Plutonium (Pu-239), that triggers in micrograms cancer, is produced in each reactor in the fission of uranium, about 250kg per year and reactor.
Radiogases which reactors emit do not disappear.
Instead they become more deadly.
Krypton 89 stays 18,4 seconds (full decay time) and becomes Strontium 89, which stays 500 days.
Krypton 90 stays 323 seconds and delievers Strontium 90 (288 years) via Rubidium 90 (1,580 seconds).
Xenon 137 stays 384 minutes and decays into Cesium 137 (300 years) which becomes stable Barium 137.
Plutonium 241 stays 147 years, becomes Americium 241 which stays 4,000 years and then becomes Neptunium 237, which stays 22 million years.
HALF-LIFE – an euphemistic term, used by the nuclear industry, to downplay and conceal.
European Continent is COMPLETELY irradiated: http://www.unscear.org/docs/JfigXI.pdf Cesium 137: Full Decay Time is 10 human generatons = 300 years.
Radionuclides decay more slowly with time.
Half life time means that only half of the radiation has disappeared. You have to multiply the half life time by ten in order to know approximately, when it disappeared from the environment. In the case of Strontium and Cesium that would be 300 years. http://www.n-tv.de/Spezial/Horrorszenario-wartet-auf-Japan-article2887296.html