What do nuclear scientists call a stick of uranium?

They call it "rod" or "fission rod". This is because the uranium is the element used for the nuclear fission, where the uranium nuclei disintegrate producing lighter nuclei and energy.

Uranium is material that has 92 protons and 92 electrons with a valence of 6. The chemical is highly used in the nuclear fission for creating nuclear weapons and ends up as radioactive waste which got its own issues of disposal. As it is used in nuclear reactor, nuclear scientist calls the stick of uranium as a fission rod.