Molten salt fluorides

Metalliding. MetaUiding, a General Electric Company process (9), is a high temperature electrolytic technique in which an anode and a cathode are suspended in a molten fluoride salt bath. As a direct current is passed from the anode to the cathode, the anode material diffuses into the surface of the cathode, which produces a uniform, pore-free alloy rather than the typical plate usually associated with electrolytic processes. The process is called metalliding because it encompasses the interaction, mostly in the soHd state, of many metals and metalloids ranging from beryUium to uranium. It is operated at 500—1200°C in an inert atmosphere and a metal vessel the coulombic yields are usually quantitative, and processing times are short controUed... 

Gaseous fluorine is also prepared by electrolysis of molten fluoride salts but simpler methods are available for the preparation of bromine and iodine. Chemical oxidation, usually with chlorine as the oxidizing agent, provides Br2 and I2 economically because chlorine is a relatively inexpensive chemical. The reactions are... 

Electrolysis in Molten Fluoride Salts or Fluoride Ion Solutions. . 199... 

V.F. Gorbunov and G.P. Novoselov, Interaction of Fluorides of Lanthanum and Cerium with Metal Oxides in the Medium of Molten Fluoride Salts, Zh. Neorg. Khim. 19 (1974) 1734-1736. 

Choice of molten fluoride salt is dependent upon multiple factors... 

Molten Fluoride Salts Are Compatible With Graphite-Based Fuels... 

X. X. Keiser, The Corrosion Resistance of Type 316 Stainless Steel to Li2BeF4, ORNL/TM-5782, and Compatibility Studies of Potential Molten Salt Breeder Materials in Molten Fluoride Salts, ORNL/TM-5783, Oak Ridge National Laboratory (1977). 

Molten fluoride salts in general are excellent coolants, with a 25% higher volumetric heat capacity than pressurized water and nearly five times that of liquid sodium. This results in more compact primary loop components like pumps and heat exchangers. They also have a lower thermal conductivity than sodium which avoids thermal shock issues. 

Demonstrated to be compatible with molten fluoride salts... 

Aluminum industry uses molten fluoride salts in graphite baths to produce aluminum (100+ years of experience)... 

Other salt compounds are added in the reactor for lowering the temperature between 4(X) °C and 900 °C below the fusion point of pure K2TaF7 [13], Another reduction mode involves the electrochemical reduction of K2Tap7 in molten fluoride salts [5]. It is well stated now that the electroreduction of Ta in fluorides proceeds in a five-electron single step directly leading to Ta metal [14], and thus current efficiency of the preparation of Ta by the electrochemical route is close to 100 %. 

Electrochemical fluorination of various aromatic compounds such as benzene, substituted benzenes toluene, and quinolines is achieved at high current densities using these molten fluoride salts in the absence of organic solvent with good to high current efficiencies (66-90 %)... 

MARS Russian Federation XXVIII Non- conven- tional 6/16 15 or 60 UO2 based TRISOin spherical graphite 10 Natural circulation Molten fluoride salts 550/750 Indirect open cycle air turbine 5-8 years 3500- 2500 ... 

MSR-FUJI Japan XXX Molten salt reactor 200/450 >30 (on-line fissile and fertile feeding) Molten fluoride salt of LiF-Bep2-ThF4- UF4 Forced circulation Molten fluoride salt 570/710 Indirect super- critical steam Rankine cycle 8 years 1584 0.012... 

The AHTR concept [XXVIII-7] proposed by ORNL (USA) uses coated particle graphite-matrix fuel and a molten-fluoride-salt coolant. The principal difference from the MARS concept is that a larger power output (2400 MW(th) and more) is considered as one of the enabling technologies to improve the AHTR economics. 

Type of fuel Molten fluoride salt LiF-BeF2-ThF4-UF4... 

Fuel Requirements for the Advanced High-Temperature Reactor Graphite Coated-Particle Fuel and Molten Fluoride Salt Coolant... 

Only one type of nuclear fuel has been fully demonstrated for use in high-temperature reactors for commercial applications the graphite-matrix coated-particle fuel. Although helium has historically been the coolant used in high-temperature reactors, graphite-based fuel is also compatible with one other type of coolant molten fluoride salts. For example, for over a century the aluminum industry has produced aluminum by electrolytic methods in graphite baths filled with molten fluoride salts at 1000°C. The AHTR uses a low-pressure molten fluoride salt with a boiling point of 1400°C. 

Fig. 3. Non Wetting Characteristics of Molten Fluoride Salts and Graphite.

The AHTR reactor core consists of coated-particle graphite-matrix fuel cooled with a molten fluoride salt. The fuel is similar to helium-cooled reactor fuel (Fig. 2). The important characteristic of these fuels is that they can operate at very high temperatures with peak temperatures of 4200 C. They are the only practical, demonstrated nuclear fuels capable of producing heat at sufficient temperatures for H2 production. 

The excellent heat transfer properties of molten fluoride salts, compared with those of helium, reduce the temperature drops between (1) the fuel and molten salt and (2) the molten salt and any secondary system. Comparable calculations for a typical prismatic geometry were made of the temperature drop between the centerline prismatic fuel temperatures and coolant for helium and molten-salt coolants. The temperature drops for helium and molten-salt coolants were 415 and 280 C, respectively. The better heat transfer capabilities of molten salts (a liquid) compared with those of helium allow reactor designs with higher coolant exit temperatures and power densities than in gas-cooled systems for the same maximum temperature limit in the fuel. 

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