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Modular Nuclear Reactors
Published in Yatish T. Shah, Modular Systems for Energy and Fuel Recovery and Conversion, 2019
Research on molten salt coolant has been revived at ORNL in the United States with the Advanced High-Temperature Reactor (AHTR). This is a larger reactor using a coated-particle graphite-matrix fuel like that in the GT-MHR and with molten fluoride salt as primary coolant. It is also known as the Fluoride High-Temperature Reactor (FHR). While similar to the gas-cooled HTR, it operates at low pressure (less than 1 atm) and higher temperature, and gives better heat transfer than helium. The FLiBe salt is used solely as primary coolant, and achieves temperatures of 750°C–1,000°C or more while at low pressure. This could be used for hydrogen production by thermochemical processes.
MSR Technology Basics
Published in Kenneth D. Kok, Nuclear Engineering Handbook, 2016
The most common carrier salts proposed are mixtures of enriched (>99.99%) 7LiF and BeF2 termed FLiBe. Mixtures with upward of 14% ThF4 and/or UF4 have melting points below a traditional limit of 525°C, which gives adequate margin for use with the high nickel alloy Hastelloy N™. This alloy is proposed for use up to 704°C with these molten salts and options exist to employ newer, ASME Section III-qualified materials such as Alloy 800H clad with Hastelloy N™. Furthermore, many forms of steel such as SS304 and SS316 have performed well in loop tests (ORNL-TM-4286, 1972; ORNL-TM-5782, 1977) and although somewhat inferior corrosion resistance to Hastelloy N™ bring many other potential advantages. It has also been suggested that carbon-based structures or refractory metals could be used throughout the primary loop including heat exchangers. This would allow higher peak temperatures and a wider set of carrier salt options with higher melting points. However, such options would require a significant development effort to prove their viability.
Fusion Reactor Materials
Published in C. K. Gupta, Materials in Nuclear Energy Applications, 1989
Liquid lithium has the advantage of combining cooling and breeding functions. One problem arises with the isotopic content of lithium. In fission nuclear reactors, where neutron economy is of paramount importance, one can use as cooling agents only those lithium compounds that are 6Li free, since this particular isotope is a powerful absorber of thermal neutrons; thus, 7Li isotope is preferred. With fusion reactors, the odds are reversed; 6Li is essential for breeding tritium. In pumping liquid conductive coolant lithium across magnetic fields, there occur magnetohydrodynamic pumping losses. From analysis on several proposed systems, one finds these losses to be as high as 18% of the gross electrical output of a 1000-MWe plant in the case of lithium and 13% for flibe. With lithium, there is also a compatibility problem with many structural materials. Stainless steels undergo thermal gradient mass transfer in flowing lithium above 500°C. Refractory metals such as niobium and vanadium alloys exhibit compatibility with lithium to such higher temperatures, but these materials must be protected from the atmosphere and from impurities in the blanket. Flibe has a comparatively high melting point (460°C). As an electrolyte, it affords the potential for galvanic attack under the influence of a magnetic field. The compatibility of flibe strongly depends on the ratio of lithium to beryllium fluoride contained in the salt. If the oxidation potential of the salt can be buffered with chromium, for instance, the more reactive element of the first wall, flibe can be used with stainless steels up to about 600°C. Flibe is compatible with nickel-based alloys up to about 800°C and with molybdenum alloys to even higher temperatures.
Evaluation of Corrosion Behavior of Various Fe- and Ni-Based Alloys in Molten Li2BeF4 (FLiBe)
Published in Nuclear Technology, 2023
Krishna Moorthi Sankar, James R. Keiser, Dino Sulejmanovic, Tracie M Lowe, Preet M. Singh
Most Generation=-IV fluoride salt–cooled high-temperature reactor (FHR) designs are considering molten salt 2LiF-BeF2, or FLiBe, as a primary coolant. FLiBe also has been used in the self-cooled liquid blanket of a fusion reactor in a Japan-U.S. collaboration project called JUPITER-II. Molten FLiBe has the required thermal properties and chemical and physical stabilities at high temperatures.[1,2] Most importantly, this salt can be heated to very high temperatures, 700°C or higher, without significant increases in vapor pressure. The containment alloy for these reactors should not only have good corrosion resistance to the molten fluoride salt coolant, but should also have good creep strength and resistance to oxidation in air at molten salt reactor operating temperatures.[2]
Direct Numerical Simulation of High Prandtl Number Fluid Flow in the Downcomer of an Advanced Reactor
Published in Nuclear Science and Engineering, 2023
Among nontraditional coolants, molten salts are promising coolants for advanced reactor designs thanks to their special thermophysical and neutronic properties.1 For example, FLiBe, a mixture of lithium fluoride (LiF) and beryllium fluoride (BeF2), is one of the most well-studied molten salts, as was successfully demonstrated in the Molten Salt Reactor Experiment2 from 1965 to 1969 at Oak Ridge National Laboratory. However, a comprehensive heat transfer characteristics database for molten salts is still not yet established, as traditional water-based correlations may not be applicable. Currently, there is a significant gap in direct numerical simulation (DNS) data that can be used to advance reactor design. Moreover, the experimental results and high-fidelity numerical simulations, which could be used to better understand the physics, are ineffectively implemented.
Fusion Blankets and Fluoride-Salt-Cooled High-Temperature Reactors with Flibe Salt Coolant: Common Challenges, Tritium Control, and Opportunities for Synergistic Development Strategies Between Fission, Fusion, and Solar Salt Technologies
Published in Nuclear Technology, 2020
Charles Forsberg, Guiqiu (Tony) Zheng, Ronald G. Ballinger, Stephen T. Lam
The FHR is a new reactor concept,18–20 less than 20 years old, that combines a clean fluoride salt coolant (no dissolved fuel or fission products), the graphite-matrix coated-particle fuel originally developed for high-temperature gas-cooled reactors (HTGRs), and passive decay heat removal systems from sodium fast reactors (SFRs). The baseline FHR uses pebble-bed fuel and 7Li2BeF4 (flibe) salt coolant enriched in 7Li to minimize tritium production. Fluoride salt coolants are used because they are chemically compatible with the carbon-based fuel and fluorine has a small neutron adsorption cross section. Flibe is the best salt coolant in terms of thermal hydraulics and neutronics performance.21,22 A schematic of the pebble-bed FHR is shown in Fig. 1c.