# Neutrons, fertile, fissile and fissioning

## Summary:

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

## Main Question or Discussion Point

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

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mfb
Mentor
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.

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
Mentor
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.

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

mfb
Mentor
What is unclear?

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.

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mfb
Mentor
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.