WASHINGTON, May 15 (Yonhap) -- North Korea may conduct a second nuclear test in the near future with weapons-grade plutonium produced from restarting its nuclear facilities, a U.S. expert said Friday."Although it must have been tempted to conduct a second test after the limited success in October in 2006, North Korea has been constrained by its meager plutonium inventory and by the threat of international sanctions," said Siegfried Hecker, a professor at Stanford University, in an article in the Bulletin of the Atomic Scientists.