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Flattop is a benchmark critical assembly that is used to study the nuclear characteristics of uranium-233, uranium-235, and plutonium-239 in spherical geometries surrounded by a relatively thick natural uranium reflector.
Flattop assemblies are used to measure neutron activation and reactivity coefficients. Since the neutron energies gradually decrease in the reflector, experiments may be run in various energy spectra based on the location in which they are placed.
The experiment was originally located at the Los Alamos National Laboratory Critical Experiments Facility (LACEF), but was moved to the National Criticality Experiments Research Center (NCERC) at the Nevada National Security Site, along with three other critical assemblies, Comet, Planet, and Godiva-IV. In 2012, these facilities were operational.
The U-233 core is no longer usable because of its high gamma-ray activity.
Controls
Flattop consists of a hemispherical fixed reflector and two movable quarter-spheres of reflector that can close down on the central core. One movable reflector is controlled by hydraulic pressure, while the other is actuated by a motor.