|Fig. 1: The Koeberg Nuclear Power Station. (Source: Wikimedia Commons)|
Continued industrial expansion, the need to speed up the development of underdeveloped countries, the deregulation of various electric utilities, the global want to reduce the amount of CO2 in the atmosphere - these factors indicate that nuclear energy will once again be in demand. The U.S. Department of Energy has led an ambitious program called "Generation IV nuclear reactors", whose goal is to revitalize the nuclear energy option.  One of the main drawbacks of nuclear options is the economic viability. Because of this, there is a need to improve the economics and efficiency of light water reactors. This is one of the Generation IV concepts.
Light water reactor development began as a military program in the US. President Eisenhower's "Atoms for Peace" signaled the movement of various military technologies, such as light water reactors, to be used for civilian purposes.  Fig.1 shows the Koeberg nuclear power station, which has two pressurized water reactors. Various reactor concepts have stemmed from the research done on light water reactors. One such reactor concept is the supercritical light water reactor. 
Supercritical light water reactor concepts were explored and evaluated during the 1950s and 1960s by companies such as GE and Westinghouse. Fig. 2 shows one of the proposed concepts for the supercritical light water reactor. The original concepts were found to be feasible, but not economically competitive. Furthermore, the operation at high pressure and high cladding temperature was seen as a safety concern.  Older concepts developed by GE and Westinghouse considered the use of heavy water or graphite moderated and light water cooled reactors. While these concepts would've yielded large reactor volume and more complex reactor systems, they would have also been extremely expensive. 
A study conducted found that most supercritical light water reactor concepts are likely to be economically non-competitive. One concept, developed by the University of Tokyo, was found to potentially be economically viable. The reasons for the economic advantage is that this reactor concept is compact - the pressure vessel, containment, reactor building, spent fuel pool, cooling tower, etc - are all smaller in this concept than in modern light water reactors. Furthermore, the supercritical light water reactor concept does not use as many parts as modern light water reactors - steam separators, steam dryers, main circulation pumps, steam generators, and pressurizer are not used in the rector concept. 
|Fig. 2: Supercritical Light Water Reactor. (Source: Wikimedia Commons)|
The current US design dictates that the supercritical light water reactor will operate with an inlet temperature of about 280°C and an inlet density of about 760 kg/m3. The inlet connects the core barrel and the reactor pressure vessel. The outlet coolant of the supercritical light water reactor is supposed to decrease the density to around 90 km/m3 and the temperature to about 500° C. The outlet coolant is what provides power to the turbine.  The pressure vessel houses the fuel in order to ensure that the fuel is separated from the surrounding environment. The reactor's coolant system is made up of 2 components. The first is a feed of water lines from the isolation valves to the reactor pressure vessel. The second is a set of steam lines from the reactor pressure vessel to the other isolation valves outside the container. 
One of the main differences between light water reactors and supercritical light water reactors is that supercritical light water reactor performs at higher pressure and temperature than light water reactors. This seemingly small difference increases the high thermal efficiency from 35% (LWR) to 45% (SCLWR) efficiency. 
There are various key advantages to the supercritical light water reactor concept. Some of the key advantages include: 
There is little research on the effects of pseudo-critical condition supercritical water and its effects on the materials used to build the supercritical light water reactor. Therefore, more research must be conducted in regards to oxidation, corrosion, stress corrosion cracking, radiolysis, water chemistry, and structural stability. 
Making accurate simulations of supercritical light water reactors is made complicated by large variations of thermodynamic properties of these reactors. Whereas light water reactors use two-phase flow thermal-hydraulics, the supercritical water reactors use supercritical fluids. This variation has large implications when it comes to modeling supercritical light water reactors. Supercritical fluids lack interfaces with surface tension and their variation of properties is continuous but discontinuous for a two-phase fluid. These differences cause the heat transfer, critical flow, and other correlations and models used by light water reactors to not be applicable to supercritical conditions. 
The supercritical light water reactor concept has great potential for a more efficient and economically advantageous electrical generation. However, due to the varying properties of the supercrtitical light water reactor, there are still several issues that need to be resolved before a working prototype can be produced. Suitable materials for the structure and cladding have yet to be identified. Heat transfer data and critical flow data still needs to be collected at supercritical water conditions.  Regulation issues have also not been made clear or resolved in order for the supercritical light water reactors to be made.  While there are various obstacles facing the development and implementation of supercritical light water reactors, large progress has been made and steps have been taken to further develop this reactor concept.
© Juan Leis-Pretto. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. 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.
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