Nuclear reactors sodium alloys

Sodium is also used, especially in alloys with potassium, as a heat-exchange liquid in fast breeder nuclear reactors. Sodium alloys with calcium, lead, copper, silver, gold, zinc, cadmium, and mercury are also industrially formed and used. 

Liquid potassium, when mixed with liquid sodium (NaK), is an alloy used as a heat-exchange substance to cool nuclear reactors. 

The industrial demand for potassium metal is much smaller than that for sodium. Potassium-sodium alloys (which are liquid at room temperature) serve as heat-exchange hquids in the cooling systems of nuclear reactors. Strong bases such as potassium amides and alkoxides are formed from the reaction of potassium with amines and alcohols, respectively. 

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Some of the most important properties of sodium and lithium for high-temperature nuclear-reactor applications are listed in Table I. Several other popular and potential heat-transfer fluids are shown for comparison purposes. The advantages and disadvantages of various coolants are considered in relation to their application at temperatures in excess of 1200 °F. The undesirable properties of a particular coolant are underlined. Water is not particularly suitable because of its very low boiling point and its poor thermal conductivity. Sodium and the sodium-potassium alloy have properties to which there are no major objections. (Any statement made in this paper concerning the corrosiveness of sodium may be considered as applicable to the sodium-potassium alloys, as differences found... 

The basic nuclear reactor fuel materials used today are the elements uranium and thorium. Uranium has played the major role for reasons of both availability and usability. It can be used in the form of pure metal, as a constituent of an alloy, or as an oxide, carbide, or other suitable compound. Although metallic uranium was used as a fuel in early reactors, its poor mechanical properties and great susceptibility to radiation damage excludes its use for commercial power reactors today. The source material for uranium is uranium ore, which after mining is concentrated in a "mill" and shipped as an impure form of the oxide UjO (yellow cake). The material is then shipped to a materials plant where it is converted to uranium dioxide (UO2), a ceramic, which is the most common fuel material used in commercial power reactors. The UO2 is formed into pellets and clad with zircaloy (water-cooled reactors) or stainless steel (fast sodium-cooled reactors) to form fuel elements. The cladding protects the fuel from attack by the coolant, prevents the escape of fission products, and provides geometrical integrity. 

Modem uses of potassium include the use of the metal in combination with other alkali metals to give a range of alloys of unusual properties. For instance, one alloy of sodium and potassium is used as a heat exchanger in nuclear reactors [2]. Potassium is essential to life and it is used in extremely large quantities in agricultural fertilizers. Potassium salts are also widely used industrially in the electroplating of metals and in photographic materials. 

Industrial uses of sodium are based primarily on its strong reducing properties. A large part of the annual sodium production is needed to produce the gasoline antiknock agents tetramethyllead and tetraethyllead. It is also employed for the reduction of titanium and zirconium chlorides to produce titanium and zirconium metals. The remaining part of sodium is used to produce compounds such as sodium hydride, sodium alkoxides, and sodium peroxide. Sodium is also used, especially in alloys with potassium, as a heat exchange liquid in fast-breeder nuclear reactors. 

For metal fuel fabrication, the actinide metals are alloyed in an injection casting furnace that melts, mixes the alloy and injects the molten metal into quartz molds. After quick cooling, the quartz mold is removed from the metal pin, which is cut to length and undergoes quality assurance measurements. These pins are placed into new fuel cladding that contains a small amount of metallic sodium, which provides a thermal bond in early irradiation in the nuclear reactor. These fuel elements are welded closed and are ready for the reactor. Recent research in this area has focused on modifying the process to minimize the volatization of americium, which is a key component in U/TRU recovered for fast reactors and has a high vapor pressure. 

Sodium metal melts at 98°C and potassium metal melts at 63°C. When the two metals are mixed to give a solution that is 20% sodium, for example, the melting point is lowered to — 10°C.This is another example of the lowering of the freezing, or melting, pioint of solutions, which we will discuss later in the chapter. Potassium—sodium alloy is used as a heat-transfer medium in nuclear reactors. 

For high-temperature operations, materials, and fuels are key technologies. There is a century of large-scale experience in the use of fluoride molten salts. Aluminum is made by electrolysis of a mixture of bauxite and sodium aluminum fluoride salts at 1000 C in large graphite baths. Fluoride salts are compatible with graphite fuels. A smaller nuclear experience base exists with molten fluoride salts in molten salt reactors. Nickel alloys such as modified Hastelloy-N have been qualified for service to 750 C. A number of metals and carbon-carbon composites have been identified for use at much higher temperatures however, these materials have not yet been fully developed or tested for such applications. 

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