The answer to this physics se question, established that 2.2Mev (Mega electron volts) of energy is emitted as a gamma ray photon when the hydrogen atom nucleus oka protium absorbs a slow moving free neutron causing the formation of a deuterium nucleus. Deuterium is heavy hydrogen with both a proton and neutron in the nucleus and weighs twice as much as regular hydrogen.
Deuterium is a more stable nucleus than protium, meaning it is more energetically favored over protium and neutron seperate. This is due to the strong nuclear force binding nucleons together. The mass of the deuteron is less than the sum of the proton and neutron. The deficit of mass in the reaction is released as energy; in this case in emission of the gamma ray photon of 2.2MeV. This is what happens in nuclear fusion where lighter element nuclei merge together into a heavier element nuclei and releasing a great deal of energy.
Now this reaction cited is a very special case because we don't need millions of degrees temperature to cause the fusion since there is no Coulomb repulsion. A slow neutron strikes a regular protium hydrogen nucleus (just a proton) and we have 2.2MeV of energy. This happens at room temperature with no energy consuming magnetic fields used to contain high temperature plasma as is done with current fusion reactors. Granted we are not getting 17.59MeV that is released when tritium and deuterium are fused, but we are not requiring 25MeV of input power to cause and maintain said reaction, resulting in net loss of power.
In the neutron-protium fusion we have a net 2.2MeV energy output with little or no energy input. My main question is why this energy is not being harnessed for power generation? 2 somewhat challenging but not insurmountable reasons come to mind.
For one, we need free neutrons. These can be obtained from a fission nuclear reactor, cyclotrons, alpha radiating radioactive isotopes with beryllium targets, and others.
If we want to avoid the fission reactor as the neutron source, we can use various other methods. This physics se question probes an exothermic beryllium target process. I had in mind a cyclotron to mildly accelerate alpha particles (helium nuclei) to the appropriate energy level to cause neutron emission when striking a beryllium target. The beryllium target would be encased in liquid or super-critical hydrogen to maximize neutron capture.
My estimation is that the energy output from the n + p = d reaction will supply enough power for the neutron generating cyclotron and then some for useful output power. My reasoning for this is the alpha particles don't need that much energy for this purpose. The process in and of itself is exothermic so we are getting free neutrons plus energy. We are also not concerned with the enormous energy required to confine a plasma.
The other hurdle is harnessing the energy from the emitted gamma ray. This SE question explores the gamma ray energy conversion topic. Then we have to figure out a way to use some of the output energy that was harnessed from the gamma ray to generate more neutrons to "keep the fire going".
While this process is indeed challenging, I think a working system can be developed for a fraction of what is being spent on ITER, the most expensive science project in history to investigate surplus energy from fusion.