The high-stakes operation at the Fukushima plant has added to Japan's unprecedented humanitarian disaster with 27,500 people dead or missing from a March 11 earthquake and tsunami.

French President Nicolas Sarkozy, who chairs the G20 and G8 blocs of nations, plans to visit Tokyo on March 31. He will be the first foreign leader in Japan since the disaster.

In further support, France flew in two experts from its state-owned nuclear reactor maker Areva and its CEA nuclear research body to assist Japan's heavily-criticized plant operator Tokyo Electric Power Co (TEPCO).

A global leader in the industry, France produces about 75 percent of its power from reactors so it has a strong interest in helping Japan get through the Fukushima disaster.

The United States is also weighing in to send some radiation-detecting robots to Japan to help explore the reactor cores and spent fuel pools, the Energy Department said.

The UN atomic agency IAEA agreed. "Concentrations reported for both, plutonium-238 and plutonium-239/240, are similar to those deposited in Japan as a result of the testing of nuclear weapons," said its latest briefing.