China on Friday urged Japan to explain its massive stockpile of nuclear material, which has allegedly exceeded the country's demand and might increase as Japan re-launches nuclear power plants. "It's not only the United States and Western society that have raised doubts and concern about Japan's nuclear stockpile, which has far exceeded its demand," Foreign Ministry spokesman Hong Lei said. Questions have also emerged from within Japan and from the international community, he said at a daily briefing. Hong made the remarks when asked to comment on Japan's nuclear stockpile and the capacity and security of Japan's Rokkasho Nuclear Fuel Reprocessing Facility, which is set to open this October. The new nuclear material factory, the fifth-largest of its kind in the world, allegedly has security risks. It will reportedly be able to produce 96 tons of plutonium metal in the next dozen years, far more than all stocks that remain in the United States as a legacy of the Cold War. "What if a massive amount of nuclear material is to be produced as Japan restarts its nuclear plants? Will it worsen the existing imbalance between its demand and supply of nuclear material?" Hong asked. "Such questions have not been addressed so far. Is there a hidden problem?" he added. Hong urged the Japanese government to face the concerns and explain in detail the imbalance of sensitive nuclear material in the country. The United States gave 331 kilograms of weapons-grade plutonium to Japan during the Cold War, Japan's Kyodo News Agency reported in late January. Japan holds another 44 tonnes of plutonium, which could be used for nuclear reactors.