Following is a factfile on plutonium, which Japan says has been found at very low levels at five locations at the stricken Fukushima nuclear plant:

WHAT IS IT?

Plutonium is a highly radioactive silvery metal that is used in nuclear bombs and nuclear power. It is a heavy and essentially man-made element, derived from the transformation of uranium through fission. Traces of two of its atomic variants, or isotopes, exist naturally.

HISTORY

Plutonium was found by US scientists in 1940. The new element was named after Pluto, the god of the Roman underworld, and assigned the abbreviation of Pu in the periodic table. It is one of the most complex elements, for it is a metal yet does not conduct heat or electricity well. Only a slight rise in temperature makes it switch from solid state similar to cast iron to plastic malleability.