neutron moderator and coolant. Above the critical point, steam and liquid become the same density and are indistinguishable, eliminating the need for pressurizers and steam generators (PWR), or jet/recirculation pumps, steam separators and dryers (BWR). Also by avoiding boiling, SCWR does not generate chaotic voids (bubbles) with less density and moderating effect. In a LWR this can affect heat transfer and water flow, and the feedback can make the reactor power harder to predict and control. Neutronic and thermal hydraulic coupled calculation is needed to predict the power distribution. SCWR's simplification should reduce construction costs and improve reliability and safety. A LWR type SCWR adopts water rods with thermal insulation and a CANDU type SCWR keeps water moderator in a Calandria tank. A fast reactor core of the LWR type SCWR adopts tight fuel rod lattice as a high conversion LWR. The fast neutron spectrum SCWR has advantages of a higher power density, but needs plutonium and uranium mixed oxides fuel which will be available from reprocessing. Control SCWRs would likely have control rods inserted through the top, as is done in PWRs. Material The temperature inside an SCWR is higher than those in LWRs. Although supercritical fossil fuel plants have much experience in the materials, it does not include the combination of high temperature environment and intense neutron radiation. SCWRs need core materials (especially fuel cladding) to resist the environment. R&D focuses on: The chemistry of supercritical water under radiation (preventing stress corrosion cracking, and maintaining corrosion resistance under neutron radiation and high temperatures) Dimensional and microstructural stability (preventing embrittlement, retaining strength and creep resistance also under radiation and high temperatures) Materials that both resist the high temperature conditions and do not absorb too many neutrons, which affects fuel economy In the once-through coolant cycles, such as SCWRs and supercritical fossil fired power plants, the entire reactor coolant is processed at low temperature after condensation. It is an advantage in managing water chemistry and stress corrosion cracking of structural materials. It is not possible in LWRs due to the recirculation of hot reactor coolant. Materials and water chemistry R&D should be done with the once-through characteristics in mind. Advantages Supercritical water has excellent heat transfer properties allowing a high-power density, a small core, and a small containment structure. The use of a supercritical Rankine cycle with its typically higher temperatures improves efficiency (would be ~45 % versus ~33 %