p+n reaction in nuclear fission reactors

There is a recent question (Proton - neutron fusion?) about the possibility of the fusion reaction $p+n$. According to @dmckee's answer, the reaction is possible, but not useful for fusion power generation as supplying free neutrons is problematic.

My question: can this reaction be observed/useful/undesirable in nuclear fission reactors, where free neutrons are aplenty? I do not have in mind reactions including protons of heavy nuclei in the reactors, but reactions with nuclei of, say, hydrogen (if for some reason there is some hydrogen inside the reactor).

• ...and it is also the reason, why LWRs need enriched uranium, while HWR do not. It is also the reason, why the heavy water was the strategic material during WWII and later, as it allows production of $^{239}Pu$ directly from natural uranium. – Poutnik Mar 28 '19 at 9:13