Critical-Assembly Experiments on a Reflector Control System for a Boiling Reactor

Research directed toward an evaluation of a reflector control scheme for a small boiling heterogeneous reactor is reported. The control method uses the ambient reactor steam pressure to adjust the height of a radial water reflector. Critical-assembly experiments were conducted to measure the reflector worth and the effect of nonhomogeneous voids. The critical-assembly core approximated a right circular cylinder 24 in. high and 21 in. in diameter. The core was made up of polyethylene strips to simulate water channels, aluminum strips, and uranium foil. The reflector consisted of polyethylene blocks and two different densities were used to simulate two different water temperatures. The coldwater, hot non- power-producing, and boiling powerproducing reactor conditions were mocked-up. These conditions were obtained by voiding the moderator plastic strips. The results indicate that sufficient reactivity is available in the reflector to compensate for large power variations and equilibrium poisons. However, the reactivity worth is not great enough in the system considered to include startup from the cold condition or long-time fuel burnup. These experiments also show that in calculatioiis of critical mass and reflector worth the assumption of uniform void distributions seems valid for corcs of the size studied. (auth