Question

Can high-energy neutrons fission U-238 without breeding or converting it to Pu-239 first?

See: “nuclear fusion: deuterium–tritium fusion produces neutrons of 14.1 MeV (1400 TJ/kg, i.e. 52,000 km/s, 17.3% of the speed of light) that can easily fission uranium-238 and other non-fissile actinides.” Here:

https://en.wikipedia.org/wiki/Neutron_temperature#Fast

Doesn’t “non-fissile” mean under all circumstances?

Answer 1

To fission U-238 to Pu-239, neutrons must have the critical energy of 5.9 MeV, which means that U-238, after capturing such fast neutrons, will form Pu-239. This is unavoidable with neutrons of such a high speed.

The corresponding "Uranium-238" article says that "non-fissile" means it cannot sustain a chain reaction in a thermal-neutron reactor. So, it is more about terms and engineering language, not about non-fissile under all circumstances. Non-fissile under all circumstances could be called "non-fissionable", but such materials do not exist.