Radioactive Boyscout: Real-Life Story of a Teenager's Quest for Nuclear Fusion

[Morbius]

Just a quick question if I may: So would spallation be the appropriate term for the fissioning of the U238 tamper on a thermonuclear warhead? The U238 in the tamper fissions from the fast neutrons, but thermal neutrons won't have any effect (other than transmuting the uranium into neptonium, which will quickly decay to Pu239), correct?

TOKAMAK,

As tehfrr stated, fissioning of U238 is not spallation - it is a fission reaction.

Or more specifically, it is a "threshold fission reaction".

An isotope like U-235 is said to be "fissile" - because it will fission with
low energy neutrons - that is the neutron need not bring with it any added
kinetic energy. Just falling into the nuclear potential well of a U-235
nucleus is enough to fission U-235.

An isotope like U-238 is said to be "fissionable". That is it will fission,
but only if the neutron has a kinetic energy above a certain threshold
value; which in the case of U-238 is about 1 MeV. If you have a lot of
high energy neutrons running around, U-238 will fission, and it is called
"fission" - same mechanism as U-235 - but just needs a little more energy.

Dr. Gregory Greenman
Physicist

 

[TOKAMAK]

Oh okay, I think I get it now. Thanks for the quick responses.