Uranium Ore is obtained by mining Uranium Ore (block). It can be compressed in a Compressor to obtain Refined Uranium, which can be further used in nuclear engineering. Refined Uranium can also be used for the creation of a Nuke.
Uranium is classified as "Fuel" in Equivalent Exchange so to transmute an item classified as "Matter" you must use an Energy Condenser or place a Klein Star in the left hand side of the Transmutation Tablet and place the uranium in the fire. This would lock it to fuel but if you remove the Klein Star then place it back you can choose an item classified as "Matter".
Uranium is almost always found in veins of one, but is occasionally found in veins of two to four. It can be found on all levels.