Uranium Nitride Fuels

For purposes of limiting the scope of the thesis, the reactor will use a directly coupled closed Brayton cycle for power conversion and will use highly enriched Uranium Nitride fuel. The sections of the thesis will be ... 

For mass-produced BN GT plants, the uranium nitride fuel with or without reprocessing could be used. 

At this stage, the key to improving the economic parameters of the fuel cycle will be extending the core lifetime (to increase fuel burn-up), as experience in operability of the fuel elements is gained. Further on, the reprocessing and recycle of the uranium could be applied, and the plutonium, minor actinides and fission products could be extracted and then stored until their recycle becomes economically efficient. The duration of the uranium stage may be extended upon a transition to the uranium nitride fuel. 

Type of fuel Transuranic nitride clad in cylindrical fuel rods nitrogen isotope = N. (Early development may use enriched uranium nitride fuel.)... 

Initially, SSTAR core loadings can be based on uranium nitride fuel or U/ transuranic/ nitride fuel using transuranics recovered from LWR spent fuel. In the longer term, for recycle of SSTAR spent fuel returns (after 20 years), development and demonstration of electrometallurgical reprocessing for transuranic nitride fuel will be required. A key requirement is to recover the enriched N [XXII-15]. Some theoretical work on fuel recycle and small-scale experiments have been conducted in Japan mainly at the Japan Atomic Energy Research Institute (JAERI). 

Based on the Ross, S.B., El-Genk, M.S., Matthews, R.B., 1988. Thermal conductivity correlation for uranium nitride fuel. Journal of Nuclear Materials 151, 313—317 and Hayes, S., Thomas, J., Peddicord, K., 1990a. Material properties of uranium mononitride-in transport properties. Journal of Nuclear Materials 171, 289—299 correlations. 

Matthews, R.B., Chidester, K.M., Hoth, C.W., Mason, R.E., Petty, R.L., 1988. Fabrication and testing of uranium nitride fuel for space power reactors. Journal of Nuclear Materials 151, 334-344. 

Ross, S.B., El-Genk, M.S., Matthews, R.B., 1990. Uranium nitride fuel swelling correlation. Journal of Nuclear Materials 170, 169—177. 

Irradiation testing of refractory metal clad uranium nitride fuel elements. 

Nitrides. Uranium nitrides are weU known and are used in the nuclear fuel cycle. There are three nitrides of exact stoichiometry, uranium nitride [2565843-9], UN U2N3 [12033-85-1/ and U4N2 [12266-20-5]. In addition to these, nonstoichiometric complexes, U2N3, where the N/U ratio ranges... 

Uranium and mixed uranium—plutonium nitrides have a potential use as nuclear fuels for lead cooled fast reactors (136—139). Reactors of this type have been proposed for use ia deep-sea research vehicles (136). However, similar to the oxides, ia order for these materials to be useful as fuels, the nitrides must have an appropriate size and shape, ie, spheres. Microspheres of uranium nitrides have been fabricated by internal gelation and carbothermic reduction (140,141). Another use for uranium nitrides is as a catalyst for the cracking of NH at 550°C, which results ia high yields of H2 (142). 

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. 

In order to be used as nuclear fuel, carbide microspheres are required. These microspheres have been fabricated by a carbothermic reduction of UO3 and elemental carbon to form UC. In addition to these uses, the carbides are also precursors for uranium nitride based fuels. 

Uranium nitride UN has a theoretical density of 14.32 g/cm and melts around 2630°C. Made with N, UN has been suggested as an advanced fuel for fast reactors because of its high U atom density, low moderation, and high melting point. UN is made by reacting UH3 with the correct proportions of nitrogen or ammonia. UN reacts rapidly with moist air or water. 

Thorium and uranium are used in cotmnercial catalytic systems. Industrially, thorium is used in the catalytic production of hydrocarbons for motor fuel. The direct conversion of synthetic gas to liquid fuel is accomplished by a Ni-Th02/Al203 catalyst that oxidatively cracks hydrocarbons with steam. The primary benefit to the incorporation of thorium is the increased resistance to coke deactivation. Industrially, UsOs also has been shown to be active in the decomposition of organics, including benzene and butanes and as supports for methane steam reforming catalysts. Uranium nitrides have also been used as a catalyst for the cracking of NH3 at 550 °C, which results in high yields of H2. 

There are a variety of advantages to using uranium nitride (UN) over the other fuel types, and a handful of drawbacks that need to be considered. Uranium nitride has a much higher thermal conductivity than uranium dioxide, resulting in a flatter temperature profile across the fuel pin. The same is likely true for uranium carbide, but less testing has been done on uranium carbide than for UN. Figure 3-3 shows the desired thermal properties of UN [Touloukian, 1979]. At the operational temperature, 1300 K, the thermal conductivity is 28.5 W/m-K and the linear expansion is roughly 1%. It is important for the different components to expand at similar rates to minimize the extra stresses. 

Increasing the thickness of both the rhenium liner and the uranium nitride pin were required in the reactor design to meet the safety conditions. During normal operations the rhenium had a negative effect on the k-effective but was countered by the extra fuel in the core. In two of the three accident scenarios, the neutron spectrum is more thermal than during normal operation due to the addition of water to the core. For the dry sand accident case, the spectrum is faster than the normal operation case. For the accident scenarios, the extra thermal absorption of neutrons from the additional rhenium dominated the effects from the additional fissionable fuel. 

Preliminary design study of lead cooled fast reactor with nitride fuel assemblies has been performed by the Japanese specialists to improve uranium resource utilization and transmutation of HLW nuclides. Plant size limitations caused by seismic resistance... 

The highest Pu consumption rates can be achieved only if uranium is eliminated from the core. Nitride appears to be a possible non-uranium fuel material, and the performance of a core fuelled with pure PuN has been studied. AEA-T have studied the vaporisation behaviour of nitride fuels, surveyed the extant data on the physical and chemical properties of PuN and (U,Pu)N, and set up a calculational model of a nitride fuel pin. Preliminary results indicate that acceptable bumups can be achieved provided potential problems of fuel swelling can be solved. 

The interest of nitride in the frame of CAPRA would be threefold as an alternative to oxide as a solution allowing access to an intermediate Pu burning range between the reference oxide and the cores based on an uranium-free fuel and, finally as a Pu compound for uranium-free fuels. At present, the last of these seems most promising. 

Plutonium, instead of is a main contributor to the nuclear reactions in FRs where mixture of plutonium and uranium is used as fuel. Simple metallic fuel is a candidate to achieve better FR core performance than oxide fuel due to its higher fuel density, which has been employed in the LWR. Carbide and nitride fuels are also considered. 

Nitride fuel with BR 1, no uranium blanket, small reactivity variations with fuel burning (optimal BR 1.05), fuel composition designed to require no U, and Pu separation in reprocessing (merely addition of... 

Another promising uranium compound that can be used in nuclear fuels is uranium carbide that has a high melting point and better thermal conductivity than the oxide and in addition does not form oxygen when radiolyzed. Uranium nitride can also be used, but formation of from N could be problematic. In addition, other uranium compounds that can be used as a fuel in a nuclear reactor, ranging from aqueous solutions to molten salts that are brought to a high temperature in order to keep them in a molten state. MOX of uranium and plutonium also serve as a nuclear fuel in some reactors. 

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