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Short Course on Nuclear Fission

BEEN a few years since you studied Nuclear Physics 101? Here's a quick review. Uranium atoms fission (split) in a nuclear reactor. Fission yields the heat that makes the steam that turns the turbine that generates electricity.

Only 0.7 percent of the atoms in uranium ore are U-235, the isotope that splits during the fission process. The other atoms are U-238, which isn't immediately good for anything.

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So to turn uranium into fuel, it must be ``enriched.'' In this process a lot of U-238 is separated out and thrown away, becoming nuclear waste. What remains - 3 percent U-235 and 97 percent U-238 - is shaped into pencil-width rods. These are placed in the nuclear reactor core, where fission occurs.

As it fissions, U-235 is transmuted into short-lived radioactive waste, stuff decays to harmless levels after about 200 years. After a few years, the waste builds up in the fuel rods, stifling the nuclear reaction.

Then it's time either to throw the rods away or to reprocess them for reuse. Current United States policy is to carve out an underground storage area where the fuel can be discarded, but also retrieved someday if desired.

During fission, some neutrons released by the U-235 are captured by neighboring U-238 atoms. When that happens, the U-238 changes into actinides - radioactive chemical elements - mostly plutonium.

This could be good or bad. Actinides are fissionable. So what was worthless U-238, which is 99.3 percent of all uranium, can now replace scarce U-235 in reprocessed fuel rods. Breeder reactors are configured to create as much or more fissionable material as they expend.

But creating actinides is bad if any will end up as waste, because they stay radioactive for eons. This is what happens in countries that reprocess waste from breeder reactors. They recover only the plutonium, while the other actinides are discarded.