How do fast breeder reactors differ from regular nuclear power plants?
P. Andrew Karam, a senior health physicist with MJW Corporation in Amherst, N.Y., explains:
In addition to generating power, as regular nuclear plants do, fast breeder reactors utilize so-called fast neutrons to produce up to 30 percent more fuel than they consume.
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Nuclear reactors generate energy through fission, the process by which an atomic nucleus splits into two or more smaller nuclei. During fission, a small amount of mass converts into energy, which a reactor harnesses by a controlled chain reaction; when a uranium nucleus splits, it produces two or more neutrons that other nuclei absorb, causing them to undergo fission as well. As more neutrons are released in turn, continuous fission occurs. Fission produces neutrons with high energies that move extremely quickly. These fast neutrons do not efficiently cause fission. To maintain the reaction, most reactors use a moderator, commonly water or helium, to slow the neutrons to optimum energies.
In contrast, a fast reactor uses a less effective coolant, such as liquid sodium, so the neutrons remain high energy. Although these fast neutrons are not as good at causing fission, they provide a side benefit: they are readily captured by the isotope uranium 238, which then becomes plutonium 239. Certain reactor designs maximize plutonium production for use as fuel or to produce nuclear weapons; reactors that can make more fuel than they consume are called breeders. They do this by exploiting the natural preponderance of uranium 238, which does not fission readily, over uranium 235, which does.
Commercial nuclear reactors typically use uranium fuel that has been enriched so it is up to 8 percent uranium 235. Although this minority isotope does the bulk of the fissioning to produce energy, most of the atoms in the fuel are uranium 238—potential future plutonium 239 atoms.
Plutonium 239 happens to be even better at fissioning than uranium 235, but this fissioning reduces the plutonium content of the fuel. This is why many breeder reactors are fast reactors. Fast neutrons are ideal for plutonium production because they are easily absorbed by uranium 238 to create plutonium 239 yet cause less fission than slower-moving neutrons.
Creating extra fuel in nuclear reactors is not without its concerns: the plutonium produced could contribute to nuclear proliferation, and the fuel reprocessing necessary for plutonium extraction creates radioactive waste and potentially high radiation exposure. For these reasons, President Jimmy Carter halted such reprocessing in the U.S. in 1977. Today only India, Russia, Japan and China have operational fast breeder programs; the U.S., the U.K., France and Germany have effectively shut down theirs.
What do butterflies do when it rains?
Michael Raupp, professor of entomology at the University of Maryland, offers this answer:
What butterflies ultimately do in the rain is avoid it. A stormis no trivial matter—for a 500-milligram monarch butterfly, getting hit by a 70-milligram raindrop would be equivalent to you or me being pelted by water balloons with twice the mass of small bowling balls.
Storms may also hinder a butterfly's mobility. In preparation for flight, these aerial acrobats expose their wings to direct sunlight, which rapidly warms their flight muscles. Overcast skies block the solar radiation they need to take wing.
Thus, when the sky darkens, butterflies seek shelter in their roosts: protected locations such as tall grasses or leafy plants. But when sunshine returns, they often resume patrolling and courting within minutes. So the next time thunder rumbles, take a cue from the butterflies: find shelter, but as soon as the sun comes back, go out and enjoy.
For a complete text of these and other answers from scientists in diverse fields, visit www.sciam.com/askexpert
