This special issue of the Journal of Nuclear Engineering and Radiation Science features selected papers from the 7th International Symposium on Supercritical Water-Cooled Reactors (ISSCWR7), held from March 15 to 18, 2015, in the Radisson Blu Royal Hotel in Helsinki, Finland, 15 years after the first International Symposium was held on this topic in Tokyo, Japan. Like the 4th Symposium (2009) in Germany, the 5th Symposium (2011) in Canada, and the 6th Symposium (2013) in China, this International Symposium attracted more than 100 participants from nuclear industry, research centers, and universities mainly from Canada, China, Europe, Japan, and Russia. Within three days, about 100 presentations were given on design, safety issues, materials, thermal-hydraulics, and qualification tests of this Generation IV nuclear energy system, providing a worldwide forum for information exchange on innovative nuclear research and technologies.

While dozens of water-cooled reactors are being built today, we should not give up challenging their design, trying to simplify them to reduce their costs without compromises, however, to their high safety standard. As an opening remark, Professor Riitta Kyrki-Rajamäki (Lappeenranta University of Technology, Finland) reminded the participants of the extreme importance of innovative technologies for the future of nuclear power. Young scientists and engineers, in particular, should not hesitate to try unconventional solutions, overcoming traditional bounds of nuclear engineering. The worldwide research on supercritical water-cooled reactors (SCWRs) is an ideal platform in this context, inspiring to develop something really new, but also to understand the huge amount of iteration work, including qualification and tests, until a feasible design concept has been reached. Different from other nuclear energy systems of the Generation IV International Forum (GIF), research and development (R&D) of SCWRs aims primarily for simple and economical solutions, which are based on the long-term experience of pressurized water reactors, boiling water reactors, and supercritical coal-fired power plants.

The ISSCWR7 was hosted by GEN4FIN, the Finnish research network for Generation IV nuclear energy systems. Initiated by a Finnish working group of VTT (Technical Research Centre of Finland Ltd), universities, STUK (Radiation and Nuclear Safety Authority), TEKES (the Finnish Funding Agency for Innovation), and industry in 2004, and in collaboration with the Nordic Network Nordic-Gen4 since 2009, GEN4FIN has participated in Euratom research projects, in bilateral collaboration with CEA, has performed a doctoral program for nuclear engineering and radiochemistry, and has arranged several scientific seminars in Lappeenranta, Finland. In the opening plenary session, Dr. Kristiina Söderholm, Chair of GEN4FIN, summarized past and future activities of this network on cross-cutting issues such as material research, development of numerical tools, risk and safety analyses and criteria for new concepts, study of new technologies for handling of nuclear waste, and training of young scientists. GEN4FIN is active globally to survey Generation IV activities, to establish cooperation and identify partners for R&D applications, to build a sustainable forum for Generation IV issues, and to inspire young scientists for R&D of Generation IV nuclear energy systems.

Motivated by early research on the SCWR concepts in the 1950s and 1960s, Professor Yoshiaki Oka started in the 1990s with a small group of young scientists to merge the latest technologies of supercritical coal-fired power plants with those of light-water reactors and to continue their development. Since the Generation IV International Forum adopted this concept as one of the six most promising nuclear energy systems, the idea quickly spread from Japan to Europe and Canada, where detailed design concepts were worked out to illustrate potential applications and to develop necessary technologies. As an example, Dr. Laurence Leung presented a recent Canadian concept with vertical pressure tubes, cooled with supercritical water and moderated with heavy water, which was developed since 2006 in Canada’s Generation IV National Program, supported by 5 laboratories and 22 universities in Canada and abroad. The second phase of this joint program is ending in 2015, and several publications of the ISSCWR7 report about results not only on design but also on technologies such as thermal-hydraulics experiments with tubes, annuli, and rod bundles, and on cladding-material research for the Canadian SCWR concept.

Meanwhile, innovative SCWR design concepts are being developed also in Russia and China. During the opening plenary session, Dr. Alexei Sedov summarized the history of SCWR development in Russia, starting from first studies in 1961, and recently continued by three VVER concepts at supercritical pressure, which were considered in R&D studies from 2008 to 2011. Besides direct-power conversion with a once-through steam cycle, favored in other countries, Russia also studied concepts with indirect power conversion using steam generators, which are producing superheated, subcritical steam for the secondary steam cycle. A fast or epithermal neutron spectrum was favored in each concept. Since July 2011, when ROSATOM signed the SCWR System Arrangement, an R&D program has been worked out focusing in particular on thermal-hydraulics and safety issues to collaborate with the SCWR development in GIF.

Quite ambitious R&D on SCWR can also be reported from China. The China Atomic Energy Authority signed the SCWR System Arrangement in GIF in May 2014, but comprehensive research on SCWR already started in China 10 years earlier. As Dr. Yanping Huang summarized at the ISSCWR7, Chinese research organizations have been contributing to material research on stainless steels, Ni-based alloys, and oxide dispersion strengthened (ODS) materials for SCWR applications, on heat-transfer tests and safety-related analyses and tests, as well as on code development. Two conceptual design studies for an SCWR are still being performed at the NPIC-CCNC and at the CGNPC, each for a target electric power of 1000MWel. Large-scale test facilities have been built and an in-pile qualification test loop is envisaged for the near future. Already, since 2010, Chinese research organizations have collaborated with Canadian or European partners in SCWR research projects, and the results were presented at the ISSCWR7.

The focus on SCWR research in Europe in recent years has been on design and analyses for a fuel qualification tests in collaboration with Chinese research organizations. Having completed their conceptual design study of the high-performance light-water reactor (HPLWR) in 2010, a European consortium has been preparing a small-scale fuel-assembly test, heated by enriched UO2, to be operated inside a research reactor in the Czech Republic. Such an integral qualification test, challenging material research, manufacturing, code development, and design tools for the SCWR, is considered as a mandatory step before an SCWR prototype can be built.

A central contact point, disseminating SCWR technologies to interested parties outside the SCWR System Arrangement, is the International Atomic Energy Agency (IAEA). Today, more than 20 member states of the IAEA have R&D projects related to SCWRs, as Dr. Katsumi Yamada reported at the ISSCWR7. Since 2008, the IAEA has launched several projects on SCWR technologies to facilitate collaboration and information exchange, including coordinated research projects (CRPs), technical meetings, and training courses on thermal-hydraulics, materials, and chemistry, and on conceptual design.

Like all previous SCWR symposia, the ISSCWR7 was open to everyone interested in this innovative technology, and the presentations given there provide an excellent overview of the worldwide status of SCWR development. I would like to thank all participants for their fascinating contributions and the technical program committee, in particular, its Chair Sami Penttilä, for organizing this symposium.

Figure 1
Thomas Schulenberg, Guest Editor
Figure 1
Thomas Schulenberg, Guest Editor
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