|Fig. 1: Supercritical Water Cooled Reactor.  (Courtesy of the DOE.)|
As part of the Generation IV initiative, the International Forum chose six proposed nuclear reactor designs to pursue research on. One of these proposed solutions was the design of the Supercritical water cooled reactor. SCWR reactors are very similar to light water reactors, see Fig. 1. "The light water reactor is a type of thermal- neutron reactor that utilizes normal water as opposed to heavy water, a form of water that contains a larger amount of the hydrogen isotope deuterium."  The difference between these reactors is that SCWRs perform with much higher pressure and temperature than the light water reactors. This seemingly small difference actually increases the high thermal efficiency from an advanced light water reactor's efficiency of 35% to about a 45% efficiency.  Along with the LWRs, SCWRs use similar technology to the super-critical coal-fired boiler with the goal of producing cheap electricity. 
As the current US design stands, the SCWR will operate with an inlet temperature of about 280°C and an inlet density of about 760 kg/m3.  This inlet flow runs between the core barrel and the reactor pressure vessel, two components that I will discuss in this section. The outlet coolant is supposed to decrease its density to about 90 kg/m3 and increase its temperature to roughly 500°C, see Fig. 2.  This outlet coolant is what provides the turbine with power to operate.
|Fig. 2: The SCWR reactor pressure vessel.  (Courtesy of the DOE.)|
Reactor Pressure Vessel The reactor pressure vessel is essentially the housing for the SCWR fuel to ensure that it is separated from the surrounding environment. This particular vessel is similar to that of the pressurized water reactors by having no objects running through the shell of the lower head, but because it operates at a much higher temperature, the vessel has a significantly thicker shell. With such high temperatures involved, the design added a thermal sleeve for the outlet nozzle to minimize loss of heat.  With these changes, the International Forum states that it expects a similar amount of damage to the reactor pressure vessel over the standard 60 year life time. One major issue with this reactor pressure vessel design is that this idealistic solution is currently out of range for the United States production capabilities because the beltline-region forged ring is larger than the largest forged rings currently made. Japan Steelworks produced the current largest and claim that with modifications to their production line that they will be able to produce a large enough forged ring for this system.  Another issue is the design of the thermal sleeve for the outlet nozzle. This design has not been tested and is absolutely necessary for the design to properly function. 
Coolant System The reactor's coolant system is a major component of this new design structure. This new design eliminates the use of reactor coolant pumps, which give the system artificial coolant circulation to remove core heat.  The new coolant system is made up of the feed water lines from the isolation valves to the reactor pressure vessel and steam lines from the reactor pressure vessel to the other isolation valves outside the container.  The feed water pumps drive coolant under normal operating conditions and therefore the same materials can be used from light water reactors. Unfortunately, the steam lines have regulation issues. The steam lines operate within the supercritical fossil plant systems, but those regulations only approve usage for a lifetime of 34 years, where the SCWR is intended to have a lifetime of 60 years. 
In this design, there are multiple benefits in terms of cost and efficiency making it a highly sought after goal for the Generation IV International Forum and a clear upgrade from the current light water reactors. Unfortunately, there are still many technological challenges that we must overcome to make this design a reality. Key elements have yet to be developed and tested under realistic conditions, designs for parts have yet to be made, and regulation issues have yet to be resolved, but all in all there is solid progress being made to develop the first supercritical water cooled reactor.
© Clay Jones. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 B. Zarubin, "Introduction To Light Water Reactors," Physics 241, Stanford University, Winter 2015.
 J. Buongiorno and P. E. MacDonald, "Supercritical-Water-Cooled Reactor (SCWR)," Idaho National Engineering and Environmental Laboratory, INEEL/EXT-03-01210, September 2003.
 M. Greger, "Forging," Technical University of Ostrava, 2014.