Two Chinese academic institutions, China Arms Control and Disarmament Association and China Institute of Nuclear Information and Economics released a report, "Study on Japan's Nuclear Materials" (local copy), that expresses concern about the amount of fissile materials in Japan.
The report estimated that "Japan has 47.8 tons of high sensitive separated plutonium, 10.8 tons of which are stored in Japan, enough to make 1350 nuclear warheads. In addition, Japan also has about 1.2 tons of highly-enriched uranium (HEU) for research reactors."
According to the report, "the large amount of nuclear materials stored by Japan and its imbalance between supply and demand constitute serious risks of nuclear proliferation, nuclear safety and nuclear terrorism." The authors suggest that
Japan [...] should take measures to properly solve the problem of the current nuclear materials inventory, such as reducing the growth of nuclear materials, and realizing the balance between supply and demand. Japan should also enhance the transparency of nuclear materials management, strengthen the safety and security of its nuclear facilities and voluntarily put them under stricter safeguards by the IAEA.