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The "Light-water reactor" Wikipedia page states that "the uranium 238 atoms also contribute to the fission process by converting to plutonium 239". But from what I've read you need fast neutrons to breed plutonium 239 from uranium 238, not thermal neutrons. The point of using water is to moderate the neutrons so a LWH must be a thermal reactor by definition, right? So how can a thermal reactor, specifically a LWR, breed plutonium? Maybe it's actually some kind of hybrid thermal-fast reactor where some neutrons get moderated and some dont?

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You need fast neutrons to fission U238.not to breed plutonium from it. The reason reactors don't use U238 as a fuel and fission it with fast neutrons is that fission of U238 would result in a runaway reaction which couldn't be controlled. To have a controlled nuclear reaction, you need delayed neutrons, which aren't present with fission of U238. As you say, fission of U235, the usual fuel of fission reactors, has to be done with slow neutrons, so the fission neutrons are slowed with a moderator. They can then be captured by U238 to transmute it into Pu239, as well as causing fission in the U235 content of the fuel rods. The daughter nuclides of U235 emit delayed neutrons at up to a second later, and this enables the reaction to be controlled.

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  • $\begingroup$ I totally read the Wikipedia page on U238 wrong. But in my defence the wording is a little tricky. Thanks! $\endgroup$ – Jonah Jul 4 at 21:40
  • $\begingroup$ So if U238 is fissionable, does it ever fission from fast neutrons coming from within the same fuel rod? $\endgroup$ – Jonah Jul 4 at 22:00
  • $\begingroup$ It probably does, but not enough to cause a chain reaction. Both U238 and U235 undergo occasional spontaneous fissions, but not enough to cause a chain reaction unless the fuel achieves a critical mass. $\endgroup$ – Michael Walsby Jul 4 at 22:23
  • $\begingroup$ This answer is not correct. U238 also emits delayed neutrons and there is a class of "fast reactors" that are designed to use fast neutrons to take advantage of U238 fission. Granted, U235 has a higher delayed neutron fraction (measured by a quantity called "beta"), but U238 has delayed neutrons and you can safely operate a reactor with U238 fissions. $\endgroup$ – pmy Jul 5 at 12:51
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Heavy nuclides can undergo several nuclear reactions including neutron capture, fission, and scattering. The probability of these reactions are given by "cross sections" and are highly dependent on the neutron energy.

The fission cross sections for U235 (and Pu239) are much higher at thermal energies, so the goal of a light water reactor (LWR) is to slow the neutrons down to thermal energies to increase the probabilities of a fission reaction happening. For example, the fission cross section of U235 is about 500 times bigger at thermal energies than fast neutron energies.

The U238 fission cross section is a "threshold reaction", which means that it can only occur with high neutron energies. There is some U238 fission in a LWR, but it is not very much. The majority of U238 reactions in a LWR are neutron capture. During a neutron capture, the U238 becomes U239, then quickly decays into Np239, then quickly decays again into Pu239. Therefore, U238 does convert into Pu239 in a LWR. Pu239 is fissionable at low energies (like U235), so the Pu239 will eventually contribute to the total power. A rough rule-of-thumb is that at the end of an LWR cycle, 20% of the power comes from plutonium isotopes.

In a "fast reactor", the neutron energies stay in the high range and U238 fission can occur directly. However, there is still a large probability of U238 capture which eventually produces Pu239.

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