Preliminary design of I-loops in ATR for irradiation testing of LWR fuels

ATR's existing LWR loop in the center flux trap provides ample flux for both prototypic and accelerated burnup accumulation in approximately 30 test rodlets, a handful of which can be comprehensively instrumented, this loop alone cannot satisfy the capabilities left by the Halden Boiling Water Reactor's (HBWR) closure. Additional LWR loops, which are not subject to some of the constraints present in flux trap-based loops, will be needed in order to address Halden capability gaps using ATR. Anticipated testing will be performed in two separate loops conditions, BWR and PWR, running base irradiations and ramp testing with instrumented specimens. While useful for other capsule-based and instrumented-lead type experiments, positions within ATR's neck shim housing and inner reflector have less desirable useable diameters to implement LWR loops. Only the Large and Medium I-positions, which reside outside of the reactivity control cylinders in the outer beryllium reflector, have adequate volume for LWR loop installation because of high availability and a neutron flux similar to the HBWR. A typical I-Loop is capable of achieving prototypic PWR fuel rod heating rates. Each I-Loop enables a 2x2 rodlet array (for a total of up to quantity 16 at 30cm long rodlet across the ATR 1.2m active core.) An I-Loop will be installed so that test train extraction and instrument leads route through the top closure plate while permanent plumbing will penetrate through the side of the reactor pressure vessel in existing side flange penetrations in a manner typical for many successful lead-out type experiments performed at ATR. The slight offset (~20cm offset over ~6m length) of these in-pile tubes will require that test trains are designed with some compliance to facilitate insertion and extraction. Preliminary design and safety evaluations have been performed and show that this effort is a viable strategy to address LWR fuel irradiation testing capability gaps left by the HBWR closure.