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Nomy-the wanderer
#6
Mar25-06, 11:55 AM
P: 186
Quote Quote by T.Duncan
2) why can U235 fission by low energy, U238 need a large energy to fission. is it relative to the critical energy for fission?
Odd numbered heavy nuclei, are naturally fissionable u don't need a lot of energy to split them, such as U235 or Pu 239, this has something to do with the repulsive and binding forces inside the nucleus. Usually natural uranium contain 0.7% only of U235 which is fissile, and the rest of U238.And that's why some times u need the enrichment process...
U-238 can capture a neutron and decay to Pu239 which is also fissile, but u need some more energy to do that.

Quote Quote by T.Duncan
3)In a chain reaction, why not only used U235?
It's a very hard chemical procedure to separate isotopes...

Quote Quote by T.Duncan
4) what are nuclear reactor and breeder reactor?
There are fast breeder reactors and thermal reactors, fast reactors use fast neutrons (energetic), and thermal reactors operate with thermal (slow) neutrons...

Thermal reactors also use moderators, which are intended to slow the neutrons until they approach the average kinetic energy of the surrounding particles. Thermal neutrons have a far higher probability of fissioning U-235, and a lower probability of capture by U-238 than the faster neutrons that result from fission do.
Examples can be Candu heavy water reactor, PWR(pressurized), LWR(light), BWR(boiling)..etc. Naval reactors can be PWRs.

For fast breeders there's hardly any need for moderators, u only need highly enriched uranium (sometimes weapons-grade), or plutonium in order to reduce the amount of U-238 that would otherwise capture fast neutrons. Some are capable of producing more fuel than they consume, usually by converting U-238 to Pu-239.
An example is sodium cooled reactor, and generally LMFBRs (liquid metal fast breeder reactors...

I hope this has answered your question, i didn't try to get into much details...