According to the report:
A Japanese foreign ministry official said the two countries had been discussing the transfer for some time as part of efforts to resolve concerns over Japan’s large stockpile of spent nuclear fuel and plutonium. The U.S. and Japan also are discussing ways to reduce the quantity and toxicity of the radioactive material, the official said.
The material designated for transfer to the U.S. has been kept for decades at a research reactor site in Tokaimura, the site of a 1999 accident that killed two workers who mishandled a highly enriched uranium solution. More than 300 people were believed to have been exposed to radiation exceeding the annual limit after a nuclear chain reaction that lasted for 20 hours, leaking radioactive gasses out of the complex.