computer simulations.
But to prove that it will work he has to demonstrate a self-sustaining
reaction in uranium-238. Working with Ivo Vasa of the Nuclear Research
Institute in Rez, in the Czech Republic, he is now looking for the money
needed to perform such an experiment, which is expected to take about two
years and cost around $2m. With luck, his new design will generate a
favourable reaction.
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