|MadSci Network: Physics|
This is one of those questions where I have to re-interpret the question, because protons are very capable of initiating nuclear reactions. The only drawback is that they have to be accelerated in some form of "atom smasher" such as a cyclotron or linear accelerator. These energetic protons can initiate many reactions depending on the target nucleus and the energy of the proton. Some of these can be: proton in, neutron out (the (p,n) reaction), proton in two neutrons out (p, 2n), up to many neutrons such as (p, 5n); spallation (where a high energy proton knocks out protons and neutrons and possibly other products); and fission of the target nucleus if it is large enough. The question may also mean: why doesn't a proton at room temperature initiate fission in U-235 like a neutron does, or why doesn't a proton at room temperature cause a reaction such as proton absorbed and gamma-ray emitted like a neutron is able to do (the (n, gamma) reaction)? This is because the positive charge on the proton causes it to be repelled by the positive charge on the nucleus. This repulsion is called Coulomb Repulsion and creates an energy barrier to the proton entering the nucleus to initiate any nuclear reaction. The proton at room temperature does not have enough energy to break through this barrier. That is why the proton has to be given additional energy to "get over" the Coulomb Barrier. The neutron has no charge, and therefore there is no Columb Barrier to stop a slow-moving (room temperature, also called thermal neutron) from entering the nucleus and initiating a nuclear reaction. This is more fully explained in such classic textbooks as "Introduction to Nuclear and Radiochemistry" by Friedlander, Kennedy, Macias and Miller or any other book on nuclear and radiochemistry available in your library.
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