Cesium

Iodine and cesium, two byproducts of the nuclear fission process that occurs in nuclear plants, were detected around the area.

In addition, radiation levels of 1,015 micro sievert have been found outside the plant, which is the equivalent to receiving the maximum amount of healthy exposure someone can get in one year, in one day, according to JibTV and the WSJ

Source: http://www.businessinsider.com/cesium-and-iodine-radiation-fukushima-ines-scale-2011-3