Neutrons, fertile, fissile and fissioning

[Flexwheeler]

TL;DR Summary

How do you turn fertile material to fissile without fissioning when they are afterwards are hit with neutrons again?

How do you turn fertile material to fissile without fissioning when they are afterwards are hit with neutrons again?

 

[mfb]

Remove it fast enough. That's how plutonium for weapons is extracted, for example. Or choose a neutron energy spectrum that breeds fissile material but doesn't lead to much fission.
More context would be helpful.

 

[Flexwheeler]

I don't understand what you meant with "choose a neutron energy spectrum that breeds fissile material but doesn't lead to much fission". Please explain.

 

[mfb]

The probability that material A is converted to B by a neutron depends on the energy of the neutron. The probability that B is converted to C also depends on the energy - but in a different way. There can be an energy range where the transition of A to B is likely but B to C is unlikely. That's where you want your neutrons.

But realistically, removing the material often enough is easier. Let's say you convert 1% of A to B in a week and 1% of B to C. Start with 100% A. After a week you'll have ~99% A, ~0.995% B and ~0.005% C - you lost less than 1% of your B to further conversion as B was in for less than a week on average. After two weeks you'll have ~98% A, ~1.98% B and ~0.02% C. Losses now reached 1%. After 10 weeks you'll have ~90% A, ~9.5% B and 0.5% C. Loss is now 5%. That's probably still fine. Separate A, B and C, put A back into the machine, use C elsewhere if possible or store it as waste.

 

[Flexwheeler]

Sorry I do not understand. Can you please explain easier.

 

[mfb]

What is unclear?

 

[Flexwheeler]

Could you please explain in simple terms what you meant. I am unfamiliar with what you explained. How do you control or choose energy spectrum? Can you tell me how it is done with the 3 fissile material (U-233, U-235 and Pu-239) choosing a neutron energy spectrum that breeds fissile material but doesn't lead to much fission.

 

[mfb]

How do you control or choose energy spectrum?

With moderators and the choice of the fission material.

I don't have a specific example with real isotopes, but here is a toy example: X can only fission with fast neutrons, but it can capture both fast and thermal neutrons. After capturing a neutron and maybe beta decays it becomes material Y. If material Y also needs fast neutrons to fission then you can collect it in an environment where you have mainly thermal neutrons: They can convert X to Y, but they can't fission Y.

 

[Flexwheeler]

I am sorry I do not understand.