Expert #7451
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Howdy. I'm currently writing a Physics research briefing for A2 on the subject of a Thorium Fuel Cycle in Nuclear Power, but I've hit an impasse at a point where I explain how the Uranium-233 in a Thorium fuel cycle may be proliferation resistant due to Uranium-232 contamination (and subsequent gamma emissions from its decay series). The problem is explaining how Uranium-232 is produced. All Uranium-232 would be produced by a (n,2n) reaction with various nuclei in the fuel; the problem is, I've never encountered an (n,2n) reaction in my Physics course before and I can't find a concrete explanation anywhere on the internet, other than it being referred to as an uncommon mode of nuclear decay which only occurs with faster neutrons (at around 10MeV).

An example of an (n,2n) reaction would be:
Name:  blah.png
Views: 115
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What I need for this is a solid physical explanation as to why this happens, and a source that I can cite in my bibliography of the briefing which backs that explanation up.

Thanks in advance!
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Cora Lindsay
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(Original post by Expert #7451)
Howdy. I'm currently writing a Physics research briefing for A2 on the subject of a Thorium Fuel Cycle in Nuclear Power, but I've hit an impasse at a point where I explain how the Uranium-233 in a Thorium fuel cycle may be proliferation resistant due to Uranium-232 contamination (and subsequent gamma emissions from its decay series). The problem is explaining how Uranium-232 is produced. All Uranium-232 would be produced by a (n,2n) reaction with various nuclei in the fuel; the problem is, I've never encountered an (n,2n) reaction in my Physics course before and I can't find a concrete explanation anywhere on the internet, other than it being referred to as an uncommon mode of nuclear decay which only occurs with faster neutrons (at around 10MeV).

An example of an (n,2n) reaction would be:
Name:  blah.png
Views: 115
Size:  772 Bytes

What I need for this is a solid physical explanation as to why this happens, and a source that I can cite in my bibliography of the briefing which backs that explanation up.

Thanks in advance!
OK. This is complex stuff but I hope this explanation will suffice for you.

When a neutron is absorbed, the resulting 'compound nucleus' is in an excited state. For many neutron absorptions, this de-excites to the ground state by gamma ray emission.

Sometimes, though, the de-excitation occurs by 'evaporation' of neutrons. Neutrons are ejected from the compound nucleus and carry away the excitation energy. This parallels the evaporation of the most energetic molecules from a hot liquid, resulting in a decrease in average energy (temperature).

On a related matter, I am not sure I agree about proliferation resistance. U-232 decay products are nasty, but plutonium gets pretty dosey from Am-241 ingrowth and that can be handled fairly well with just light shielding. If you get your timing right so the decay products don't have time to form, or treat your workforce as expendable, U-233 fabrication is do-able. The Americans tested a composite U-233/Pu weapon back in the 1950s and, from memory, India tested a low yield U-233 device too.
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Expert #7451
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Thanks for the response! Would I be right to say then that the ejection of the neutron is merely an unusual way of the nucleus conserving energy whilst going down to its ground state? I'm looking for ways to keep my word count down.

You're right in that U-233 can still be used for weaponry, but of what I'm aware from my research is that the U-232 makes it much less appealing in a weapon due to a greater amount of gamma emissions than Plutonium, and the inability to chemically separate the U-232 from the U-233. Whilst you can time weapon fabrication around the decay cycle or treat workers as expendable, this is significantly less appealing than bomb fabrication using other fissile materials.

An economic cost-benefit analysis rules Uranium-233 as being a pretty awful fuel. It'd probably be easy for a terrorist cell/rogue state to try to highly enrich natural Uranium rather than deal with transmuting Thorium and then worry about copious amounts of radiation from the core.
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Cora Lindsay
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(Original post by Expert #7451)
Thanks for the response! Would I be right to say then that the ejection of the neutron is merely an unusual way of the nucleus conserving energy whilst going down to its ground state? I'm looking for ways to keep my word count down.
Sorry for delayed reply. I'd call it an uncommon way of the nucleus dissipating energy.

(Original post by Expert #7451)
You're right in that U-233 can still be used for weaponry, but of what I'm aware from my research is that the U-232 makes it much less appealing in a weapon due to a greater amount of gamma emissions than Plutonium, and the inability to chemically separate the U-232 from the U-233. Whilst you can time weapon fabrication around the decay cycle or treat workers as expendable, this is significantly less appealing than bomb fabrication using other fissile materials.

An economic cost-benefit analysis rules Uranium-233 as being a pretty awful fuel. It'd probably be easy for a terrorist cell/rogue state to try to highly enrich natural Uranium rather than deal with transmuting Thorium and then worry about copious amounts of radiation from the core.
I agree U-233 would be a rubbish bomb material but I do get hacked off with Th evangelsists saying there is no proliferation problem with the Th fuel cycle. There is!

Making HEU is VERY VERY hard without the resources of a nation state, and so is manufacturing Pu. A non-state group would realistically be looking to buy or steal fissile material rather than make their own, which is why fissile materials stores are generally very heavily guarded (apart from the Y-12 example where an 80 year old nun got through most of the security unchallenged).

Even if you get fissile material, you might have to convert it into metal (not easy) and fabricate the weapon components (hard for Pu; easier for HEU), and build a credible weapon, which is tough for Pu but plausible for HEU. That's why the Americans are putting so much pressure n people to take HEU out of circulation.
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