Like the title already suggest, a text overlay such as the Minecraft Clock has as shown in the below image but then with a geiger counter. ( I am not entirely sure if this is a vanilla feature or added by some mod of the modpack but I figured this could be modded into nuclearcraft in either cases )

Without knowing which mod does this, as someone who is not well-versed in MC rendering, I'm afraid that I don't think I am going to be able to recreate the effect :P