b'Figure 4. Change in thermal neutron flux due to the presence of gadolinium-lined capsules in HFIRs central flux trapis expected that fueled capsules willlinium-lined capsules were modeled probably avoid the irregular powerin HFIR which showed a very and temperature history they wouldgood fast flux environment in the experience in this part of the reactor.capsules, but also showed that one Materials test capsules, however,should expect a modest reduction in should be less sensitive to thesecycle length and a fairly significant changes and could benefit from thedecrease in thermal flux to HFIRs increased flux. The spectral environ- numerous isotope targets. Thus, ment was predicted for cadmiumuse of thermal spectrum filtering in baskets in these positions and foundHFIR probably creates an untenable to be respectable. This study alsoimpact on other users. HFIR replaces showed the viability of modestlyits entire driver fuel load every cycle increasing the uranium loadingand cannot shuffle or arrange the in ATRs driver fuel, but still wellcore differently to compensate for within the range shown viable inthese changes. The key conclusion historic tests, to enable HTSS cyclesis that HFIR offers the best fast flux to operate longer and reduce reactorenvironment available, but only for downtime for refueling outages. small specimens made from mate-Other calculations were performedrials not overly sensitive to very high to evaluate HFIRs flux trap. HFIRsthermal flux.innate fast flux exceeds even that ofReferences:BEAST, but the challenge lies with[1.]N. Woolstenhulme, J. Brookman, taming its extremely high fast fluxC. Jesse, C. Downey, C. Murdock, since many fast reactor specimenScoping Study for Fast Flux Test-materials can be sensitive to spuriousing in the Advanced Test Reactor, effects from thermal neutron captureApril 2023, INL/RPT-23-72212.and transmutation. Small gado-Location Thermal Flux Fast Flux Fast-to-Thermal (0.625 eV) (n/cm2-s) (0.1 MeV) (n/cm2-s) RatioATR BEAST 1.39E+13 7.46E+14 53.7ATR Outboard-A in1.47E+13 5.55E+14 45.6Optimized HTSS CyclesHFIR Central Flux Trap2.82E+13 1.17E+15 41.6with Gd Capsule212 2023|AFC ACCOMPLISHMENTS'