The thermal neutron absorption cross-section for Sodium is about 0.400 barns, compared to Oxygen at 0.002 and Boron (Used in control rods to catch neutrons) at 758.86 barns. (7k barns for Cd and 180 for In). That's for reference.
For fissioning U-235 you need thermal (slow) neutrons. Water happens to be good at this neutron slowing. Sodium not so much, but to breed U-238 and to fission Pu-239 you need fsst neutrons. Since sodium transfers heat very well but doesn't slow neutrons it's great for breeders (and burning Pu). But if water were around it would slow them down too much and the reaction couldn't happen.
Conversely, if Sodium was the moderator on a reactor using U-235 the neutrons would be too fast, and it wouldn't fission.
While I would suggest something similar for the Plutonium I'm pretty happy with that change.
On a side note, you could have a thermal rector modified by graphite like the Russian RMBK reactors (i.e. Chernobyl), which could allow you to use other coolants. Essentially, the moderator doesn't have to be the coolant.
Love your work Reika, keep it up!