Nitride fuel elements

Six mixed nitride fuel pins were fabricated for irradiation tests in JMTR and JOYO [7.56. 7.61], The irradiation in JOYO started in August, 1994, for a target bura-up of about 4.5 at. % (2 fuel pins). With regard to the fuel cycle, an innovative concept has been proposed, involving fused salt electroreflning and refabrication of sphere-pac fuel. At present the results of PIE of 4 pins are under analysis, but the features of fuel behavior observed in irradiation tests are briefly summarized as follows [7.56]  

Russia. At present the BR-10 core is loaded with mononitride fuel. Up to now 667 fuel pins have been irradiated with the maximum bum-up being 9 at.%. All the fuel pins have remained intact. Currently another 590 fuel pins (the second loading) are under irradiation [7.14]. For the major part of the experimental work it has been assumed that dense fuels should operate at high linear ratings, and in this connection in most of the experiments the fuel temperatures are rather high. For fuel temperatures below 1200 C, limited data are available. 

It should be noted in conclusion, that the irradiation results provide favourable expectations for the performance of nitride fuels. However they have to be confirmed at higher bum-up levels and to be extended to a larger statistic of scale. In addition, several other issues have to be addressed, including fabrication of -enriched fuels (to avoid production of radioactive C), transient performance and high temperature dissociation, and reprocessing. 

---

Irradiation testing of refractory metal clad uranium nitride fuel elements. 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

Fast breeder reactor fuel rods consist of stainless-steel-clad mixed oxide (U,Pu)02 fuel however, more stable alloys for cladding and in-core structural materials, with resistance to swelling and embrittlement under fast neutron irradiation, and more efficient fuels (carbide see 17.3.12.1.2) or nitride (see 17.3.12.3)] are needed h The mechanical, metallurgical, and chemical processes in fuel element irradiation are depicted in Figure 1. Figure 2 shows the PFR (U.K.) fast breeder fuel element, and Figures 3 and 4 illustrate the Fast Flux Test Facility (FFTF) fuel system. 

The carbide and nitride elements have higher ( 25%) costs of fabrication than oxide fuels. However, the total fuel element fabrication costs for a core loading may be lower because fewer elements of larger diameter are required. 

In recent years a wide variety of inorganic, non-metallic materials has been developed for the electrical, nuclear power, and engineering industries. In the shaping and processing of these products some form of heat treatment is involved, and they too are regarded as ceramic materials. Examples are rutile, a form of titanium dioxide used for making ferroelectric materials steatite or talc, for electrical insulators alumina, zirconia, thoria and beryllia as refractories and electrical insulators, uranium oxide as a nuclear-fuel element, and nitrides and carbides as abrasives or insulators. 

According to the thermodynamic calculation, it is possible to make a conclusion that all presented SHS-Az systems can be self-igniting. Moreover, combusting temperatures and reaction heat are sufficient for base products formation. The presence of the active nitrogen and fuel elements without oxide layer is a supposition for the positive result of nitride synthesis by azide technology. At that, formation reactions of some nitrides are carried out in the gas-vapor phase, that allows to get nano-sized nitride powders. 

Boron-carbon-nitride ceramic is deposited on iron-based sliding parts by chemical vapor deposition (CVD) it is used as rotary compressor shafts, in order to improve the wear resistance [1 to 5]. Such B-C-N coatings have also been applied to dynamic pressure air bearings [6]. In gas-cooled nuclear reactors, °B-enriched B-C-N can be deposited by CVD in the fluid channels of the fuel elements for permanent deactivation in case of an emergency [7]. Radiofrequency or microwave-enhanced CVD is employed in order to deposit a diamond carbon and (3-BN superlattice structure [8]. 

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. 

The use of nitride allows the possibility of pure plutonium fuel without uranium diluent. The thermal rating of such fuel would be very high, which means either that some other diluent material must be found, or the design of the fuel elements must be altered radically. Whichever direction is chosen the design will be different from the familiar pins... 

Operational validation of a core employing fuel elements with nitride and metallic fuels... 

Mastering fabrication of the nitride and metallic fuel and fuel elements... 

Fuel type Cylindrical fuel elements with fuel pellets made of uranium-plutonium oxide, nitride or metallic fuel in steel claddings ... 

Validation of operability for a core based on fuel elements with innovative nitride and metallic fuel. 

Schematic views of the typical core configurations for SFRs are shown in Fig. 5.1. The core consists of core fuel, control rods, blanket fuel, and shields. In general, the core fuel is a mixture of plutonium and depleted uranium. The blanket fuel is depleted uranium. The chemical forms of fuel element, close to its final stage of development, are oxide and metal (U-Pu-Zr alloy). Nitride fuel is also available. The neutron absorber used in control rods is boron carbide (B4C).

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. 

For a large number of applications involving ceramic materials, electrical conduction behavior is dorninant. In certain oxides, borides (see Boron compounds), nitrides (qv), and carbides (qv), metallic or fast ionic conduction may occur, making these materials useful in thick-film pastes, in fuel cell apphcations (see Fuel cells), or as electrodes for use over a wide temperature range. Superconductivity is also found in special ceramic oxides, and these materials are undergoing intensive research. Other classes of ceramic materials may behave as semiconductors (qv). These materials are used in many specialized apphcations including resistance heating elements and in devices such as rectifiers, photocells, varistors, and thermistors. 

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. 

Molten-Tin Process for Reactor Fuels (16). Liquid tin is being evaluated as a reaction medium for the processing of thorium- and uranium-based oxide, carbide, and metal fuels. The process is based on the carbothermic reduction of UO2 > nitriding of uranium and fission product elements, and a mechanical separation of the actinide nitrides from the molten tin. Volatile fission products can be removed during the head-end steps and by distilling off a small portion of the tin. The heavier actinide nitrides are expected to sink to the bottom of the tin bath. Lighter fission product nitrides should float to the top. Other fission products may remain in solution or form compounds with... 

---