North Korea seems to be expanding its nuclear program at its Yongbyon nuclear site and very actively too.The U.S.-based Institute for Science and International Security said Wednesday, that satellite imagery from April and June shows there was an increase in activity surrounding a building known to store centrifuges, which are used to enrich uranium.This data suggests Pyongyang might be building more centrifuges inside the building to produce weapons-grade plutonium *and enriched uranium for its nuclear weapons program.The institute did add though that it wasn't sure whether the construction of the centrifuges had been completed.