Cs

cesium (aternative spelling of caesium).

atomic number 55.

one of three metals found in a liquid state at room temperature (makes you wonder what the other two are, doesn't it?)

it is soft and silvery white.

most of the world's cesium comes from minerals and nuclear fission.

NY Times article explains:

In the form found in reactors, radioactive cesium is a fragment of a uranium atom that has been split. In normal operations, some radioactivity in the cooling water is inevitable, because neutrons, the sub-atomic particles that carry on the chain reaction, hit hydrogen and oxygen atoms in the water and make those radioactive. But cesium, which persists far longer in the environment, comes from the fuel itself.

cesium-137 is the most common radioactive isotope of the element cesium.
it emits highly penetrating gamma radiation.

(gamma rays are similar to X rays rays, but X rays generally have lower energy. A dose of three gray of gamma rays delivered briefly to the total body would be lethal to about 50% of humans exposed because of severe damage to the hematopoietic system. deaths would be expected to occur within about 60 days.)

in any event, even the non-radioactive form of cesium is highly poisonous.

japanese television yesterday reported that the country’s nuclear and industry safety agency detected cesium near one of the reactors damaged by the earthquake.

As of this morning's NY Times:

Naoto Sekimura, a professor at Tokyo University, told NHK, Japan’s public broadcaster, that “only a small portion of the fuel has been melted. But the plant is shut down already, and being cooled down. Most of the fuel is contained in the plant case, so I would like to ask people to be calm.”