b'specimen holder with various thermalwork together to yield an irradiation neutron filtering materials (cadmiummethod far more representative of and europium options were investi- true fast reactors. The europium oxide gated). A typical sodium fast reactorand cadmium filter options were both type seven-pin hexagonal arrangementshown to effective at hardening the of U-10Zr metallic fuel was used asspectrum, with europium performing the specimen bundle. slightly better, as shown in Figure 2. These predictions showed that theVarious thermal finite element calcula-total power output and surface heattions were also performed near the flux from the BR2 booster assemblycompletion of this project to deter-would be well within both ATRmine the best method for managing thermal hydraulics safety limits andfuel pin temperatures. Several concepts the qualified performance envelopewere investigated and two emerged for the booster fuel. Most impor- as viable candidates in achieving tantly, predictions of various designdesired conditions. The first method candidates showed that the fuel pinsused a seven-hole molybdenum insert could achieve fission rates typicalwithin a capsule with thin liquid of a sodium fast reactor in neutronsodium thermal bonds between clad-flux spectra with fast ( 100 keV) toding and capsule surface gaps.This thermal ( 0.625 eV) ratios rangingmethod minimized moderator and from 50150. This would be ayielded the highest fast-to-thermal marked improvement compared toneutron flux ratios (~100-150) but the same ratio of ~20 seen in theshowed modest temperature gradients current state-of-the-art method wheretransversally across the fuel pins only thermal flux filtering is used.which could affect fuel constituent This finding is important and showsredistribution in and non-prototypic that ARCTIC will increase claddingway. This concept was hence deter-atom displacement damage, reducemined to be the most favorable for thermal neutron transmutation effectstests designed to investigate cladding in cladding, and flatten radial powerperformance in environments with the peaking in the fuel. All these effects 130 2021|AFC ACCOMPLISHMENTS'