Heat exchanges and control rod required. Heat transfer only moderately efficient. Fuel reprocessing a difficult task. Reactors must be shut down to unload and reload core. Heavy water would flush to steam in case of rupture of primary loop.
Heavy water reacts with uranium, thorium and structural metals.
Low thermal heads make heat exchangers, pumps, and piping large. Hot channel factors are significant. Boiling point of diphenyl is very high. Extremely good moderator. Fission products are contained, not circulated.