This is too primitive question. but I cant find a definitive answer anywhere.

Everywhere its mentioned that Binding energy is released in the good-ol uranium 235 and uranium 238 nuclear fission.

Where does this energy go?

Is it distributed to all the constituents( neutrons, reactants, products) as kinetic energy?

Or, Is it released as high frequency photons i.e radiation?

Or any other form?

(From what I can gather, the particles present in the reactor gain kinetic energy and they hit walls which produce heat that is transferred to water flowing around the walls)

But it was mentioned somewhere that only some fraction of energy is acquired by the products as kinetic energy, rest of the energy is used for human consumption, Is this statement correct? What is this 'rest of the energy'?


2 Answers 2


The energy output appears as kinetic energy of the produced neutrons and nuclei, and as $\gamma$ rays.
Quoted from Wikipedia: Nuclear fission - Physical overview - Energetics - Output:

When a uranium nucleus fissions into two daughter nuclei fragments, about $0.1$ percent of the mass of the uranium nucleus appears as the fission energy of ~$200$ MeV. For uranium-$235$ (total mean fission energy $202.79$ MeV), typically ~$169$ MeV appears as the kinetic energy of the daughter nuclei, which fly apart at about $3$% of the speed of light, due to Coulomb repulsion. Also, an average of $2.5$ neutrons are emitted, with a mean kinetic energy per neutron of ~$2$ MeV (total of $4.8$ MeV). The fission reaction also releases ~$7$ MeV in prompt gamma ray photons. The latter figure means that a nuclear fission explosion or criticality accident emits about $3.5$% of its energy as gamma rays, less than $2.5$% of its energy as fast neutrons (total of both types of radiation ~$6$%), and the rest as kinetic energy of fission fragments (this appears almost immediately when the fragments impact surrounding matter, as simple heat).

In a nuclear power plant heat energy is converted to mechanical energy (like flowing water or steam), and then to electric energy. However, the conversion from heat energy to mechanical energy is not $100$%. This is because of the limited energy conversion efficency (a limit applying to all kinds of thermal power plants, not only nuclear power plants). It means that a considerable part of the heat energy is necessarily "wasted" (like to heat the cooling water).


During nuclear fission, the binding energy of the initial nuclide, aka the educt, is converted into kinetic energy for the nuclei of the products and neutrons, as well as high-energy quanta ($\gamma$-photons) and other particles. At least that is the ideal process.

In reality, some energy is also "lost" in the form of Thermal Energy. This is known as Decay Heat. However, the Decay Heat does not have to be implied directly from the nuclear fission. While it contributes approximately 7% of the energy during operation, it continues to be produced after the fission process and energy conversion process is stopped as the daughter fragments continue to decay. This heat must still be removed. It does decrease with time; the initial predominant decay contributors have a 56 second half-life, resulting initially in the 7% reducing with approximately one hour half-life. Once those decays occur, at about 1% power, the reduction then asymptotically approaches zero over much, much longer period. Some sensible heat will still be released over many years, and some decay takes thousands of years, even though the heat contribution is insignificant.

The kinetic energy arises from the direct conversion of the binding energy. We can easily recognize this kinetic energy from the speed of the nuclei and neutrons. The $\gamma$-photons can also arise directly from the conversion of the binding energy. However, they can also from the further radioactive decay of the products. The products of nuclear fission can also decay radioactively and other particles such as $\beta^{-}$ (electrons), $\beta^{+}$ (positrons), neutrinos, ... But the starting material itself can also decay radioactively and therefore emit particles alongside these. However, this process itself is no longer part of Fission. However, it should not be forgotten that they also take kinetic energy from the initial nuclide.


Source Average energy released $\left[ \mathrm{MeV} \right]$
Instantaneously released energy: Sum: $180.9$
Kinetic energy of fission fragments $169.1$
Kinetic energy of prompt neutrons $4.8$
Energy carried by prompt $\gamma$-photons $7.0$
Energy from decaying fission products: Sum: $21.6$
Energy of $\beta^{-}$ particles $6.5$
Energy of delayed $\gamma$-photons $6.3$
Energy released when those prompt neutrons which do not (re)produce fission are captured $8.8$
Energy of anti-neutrinos $8.8$

Source: Wikipedia


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