b'heating rate comparison of a fuel rod in HBWR and a PWR vs. in the ATR I-Loop position is shown in Table 1. Early results show that the I-Loop is capable of matching and even exceeding a HWBR power rate.An annular loop layout using nuclear grade Zr-2.5Nb alloy (which has been used extensively in high fluence pressurized water conditions in CANDU reactors) works to accom-plish this aim in the I-Loop design. Figure 4.Modified ATR top headThermal neutron flux in Medium-I closure plate. positions are slightly higher than the Large-I positions at 41013 n/cm2s during typical ~50 day ATR cycle in the Large-I position. The holderpowers (five lobes each at ~25 MW is composed of aluminum structurallobe power, ~125MW total core materials with the inner filler ofpower); making medium-I positions beryllium for neutron reflection andthe preferred location for I-Loops. moderation. Each I-Loop enables a 22 rodlet Figure 3. I-Loop reactor pressurearray (giving up to 16 total 30 cm vessel layout. Monte Carlo investigations have been performed and have demonstratedrodlets across ATR 1.2 m active core that inclusion of a standard ATR driverlength per loop). The I-Loop design is fuel assembly in a Large-I positionalso compatible with other test train can increase neutron population inconfigurations such as a cross section the outer reflector for a 10% thermalwith two or three individual rodlets neutron flux increase in the fuelin discrete flow tubes for varying specimen. This novel, yet straightfor- thermal hydraulic conditions within wardly achievable, approach to ATRa single test assembly, or a single core management can elevate thermalrodlet cross section to reduce rod-neutron flux to the same level avail- to-rod self-shielding for increased able in the HBWR at 513 n/cm2s [3]nuclear heating with additional during routine ATR cycles. Averagevolume for instrumentation.Table 1. Reactor heating rate comparison.Reactor PWR HBRW ATR I-LoopkW/m ~20 ~15 ~18kW/ft ~6 ~4.5 ~5.5128 2019|AFC ACCOMPLISHMENTS'