The shorter the half-life of an isotope is, the more radioactive and thus dangerous it is.
The longer the half-life of an isotope is, the less radioactive and thus dangerous it is.
Fukushima happened in 2011, almost 8 years ago.
Almost everything dangerous had since decayed.
What we can detect long-living and thus not-so-dangerous isotopos, or detect minute traces of short-living ones because we have very sensitive equipment means exactly nothing besides the fact that we can.
> The longer the half-life of an isotope is, the less radioactive and thus dangerous it is.
> Fukushima happened in 2011, almost 8 years ago. Almost everything dangerous had since decayed.
Plutonium-239 has a half-life of 24,100 years and is pretty dangerous AFAIK [1]:
> Radioactive contaminants are dangerous for 10 to 20 times the length of their half-lives, meaning that dangerous plutonium released to the environment today will stick around for the next half a million years.
[1] https://www.livescience.com/33127-plutonium-more-dangerous-u...
edit: if you've got granite in the basement, be vefy afraid :)
Its release was mostly I and Cs isotopes with half-lifes in days. And Cs-137 with 30-ish year half-life which we have to thank for this discussion.
> Almost everything dangerous had since decayed.
This is where I take issue with your comment. Frankly because long lasting isotopes still can be dangerous, even if their levels are low. Excluding the effects of radiation, most of these are heavy metals. With the difficulty of the subject matter I think it is easy for people to pick apart generalizations. The subject matter is just really convoluted and required a high degree of knowledge to even feel like you can understand what scientists are talking about. Plus combine all the fear associated with nuclear.
I think you could reword it as
> Most of the material with high activity has decayed and we're mostly left with long living isotopes that tend to be less dangerous.
Also, for data I like this map:
Is not so simple. In the decay process new radiactive compounds are being created. And this is happening since 8 years.
To have a half-life of 24.000 years is not equal to "this atoms will sleep for 24.000 years" or we could just save all that plutonium in a mayonaise jar in our bedroom. It does not work like that.
You can have a fair change to observe a case with very low probability if you have a population big enough. Each single plutonium atom can decay anytime in the next 24.000 years splitting in other dangerous compounds and treatening life and humans around.