Molten salt coolants

The various high temperature concepts propose three coolants - molten salt Pb or Pb-Bi and gas. The fuel is nitride for fast spectrum concepts (BGR-300 (28) suggests the use of advanced porous fuel) and TRISO derivative for thermal or intermediate spectrum concepts. Two of the 5 concepts are reactors intended for operation in U-Th fuel cycle. 

Oxidation Step. A review of mechanistic studies of partial oxidation of propylene has appeared (58). The oxidation process flow sheet (Fig. 2) shows equipment and typical operating conditions. The reactors are of the fixed-bed shell-and-tube type (about 3—5 mlong and 2.5 cm in diameter) with a molten salt coolant on the shell side. The tubes are packed with catalyst, a small amount of inert material at the top serving as a preheater section for the feed gases. Vaporized propylene is mixed with steam and ak and fed to the first-stage reactor. The feed composition is typically 5—7% propylene, 10—30%... 

The preheated gases react exothermically over the first-stage catalyst with the peak temperature ia the range of 330—430°C, depending on conditions and catalyst selectivity. The conversion of propylene to waste gas (carbon dioxide and carbon monoxide) is more exothermic than its conversion to acroleia. At the end of the catalyst bed the temperature of the mixture drops toward that of the molten salt coolant. 

A variety of graphite moderated reactor concepts have evolved since the first aircooled reactors of the 1940s. Reactors with gas, water, and molten salt coolants have been constructed and a variety of fuels, and fissile/fertile fuel mixtures, have been used. The evolution and essential features of graphite moderated power producing reactors are described here, and details of their graphites cores are given. 

In contact with molten salts, the nickel-base alloys behave much more satisfactorily than is the general experience with molten metals. For this reason they are considered as structural materials in atomic reactors using fluoride mixtures as coolants and are used as vessels for heat-treatment salt baths, as thermocouple sheaths and in similar applications. 

Events of this nature have been described by various terms, e.g., rapid phase transitions (RPTs), vapor explosions, explosive boiling, thermal explosions, and fuel-coolant interactions (FCIs). They have been reported in a number of industrial operations, e.g., when water contacts molten metal, molten salts, or cryogenic liquids such as liquefied natural gas (LNG). In the first two examples noted above, water is the more volatile liquid and explosively boils whereas, in the last example, the cryogenic liquid plays the role of the volatile boiling liquid and water is then the hot fluid. 

The above reactions, which arc highly Exothermic, take place in the presence of a catalyst This catalyst may be in a fixed bed in multi-tube reactors with 5,000 to 20,000 tubes. Heat is removed by means of a coolant fluid consisting either of a bath of molten salts (nitrate/nitrite), or the sodium potassium eutectic opmating between 360 and 420 C. The catalyst may also be used in a fluidized bed traversed by cooling systems (coils, etc.). The heat generated is used to produce high-pressure steam. 

Constant-temperature baths or furnaces are needed to maintain the uniform temperature environment for comparison calibration. Stirred-liquid baths and temperature controllers have good characteristics as constant-temperature media due to their ability to maintain temperature uniformity [100]. The liquids used include refrigerants, water, oils, molten tin, and molten salt. Water can be used for temperatures between 0 and 100°C and oils above 300°C. Refrigeration units are available for commercial constant-temperature baths with alcohol as the working fluid and temperatures as low as -80°C. At NIST, special cryostats [8, 25] with liquid nitrogen or liquid helium as the coolant are used for comparison calibration at low tem-... 

Because the reactor can be designed with very little excess reactivity in the core and the molten salt has very good heat transport characteristics, the potential for achieving passive safety objectives also exists. The fact that (1) the molten-salt reactor is a low pressure system, and (2) the coolant is very chemically stable and does not react with air or water also support the passive safety characteristics of this concept. 

AHTR Molten Salt Coolants Have Several Advantages Good Heat Transfer, Low Pressure, and Transparent... 

Coolant environment Water Hehum Helium Carbon dioxide Sodium Molten salt Liquid salt... 

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