Japan’s Chief Cabinet Secretary Yukio Edano says that the small amount of plutonium detected in the soil at the Fukushima nuclear power plant’s No.1 reactor shows that there was a partial meltdown of the fuel rods inside the reactor vessel.

Edano told reporters on Tuesday that the situation is very grave, adding that the government is doing its utmost efforts to contain the damage.

The official stressed that Tokyo will enhance the monitoring of plutonium at the site.