- Account for over 95% of all operating civilian power reactors worldwide. - Majority of nuclear reactors under development and construction are water-cooled - Light water reactors (LWRs) are the most common type of WCRs, divided into: - Pressurized Water Reactors (PWRs) - which produce steam for the turbine in separate steam generators - Boiling Water Reactors (BWRs) - which use the steam produced inside the reactor core directly in the steam turbine - Heavy Water Reactors (HWRs) use enriched water with a higher concentration of deuterium - To improve plant thermal efficiencies and economics, research and development for Supercritical Water Reactors (SCWRs) is being pursued. Supercritical water exists at temperatures and pressures above its critical point, where the liquid and vapor states are indistinguishable. Plant efficiencies of SCWRs are expected to be around **1.3 times higher** than conventional WCRs.