What key characteristic of H2O makes it a better coolant/moderator compared to D2O?

The key characteristic that makes H2O a better coolant/moderator compared to D2O is its higher capture cross-section for thermal neutrons. In nuclear reactors, materials with a high neutron capture cross-section effectively slow down neutrons, facilitating the fission process and maintaining a sustained chain reaction.

H2O's ability to effectively slow down (moderate) neutrons is due to its lighter mass compared to deuterium (D2O), which contains one proton and one neutron. The lighter hydrogen in water interacts more favorably with thermal neutrons, enhancing the likelihood of neutron absorption and scattering. This property contributes significantly to maintaining a critical reactor state while optimizing reactor efficiency through increased neutron moderation.

In contrast, while D2O is a viable neutron moderator and coolant, its higher mass results in a lower neutron capture cross-section compared to H2O, which impacts its effectiveness in promoting fission reactions in certain reactor designs. Thus, the ability of H2O to have a higher capture cross-section directly benefits the reactor's operational efficiency and neutron economy.