Several advanced nuclear fuel concepts are being considered throughout the nuclear industry. Implementation of these concepts could increase accident tolerance or enhance performance beyond the capabilities of the UO2-based nuclear fuel currently in use. Qualification and deployment of any new fuel requires rigorous irradiation testing to demonstrate performance under representative normal and offnormal operating conditions. The traditional approach for qualifying new fuels requires exhaustive execution of many integral fuel tests. However, due to the long timeframe for executing these integral tests and the limited number of available materials test reactors, this approach is becoming impractical. To accelerate fuel qualification, Oak Ridge National Laboratory developed the MiniFuel irradiation vehicle for use in conducting accelerated separate effects irradiation testing of a wide range of nuclear fuel materials in the High Flux Isotope Reactor (HFIR). The first MiniFuel irradiations performed in the facility tested sol gel-derived uranium nitride kernels and tristructural isotropic (TRISO)-coated particle fuels. This report describes the preparation and assembly of the first set of monolithic MiniFuel irradiations conducted to support accident-tolerant fuel (ATF) development and accelerated fuel qualification. Two irradiation targets containing a variety of UO2 and U3Si2 disk fuel specimens were fabricated and assembled for irradiation to burnups of 8−10 and 28−40 MWd/kg U. The target irradiation temperature is 450−550°C. Irradiation of U3Si2 will provide new data regarding the irradiation performance of a candidate ATF to complement current ATF-1 integral experiments being performed in the Advanced Test Reactor. UO2 samples were included as a reference so that the results from the MiniFuel experiments can be compared with the extensive UO2 fuel performance database. The monolithic MiniFuel capsules were successfully assembled, welded, and tested per HFIR requirements and are ready for insertion into the reactor. Pictures of the assembly process are included in this report. The experiment is planned for insertion into the HFIR during cycle 487 in April 2020.
Within the Advanced Fuels Campaign, the Oak Ridge National Laboratory is looking into the fabrication of high-performance UO2 fuel candidates. The aim of this study was to develop a suitable fabrication avenue to synthesize UO2 feedstock with controlled dopants to enhance UO2 grain growth within these fuel candidates. Enlarged UO2 grains in a nuclear fuel pellet are expected to slow down fission gas diffusion, which would be a preferred property for a high-performance fuel compared to fission gas diffusion in standard UO2. The unique fabrication processes for microspherical feedstocks via the internal gelation approach at the Oak Ridge National Laboratory have been developed further to enable the dopant uptake within the UO2 microspheres. Mn and Cr were added via infiltration and/or addition of the dopant to the starting broth of an internal gelation experiment. The dopants were added in various amounts (ppm) into UO2. Even though large amounts of these dopants volatized during sintering, scanning electron microscopy images of the microstructure revealed large UO2 grains of up to 80 µm for the pellets that were prepared from doped microspherical feedstocks.
MiniFuel compacts containing 20 individual tristructural isotropic (TRISO)-coated fuel particles were characterized to provide supporting preirradiation characterization data. The preirradiated characterization data supports irradiation capsule design and safety analysis, as well as downstream postirradiation examination (PIE) for a planned MiniFuel irradiation to explore high particle powers. The analysis included dimensional inspection, x-ray radiography and tomography, as-fabricated defect fraction analysis, and a matrix impurity analysis.
The Oak Ridge National Laboratory (ORNL) is looking into the fabrication of high performance UO2 fuel candidates via the addition of dopants to the UO2. The addition of dopants is proposed to cause enlarged grains that will slow down fission gas release from the irradiated fuel pellet candidates. The Cr-doped UO2 pellets were fabricated from a microspherical feedstock, and enlarged grains were observed during the microstructural characterization. Irradiation under the Miniature Fuel program in the High Flux Isotope Reactor at ORNL will enable the study of irradiation response of such potential fuel candidates and will allow fission gas release benchmarking for these fuel candidates.
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