The transition region between short, but controlled, pile periods and uncontrolled, highly-supercritical conditions has been investigated with an unreflected, spherical uranium assembly (Godiva). This study was undertaken because knowledge of the behavior of supercritical systems is important in evaluation of reactor hazards and also the hazards of less standard operations involving possibly critical quantities of active material.

A spherical, unreflected U-235 critical assembly (52.6 kg) has been in operation since August, 1951. A remotely controlled mechanical system is used to assemble subcritical components of the sphere, and reactivity is adjusted with U-235 control rods positioned in the sphere. The maximum power level during sustained operation is about 1 kilowatt. In addition to investigations of the neutron spectrum of the assembly, observation of the changes of reactivity produced by inserting foreign materials into the assembly, and the dletermination of parameters such as the temperature coefficient of reactivity, studies have been made of the behavior of the assembly at reactivities above prompt critical.