To improve <math>P_{fission}</math> and enable a chain reaction, uranium-fueled reactors must include a [[neutron moderator]] that interacts with newly produced [[fast neutrons]] from fission events to reduce their kinetic energy from several [[MeV]] to thermal energies of less than one [[Electronvolt|eV]], making them more likely to induce fission.
Hydrogen moderators include [[water]] (H<sub>2</sub>O), [[heavy water]]([[deuterium|D]]<sub>2</sub>O), and [[zirconium hydride]] (ZrH<sub>2</sub>), all of which work because a hydrogen nucleus has nearly the same mass as a free neutron: neutron-H<sub>2</sub>O or neutron-ZrH<sub>2</sub> impacts excite [[rotational mode]]s of the molecules (spinning them around).