When a hydrogen-1 atom (a proton) absorbs a neutron, it can form deuterium, which is an isotope of hydrogen with one proton and one neutron. This process is called neutron capture.
The formation of deuterium from hydrogen-1 and a neutron is an exothermic reaction, meaning that energy is released in the process. The reason for this energy release is the binding of the neutron to the hydrogen nucleus, which results in a more stable configuration. The additional neutron contributes to the stronger binding of the nucleus, releasing energy in the form of gamma radiation.
In the context of a nuclear reactor, the absorption of neutrons by coolant is not a significant source of heat gain. The primary purpose of the coolant in a nuclear reactor is to remove excess heat produced by nuclear reactions and carry it away from the reactor core. The heat gain in a nuclear reactor primarily comes from the fission reactions taking place within the nuclear fuel, where the splitting of heavy atomic nuclei releases a significant amount of energy.
The absorption of neutrons by the coolant does occur in a reactor, but it generally does not contribute significantly to the overall heat output. The primary heat generation in a nuclear reactor is from the fission reactions themselves, where the splitting of fuel nuclei releases a large amount of energy in the form of heat. The coolant then carries this heat away from the reactor core to be used for various purposes, such as generating electricity or heating water for industrial or residential use.