Answer to Question #103826 in Atomic and Nuclear Physics for John O'Renick

Question #103826

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?

Expert's answer

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.

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28.02.20, 17:23

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John O'Renick

27.02.20, 20:12

But a U-238 has to capture that neutron, do a beta decay, and become a Pu-239 before it can capture another neutron, and fission? And that first neutron has to be at 5.9 MeV or the U-239 can't capture it? I've been reading that 14.1 MeV neutrons from D-T fusion can be used to make minor actinides fission. Can a high-enough energy neutron make U-238 fission without transmuting to Pu-239 first?

Answer to Question #103826 in Atomic and Nuclear Physics for John O'Renick

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