Supercritical water reactor

Supercritical water reactor scheme.

The supercritical water reactor (SCWR) is a concept Generation IV reactor,[1] mostly designed as light water reactor (LWR) that operates at supercritical pressure (i.e. greater than 22.1 MPa). The term critical in this context refers to the critical point of water, and must not be confused with the concept of criticality of the nuclear reactor.

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. 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.[2]


The super-heated steam cooled reactors operating at subcritical-pressure were experimented with in both Soviet Union and in the United States as early as the 1950s and 1960s such as Beloyarsk Nuclear Power Station, Pathfinder and Bonus of GE's Operation Sunrise program. These are not SCWRs. SCWRs were developed from the 1990s onwards.[3] Both a LWR-type SCWR with a reactor pressure vessel and a CANDU-type SCWR with pressure tubes are being developed.

A 2010 book includes conceptual design and analysis methods such as core design, plant system, plant dynamics and control, plant startup and stability, safety, fast reactor design etc.[4]

A 2013 document saw the completion of a prototypical fueled loop test in 2015.[5] A Fuel Qualification Test was completed in 2014.[6]

A 2014 book saw reactor conceptual design of a thermal spectrum reactor (Super LWR) and a fast reactor (Super FR) and experimental results of thermal hydraulics, materials and material-coolant interactions.[7]



The SCWR operates at supercritical pressure. The reactor outlet coolant is supercritical water. Light water is used as a neutron moderator and coolant. Above the critical point, steam and liquid become the same density and are indistinguishable, eliminating the need for pressurizers and steam generators (PWR), or jet/recirculation pumps, steam separators and dryers (BWR). Also by avoiding boiling, SCWR does not generate chaotic voids (bubbles) with less density and moderating effect. In a LWR this can affect heat transfer and water flow, and the feedback can make the reactor power harder to predict and control. SCWR's simplification should reduce construction costs and improve reliability and safety. The neutron spectrum will be only partly moderated, perhaps to the point of being a fast neutron reactor. This is because the supercritical water has a lower density and moderating effect than liquid water, but is better at heat transfer, so less is needed. In some designs with a faster neutron spectrum the water is a reflector outside the core, or else only part of the core is moderated. A fast neutron spectrum has three main advantages:


The fuel will resemble traditional LWR fuel, likely with channelized fuel assemblies like the BWR to reduce the risk of hotspots caused by local pressure/temperature variations. The enrichment of the fuel will have to be higher to compensate for the neutron absorption by the cladding, which can't be made from the zirconium customary in LWRs, as zirconium would corrode rapidly. Stainless steel or nickel alloys may be used. The fuel rods must withstand the corrosive supercritical environment, as well as a power surge in case of an accident. There are four failure modes considered during an accident: brittle failure, buckling collapse, overpressure damage and creep failure. To reduce corrosion, hydrogen can be added to the water.

At least one concept uses high temperature gas cooled reactor fuel particles, BISO.[8]

This uses corrosion resistant silicon carbide coatings on uranium fuel particles, solving the challenge of the cladding using an innovative yet proven fuel.


SCWRs would likely have control rods inserted through the top, as is done in PWRs.


The conditions inside an SCWR are harsher than those in LWRs, LMFBRs and supercritical fossil fuel plants (with which much experience has been gained, though this does not include the combination of harsh environment and intense neutron radiation). SCWRs need a higher standard of core materials (especially fuel cladding) than either of these. In addition, some elements become very radioactive from absorbing neutrons, e.g. cobalt-59 captures neutrons to become cobalt-60, a strong gamma emitter, so cobalt-containing alloys are unsuitable for reactors. R&D focuses on:



The coolant greatly reduces its density at the end of the core, resulting in a need to place extra moderator there. Most designs 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. Again a pressure-tube design has potentially fewer issues, 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.[10]

See also


  1. |accessdate=7 Apr 2016
  2. Buongiorno, Jacopo, "The Supercritical Water Cooled Reactor: Ongoing Research and Development in the U.S", 2004 international congress on advances in nuclear power plants, American Nuclear Society - ANS, La Grange Park (United States), OSTI 21160713, retrieved 10 Nov 2012
  3. Oka, Yoshiaki; Koshizuka, Seiichi (2001), "Supercritical-pressure, Once-through Cycle Light Water Cooled Reactor Concept" (PDF), Nuclear Science and Technology, 38 (12): 1081–1089
  4. Oka, Yoshiaki; Koshizuka, Seiichi; Ishiwatari, Yuki; Yamaji, Akifumi (2010). Super Light Water Rectors and Super Fast Reactors. Springer. ISBN 978-1-4419-6034-4.
  7. Yoshiaki Oka; Hideo Mori, eds. (2014). Supercritical-Pressure Light Water Cooled Reactors. Springer. ISBN 978-4-431-55024-2.
  8. 1 2 Tsiklauri, Georgi; Talbert, Robert; Schmitt, Bruce; Filippov, Gennady; Bogoyavlensky, Roald; Grishanin, Evgenei (2005). "Supercritical steam cycle for nuclear power plant" (PDF). Nuclear Engineering and Design. 235 (15): 1651–1664. doi:10.1016/j.nucengdes.2004.11.016. ISSN 0029-5493.
  9. MacDonald, Philip; Buongiorno, Jacopo; Davis, Cliff; Witt, Robert (2003), Feasibility Study of Supercritical Light Water Cooled Reactors for Electric Power Production - Progress Report for Work Through September 2003 - 2nd Annual Report and 8th Quarterly Report (PDF) (INEEL/EXT-03-01277), Idaho National Laboratory
  10. 1 2 Chow, Chun K.; Khartabil, Hussam F. (2007), "Conceptual fuel channel designs for CANDU-SCWR" (PDF), Nuclear Engineering and Technology, 40 (2)
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