The APRFR is an advanced version of the Health Physics Research Reactor (HPRR) at Oak Ridge National Laboratory (ORNL), which has been operating since 1962. ORNL has played a key role in the design and testing of the APRFR. In pulse operation, the power level may rise on periods as short as 10 microseconds. Electro-mechanical scram systems are too slow to terminate such an excursion. Shutdown results from increased neutron leakage due to fuel expansion, resulting in a large prompt negative temperature coefficient of reactivity. This self-limiting feature depends almost entirely on the thermal expansion of the fuel alloy, and thus it is regarded as completely reliable and safe. Following a pulse, additional reactor shutdown capability is provided by a safety block which, when ejected from the core, reduces the reactivity to about 20 dollars below delayed-critical. At lower yield pulses, below about 6 × 10¹⁶ fissions, the safety block is ejected by the electro-mechanical scram system in about 0.1 seconds after a pulse. At higher yield pulses, the safety block is ejected in much shorter times due to thermo-mechanical shock forces which cause the safety block to bounce out. The large shutdown margin provided by the safety block is also the primary design device for preventing accidental criticalities during periods of reactor shutdown. The APRFR core is an unmoderated cylindrical assembly containing about 125 kilograms of an alloy of uranium 235 containing 10% molybdenum. The actual core mass varies with the experiment. The core is cylindrical and consists of two concentric annuli: a fixed outer shell of stacked fuel discs bolted together with nine fuel bolts and Inconel nuts and a movable inner safety block, also of fuel alloy. The 1½-inch OD “glory hole” runs vertically through the center of the safety block. Key reactor data is summarized in Tables I and II. The APRFR has been operated during tests at ORNL at more than twice its design yield. Table I. APRFR Core Design Data Core Diameter 8.90 inches Core Height[1] 8.0 inches Fuel Alloy 90 wt % uranium - 10 wt % molybdenum Uranium-235 Enrichment 93.14% Total Fuel Mass[2] 125 Safety Block Mass 15.7 kg Safety Block Height 8.06 inches Safety Block Diameter 4.00 inches Glory Hole Diameter 1.50 inches Number of Control Rods Three Core Cooling Forced Air Number of Core Bolts Nine Safety Block Reactivity Worth ~$20 Pulse Rod Reactivity Worth ~$1.15 Core Environment During Pulse Dry Nitrogen Core Cooling Forced Air [1] This value varies with experimental environment of core.