I know that when a neutron bombards a U235 nucleus, its fissions because it's unstable, but U238 doesn't. Or at least it takes more energy for it to fission. How? Shouldn't it be easier since it is bigger, and the nuclear force is weaker? Isn't it?
2007-09-26 13:31:21 · 2 answers · asked by shish2kabob 2 in Science & Mathematics ➔ Physics
Nuclei "like" to have certain numbers of protons and neutrons and "dislike" having other numbers. U-235 has an unstable arrangement, more easily split.
A crude analogy is a big pile of balls, very stable as a pyramid, not so stable with some balls taken out of the base of the pyramid.
2007-09-26 13:38:00 · answer #1 · answered by Howard H 7 · 0⤊ 0⤋
U238 does not technically fission at all. If it absorbs a neutron it converts to Plutonium 239 (which is just as fissionable as U235). The main difference is the energy required to either be absorbed or cause fission. U235 will fission with a relatively slow neutron, whereas U238 will only absorb a relatively fast neutron (slow neutrons essentially bounce off). In a nuclear reactor, this principle is used in choosing a "moderator" which is a medium such as carbon, water, or heavy water, which slows down the neutrons sufficiently that they are not easily absorbed by the U238 (which is far more common in the fuel than U235), but still can be absorbed by U235 to cause further fission. Each fission event absorbs one neutron and releases two (this defines the meaning of fission).
2007-09-26 13:41:24 · answer #2 · answered by Patrick M 3 · 0⤊ 0⤋