The extensive use of water as a reactor coolant is related to the relatively low pressure drops accompanying flow at significant rates, and the relatively high heat transfer coefficients. Figure shows the relationship of heat flux to the temperature difference between the fuel element surface and the water. The nucleate boiling stage is attractive for reactor operation, because of the favorable heat fluxes that can be obtained. Water technology well known. Water is cheap.
Water is very effective moderator of neutron energy core is compact.
Water has high heat capacity. Negative temperature coefficient. Ordinary leakage can be tolerated. Fission products are contained, not circulated. Radioactivity of coolant is short-lived if kept pure.