The p + n --> D and D + n --> T reactions are the basis of the difference between light and heavy water used as the moderator in fission reactors. Because the former reaction has bigger cross section and steals more neutrons from the chain reaction neutron economy.
Because of the optimal proton/neutron mass ratio, light water would be better moderator than heavy water, as it faster slows down neutrons to the thermal speed.
It would be, but is not, as this effect is overruled by higher neutron absorption.
The result is, that while heavy water moderator allows to build fission reactors with unriched uranium, reactors with light water moderator require enriched uranium. ( typically 2-3 % of 235U, instead of the natural 0.7% )