Critical Experiments with Concrete-Reflected Fast Test Reactor Fuel Pins in Water

Criticality experiments with Fast Test Reactor (FTR) fuel pin lattices in water and reflected with concrete have provided benchmark-type data for lattice pitches of 9.53, 12.63, 15.41, and 19.06 mm (water-to-fuel volume ratios of 3.34, 6.92,10.99, and 17.55, respectively). At these center-to-center fuel pin spacings, 554, 260, 191, and 152 fuel pins were required for criticality in rectangular lattices 28, 18, 18, and 14 fuel pins wide, respectively. This corresponds to a decrease of ∼7% from that previously observed in similar lattices with full water reflection. However, the FTR fuel pin design essentially limited the concrete reflection to only the four sides of each fuel lattice.