A U.S. nuclear expert, in Japan, stated that no one should get to worried about the Fukushima Daiichi nuclear plant disaster, until cesium starts showing up.

Cesium (aka Caesium) is the main reason people can not live near Chernobyl, even 25 years after that disaster. The expert said cesium was spread all over, and in huge amounts, by the Chernobyl reactor melt down. Cesium-137 has a half life of 30 years.

Cesium-137, and many other cesium isotopes, are used in nuclear power plants (as well as other applications, like dirty bombs).

Has cesium been detected following the Fukushima Daichi disaster? Yes. The University of Nevada has detected cesium-137 between 17-21 March.  The levels were small, but they represent the early days of the nuclear disaster in Japan, things have gotten worse since.  On 29 March, South Korea reported that they have detected cesium-134 & 137.

Cesium builds up in soil and plants. Cesium ingested by people can kill. Tests were done on dogs, and it killed them within three weeks.

Besides Chernobyl, there was another nuclear disaster involving cesium.  It did not involve a nuclear power plant. It was outdated radioactive medical cesium chloride, in Brazil in 1987.  Four people were killed, 245 were contaminated, by a thimble sized amount.