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.
Pressure is lower for given steam output conditions than in pressurized water reactor.
Metal surface temperatures are lower for given steam output conditions than in pressurized water reactor. Power excursion quickly damped by formation of steam. Overall thermal efficiency quite high. Water is cheap. Core is compact if void coefficient is low.