Another article from NHK News :
" Japan's top government spokesman says the detection of trace amounts of plutonium in ground at the Fukushima Daiichi nuclear power plant means the situation there is extremely serious.
Chief Cabinet Secretary Yukio Edano said Tuesday that the density of plutonium found in soil samples taken from the plant a week ago was about the same as that found in the environment from past nuclear tests abroad.
But he said 2 of the samples appeared to contain the type of plutonium used in nuclear fuel, making it most likely that reactor fuel rods were the source.
Edano said that the traces of plutonium, combined with the detection of highly radioactive water, back up the view that nuclear fuel rods have partially melted.
He said the government is doing all it can to control the impact of the contamination and contain the situation."