Molten uranium

Even though not molten, uranium rapidly attacks Nimonic 80A (Ni-20Cr-2-5Ti-l -5A1) at temperatures above 740°C, at which temperature the nickel-uranium eutectic melts. 

This comprehensive survey of the title topic is in three parts, the first dealing with the theoretical background and laboratory studies, with 29 references. The second part, with 21 references deals with case histories and experimental studies of industrial vapour explosions. These involved the systems molten titanium-water, molten copper-water, molten aluminium-water, smelt-water, water-various cryogenic liquids, molten salt-water and molten uranium dioxide-liquid sodium. In the third part (with a further 26 references) is discussion of the various theories which abound, and the general conclusion that superheated liquids most likely play a major role in all these phenomena [1]. A further related publication covers BLEVEs and pressure let-down explosions [2],... 

Two uranium carbides are known, the monocarbide, UC, and the dicarbide, UC2. These can be prepared by direct reaction of carbon with molten uranium, or by reaction of carbon monoxide with metallic uranium at elevated temperatures. The scsqutcarbidc, U,Oj, has been found to exist as a stable compound below about 18 ") U and can be produced by heating a mixture of UC and UC2 between 1.250 and 1,800°C. 

Barney, R., and F. Keneshea The Distribution of Pu and Fission Products between Molten Uranium and Magnesium, Report NAA-SR-1324,1956. 

The reaction of Ca° with UF4 is sufficiently exothermic that both Cap2 and U° reaction products are molten the equivalent Mg reaction is less exothermic, so the reaction must be initiated at an elevated temperature to result in molten products. CaF2 and Mgp2 are not soluble in molten uranium, so the reaction produces a dense metallic uranium phase and a lower-density salt phase unreacted starting materials are more soluble in the salt phase and concentrate there. After cooling, the salt cake can be physically separated from the metal product and discarded. 

Uranium carbides UC and UCj can be produced by reaction of carbon monoxide with molten uranium. These gray crystals have high melting points of 2790°C and 2350°C, respectively. U2C3 also exists but decomposes at 1700°C. In principle, uranium carbide can replace uranium dioxide in nuclear fuels. 

There vas no positive visual Indication of uay penetration of the melt through the zirconium during the tests. Metallographlc examination after the highest teinperature tests revealed that zirconium alloy vas destroyed in the area of contact with the molten uranium and that some zirconium was dispersed through the uranium mass. 

Another set of calculations were made assuming 100 per cent meltdown of the reactor core but with the same plate out, fog spray and filter efficiencies and the same percentage escape of fission products ftrom the molten uranium Consequences are shown In TAble 9-10 ... 

Much disagreement exists on whether a sodium vapor explosion of the kind postulated above can in fact take place when the molten core mixes with the coolant following the initial explosive dispersal of the core. According to the superheat theory of Fauske (see Bibliography), it seems likely that the mixing of molten uranium or plutonium oxide with liquid sodium will not in fact produce a sodium vapor explosion. On the other hand, it is possible that the much more rapid transfer of heat to the sodium from the molten steel of the fuel cladding could either trigger a sodium vapor explosion directly or set up the conditions under which the slower heat transfer from the fuel itself could do so. 

Okamoto Y, Madden PA, Minato K (2005) X- ray diffraction and molecular dynamics simulation studies of molten uranium chloride. J Nucl Mater 344 109-114... 

Leibowitz L, Fischer DF, Chasanov MG (1974) Enthalpy of molten uranium- plutonium... 

The fuel elements were unusual in that they consisted of hollow cylinders, which were fabricated as a form of sandwich on the outside was niobium metal and on the inside vanadium metal, with the uranium sandwiched between the two. The reasoning behind this was that if there were a catastrophic loss of coolant, the vanadium would fail first, meaning that the molten uranium could flow down inside the outer niobium tube. Making the cylinder hollow improved the head transfer as the coolant could flow down the middle of the tube as well as the outside. 

Plutonium Is concentrated by vacuum distillation from molten uranium at 1500-lB00"C. 

Plutonium Is extracted from molten uranium by salts such as or MgClp. Uranium remains in the metallic state. Plutonium la recovered as a halide salt. 

Plutonium Is extracted by molten metals, such as silver or magnesium, that are immiscible with molten uranium. Fission products are also extracted. 

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