This radioactivity chart 60 km from Fukushima Daiichi suggests that much of the radiation originates from short-lived isotopes, like iodine-131. The presence of longer-lived isotopes (like Cs-134 and Sr-90) with a half-life of 181 days, means that the radiation levels at Fukushima will remain at 1.5 times the pre-quake levels for a very long time to come.
