ing temperatures than does water. These include liquid metals such as sodium or lead, gases such as helium and carbon dioxide, molten salts, and operating coolants at supercritical conditions such as supercritical water or carbon dioxide. The major R&D challenges that need to be addressed include the associated chemical and metallurgical effects of these coolants on wetted materials, the fluid properties of these coolants, their radiation resistance, industrial scale handling, and safety. There is active international R&D in all of these areas. Test reactors and the first prototypes of new reactors using gas and liquid metal coolants are likely to be operable in some countries by 2020 or shortly thereafter.

Efficiency improvements in currently operating and evolutionary LWRs may be able to be gained by using coolant additives. The use of such additives could enhance heat transfer and potentially suppress phenomena that currently limit heat transfer and power density. Work has begun into the use of very dilute additions of nanoparticles to coolant water, and initial tests have been encouraging, suggesting that their use allows higher heat fluxes to be tolerated. Many R&D opportunities remain, including characterization of the enhanced heat transfer effect under realistic operating and transient conditions, metallurgical and chemical capability with other materials wetted by the coolant, radiation resistance, neutron absorption properties, and safety and environmental issues. Coolant additives (along with the associated redesign of reactors to adopt their use) are likely to be ready for commercial deployment after 2035.

Improved Heat Transfer Materials

As just noted, higher temperatures generally improve efficiency. At higher temperatures, improved materials are needed to contain the coolant and act as heat-transfer surfaces. High-temperature metal alloys developed for use in other applications such as combustion facilities and ceramics are being considered for improved heat transfer materials. Remaining R&D challenges for these materials include producing large quantities in the needed product form; improving fabricability, acceptance, and in-service inspection; understanding radiation effects; and fragility. Work is currently under way to use these materials with the alternative coolants previously described and to replace materials used in existing LWRs—for example, replacing the metallic tubes currently used for fuel cladding with ceramic tubes. After attractive advanced materials are identified, typically 15–20 years are required before the materials are commercially deployed. Since some materials of interest have been identified now, some new materials may be available after 2025, with a higher probability of successful widespread application after 2035.

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