rises across the core (compared to PWR/BWRs) result in increased mechanical and thermal stresses on vessel materials that are difficult to solve. However, a LWR type design, reactor pressure vessel inner wall is cooled by the inlet coolant as PWR. Outlet coolant nozzles are equipped with thermal sleeves. A pressure-tube design, where the core is divided up into smaller tubes for each fuel channel, has potentially fewer issues here, as smaller diameter tubing can be much thinner than massive single pressure vessels, and the tube can be insulated on the inside with inert ceramic insulation so it can operate at low (calandria water) temperature. • The coolant greatly reduces its density at the end of the core, resulting in a need to place extra moderator there. However, a LWR type SCWR design adopts water rods in the fuel assemblies as BWRs. The coolant density in water rods is kept high with thin thermal insulation, not fully insulated. Most designs of CANDU type SCWR use an internal calandria where part of the feedwater flow is guided through top tubes through the core, that provide the added moderation (feedwater) in that region. This has the added advantage of being able to cool the entire vessel wall with feedwater, but results in a complex and materially demanding (high temperature, high temperature differences, high radiation) internal calandria and plena arrangement. A pressure-tube design has the characteristics as most of the moderator is in the calandria at low temperature and pressure, reducing the coolant density effect on moderation, and the actual pressure tube can be kept cool by the calandria water. • Extensive material development and research on supercritical water chemistry under radiation is needed. However, the entire SCWR coolant is cleaned after condensation. This is an advantage in managing water chemistry and Stress corrosion cracking of structural materials. It is not possible in LWRs where hot coolant circulates. • Special start-up procedures needed to avoid instability before the water reaches supercritical conditions. However, Instability is managed by power to coolant flow rate ratio as a BWR. The coolant density change is smaller in SCWRs than BWRs. • A fast SCWR needs a relatively complex reactor core to have a negative void coefficient. However, single coolant flow pass core is feasible. • As with all alternatives to currently widespread designs (mostly subcritical water cooled, water moderated thermal reactors of some kind) there will be fewer