Two ships carrying uranium-plutonium oxide fuel for the Takahama nuclear power plant in Fukui Prefecture, will arrive on June 27 — the first batch of MOX to be sent to Japan since the Fukushima disaster in March 2011.

The controversial fuel is destined for reactor 3 at Kansai Electric Power Co.'s Takahama plant. But with the reactors still idle and stricter safety regulations taking effect next month, it's unclear how long the fuel might sit in storage. The Nuclear Regulation Authority still has to clear the plant before it can be restarted.

The U.K.-registered Pacific Heron and Pacific Egret left France in mid-April with 20 MOX fuel assemblies. While Kepco and French nuclear firm Areva SA, which made the fuel, have not released figures, Greenpeace estimates the two ships are carrying about 10 tons of MOX, which is made with weapons-grade plutonium.