Uraninite is an ore added by Atomic Science. (It is also called Uranium).
Uranium Ore is found underground in layer 20 and below, and can spawn in veins of 1-4. Be careful, if you aren't wearing a Hazmat Suit you will receive radiation poisoning if you step on the ore.
Note: the Uraninite Ore/Uranium Ore is turned off by default, so your world will will not generate this ore. To turn it on, you have to edit the config file, AtomicScience.cfg, located by default in C:\Users\User\AppData\Roaming\.technic\modpacks\bigdig. Line 71 should be changed to "true" instead of "false" (B:"Generate Uranium Ore"=true). However this does not seem to effect the ore actually spawning in the world.