Steric / kinetic factors then usually favour the formation of a single product.
2.
One kinetics factor is the tendency of most void coefficients of reactivity . ( Due to the low density of steam, pockets of water vapor are known as " voids " in a LWR . ) A negative reactivity coefficient means that as the water moderator heats up, molecules move farther apart ( water expands and eventually boils ) and neutrons are less likely to be slowed by collisions to energies favorable for inducing fission in the fuel.