![]() So if you don't mind trading a little bit of "stopping power" for lower absorption, it can be feasible to design your reactor with heavy water (D* 2*O) as the moderator. However, deuterium has a much lower absorption cross section than hydrogen-1. A = 2 (deuterium) is pretty good, although not as good as hydrogen-1. So materials with a high natural hydrogen content (natural hydrogen is almost entirely hydrogen-1) is best for that purpose. That means that material with A = 1 (hydrogen-1) is the best at slowing down neutrons. When a neutron scatters elastically off of a nucleus with mass number A, the lowest possible energy of the neutron after the scattering is 2 times its original energy. In the case of heavy water moderators, heavy water is less good at slowing neutrons down than light water. ![]() How exactly do moderators help with this? A reactor fueled by 100% uranium-238 couldn't reach criticality, some fissile material is necessary. 0.7% of natural uranium is 235, which is fissile. Like you said, uranium-238 can only undergo fission with fast neutrons, not thermal neutrons.Īnd also I'll add that even when a reactor is operating on low or zero enrichment, there is still fissile material present. The choice of moderator influences the necessary enrichment of the fuel but it's not because of the moderation that fission of 238 happens. I also know that moderators such as heavy water and graphite can allow for natural, unenriched uranium (mostly 238) to be used as fuel. Does U-238 actually fission with thermal neutrons as well but only with a very small cross-section and moderators just drastically increase the frequency of collisions? That's the only answer I can come up with, but I'm no nuclear physicist. I've searched so many resources to find an answer to this, but everyone just seems to say "moderators allow natural uranium to be used as fuel" without specifying how when (based on my reading) U-238 requires fast neutrons to fission. However, I've also read that U-238 only tends to fission from fast neutron capture and that its own fission product neutrons are not typically fast enough to sustain a chain reaction. However, I don't understand how to reconcile these two ideas.Īs I understand it, moderators lower the kinetic energy of fast neutrons until they are thermal neutrons and are more likely to fission various nuclides. I understand the premise of neutron moderators and that U-238 is fissionable but not fissile.
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