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I am just wondering why are protons (as opposed to neutrons) not used to mediate nuclear fission? Is it because it is charged, so we will have to input more unnecessary energy to overcome the Coulomb force?

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I think, you are wrong in saying coulomb force instead of nuclear force. – Vinaykumar Apr 16 '14 at 10:20
@GODPARTICLE He means the Coulomb force ... the electrostatic interaction between the positively charged nucleus and the positively charged proton is repulsive and quite significant on the scale of kinetic energies in a nuclear reactor. – dmckee Apr 16 '14 at 13:46

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I presume you are referring to process of nuclear fission of uranium-235, which has the equation: $$^1_0\text{n}+^{235}_{\ \ 92}\text{U}\longrightarrow ^{236}_{\ \ 92}\text{U}$$ However, a subsequent reaction is: $$^{236}_{\ \ 92}\text{U}\longrightarrow^{144}_{\ \ 56}\text{Ba}+^{89}_{36}\text{Kr}+3^{1}_{0}\text{n}$$ The production of neutrons is a feature of fission reactions, and they can be used to collide with other nuclei of uranium-235 producing more fission, energy and neutrons. The reaction is thus self-sustaining.

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Correct. The observation about charge in the question is also correct, that a proton has to overcome the coulomb force, but of course one wants the chain reaction to be self sustained in order to have a reactor. – anna v Apr 16 '14 at 15:49

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