Posted on 02/02/2003 12:52:39 PM PST by jaime1959
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 (“clean and environmentally safe advanced reactor”) 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 “moderator” 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.
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 “delayed neutrons”. 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 “spent” 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
Anyone know exactly what energy this is (in MeV)?
This is the rub. U238 doesn't fission, according to my references, it is a great absorber of "fast" newtrons, hence a good shielding material.
If the conditions are just right, the U238 becomes a U239 isotope and CAN, with the right conditions, become Pu239, Plutonium, which fissions quite well.
I doubt that this guy will ever demonstrate a self-sustaining reaction with the uranium left over in spent fuel. There are too many fission fragments in spent fuel acting as impurities that mess up the process.
Delayed neutrons are born in the keV (thousands of electron volt) range. They are born below the fast fission threshold of U-238.
Based on this article, I don't have a clue as to what they are trying to accomplish.
Delayed neutrons make up about 0.6% of all neutrons. Conventional thermal reactors, which include almost all of the power reactors now operating, depend on delayed neutrons for control of the reactor.
This sounds a bit like the U238 reactor described.
I think Proto-Actinium would work, but may be on the rare side.
I cannot run a reactor using natural uranium as the fuel and steam as the moderator / coolant.
I note that roughly 40% of the reactors in the USA are General Electric Boiling Water Reactors, which produce steam directly in the reactor core. They do require enriched uranium as the fuel.
There is a reactor concept called "spectral shift" in which the moderation (slowing down of the neutrons) is intentionally varied to improve the fuel utilization. I suspect that the reactor discussed in this article is a spectrail shift reactor; the author of the article simply doesn't understand the underlying physics and technology and so has badly botched the description and explanation of it.
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