The United States and Japan are reported to have reached an agreement to return plutonium that is used in the Fast Critical Assembly (FCA) in JAERI Tokai Research Establishment, Tokai-mura, Ibaraki Prefecture. The formal agreement is expected to be concluded at the Nuclear Security Summit in the Netherlands in March 2014.

According to a DoE document obtained by Paul Leventhal, the FCA facility stockpile initially consisted of 291.4 kg of "fissile plutonium" (Pu-239 + Pu-241), of which 231.0 kg was in Pu-92%, 25 kg - in Pu-81%, and 35.4 kg - in Pu-75%. This amounts to the total of about 327.7 kg of plutonium. In its 2012 plutonium report, Japan stated that the FCA facility has the total of 331 kg of plutonium, of which 293 kg is fissile plutonium.

A substantial fraction of plutonium for FCA was supplied by the United Kingdom (although it is possible that some of the plutonium was included in the barter exchange under the Mutual Defense Agreement between the United States and the United Kingdom). Accordingly, the United States is seeking authorization from the United Kingdom to take custody of the material.

In addition to plutonium, the initial FCA stockpile included 199.5 kg of U-235 in HEU enriched to 93% (214.5 kg of HEU) and 349.9 kg of U-235 in 20% enriched uranium (1749.5 kg of material). All 93% HEU was supplied by the United Kingdom, all 20% uranium by the United States.