The water heated in the reactor core becomes a supercritical fluid above the critical temperature of 374 °C, transitioning from a fluid more resembling liquid water to a fluid more resembling saturated steam (which can be used in a steam turbine), without going through the distinct phase transition of boiling.
In contrast, the well-established pressurized water reactors (PWR) have a primary cooling loop of liquid water at a subcritical pressure, transporting heat from the reactor core to a secondary cooling loop, where the steam for driving the turbines is produced in a boiler (called the steam generator). Boiling water reactors (BWR) operate at even lower pressures, with the boiling process to generate the steam happening in the reactor core.
The supercritical steam generator is a proven technology (used in dozens to hundreds of commercial coal plants). The development of SCWR systems is considered a promising advancement for nuclear power plants because of its high thermal efficiency (~45 % vs. ~33 % for current LWRs) and simpler design. As of 2012 the concept was being investigated by 32 organizations in 13 countries
A supercritical nuclear plant could be cheaper than coal plants. In China these type of reactors could achieve costs that are up to half the cost of current reactors and have higher efficiency. They could be low cost enough to displace all future coal plant construction in China starting in 2025-2030. $900 per kilowatt is over three times cheaper than the estimated overnight cost of advanced nuclear reactors ($3100 per kilowatt) estimated by the US department of energy
- This higher efficiency would lead to better fuel economy and a lighter fuel load, lessening residual (decay) heat.
- SCWR is typically designed as a direct-cycle, whereby steam or hot supercritical water from the core is used directly in a steam turbine. This makes the design simple. As a BWR is simpler than a PWR, a SCWR is a lot simpler and more compact than a less-efficient BWR having the same electrical output. There are no steam separators, steam dryers, internal recirculation pumps, or recirculation flow inside the pressure vessel. The design is a once-through, direct-cycle, the simplest type of cycle possible. The stored thermal and radiologic energy in the smaller core and its (primary) cooling circuit would also be less than that of either a BWR's or a PWR's.
- Water is liquid at room temperature, cheap, non-toxic and transparent, simplifying inspection and repair (compared to liquid metal cooled reactors).
- A fast SCWR could be a breeder reactor, like the proposed Clean And Environmentally Safe Advanced Reactor, and could burn the long-lived actinide isotopes.
- A heavy-water SCWR could breed fuel from thorium (4x more abundant than uranium), with increased proliferation resistance over plutonium breeders.
In 2014, Russian researchers looked at refining the previously developed concept of the 1700 MW(e) VVER-SKD reactor. They recommended a plan for top-priority supercritical reactor research, formulation of a technical mission and proceeding to the design of an experimental 30 MW reactor.
There was a 2016 review of Russia's supercritical water nuclear reactor designs.