|Fig. 1: Illustration of a supercritical water-cooled reactor.  (Courtesy of the DOE.)|
Nuclear technology is responsible for producing 20% of the United States' electricity and 15% of the world's electricity annually.  Nuclear power provides the largest share of electricity of any greenhouse-free source. As such, it represents a promising technology to expand and refine to simultaneously better meet global power demands and decrease greenhouse gas emissions. The Generation IV International Forum (GIF) has designated six nuclear technologies as sufficiently promising to fulfill its vision for reduced environmental impact and energy security.  Of these, supercritical water-cooled reactors (SCWR) present an interesting opportunity.
SCWRs (see Fig. 1) are a modified version of light water reactors (LWR), the primary difference being in the working fluid within each power plant. LWRs utilize the energy released from fission to, ultimately, boil water, allowing the steam to turn a series of turbines connected to a transformer to generate electricity.  Currently, steam generators in LWRs have an energy efficiency of nearly 30%.  The energy efficiency cap is, fundamentally, a thermodynamic issue, as the Carnot efficiency is dependent upon the maximum temperature the steam can reach. Higher steam temperature and pressure should, theoretically, result in higher efficiency.  SCWRs attempt to increase both: the working fluid (supercritical water) has an operating temperature of up to 625°C and a nominal pressure of 25 MPa, compared to 275°C and 15 MPa in a subcritical steam generator. 
If successfully deployed, SCWRs' increased operating temperatures will allow the reactors to approach a thermal efficiency of approximately 45% (compared to the current 33% efficiency in modern LWRs), an increase in efficiency of roughly 136% over LWRs.  The increased efficiency offers improved economics as well as potential for cutting costs by eliminating the need for plant parts (such as boilers and condensers) used in subcritical steam generators. 
At face-value, there appears to be little risk in developing SCWR technology - increased efficiency and a reduction of plant cost offer promising rewards in reducing cost and producing more clean energy. However, a number of technical issues must be addressed before SCWRs can be reliably deployed. Chief among these are materials challenges. The operating temperature of SCWRs is approximately the same at which many of the steel alloys begin to display "creep", a rapid drop-off in maximum allowable stress on the structure.  This can be counteracted by adding more material to the structure or by using higher-grade alloys. Both of these options present added cost to the building of the reactor structure, offsetting the cost savings gained by eliminating other system components. Moreover, the chemical properties of supercritical water are markedly different than subcritical water, causing significant corrosion and erosion to the cladding materials used in the reactor.  Finally, most data associated with materials' behavior in supercritical plants is gathered from coal-fired plants, not nuclear plants. As such, there is little experience with how these materials (steel alloys, nickel cladding, etc) behave when subject to both supercritical water and radiation. 
While SCWRs offer a promising future for nuclear technology (increased efficiency, reduction of plant components, overall reduction of greenhouse gas emissions), there remains a number of challenges that stand in the way of full deployment. The GIF aims to begin preparing a prototype plant after 2020, so the coming years will witness the results of a global engineering initiative to improve nuclear reactor technology. 
© Thomas Blackwood. 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.
 "GIF R&D Outlook For Generation IV Nuclear Energy Systems," Generation IV International Forum, August 2009.
 M. A Rosen, "Energy- and Exergy-Based Comparison of Coal-Fired and Nuclear Steam Power Plants," Exergy. 1, 180 (2001).
 P. L. Joskow, and R. Schmalensee, "The Performance of Coal-Burning Electric Generating Units in the United States: 1960-1980", J. Appl. Econom. 2, 85 (1987).
 P. MacDonald et al., "Feasibility Study of Supercritical Light Water Cooled Reactors for Electricity Production," Idaho National Engineering and Environment Laboratory, INEEL/EXT-03-01277, September 2003.
 ASME Boiler And Pressure Vessel Code, 2007 Ed., Section II-D: Tables 1A, 1B., pp. 6-273. (American Society of Mechanical Engineers, 2007).
 Y. Oka and S. Koshizuka, "Supercritical-Pressure, Once-Through Cycle Light Water Cooled Reactor Concept," J. Nucl. Sci. Technol. 38, 1081 (2012).
 J. Buongiorno and P. E. MacDonald, "Progress Report for the FY-03 Generation-IV R&D Activities for the Development of the SCWR in the U.S.," Idaho National Engineering and Environment Laboratory, INEEL/EXT-03-01210, September 2003.