Which substance is the best moderator for fission reactors, if there is even a definition for "best"? My current physics book says heavy water. Do you agree? Please explain.
Protons are the best at slowing down neutrons, however they can also absorb neutrons (and form deuterons) in competition with the fuel. Heavy water or D2O slows down neutrons but does not readily absorb thermal neutrons, there it is considered a better moderator for thermal fission reactors.
Moderating ratio is a figure of merit for moderators.
But that wouldn't work for a hot reactor. D2 doesn't compress to a liquid but for pressures well beyond what we normally achieve terrestrially.It's not "the best".
Definition of "the best" is moderating ratio, so liquid D2 maybe "the best". D2 is gaseous state in environmental temperature. When D2 is compressed into liquid state, it would be "the best".
Supercritical fluid would seem more appropriate. The moderator needs a certain density.Above critical temp (IIRC close to 40K) D2 has no liquid state per se.
Since you brought up the idea of solid hydrides, which is being considered in some high temperature concepts, e.g., SCWR, the other complication of high pressure deuterium would be the propensity for the hydrogen to diffuse into the structural metals and forming hydrides. This would be bad news for steels or Inconels as it would undermine their structural integrity, especially when it comes to shutdowns. Probably the primary system would have to be cladding in Al or Cu or some other element that does not readily allow diffusion of hydrogen into the metal or formation of hydrides. However at the pressure required to maintain a supercritical deuterium fluid, I'm not sure there is a metal that could handle it.A dense solid compound of deuterium would be good, i.e. "heavy paraffin" or perhaps ZrD2. Remember that the moderator can be outside the reactor and doesn't have to also be the coolant (e.g. CANDU, RBMK, MAGNOX, etc).