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Water is used as a moderator to slow down neutrons in a light water nuclear reactor. By slowing down the neutrons they have more of a window to strike fission ready nuclei thus promoting the self perpetuating chain reaction when more neutrons are released from the fissions and slowed neutrons.

All fine and good so far. I got it. Now, what I don't get is why fast neutrons are needed for the breeder nuclear reactor. According to my research, the main reason breeder reactors have not met expectations is the problems encountered with the liquid sodium cooling system. Why liquid sodium instead of water? Because water is a moderator that slows neutrons, while liquid sodium does not slow down neutrons.

Why do we need fast neutrons to breed Pu 239 from U 238. Wouldn't the same slow neutrons used in fission chain reactions also increase the probability of the neutron encountering and hitting the U238 nucleus?

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  • $\begingroup$ My understanding is that when you have a neutron moderator, the moderator itself tends to absorb a fair amount of neutrons. If your goal is just to sustain a chain reaction, then fission of enriched uranium produces more neutrons than you need anyway, so this isn't a problem. If, instead, your goal is to create more much fuel than you burn, then wasting neutrons that could be captured is a major downside of using a moderator. Fast neutrons also have the benefit of being able to fission things like actinides, which would otherwise be long-lived nuclear waste products. $\endgroup$
    – Chris
    Commented Sep 27, 2017 at 1:30
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    $\begingroup$ You need to look at the entire reaction sequence to get from 238U to 239Pu - it is not just a simple absorption of a neutron. You need to trace through 239U -> 239Np -> 239Pu. $\endgroup$
    – Jon Custer
    Commented Sep 27, 2017 at 2:38
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    $\begingroup$ What do you mean by "more of a window?" A better statement would be "the fission cross-section of U-235 is higher for low energy neutrons than high energy neutrons." $\endgroup$
    – Bill N
    Commented Sep 27, 2017 at 14:07

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I'll take a shot at this. Head over to the National Nuclear Data Center hosted at Brookhaven National Laboratory. Go to the Evaluated Nuclear (reaction) Data File section (ENDF) where you can get cross section for various neutron reactions.

Below I have put in a screenshot of the result of plotting 4 different reactions.

The first, topmost, black line is the total cross section for neutrons interacting with $^{238}$U.

The second, green line is the elastic scattering cross section, that is $^{238}$U(n,n)$^{238}$U - the neutron goes in and bounces off.

The grey line is the cross section for neutron-induced fission - the neutron goes in and parts come flying apart. This of course does nothing to get to $^{239}$U and the path to $^{239}$Pu.

Finally, the (mostly) lowest red line is the cross section for absorption of the neutron (and release of a gamma), i.e. $^{238}$U(n,g)$^{239}$U. This is what you want in a breeder reactor.

One can readily see that below 1MeV, neutrons are (far) more likely to fission the $^{238}$U nucleus then make $^{239}$U. So, moderated neutrons are of no use at all for breeding Pu.

ENDF scattering cross sections

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  • $\begingroup$ Thank you for great answer. I am surprised at the wild fluctuations in cross section for incremental changes in incident energy. What would be the sweet spot incident energy for neutrons in a breeder reactor? The incident energy should be high enough to transmute U238 to Pu but not so high as to decrease cross section. From the chart above it looks like the barns start leveling off after 1Mev but since this is a logarithmic graph I am assuming 5 to 10 Mev. If the graph continued I think we would see the red line start to drop. I'm curious what the inflection point is. $\endgroup$ Commented Sep 27, 2017 at 15:07
  • $\begingroup$ All the wild fluctuations are from nuclear energy levels in the compound nucleus (i.e. $^{239}$U). The levels determined from gamma emission can be seen in the Evaluated Nuclear Structure section of the NNDC. As for energies beyond ~15MeV, one might have to search the literature (which in theory the NNDC folks are doing). Since D-T is a 14MeV neutron, that tends to be a standard limit for meany experiments. Otherwise you are looking at spallation neutrons. $\endgroup$
    – Jon Custer
    Commented Sep 27, 2017 at 18:01

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