Carbide Kernels Research Articles

Production of 28 μm zirconium carbide kernels using the internal gelation process and microfluidics

Studies using internal gelation and microfluidics have shown that the production of sintered spheres with diameters below 200 μm is possible. The addition of carbon can be problematic because of slow flow rates (100 μL/min) and long run times (4 h). However, a solution of dispersed carbon, Cabot’s TPX-101, was successfully used with solutions of zirconyl nitrate, urea, and hexamethylenetetramine to make zirconium microspheres with carbon. A simple carbothermic reduction using a maximum temperature of 2073 K and ultrahigh purity argon produced zirconium carbide kernels with an average diameter of 28 μm and a standard deviation of 2.3 μm.

Pyrolytic Carbon Coating Effects on Oxide and Carbide Kernels Intended for Nuclear Fuel Applications

The coating of nuclear fuel kernels with pyrolytic carbon (PyC) is a well-understood practice dating back over half a century. In spite of decades of studies related to these coatings, no study has yet investigated the effect of the PyC deposition coating process on the kernels themselves. In this study, the composition and crystallographic phase of kernel materials were observed to change after exposure to the thermal and chemical environment of the PyC coating process. Specifically, the coating process increased the fraction of high carbon content phase within carbide microsphere kernels, with W2C containing microspheres driven toward WC, and UC containing microspheres driven toward UC2. Oxide microspheres consisted of a mixture of two crystalline phases. The monoclinic phase within yttria-stabilized zirconia microspheres was eliminated by the coating process resulting in a purely tetragonal phase. Hafnium oxide microspheres were more stable showing no detectable change in composition or crystal structure after coating.

Ceramography of irradiated TRISO fuel from the AGR-2 experiment

Ceramography was performed on cross sections from four tristructural isotropic (TRISO) coated particle fuel compacts selected from the AGR-2 experiment, which was irradiated between June 2010 and October 2013 in the Advanced Test Reactor (ATR). The individual fuel specimens examined in this study contained coated particles with either uranium oxide (UO2) kernels or uranium oxide/uranium carbide (UCO) kernels that were irradiated to final burnup values between 9.0 and 11.5% FIMA. These examinations were intended to explore kernel and coating morphology evolution during irradiation. This included observations of kernel porosity, kernel swelling, and irradiation-induced TRISO coating layer fracture and separation. Variations in behavior within a specific cross section, which could be related to temperature or burnup gradients within the fuel compact, were also explored. Results were compared with similar investigations performed as part of the earlier AGR-1 irradiation experiment. The criteria for categorizing AGR-1 particle post-irradiation morphologies during ceramographic exams were applied to the AGR-2 compact particles examined. This paper presents the results of the AGR-2 examinations and discusses the key implications for fuel irradiation performance.

Carbothermic synthesis of 820 μm uranium nitride kernels: Literature review, thermodynamics, analysis, and related experiments

The US Department of Energy is developing a new nuclear fuel that would be less susceptible to ruptures during a loss-of-coolant accident. The fuel would consist of tristructural isotropic coated particles with uranium nitride (UN) kernels with diameters near 825μm. This effort explores factors involved in the conversion of uranium oxide–carbon microspheres into UN kernels. An analysis of previous studies with sufficient experimental details is provided. Thermodynamic calculations were made to predict pressures of carbon monoxide and other relevant gases for several reactions that can be involved in the conversion of uranium oxides and carbides into UN. Uranium oxide–carbon microspheres were heated in a microbalance with an attached mass spectrometer to determine details of calcining and carbothermic conversion in argon, nitrogen, and vacuum. A model was derived from experiments on the vacuum conversion to uranium oxide–carbide kernels. UN-containing kernels were fabricated using this vacuum conversion as part of the overall process. Carbonitride kernels of ∼89% of theoretical density were produced along with several observations concerning the different stages of the process.

Improvements in the fabrication of HTR fuel elements

The application of High Temperature Reactor (HTR) Technology in the course of the continuously increasing world wide demand on energy is taken more and more under serious consideration in the power supply strategy of various countries. Especially for the emerging nations the HTR Technology has become of special interest because of its inherent safety feature and due to the alternative possibilities of applications, e.g. in the production of liquid hydrocarbons or the alternative application in H2 generation.The HTR fuel in its various forms (spheres or prismatic fuel blocks) is based on small fuel kernels of about 500μm in diameter. Each of these uranium oxide or carbide kernels are coated with several layers of pyrocarbon (PyC) as well as an additional silicon carbide (SiC) layer. While the inner pyrocarbon layer is porous and capable to absorb gaseous fission products, the dense outer PyC layer forms the barrier against fission product release. The SiC layer improves the mechanical strengths of this barrier and considerably increases the retention capacity for solid fission products that tent to diffuse at these temperatures. Especially the high quality German LEU TRISO spherical fuel based on the NUKEM design, has demonstrated the best fission product release rate, particular at high temperatures. The ∼10% enriched uranium triple-coated particles are embedded in a moulded graphite sphere. A fuel sphere consists of approximately 9g of uranium (some 15,000 particles) and has a diameter of 60mm.As the unique safety features, especially the inherent safety of the HTR is based on the fuel design, this paper shall reflect the complexity but also developments and economical aspects of the fabrication processes for HTR fuel elements.

The addition of silicon carbide to surrogate nuclear fuel kernels made by the internal gelation process

The US Department of Energy plans to use the internal gelation process to make tristructural isotropic (TRISO)-coated transuranic (TRU) fuel particles. The focus of this work is to develop TRU fuel kernels with high crush strengths, good ellipticity, and adequately dispersed silicon carbide (SiC). The submicron SiC particles in the TRU kernels are to serve as getters for excess oxygen and to potentially sequester palladium, rhodium, and ruthenium, which could damage the coatings during irradiation. Zirconium oxide microspheres stabilized with yttrium were used as surrogates because zirconium and TRU microspheres from the internal gelation process are amorphous and encounter similar processing problems. The hardness of SiC required modifications to the experimental system that was used to make uranium carbide kernels. Suitable processing conditions and equipment changes were identified so that the SiC could be homogeneously dispersed in gel spheres for subsequent calcination into strong spherical kernels.