Factbox: Japan’s nuclear crisis and radioactive half-lives

(Reuters) – The passage of time has sharply reduced radiation from some elements with a short half-life at Japan’s quake-crippled Fukushima nuclear power plant but others will stay radioactive almost indefinitely.

Scientists measure radioactive materials in half-lives, or the time it takes to halve the radiation through natural decay. Half-lives range from fractions of a second to billions of years.

Major radioactive elements, with half-lives:

* Iodine 131, 8 days. It is produced by the fission of uranium atoms and has been blamed for causing thyroid cancer among young people after the 1986 Chernobyl disaster in the former Soviet Union.

“In Japan, the iodine 131 is mostly gone. The concern is more with elements with longer half-lives, like cesium and strontium,” said Ole Reistad of the Norwegian Radiation Protection Authority.

* Cesium 134, two years, cesium 137, 30 years. Cesium can be absorbed in food and water or inhaled as dust. It is easily taken up plants and animals.

* Ruthenium 103, 39 days; Ruthenium 106, about a year

* Strontium 90, 30 years

* Plutonium 239, 24,100 years

Japan has found plutonium at the reactor but said that the levels were similar to those found at a location far from an atmospheric nuclear test.

* Uranium 234, one of the three natural uranium isotopes, 247,000 years.

* Uranium 235, 710 million years.

* Uranium 238, 4.5 billion years.

(Sources: International Atomic Energy Agency, U.S. Environmental Protection Agency, U.S. National Center for Biotechnology Information; Georgia State University)

(Compiled by Gwladys Fouche and Alister Doyle)

Factbox: Japan’s nuclear crisis and radioactive half-lives