Separating Plutonium Isotopes via Laser?

[neutrino_cannon]

So, I'm given to understand that the 1977 cessation of nuclear fuel reprocessing in the USA was based on the idea that plutonium might be harvested from spent fuel and used to make a bomb. Furthermore, I am given to understand that given the technology of the time, this was nonsense on stilts; there's too much plutonium 240 contamination.

Plutonium 240 is particularly effective at screwing up nuclear weapons cores because it undergoes spontaneous fission, so it's very good at causing "fizzles." Furthermore, because the mass ratios of plutonium 240 and plutonium 239 are so close; much closer than uranium 235 and uranium 238, it's quite difficult to enrich commercial reactor plutonium using centrifuges or gasseous diffusion.

But enrichment technology marches on. Laser-based methods are on the horizon. I don't claim to understand laser-based enrichment methods that well, because they sound like magic to me, but my understanding is that because a laser emits a beam of light where every photon is exactly the same frequency, it is possible to tune a laser precisely enough that it ionizes only the desired isotope. Therefore, unless I misunderstand it, which is possible, laser-based enrichment doesn't rely on isotope mass ratios. It's basically 100% selective.

So, would it be possible to separate plutonium 240 and plutonium 239 using laser-based enrichment?

 

[geni]

So, I'm given to understand that the 1977 cessation of nuclear fuel reprocessing in the USA was based on the idea that plutonium might be harvested from spent fuel and used to make a bomb. Furthermore, I am given to understand that given the technology of the time, this was nonsense on stilts; there's too much plutonium 240 contamination.

If by nonsense on stilts you mean "its been done" then yes. The trick is in the reactor design. The british magnox design is usable for producing weapons grade plutonium. If you look at the british nuclear program this is not supprising.

So, would it be possible to separate plutonium 240 and plutonium 239 using laser-based enrichment?

In theory yes. If it is practical to well thats classifed. In theory you could do it using a calutron however the publicaly avaible data suggests it is impractical.

 

[neutrino_cannon]

If by nonsense on stilts you mean "its been done" then yes. The trick is in the reactor design. The british magnox design is usable for producing weapons grade plutonium. If you look at the british nuclear program this is not supprising.

I wrote the post in a hurry and sacrificed some clarity. Yes; obviously there are plenty of nuclear weapons that use plutonium cores. However, the fuel cycle for breeding nuclear weapons material is rather different from the regular fast neutron breeder cycle. Specifically, you don't leave the stuff in for nearly as long so that there's less chance for PU-239 to absorb a neutron and become PU-240.

My understanding is that plutonium bomb cores are below 7% PU-240, preferably lower, and that the plutonium isotope ratios are kept that way not by enrichment, but by not breeding that much PU-240 in the first place.

The "nonsense on stilts" was the idea that you could pull plutonium that's been sizzling in a typical commercial reactor for a long time and make a bomb about it.

The general understanding is that the bomb made from "reactor grade plutonium" was made from plutonium that had only a little bit more PU-240 than is typical for weapon cores.

In theory yes. If it is practical to well thats classifed. In theory you could do it using a calutron however the publicaly avaible data suggests it is impractical.

Calutrons, according to my handbook, are generally less efficient than gaseous diffusion and centrifuges.

Calutrons, gaseous diffusion and centrifuges all depend on the mass ratios of the isotopes to be separated. The further apart they are, the better it will work.

The reason I ask about whether lasers might work better is because, at least it sounds to me, the laser enrichment method uses some sort of resonance voodoo that only ionizes the interesting isotopes, so the fact that PU-240 and PU-239 have very close mass ratios might not be a problem.

 

[geni]

I wrote the post in a hurry and sacrificed some clarity. Yes; obviously there are plenty of nuclear weapons that use plutonium cores. However, the fuel cycle for breeding nuclear weapons material is rather different from the regular fast neutron breeder cycle. Specifically, you don't leave the stuff in for nearly as long so that there's less chance for PU-239 to absorb a neutron and become PU-240.

My understanding is that plutonium bomb cores are below 7% PU-240, preferably lower, and that the plutonium isotope ratios are kept that way not by enrichment, but by not breeding that much PU-240 in the first place.

The "nonsense on stilts" was the idea that you could pull plutonium that's been sizzling in a typical commercial reactor for a long time and make a bomb about it.

Magnox where at one point the UK's standard commercial reactor design and there was nothing to stop you from taking the plutonium out before the 240 built up.

Calutrons, according to my handbook, are generally less efficient than gaseous diffusion and centrifuges.

However they produce a very high purity product. Thats why the US thought the thin man bomb design might be viable.

 

[elgarak]

I've read the Wikipedia entry on Atomic Vapor Laser Isotope Separation (AVLIS). It relies on the hyperfine structure (of the electrons) being slightly different because of the slightly different potential of the nucleus, therefore slightly different absorption lines for different isotopes.

What I mean by slightly: The article doesn't have details on Pu, only on U235 vs. U238, and the difference in wavelength of the absorption lines is 502.73 nm vs 502.72 nm. That's pretty small, and I expect the difference between Pu239 and Pu240 to be even smaller. It's a very tricky application.

ETA:
I read also the Wiki entry on Laser Linewidth, and a simple traditional laser with little optics has typically line widths in the nm range (1-10 nm), which is the number I am familiar with. However, in 1992 they found that employing quantum optics (an area where I know next to nothing about) can deliver (pulsed) lasers with line widths down to 0.0004 nm, which is near the theoretical limit. So the part that I thought was the most tricky (getting a laser with narrow enough line width) isn't that tricky at all.

 

[Roboramma]

I don't know anything about this, but I would suspect that the differences in resonance are actually related to (and proportional to?) the differences in mass. Given that the structure of the atom is almost identical and the only difference is the number of neutrons (which may cause some slight difference in the structure of the electrons?), that difference should at least be related to the difference in the mass of the atoms.

Of course it's possible that the tiny difference can lead to big differences in the resonance of the atom that not proportional to the difference in mass in any straightforward way, but that's my guess.

 

[elgarak]

I don't know anything about this, but I would suspect that the differences in resonance are actually related to (and proportional to?) the differences in mass. Given that the structure of the atom is almost identical and the only difference is the number of neutrons (which may cause some slight difference in the structure of the electrons?), that difference should at least be related to the difference in the mass of the atoms.

Of course it's possible that the tiny difference can lead to big differences in the resonance of the atom that not proportional to the difference in mass in any straightforward way, but that's my guess.

After reading on the hyperfine structure on Wiki, I don't think that the mass difference has any influence, or that the influence of the 3 neutrons for U325 vs U238 is significantly larger that the influence of the 1 neutron for Pu239 vs Pu240. The main term in the hyperfine structure is magnetic, and in this case is due to the spin and associated magnetic moment of the additional neutron(s) in the nucleus. Two neutron spins should cancel each other out, so in both cases there is one additional neutron spin with associated magnetic moment causing the hyperfine structure split. (That's a first order approximation by squinting, so I could be completely wrong).

It would also mean that one can probably increase the wavelength difference of the absorption lines by putting the thing in a large electro magnet, but, given what I read about the line width of tunable lasers, I don't think it's necessary.

 

[neutrino_cannon]

I didn't really understand any of that, which underlines my conviction that laser isotope enrichment is witchcraft.