The Nuclear Security Summit is ongoing this week at The Hague in Netherlands and one of the biggest successes is that Japan has agreed to allow the United States to take over a stockpile of weapons-grade nuclear material in Japan. This material is at the Fast Critical Assembly at the Japan Atomic Energy Agency and was used/produced in research for fast reactor development, not for nuclear weapons. However, the material is weapons-grade and thus its security is a concern.
This agreement with Japan says that the United States will remove and dispose of the nuclear material. Having a MOX facility to put weapons-grade plutonium into commercial fuel and burn it in a reactor is the best way to dispose of it because it becomes too radioactive to use in a weapons. What is the point of taking control of the nuclear material in Japan if we can't dispose of it?