ATR’s existing LWR loop in the center flux trapprovides ample flux for both prototypic and acceleratedburnup accumulation in approximately 30 test rodlets, ahandful of which can be comprehensively instrumented,this loop alone cannot satisfy the capabilities left by theHalden Boiling Water Reactor’s (HBWR) closure.Additional LWR loops, which are not subject to some ofthe constraints present in flux trap-based loops, will beneeded in order to address Halden capability gaps usingATR. Anticipated testing will be performed in twoseparate loops conditions, BWR and PWR, running baseirradiations and ramp testing with instrumentedspecimens. While useful for other capsule-based andinstrumented-lead type experiments, positions withinATR’s neck shim housing and inner reflector have lessdesirable useable diameters to implement LWR loops.Only the Large and Medium I-positions, which resideoutside of the reactivity control cylinders in the outerberyllium reflector, have adequate volume for LWR loopinstallation because of high availability and a neutronflux similar to the HBWR. A typical I-Loop is capable ofachieving prototypic PWR fuel rod heating rates. Each ILoopenables a 2x2 rodlet array (for a total of up toquantity 16 at 30cm long rodlet across the ATR 1.2mactive core.) An I-Loop will be installed so that test trainextraction and instrument leads route through the topclosure plate while permanent plumbing will penetratethrough the side of the reactor pressure vessel in existingside flange penetrations in a manner typical for manysuccessful lead-out type experiments performed at ATR.The slight offset (~20cm offset over ~6m length) of thesein-pile tubes will require that test trains are designedwith some compliance to facilitate insertion andextraction. Preliminary design and safety evaluationshave been performed and show that this effort is a viablestrategy to address LWR fuel irradiation testingcapability gaps left by the HBWR closure.
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