GCRE critical-assembly studies

Critical-assembly studies were made to provide engineering and physics data to aid in developing the Gas Cooled Reactor Experiment-1 (GCRE-l). Measurements of critical mass, flux and power distributions, and shutdown worth of the GCRE-1 mock-up safety and control blades were obtained. The critical assembly consists of aluminum tubes containing four concentric stainless steel cylinders wrapped wtth highly enriched uranium foil. These tubes are supported at each end by grid plates aiid arranged to approximate a right circular cylinder. The entire core structure is supported within a tank which can be filled remotely with moderator water. A void beneath the core structure and the air above it represent the gas plenums of the GCRE-1 reactor. Critical mass and flux power distributions were determined for several cases of four basic cores. The thermal utilization, measured in two core configurations, was 0.780 for the reactor wtthout burnable poison or a lead reflector. The temperature coefficient of reactivity, measured in two cores, was positive at room temperature but negative in the proposed operatingtemperature range. The total reactivity effect in going from 20 to 80 deg C was a positive 0.22% DELTA k/k. The worth of control and safety-shutdown blades of several compositions and sizes in various locations was determined for a number of core configurations; blades having a sandwich construction of cadmium, tungsten, and indium had the greatest shutdown worth. To date these studies have resulted in a 61-element core (critical with 54 elements) with a 4-in. lead reflector which has 2.5% DELTA k/k excess reactivity for startup and for override of poison and burnup. A mock-up blade configuration has been determined which will control the excess reactivity introduced by flooding the elements with mineral oil (to simulate water flooding when fuel elements are changed in GCRE-1). (auth)