fissile versus fissionable

[dj_strom@ccmail.pnl.gov]

Gary Damschen has it right: fissile material can have fission induced by 
thermal neutrons, while fissionable materials require fast neutrons for 
incuded fission.  Here's some insight, though: for even-Z nuclides (Th, 
U, Pu, ...), an odd A (mass number) implies an odd N (number of 
neutrons).  For odd-N nuclei, the addition of another neutron completes 
a Pauli spin pair (spin up, spin down), which is an energetically more 
favored quantum state than adding a neutron to an even-N nucleus.  Thus, 
the odd-A nuclei for U (233, 235) and Pu (239, 241, 243) tend to be 
fissile, while the even A nuclei are merely fissionable.  A look a the 
neutron-induced fission cross-section for U-238 shows a threshold of 1 
MeV.

The opinions expressed above are my own, and have not been reviewed or 
approved by Battelle, the Pacific Northwest National Laboratory, or the 
U.S. Department of Energy.

Daniel J. Strom, Ph.D., CHP
Staff Scientist
Health Protection Department K3-56
Pacific Northwest National Laboratory
Battelle Boulevard, P.O. Box 999
Richland, WA 99352-0999 USA
(509) 375-2626
(509) 375-2019 fax
dj_strom@pnl.gov