In a nuclear power plant, does the fission reaction itself produce thermal energy? Or, does the energy of fission reaction mainly come in the form of the kinetic energy of daughter nuclei and free neutrons, and the kinetic energy of free neutrons is transformed into thermal energy as it passes through the moderator?
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$\begingroup$ What do you think is the difference between "thermal energy" and the kinetic energy of daughters and neutrons? According to Sears book on thermodynamics, there isn't a separate "thermal energy." $\endgroup$– Bill NMar 7, 2021 at 19:30
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$\begingroup$ @BillN That's true: temperature is a measure of the mean kinetic energy of a collection of particles (measured in the collection's rest frame), an individual particle doesn't really have a temperature, just kinetic energy & momentum (relative to some frame). We say a collection has some specific temperature when the collection has achieved thermodynamic equilibrium, as discussed here: physics.stackexchange.com/q/294813/123208 $\endgroup$– PM 2RingMar 8, 2021 at 6:30
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2 Answers
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The energy from a uranium fission reactor is mostly due to the kinetic energy of the fission fragments. There's also the kinetic energy of the neutrons, and the various particles emitted by the uranium, the fission fragments, and the isotopes produced by the natural decay chain of the uranium (i.e., when the uranium decays via alpha emission, rather than through fission). There are some photons produced as well, but they aren't blackbody thermal photons, they're gamma rays. And just like the other particles, their kinetic energy gets converted to heat through colliding with the material in the reactor.
Here's a breakdown of typical reactor energies, courtesy of Wikipedia:
| Source | Average energy released (MeV) |
|---|---|
| Instantaneously released energy | |
| Kinetic energy of fission fragments | 169.1 |
| Kinetic energy of prompt neutrons | 4.8 |
| Energy carried by prompt γ-rays | 7.0 |
| Energy from decaying fission products | |
| Energy of β− particles | 6.5 |
| Energy of delayed γ-rays | 6.3 |
| Energy released when those prompt neutrons which don't (re)produce fission are captured | 8.8 |
| Total energy converted into heat in an operating thermal nuclear reactor | 202.5 |
| Energy of anti-neutrinos | 8.8 |
| Sum | 211.3 |
The gamma rays carry about 6.3% of the total energy produced.
Note that virtually all of the energy carried by the anti-neutrinos is lost, since they have a very small probability of interacting with any of the matter in the reactor.
Also note that the table doesn't bother listing the energy carried by alpha particles, because it's quite small, which is consistent with the long half-life of the alpha emission processes in a uranium reactor. In a plutonium reactor, such emissions are more significant, especially from the isotopes with shorter half-lives, eg plutonium-238.
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As you correctly point out, the reaction of fission itself does not produce thermal energy. Instead, thermal energy is produced when the high kinetic energy of the reaction products (daughter nuclei and free neutrons, along with some radiation) is dissipated into the medium after several collisions.
There is indeed a close connection between kinetic energy and thermal energy. Kinetic energy is a "microscopic" quantity carried by individual particles, while thermal energy is a "macroscopic" quantity, which is -loosely- defined by averaging the kinetic energies of a lot of constituents
