The supercritical water-cooled reactor (SCWR) is one of the six reactor technologies selected for researchand development under the Generation IV program. SCWRs are promising advanced nuclear systemsbecause of their high thermal efficiency (i.e., about 45% versus about 33% efficiency for current LightWater Reactors [LWRs]) and considerable plant simplification. SCWRs are basically LWRs operating athigher pressure and temperatures with a direct once-through cycle. Operation above the critical pressureeliminates coolant boiling, so the coolant remains single-phase throughout the system. Thus, the need fora pressurizer, steam generators, steam separators, and dryers is eliminated. The main mission of theSCWR is generation of low-cost electricity. It is built upon two proven technologies: LWRs, which arethe most commonly deployed power generating reactors in the world, and supercritical fossil-firedboilers, a large number of which are also in use around the world. The reference SCWR design for the U.S. program is a direct cycle system operating at 25.0 MPa, withcore inlet and outlet temperatures of 280 and 500 C, respectively. The coolant density decreases fromabout 760 kg/m3 at the core inlet to about 90 kg/m3 at the core outlet. The inlet flow splits with about 10%of the inlet flow going down the space between the core barrel and the reactor pressure vessel (thedowncomer) and about 90% of the inlet flow going to the plenum at the top of the rector pressure vessel, to then flow down through the core in special water rods to the inlet plenum. Here it mixes with thefeedwater from the downcomer and flows upward to remove the heat in the fuel channels. This strategy isemployed to provide good moderation at the top of the core. The coolant is heated to about 500 C anddelivered to the turbine. The purpose of this NERI project was to assess the reference U.S. Generation IV SCWR design andexplore alternatives to determine feasibility. The project was organized into three tasks: Task 1. Fuel-cycle Neutronic Analysis and Reactor Core Design Task 2. Fuel Cladding and Structural Material Corrosion and Stress Corrosion Cracking Task 3. Plant Engineering and Reactor Safety Analysis.moderator rods.materials.