Super Critical Water Reactors (SCWR)
Supercritical Water-Cooled Reactors (SCWRs) are advanced nuclear reactors operating at temperatures and pressures above water's critical point (374°C, 22.1 MPa). SCWRs can feature thermal, fast, or mixed neutron spectra and are designed in two configurations: pressure vessel (similar to BWRs and PWRs) and pressure tubes (like CANDU reactors). Combining design insights from existing water-cooled reactors and supercritical fossil-fired plants, SCWRs achieve higher thermal efficiencies of 44-48%, significantly improving over the current 34-36%. These reactors offer economic benefits through increased efficiency and simpler designs. Representing an evolutionary step from existing technologies, SCWRs have yet to be built or operated.
Presentation of the Super Critical Water Reactor System
Supercritical Water-Cooled Reactors (SCWRs) are advanced water cooled and moderated nuclear reactor design concepts operating at high temperature, high pressure operating above the thermodynamic critical point of water (374°C, 22.1 MPa). The reactor core may have a thermal, fast or mixed neutron spectra depending on the core design. Two main configurations are looked into for the SCWR concept: a pressure vessel one (like in Boiling Water Reactors (BWR) and Pressurised Water Reactors (PWR)) and a pressure tubes one (similar to current CANDU designs).
The SCWR concepts combine the design and operation experiences gained from hundreds of water-cooled reactors with those experiences from hundreds of fossil-fired power plants operated with supercritical water (SCW). SCWRs provide higher thermal efficiency (44-48% or more) in comparison with the current generation of water-cooled reactors (34-36 %). The main improvement in the SCWR in economics is due to this high thermal efficiency and simplifications that result from the use of supercritical water as a coolant.
In contrast to some of the other Generation IV nuclear systems, the SCWR can be seen as developed incrementally step-by-step from current water-cooled reactors and can thus be seen as an evolutionary approach rather than a revolutionary one. No SCWR has been built and operated yet.
Attributes of the SCWR
| SCWR Reactor Parameters | Reference Value |
|---|---|
| Spectrum | Thermal or Fast or Mixed |
| Core Outlet Temperature range | 625°C |
| Coolant | Supercritical Water |
| Primary pressure | Above 22.1MPa (Thermodynamic critical pressure of water) |
| Power Range | 2300MWth (GIF reference design) |
| Fuel | Thermal spectrum: UO2 or Mixed Plutonium-Thorium Oxides Fast spectrum: Mixed Uranium Plutonium Oxides (MOX) |
How does Super Critical Water Reactors meet Generation IV Criteria?
SCWRs contribute to sustainability by utilizing a wide range of fuel options, including thorium and plutonium that can be extracted from spent fuel from current Light Water Reactor (LWR). This helps reduce the amount of nuclear waste and improves resources utilisation.
In a similar fashion, in the case of pressure tube designs using heavy water as a moderator, the high neutron economy facilitated by the heavy water moderator, enhances fuel and thus resources utilisation.
- SCWRs offer increases in thermal efficiency relative to current-generation water-cooled reactors. The efficiency of a SCWR can approach 44% or more, compared to 34-36% for current reactors.
- SCWRs offer economic advantages due to their simplified design.
- For instance the steam generators used in pressurized water reactors and the steam separators and dryers used in boiling water reactors can be omitted since the coolant is superheated in the core.
- Reactor coolant pumps are not required. The only pumps driving the coolant under normal operating conditions are the feed water pumps and the condensate extraction pumps.
- Containment, designed with pressure suppression pools and with emergency cooling and residual heat removal systems, can be significantly smaller than those of current water-cooled reactors.
- The direct cycle adopted by some SCWR concepts, like the Canadian SCWR, contributes to increased thermal efficiency, enhancing the economic viability of electricity generation.
- The higher steam enthalpy allows to decrease the size of the turbine system and thus to lower the capital costs of the conventional island.
These general features offer the potential of lower capital costs for a given electric power of the plant and of better fuel utilization, and thus a clear economic advantage compared with current light water reactors.
SCWR safety features are different between pressure vessel and pressure tube types, and they also depend on the neutron spectrum of the reactor.
SCWRs incorporate inherent safety features, including the absence of chaotic voids (bubbles) due to the avoidance of boiling. This reduces the likelihood of power excursions and enhances overall reactor stability.
The simplified design and fewer components reduce the potential points of failure, enhancing the safety and reliability of SCWRs.
In case Zirconium (Zr) is excluded as a fuel cladding material due to high operating temperatures, this eliminates the possibility of hydrogen (H2) gas production from Zr-water reactions during severe accidents. As a result, the risk of H2 gas explosion is significantly reduced.
SCWRs, particularly when using fuel compositions like a mix of thorium and plutonium, contribute to proliferation resistance by minimizing the risk of nuclear materials being diverted for non-peaceful purposes.
The closed fuel cycle and utilization of recycled plutonium contribute to physical protection by minimizing the availability of materials that could be used for nuclear weapons.
Compared with liquid metal cooled reactors, the SCWR has a number of general features different with respect to PR&PP.
- First, water is a clear liquid which allows optical surveillance of the fuel at any position, in the opened reactor, during fuel handling or in the storage pool, no matter if it is fresh fuel or spent fuel. The fuel assemblies and even each fuel rod can be numbered, labelled and thus identified without the need to remove them from the coolant or from the core.
- Second, water is chemically inert in the containment, such that a leak of coolant does not cause any secondary damage, and safety systems simply need to replace missing coolant. Compared with gas cooled fast reactors, water stored inside the containment can provide a heat sink at least for several hours without the need for outside heat removal.
Regarding thermal SCWRs, the use of low enriched fuel, which is less attractive for diversion, represents an additional advantage.
Finally, as a consequence of more than 50 years of experience with more than 500 water cooled reactors, methods for safeguards related surveillance and physical protection are well proven. Most of them can directly be applied to the SCWR.
Benefits and Applications of Super Critical Water Reactors
SCWRs are evolved version of the current water-cooled reactors, combining the advantages of the nuclear reactor with the balance of plant of fossil-fuelled power plant utilising SCW as a coolant.
Most of the benefits come from the capacity of SCWR concepts to meet Generation IV goals. This is notably possible through their simplified designs and improved thermal efficiency compared to current Generation III reactors.
Another strong attribute of the SCWR design path is its capacity to leverage the significant experience gained in the design, construction and operation of conventional Light Water Reactors in the last 70+ years and, for the energy conversion system, of fossil fuelled plants using SCW as a coolant. This also means that these SCWR concepts can rely on mature and diversified supply chains.
Lastly from a licensing perspective, they depart less from currently licensed reactors and rely on very similar safety approaches. This means that regulators will benefit from their extensive experience with water cooled reactors currently operating when faced with licensing applications for SCWRs.
Main Challenges for the Deployment of SCWR
There are several technological challenges associated with the development of the SCWR concept, and particularly:
- the need to validate transient heat transfer models (for describing the depressurization from supercritical to sub-critical conditions),
- Supercritical water is significantly different by its thermos-physical properties and behaviour from those of ordinary liquids or gases, causing a challenge for heat-transfer predictions
- qualification of materials (namely cladding materials (e.g. advanced steels) and their water-chemistry requirements that are among key technology challenges for SCWRs, and
- demonstration of the proposed passive safety systems.
How is GIF working to solve those
GIF has established System Steering Committees (SSC) to implement the research and development (R&D) for each Generation IV Reactor Concept, with participation from GIF Members interested in contributing to collaborative R&D. Each System Steering Committee plans and integrates R&D projects contributing to the development of a system.
The SCWR SSC was established in 2006 with Canada and Euratom as first participants later joined by Japan in 2007, Russia in 2011 and China in 2014. It now supervises two projects another provisional one. Each of those activities is governed by a (provisional) Project Arrangement and managed by a Project Management Board (PMB). Those cover areas that have been identified as key to move the SCWR concept to the next stage and bring it closer to a deployable Gen IV nuclear reactor.
To learn more about the progress made by each of these joint initiatives please see the GIF annual reports.
SCWR Projects History - Past projects
Valuable past experiences
The operating experience using nuclear steam reheat at the Beloyarsk nuclear power plant in Russia in its two first reactors (boiling water reactors moderated by graphite with a re-heating of the steam through the reactor to bring it to supercritical state): AMB 100 (108 MWe, operational 1964–1983) and 200 (160 MWe, operational 1967–1989) is extremely valuable. This is especially true in the key SCWR R&D areas of predicting and controlling water radiolysis and corrosion product transport (including fission products).
As mentioned earlier, the experience gained in supercritical fossil fuelled plants is also a very relevant source of experience and knowledge combined with the extensive operational experience of conventional water-cooled nuclear reactors.
Early Concepts (1990s)
The concept of SCWRs emerged in the 1990s as a potential evolution of traditional water-cooled reactor designs. Initial studies and research were conducted to explore the feasibility and advantages of using supercritical water as a coolant.
International Collaboration (2000s)
In the early 2000s, international collaboration played a significant role in advancing SCWR research. Organizations from Canada, China, Europe, Japan, and Russia actively participated in collaborative efforts to exchange knowledge and share research findings.
European Research (2006–2010)
A notable milestone in SCWR development was the High-Performance Light Water Reactor (HPLWR) project undertaken by a European consortium between 2006 and 2010. The HPLWR aimed to design a pressure vessel-type SCWR with a thermal neutron spectrum. A pre-conceptual design of a pressure-vessel-type reactor with a 500°C core outlet temperature and 1000 MW electric power has been developed in Europe, as summarized by Schulenberg and Starflinger. The core design is based on coolant heat-up in 3 steps. Additional moderator for the thermal neutron spectrum is provided in water rods and in gaps between assembly boxes. The design of the nuclear island and of the balance of the plant confirms results obtained in Japan, namely an efficiency improvement up to 43.5% and a cost reduction potential of 20 to 30% compared with latest boiling water reactors. Safety features as defined by the stringent European Utility Requirements are expected to be met.
Canadian SCWR Concept
Canada has been actively involved in SCWR research, with the Canadian Nuclear Laboratories (CNL) leading efforts in developing a pressure-tube type SCWR concept. The Canadian SCWR concept includes a direct cycle with steam reheat and aims for improved thermal efficiency.
SCWR Current Developments
Currently Operating
There are no SCWRs operating commercially or under construction. The development of SCWR technology is in the research and conceptual design phases, with various countries and organizations exploring the feasibility and potential applications of this advanced nuclear reactor concept.
Under Development
Currently several Research and Development activities are carried out around the world, they are focused on the main challenges identified for the SCWR concept, namely:
- the chemistry of supercritical water under neutron irradiation and materials properties related to microstructural stability, embrittlement, and creep resistance.
- Fuel cladding is also a challenge for SCWRs in case of a loss of coolant accident, because the low water inventory would allow for transient temperatures not compatible with the conventional metallic cladding used in current LWRs.
Several reactor concepts are being investigated and developed as explained below:
Canada:
Canada is developing a pressure-tube-type SCWR concept with a 625°C core outlet temperature at the pressure of 25 MPa. The concept is designed to generate 1200 MW electric power (a 300 MW concept is also being considered). It has a modular fuel channel configuration with separate coolant and moderator. A high-efficiency fuel channel is incorporated to house the fuel assembly. The heavy-water moderator directly contacts the pressure tube and is contained inside a low-pressure calandria vessel. In addition to providing moderation during normal operation, it is designed to remove decay heat from the high-efficiency fuel channel during long-term cooling using a passive moderator cooling system. A mixture of thorium oxide and plutonium is introduced as the reference fuel, which aligns with the GIF position paper on thorium fuel. The safety system design of the Canadian SCWR is similar to that of the ESBWR. However, the introduction of the passive moderator cooling system coupled with the high-efficiency channel could significantly reduce the core damage frequency during postulated severe accidents such as large-break loss-of-coolant or station black-out events.
China:
Two conceptual SCWR designs with thermal and mixed neutron spectrum cores have been established by some research institutes in China under framework of the Chinese national R&D projects from 2007-2012, covering some basic research projects on materials and thermohydraulics, the core/fuel design, the main system design (including the conventional part), safety systems design, reactor structure design and fuel assembly structure design. The related feasibility studies have also been completed, and show that the design concept has promising prospects in terms of the overall performance, integration of design, component structure feasibility and manufacturability.
The Nuclear Power Institute of China (NPIC) has been developing the CSR1000 (2300 MWth/100MWe) that is a pressure vessel type, thermal spectrum SCWR.
Japan:
Pre-conceptual core design studies for a core outlet temperature of more than 500°C have been performed in Japan, assuming either a thermal neutron spectrum or a fast neutron spectrum. Both options are based on a coolant heat-up in two steps with intermediate mixing underneath the core. Additional moderator for a thermal neutron spectrum is provided by feed water inside water rods. The fast-spectrum option uses zirconium-hydride (ZrH2) layers to minimize hardening of the neutron spectrum in case of core voiding. A pre-conceptual design of safety systems for both options has been studied with transient analyses.
A pre-conceptual plant design, the JSCWR, with 1700 MW net electric power based on a pressure-vessel-type reactor has been studied by Yamada et al. and has been assessed with respect to efficiency, safety and cost. The study confirms the target net efficiency of 44% and estimates a cost reduction potential of 30% compared with current pressurized water reactors. Safety features are expected to be similar to advanced boiling water reactors.
Russia:
Pre-conceptual designs of three variants of pressure vessel supercritical reactors with thermal, mixed and fast neutron spectrum have been conceptually developed in Russia, which joined the SCWR System Arrangement in 2011.
GIF SCWR Related Publications
GIF has produced several reports and conducted analysis on SCWR Systems produced by cross cutting methodological working groups (Risk and Safety WG, Proliferation Resistance & Physical Protection Working Group). GIF's annual reports, technology roadmap and R&D Outlooks provide more information on the progress made by GIF's SCWR System Steering Committee and Project Management Boards.
GIF GFR Related Webinars
References
International Atomic Energy Agency Advanced Reactors Information System (ARIS) Online Database
A review of existing SuperCritical Water reactor concepts, safety analysis codes and safety characteristics, Pan Wu, Yanhao Ren, Min Feng, Jianqiang Shan, Yanping Huang, Wen Yang, Progress in Nuclear Energy, Volume 153, 2022
Cheng, L.-Y & Cojazzi, Giacomo & Renda, Guido & Cipiti, B. & Boyer, B & Edwards, G. & Hervieu, E. & TD, deWit & Hori, K. & Shiba, T. & Kim, H. & Nguyen, Frédéric & Hesketh, K. & Ende, B.. (2021). White Papers on Proliferation Resistance and Physical Protection Characteristics of the Six GEN IV Nuclear Energy Systems.
Caciuffo, Roberto & Fazio, C. & Guet, Claude. (2020). Generation-IV nuclear reactor systems. EPJ Web of Conferences. 246. 00011. 10.1051/epjconf/202024600011.
I.L. Pioro, R.B. Duffey, Heat transfer and hydraulic resistance at supercritical pressures in power engineering applications, ASME Press 2007.
Y. Oka, S. Koshizuka, Y. Ishiwatari, A. Yamaji, Super light water reactors and super fast reactors, Springer 2010.
S.B. Ryzhov, V.A.Mokhov, M.P.Nikitenko, A.K.Podshibyakin, I.G. Schekin, A.N. Churkin, Advanced designs of VVER reactor plant, The 8th International Topical Meeting on Nuclear Thermal-Hydraulics, Operation and Safety (NUTHOS-8), 10-14 October 2010, Shanghai, China, Paper N8P0184.
S. B. Ryzhov, P. L. Kirillov, et al., Concept of a Single-Circuit RP with Vessel Type Supercritical Water-Cooled Reactor, Proc. ISSCWR-5, Vancouver, Canada, 13-16 March 2011.
J. Kaneda, S. Kasahara, F. Kano, N. Saito, T. Shikama, H. Matsui, Material development for supercritical water-cooled reactor, Proc. ISSCWR-5, Vancouver, Canada, 13-16 March 2011.
K. Yamada, S. Sakurai, Y. Asanuma, R. Hamazaki, Y. Ishiwatari, K. Kitoh, Overview of the Japanese SCWR concept developed under the GIF collaboration, Proc. ISSCWR-5, Vancouver, Canada, 13-16 March 2011.
M. Yetisir, W. Diamond, L.K.H. Leung, D. Martin and R. Duffey, Conceptual Mechanical Design for A Pressure-Tube Type Supercritical Water-Cooled Reactor, Proc. 5th International Symposium on Supercritical Water-cooled Reactors, Vancouver, Canada, 13-17 March 2011.
T. Schulenberg, J. Starflinger, High performance light water reactor – design and analyses, KIT Scientific Publishing 2012.
D. Guzonas, F. Brosseau, P. Tremaine, J. Meesungnoen, J.-P. Jay-Gerin, Water chemistry in a supercritical water–cooled pressure tube reactor, Nuclear Technology Vol. 179, 2012.
Special Issue: Journal of Nuclear Engineering and Radiation Science with selected "augmented and revised" papers from the 8th International Symposium on Supercritical Water-Cooled Reactors (ISSCWR-8), edited by Prof Thomas Schulenberg, July 2018.
June 2004, Pre-design studies of SCWR in fast neutron spectrum: evaluation of operating conditions and analysis of the behaviour in accidental situations - https://cathare.cea.fr/Lists/Publications/StructuredDisplayForm.aspx?ID=4957&ContentTypeId=0x010010995605D5DC4A16AA9AA61FBA54D1B200F308BA1E419F58438E6E39DA4B08D4C6 –
News related to the SCWR System
December 2014, Supercritical Water Reactor - Fuel Qualification Test - https://cordis.europa.eu/project/id/269908/reporting/fr