Disposition of Pu240 in Spent/Used Nuclear Fuel

[eXorikos]

I just finished an article on the implications of separating Pu-240 from SNF. The implications are pissvly huge, but the article does not consider the technical issues for laser separation.

I am familiar with the concept of resonance laser ionization. I am not aware of any real initiative that areclose to cracking this problem on a scale that is appropriate for separating SNF. Perhaps somebody here can give an overview.

 

[QuantumPion]

What is the purpose of separating Pu-240 from spent fuel? If you wanted weapons material, and you have the means to separate Pu-240, you could separate U-235 much more easily, and not having to deal with extreme radioactive materials in hot cells. Or just make clean Pu from a production reactor to begin with.

 

[eXorikos]

This has several advantages. One being less production of Am-241. This makes reacivity control easier and less expensive as you need less neutronflux, because there is less poison.

Secondly, it also has avantages from a fuel repository perspective. The decay heat from this type of MOX will be less than that of current MOX. Decay heat is one of the most determining factors for final repository loading. Current MOX although recycling SNF has more decay heat on the longer term. It therefore does not decrease the volume required for final repository. MOX with reduced Pu-240 content would reduce the higher actinides being produced and therefore the decay heat on the long term.

Edit: here is the article: http://www.ans.org/pubs/journals/nt/a_36800

 

[QuantumPion]

This has several advantages. One being less production of Am-241. This makes reacivity control easier and less expensive as you need less neutronflux, because there is less poison.

Separation of Pu-240 from spent fuel would be way, way more expensive then just buying more fresh fuel even if that were true. But neutron absorption by Am-241 is pretty much completely negligible. The main absorbers in spent fuel are fission products - gadolinium, samarium, cadmium, europium, etc.

Secondly, it also has avantages from a fuel repository perspective. The decay heat from this type of MOX will be less than that of current MOX. Decay heat is one of the most determining factors for final repository loading. Current MOX although recycling SNF has more decay heat on the longer term. It therefore does not decrease the volume required for final repository. MOX with reduced Pu-240 content would reduce the higher actinides being produced and therefore the decay heat on the long term.

Edit: here is the article: http://www.ans.org/pubs/journals/nt/a_36800

Separating out all plutonium from spent fuel is one thing - but isotope separation of Pu-240 is a whole different game.

I'm not an expert on MOX but I presume MOX fuel is created from downblended weapons grade Pu and already has low Pu-240 content. It is not created from spent fuel.

 

[eXorikos]

Separation of Pu-240 from spent fuel would be way, way more expensive then just buying more fresh fuel even if that were true. But neutron absorption by Am-241 is pretty much completely negligible. The main absorbers in spent fuel are fission products - gadolinium, samarium, cadmium, europium, etc.
Separating out all plutonium from spent fuel is one thing - but isotope separation of Pu-240 is a whole different game.

I'm not an expert on MOX but I presume MOX fuel is created from downblended weapons grade Pu and already has low Pu-240 content. It is not created from spent fuel.

It does not go by the economics because there is simply not an economic way to do this separation. It simply discusses the technical consequences. This is the discussion I want to have. Purely academic.

Am-241 is a poison in your fuel. We even have to adjust for its concentrations in our core calculations, since the core package we use underestimates its concentration. I don't have an order of magnitude in mind, but I can ask at work tomorrow.

MOX fuel is produced from reprocessed SNF. This is was done in Belgium in the past, Belgium has moved to a fully open fuel cycle since.

 

[QuantumPion]

Am-241 captures are very small contribution - less than U-234 or Pu-238. It has a decent cross section but its concentration is small. Its contribution to reactivity is not significant. You may be thinking of decay of Pu-241 into Am-241, which does have a significant effect on reactivity as your fuel is decaying away.

 

[eXorikos]

Am-241 captures are very small contribution - less than U-234 or Pu-238. It has a decent cross section but its concentration is small. Its contribution to reactivity is not significant. You may be thinking of decay of Pu-241 into Am-241, which does have a significant effect on reactivity as your fuel is decaying away.

I indeed mixed a few things. Am-241 is one of the main contributers to decay heat. Its concentration would be reduced by 1/3 by eliminating the Pu-240 content through recycling.

Replacing the Pu-240 by U-238 is better for reactivity purposes. Pu-241 decays to Am-241 which indeed has low absorption cross section, while U-238 is fertile to Pu-239 which is fissile.

I am still a bit confused on the influence of the Am-241 on the reactivity, but I will check on that later.

 

[QuantumPion]

Why remove Pu-240 by isotope separation, which is very very hard, when you can just remove Am-241 by chemical separation, which is comparatively easy as pie?

 

[eXorikos]

Why remove Pu-240 by isotope separation, which is very very hard, when you can just remove Am-241 by chemical separation, which is comparatively easy as pie?

But you still have the Am-241 you have to store... It is produced less when you replace the Pu-240 by U-238.

 

[QuantumPion]

But you still have the Am-241 you have to store... It is produced less when you replace the Pu-240 by U-238.

Well you would still have to store the Pu-240. I don't see how that is much of an improvement.

 

[jim hardy]

I am still a bit confused on the influence of the Am-241 on the reactivity, but I will check on that later.

i too am curious about that isotope's contribution. From what i understand it accumulates in MOX fuel if you don't start burning the MOX within a decade or so.? And it yields over 3 neutrons ?

 

[QuantumPion]

i too am curious about that isotope's contribution. From what i understand it accumulates in MOX fuel if you don't start burning the MOX within a decade or so.?

Yes, from decay of Pu-241 (half life 14 y).

And it yields over 3 neutrons ?

Am-241 is fissionable but its capture cross section is much higher.

 

[jim hardy]

Am-241 is fissionable but its capture cross section is much higher.

Thanks ! You gave me the clue what to put in search..

old jim

 

[eXorikos]

Well you would still have to store the Pu-240. I don't see how that is much of an improvement.

Have you read the article?

I really wanted to discuss the current state of laser isotope separation techniques.

 

[Astronuc]

The article requires subscription or purchase. Not everyone might have access to it.

Laser isotopic separation works better as the difference in mass increases.

 

[eXorikos]

Am I allowed to copy it here or is that illadvised?

Doesn't it depend more on the ionizationscheme than on the mass difference?

I know this works on the nuclear level for example in CERN ISOLDE, by using the hyperfine interactions. But doing this on an industrial scale, I guess they use molecules. At least they do for U. Is the principle the same for Pu?

 

[nikkkom]

It does not make much sense to think about technical side of Pu-240 isotope separation since this won't be economical. Whatever problem you are trying to fix with that, there are less expensive fixes than isotope separation of Pu.

 

[Astronuc]

I know this works on the nuclear level for example in CERN ISOLDE, by using the hyperfine interactions. But doing this on an industrial scale, I guess they use molecules. At least they do for U. Is the principle the same for Pu?

It would be same principle, but for U-enrichment, there is a 3 amu difference between U-235 and U-238, whereas with Pu, one would be trying to separate Pu-240 from Pu-239 and Pu-241, so the difference in frequency is not as great. Isotopes are very similar chemically.

 

[eXorikos]

It does not make much sense to think about technical side of Pu-240 isotope separation since this won't be economical. Whatever problem you are trying to fix with that, there are less expensive fixes than isotope separation of Pu.

It is called academic curiosity.

 

[eXorikos]

It would be same principle, but for U-enrichment, there is a 3 amu difference between U-235 and U-238, whereas with Pu, one would be trying to separate Pu-240 from Pu-239 and Pu-241, so the difference in frequency is not as great. Isotopes are very similar chemically.

Which molecule could you use? They use UF6 to UF5, which precipitates. Would this work for Pu as well?

Recent advances have made it possible to use two-step laser ionization at much higher power while keeping the bandwith of the laser very low (~10MHz).

 

[Astronuc]

Which molecule could you use? They use UF6 to UF5, which precipitates. Would this work for Pu as well?

Recent advances have made it possible to use two-step laser ionization at much higher power while keeping the bandwith of the laser very low (~10MHz).

In theory it would work, but with a 1 amu difference, it is likely uneconomical. Reprocessing of spent fuel is already expensive, and the separation of Pu-240 would not improve the economics.

 

[snorkack]

What is the purpose of separating Pu-240 from spent fuel? If you wanted weapons material, and you have the means to separate Pu-240, you could separate U-235 much more easily, and not having to deal with extreme radioactive materials in hot cells.

Then you are still stuck with critical mass thrice that of Pu-239.
The one useful alternative would be U-233, which also is less hot than Pu-239.