WASHINGTON — Experts warn that Northeast Asia could see a dangerous growth in stocks of weapons-usable plutonium — and U.S. lawmakers say Obama administration policies could be making matters worse. Japan plans to open as early as next spring a plant that could reprocess enough spent reactor fuel to make as many as 1,000 nuclear bombs a year. The plutonium that is produced is supposed to be for generating electricity, but Japan already has tons on hand and no use for it, with its reactors at a virtual halt after the 2011 Fukushima Dai-ichi disaster. “We should not leave doors open for our partners to use U.S.