Why does fusion generate such large amounts of energy. If fission releases energy when some mass is lost as energy, then shouldn't the fusion process absorb energy to fuse nuclei together?

termegolz6
2022-07-22
Answered

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Abraham Norris

Answered 2022-07-23
Author has **16** answers

Fission is exothermic only for heavy elements, while fusion is exothermic only for light elements. Intermediate nuclei, in the iron/nickel range, are the most tightly bound, and so you generally release energy moving in that direction.

Fusing stable elements into uranium would consume energy, as would trying to break helium into hydrogen.

Fusing stable elements into uranium would consume energy, as would trying to break helium into hydrogen.

Emmanuel Pace

Answered 2022-07-24
Author has **6** answers

The strong force is attractive at short range and wins over the electromagnetic repulsion between protons. Pull the protons apart a little and you will get fission because the electromagnetic force wins. Conversely, push protons together and you will eventually get fusion when the strong force takes over from electromagnetic. For large nuclei the electromagnetic energy required for fusion is larger than the energy returned by the strong force - so you will only get net energy out from fission. For small nuclei it's the other way round - the strong force releases more energy than the electromagnetic force takes to trigger fusion. In this case you will only get net enerrgy out from fusion.

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The energy released by one fission reaction is 190MeV and the energy released by one fusion reaction is 24.7MeV. Then by my common sense 190>24.7 so energy released by fission>fusion but the energy released by fusion reaction is 3 to 4 times greater than fission. Please explain me how. I hope 190>24.7

The energy released by one fission reaction is 190MeV and the energy released by one fusion reaction is 24.7MeV. Then by my common sense 190>24.7 so energy released by fission>fusion but the energy released by fusion reaction is 3 to 4 times greater than fission. Please explain me how. I hope 190>24.7

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A calculation which just shows that the mean energy will be in the range of "fast" neutrons (> 1MeV) would be enough for me.

How can I calculate the mean kinetic energy of an emitted neutron in a nuclear fission. Take for example the fission of U-235 to Ba-141 and Kr-92.

A calculation which just shows that the mean energy will be in the range of "fast" neutrons (> 1MeV) would be enough for me.

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Consider this nuclear fission reaction:

${}^{235}\mathrm{U}+{}^{1}\mathrm{n}\to {}^{236}\mathrm{U}\text{(excited)}\to {}^{92}\mathrm{K}\mathrm{r}+{}^{141}\mathrm{B}\mathrm{a}+3{}^{1}\mathrm{n}$

I have not understand why a thermal neutron (${}^{1}$n should be captured by the nuclei of ${}^{235}$U.

What are the conditions under which this neutron is captured?

My reasoning is the following: the energy that the neutron must have to be captured by the ${}^{235}$U should be equal to the ${S}_{\mathrm{n}}$ (separation energy for a neutron) of ${}^{236}$U which I found is 6.34 MeV, higher than the energy of the thermal neutron. So my reasoning must be wrong but I cannot understand where I make mistakes...

I looked on the Povh Rith Particles and Nuclei but it does not explain the reaction well. Could anyone give me a reference or explain here the fission reaction ?

Consider this nuclear fission reaction:

${}^{235}\mathrm{U}+{}^{1}\mathrm{n}\to {}^{236}\mathrm{U}\text{(excited)}\to {}^{92}\mathrm{K}\mathrm{r}+{}^{141}\mathrm{B}\mathrm{a}+3{}^{1}\mathrm{n}$

I have not understand why a thermal neutron (${}^{1}$n should be captured by the nuclei of ${}^{235}$U.

What are the conditions under which this neutron is captured?

My reasoning is the following: the energy that the neutron must have to be captured by the ${}^{235}$U should be equal to the ${S}_{\mathrm{n}}$ (separation energy for a neutron) of ${}^{236}$U which I found is 6.34 MeV, higher than the energy of the thermal neutron. So my reasoning must be wrong but I cannot understand where I make mistakes...

I looked on the Povh Rith Particles and Nuclei but it does not explain the reaction well. Could anyone give me a reference or explain here the fission reaction ?

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