Flattop (critical assembly)
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 neutron reflector. Flattop assemblies are used to measure neutron activation and reactivity coefficients.
Source: Wikipedia — Flattop (critical assembly) (CC BY-SA 4.0)