Large excess reactivity at operating temperature. Heat transfer only moderately efficient. Fuel reprocessing a difficult task. Rector must be shut down to unload and reload core. Water would flash to steam in case of rupture of primary loop.
Water reacts with uranium, thorium, and structural metals under certain conditions.
Low thermal heads make heat exchanger, pumps and pipins large. Hot-channel factors are significant. Although reactors using boiling heat transfer had been considered during the World War II Manhattan project in the United States, it was generally believed at that time such reactors would be unstable in operation. During the planning of the water cooled submarine reactors in 1947, the stability question was still unresolved and high pressure non-boiling reactors were chosen for the submarine program. The first test of basic heat transfer phenomenon was made at Argonne National Lab in 1952.