[chimera]

As I recall, you get a “bonus” of 1 MeV when you go from odd to even numbers of nucleons in the neutron capture event. The binding energy gets you about 5 MeV, the “bonus” of 1 MeV going from odd to even gets you one more, for a total of 6 MeV, just the amount you need to add to the nucleus to induce the fission reaction. U238 neutron capture goes from even to odd, so you don’t get the bonus. The 5 MeV you get from the binding energy isn’t enough to do it for thermal neutron-induced fission. You need an incoming “fast” neutron (1 MeV or more) to fission the U238 nucleus. So it is a threshold reaction rather than a thermal neutron capture. Do I have the numbers right (it’s been awhile)?

[The_Media_never_lie]

As I recall, you get a “bonus” of 1 MeV when you go from odd to even numbers of nucleons in the neutron capture event. The binding energy gets you about 5 MeV, the “bonus” of 1 MeV going from odd to even gets you one more, for a total of 6 MeV, just the amount you need to add to the nucleus to induce the fission reaction. U238 neutron capture goes from even to odd, so you don’t get the bonus. The 5 MeV you get from the binding energy isn’t enough to do it for thermal neutron-induced fission. You need an incoming “fast” neutron (1 MeV or more) to fission the U238 nucleus. So it is a threshold reaction rather than a thermal neutron capture. Do I have the numbers right (it’s been awhile)

Please, don't even think about a trip to Iran any time soon.

[RadioAstronomer]

Here is a good chart showing Isotope Netron Cross Sections including 235 and 238. :-)

http://ie.lbl.gov/ngdata/sig.htm

(Look at sigma (F)