2017 | AFC ACCOMPLISHMENTS 194 The testing of fast reactor fuels in the thermal spectrum of the AdvancedTest Reactor using cadmium-filtering has been underway since 2003.The objective of this experimental approach is to create a thermal profile inside of test fuel rodlets that is nearly prototypic of corresponding fast reactor conditions. By doing so, this experimental approach should allow for the study of fast reactor fuel performance phenomena that depend primarily on the conditions of temperature and/or temperature gradient inside the fuel to be possible using a thermal test reactor. Project Description: Validation of the experimental approach has been undertaken by comparing fuel performance phenomena observed in fuels irradiated in cadmium-filtered positions in ATR to similar or identical fuels irradiated under similar conditions in genuine fast reactors. The results of these comparisons and associated analyses were documented in a Comparison Report. Accomplishments: The Comparison Report documents a preliminary comparison of limited fuel performance data available from the testing of metallic, oxide, and nitride fuels in cadmium-filtered positions in the ATR to the performance of identical or similar fuels irradiated in genuine fast reactors.Although a variety of fuel performance metrics were presented, particular attention was paid to those Comparison Report on Fast vs. Filtered-Thermal Spectrum Irradiations Principal Investigator: Jason Harp Collaborators: Steven Hayes, Pavel Medvedev, Douglas Porter, Luca Capriotti positions in ATR as part of the AFC-1 test series were compared with similar fuels irradiated in genuine fast reactors. Results from neutron radiographs that imaged the general conditions of the fuel columns after irradiation revealed good qualitative agreement. Results from cladding profilometry agreed in no measurable diametral strain. Results from gamma ray spectrometry and tomography revealed no essential differences, and appear to confirm that the transport of important fission products like cesium and fuel constituents such as zirconium behave the same in both test environments. Results from fission gas/helium release measurements, when acknowledging the considerable variation observed in this phenomenon even from identical pins irradiated under identical conditions, show very good agreement. And finally, results from burnup analyses show good agreement under both test environments. phenomena that are primarily, or significantly, dependent on conditions of temperature and temperature gradient with the fuel. Comparisons show how formation of a columnar grain region in oxide fuels is impacted by the radial power distribution in the fuel. In an unfiltered ATR neutron spectrum, significant power is shifted to the fuel pellet periphery, requiring a much higher power to achieve formation of the columnar grain region. Use of cadmium-filtering results in removal of the vast majority of thermal neutrons seen by the experimental test fuels, which tends to level out the radial power distribution similar to the essentially flat radial power distribution that would exist in a fast reactor.This effect was demonstrated by modeling formation of the columnar grain region in both cadmium filtered ATR and typical fast reactor neutron spectra, which confirmed the similarity of oxide fuel behavior in these two conditions. Additionally, data obtained from postirradiation examinations performed on metallic and nitride fuels irradiated in cadmium-filtered According to one of the external peer reviewers of this work, “the data comparisons and analysis show that the ATR irradiations with the Cd shroud system is sufficiently prototypic that it can be used to develop fast reactor fuels with confidence.”