Fabrication of uranium oxycarbide kernels and compacts for HTR fuel

As part of the program to demonstrate tristructural isotropic (TRISO)-coated fuel for the Next Generation Nuclear Plant (NGNP), Advanced Gas Reactor (AGR) fuel is being irradiation tested in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL). This testing has led to improved kernel fabrication techniques, the formation of TRISO fuel particles, and upgrades to the overcoating, compaction and heat treatment processes. Combined, these improvements provide a fuel manufacturing process that meets the stringent requirements associated with testing in the AGR experimentation program. Researchers at INL are working in conjunction with a team from Babcock and Wilcox (B&W) and Oak Ridge National Laboratory (ORNL) to (a) improve the quality of uranium oxycarbide (UCO) fuel kernels, (b) deposit TRISO layers to produce a fuel that meets or exceeds the standard developed by German researches in the 1980s, and (c) develop a process to overcoat TRISO particles with the same matrix material, but apply it with water using equipment previously and successfully employed in the pharmaceutical industry. A primary goal of this work is to simplify the process, making it more robust and repeatable while relying less on operator technique than prior overcoating efforts. A secondary goal is to improve first-pass yields to greater than 95% through the use of established technology and equipment. first test, called “AGR-1,” graphite compacts containing approximately 300,000 coated particles were irradiated from December 2006 to November 2009. The AGR-1 fuel was designed to closely replicate many of the properties of German TRISO-coated particles thought to be important for good fuel performance. No release of gaseous fission product, indicative of particle coating failure, was detected in the nearly 3-year irradiation to a peak burn up of 19.6% at a time-average temperature of 1038–1121 °C. fabricating AGR-2 fuel, each fabrication process was improved and changed. Changes to the kernel fabrication process included replacing the carbon-black powder feed with a surface-modified carbon slurry and shortening the sintering schedule. AGR-2 TRISO particles were produced in a 6-inch diameter coater using a charge size about 21-times that of the 2-inch diameter coater used to coat AGR-1 particles. The compacting process was changed to increase matrix density and throughput by increasing the temperature and pressure of pressing and using a different type of press. AGR-2 fuel began irradiation in the ATR in late spring 2010. gh quality UCO kernels used in the AGR fuel tests at ATR were produced by B&W. Fuel for the AGR-1 test (350 μm, 19.7% 235U-enriched UCO kernels) was produced in 2005. Fuel for the AGR-2 test (425-μm, 14% enriched UCO kernels and 500-μm, 9.6% enriched UO2 kernels) was produced in 2008. Fuel of the same size and enrichment as AGR-1 kernels were produced for the AGR-3/4 experiment, yet to be irradiated. B&W has also produced more than 100 kg of natural uranium UCO kernels, which are being used in coating development tests. Successive kernel lots also demonstrate consistent high quality and allow for fabrication process improvements. Improvements in kernel forming were made subsequent to AGR-1 kernel production. Following fabrication of AGR-2 kernels, incremental increases in sintering furnace charge size have been demonstrated. Recently, small-scale sintering tests using a small development furnace equipped with a residual gas analyzer (RGA) have increased understanding of how kernel sintering parameters affect sintered kernel properties. The steps taken to increase throughput and process knowledge have reduced kernel production costs. Other modifications have been studied to increase the current fabrication line capacity for producing first core fuel for the NGNP and to provide a basis for the design of a full-scale fuel fabrication facility. d B&W are evaluating an alternative hot-press compaction methodology to yield higher matrix densities, generate less waste, and increase compaction rates without any compromise in compact quality. A variety of process approaches are available and have been historically used to manufacture cylindrical fuel compacts. Jet milling, fluid-bed overcoating, and hot-press compacting approaches being adopted by the U.S. Advanced Gas Reactor Fuel Development Program for scale-up of the compacting process involve significant paradigm shifts from historical approaches. These new methods are being pursued simply to increase yields and eliminate process mixed waste. Recent advances in jet-milling technology simplify dry matrix powder preparation. The matrix preparation method is well matched with patented fluid-bed powder overcoating technology, recently developed for the pharmaceutical industry and directly usable for overcoating high-density fuel-particle-matrix. High-density overcoating places fuel particles as close as possible to their final position in the compact and is matched with hot-press compacting, which fully fluidizes matrix resin to achieve die fill at low compacting pressures and without matrix end caps. Overall, the revised methodology provides a simpler process that should provide very high yields, improve homogeneity, further reduce defect fractions, eliminate intermediate grading and quality control steps, and allow further increases in fuel packing fractions. The compacting process for AGR fuel is being scaled-up from a laboratory process to an engineering-scale process ready for replication to meet production requirements. The scale-up effort was started after a year of planning and evaluation. It continues today on the baseline premise of designing a safe, efficient, and cost effective compacting system and adjusting graphite and resin material characteristics as needed to achieve all specification requirements, high yields, and an optimum product within that design. At this writing, the scale-up effort is 20 months into the execution phase. The B&W compacting facility was readied, and the required scale-up process equipment purchased, factory tested, and received. Its installation is nearing completion. By the end of March 2011, overcoating of natural uranium oxycarbide (NUCO) particles for process finalization testing can begin. The fuel qualification test of AGR-5/6 compacts is scheduled in approximately 23 months, so finalizing the process is gaining urgency. Process engineering work on unit operations for compacting has streamlined the fabrication approach, eliminated waste management issues, and allowed specification, design, and purchase of engineering-scale equipment appropriate for efficient compact manufacturing. A set of process-detail surrogate tests were initiated in the summer of 2010 to determine the components and process parameters appropriate for production compacting.