Uranium carbide systems

As previously stated, uranium carbides are used as nuclear fuel (145). Two of the typical reactors fueled by uranium and mixed metal carbides are thermionic, which are continually being developed for space power and propulsion systems, and high temperature gas-cooled reactors (83,146,147). In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO and elemental carbon to form UC (148,149). In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Unfortunately, the system selected was not in physico-chemical equilibrium uranium carbide was gradually going into solution in liquid uranium, and intergranular penetration and erosion of UC occurred to an extent which increased with temperature that was varied between 1180° and 1720 °C. No wonder then, that the 6 observed in different tests was as low as 37° and as high as 110°. Also for the surface tension 7 of liquid uranium values from 780 to 1510 dyne/cm have been obtained at a constant temperature of 1600 °C. The ys of UC in argon was estimated to be approximately 730 erg/cm2 at 1325 °C, and 7si (at the boundary U — UC) appeared to be near 140 erg/cm2 at 1100 °C. 

The reactor configuration, shown in Fig. t, consists of a large outer cylindrical reflector shell of beryllium, into which the fuel (in the form of a cylindrical shell Of uranium carbide fuel elements) and an inner beryllium reflector are assembled by a ram which lifts them into the reflector. Originallyi the reactor control and safety systems consisted of rotary drums with a 120-deg boron sector running the full length of the core, which resided permanently in the outer beryllium reflector and were spring loaded for fail-safe control. ... 

To summarize, the suitability of UO2 as a reactor fuel for both thermal and fast reactors has been confirmed by a wealth of experience, and the failure rate of fuel pins can be made acceptably low (down to about 1 in 10" ) by careful quality control in manufacturing. Particular problems which have arisen, such as fuel densification and damage due to pellet-clad interaction, have been solved by adjustment of the initial fuel density and pin gas pressure, and by limiting the rate at which large power increases are allowed to occur. Alternatives to the oxide fuel, such as uranium carbide and uranium nitride, have been investigated less extensively, but hold promise of superior performance in fast reactor systems. 

As a solvent for liquid-metal fuels, bismuth is a natural choice because it dissolves uranium and has a low cross section for thermal neutrons. As a result, research work at Brookhaven National Laboratory has centered on bismuth-uranium fuels. Other po.s.sible liPlutonium System (LAIMPRE) [14] and dispersions of uranium oxide in liquid metals, NaK [15] or bismuth [10]. The limited solubility, of uranium in bismuth is trouble.some in some designs. More concentrated fuels can be obtained by using slurries or dispersions of solid uranium compounds in bismuth. Among the. solids which have been suggested are intermetallic compounds [10] uranium oxide [10], uranium carbide, and uranium fluoride. Use of a dispersion avoids the limited concentration but introduces other problems of concentration control, stability, and erosion. 

The fuel for the Peach Bottom reactor consisted of a uranium-thorium dicarbide kernel, overcoated with pyrolytic carbon and silicon carbide which were dispersed in carbon compacts (see Section 5), and encased in graphite sleeves [37]. There were 804 fuel elements oriented vertically in the reactor core. Helium coolant flowed upward through the tricusp-shaped coolant channels between the fuel elements. A small helium purge stream was diverted through the top of each element and flowed downward through the element to purge any fission products leaking from the fuel compacts to the helium purification system. The Peach... 

In 1964 Union Carbide purchased a Canadian mattress bedding company. Two years later, it obtained a maker of lasers and laser systems. By investing in uranium mining and processing equipment, it expanded the nuclear division it had created to operate the U.S. government s facilities at Oak Ridge. 

The binary systems actually and potentially important as nuclear fuel include oxides, carbides, nitrides, phosphides, and sulfides of uranium, plutonium, and thorium. An increasing amount of detailed information is becoming available on the phase equilibria of these compounds, but the relations existing between the composition (especially nonstoichiometric) and the vapor pressure (or activity) of each component are known only for a limited number of systems. 

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