system
Given the following post, respond as if you're commenting on Reddit:
human
Why is it going to take 40 years to clean-up the Fukushima nuclear plant?

I know the obvious answer is that they are cleaning up nuclear material and have to be extremly careful and diligent. I'm interested in the technical reasoning behind the years vs. months? Why can't the clean-up process be done faster and why does it take 40 years to clean-up a nuclear site? (I found it interesting that 40 years is being potrayed as ambitious.)
gpt
Radioactive elements have a probability of decay that is often referenced as their "half-life". The half-life denotes the amount of time for a radioactive particle to have a 50% chance to have decayed by that point. 

A simple example is this: Given 1000 radioactive particles, approximately 500 of them will have decayed by the half-life time. In 2xHL, ~250 remain. In 3xHL, ~125 remain. In 4xHL, ~63 remain. In 5xHL, ~31 remain. In 6xHL, ~16 remain. In 7xHL, ~8 remain. In 8xHL, 4 remain. If the accepted level for human safety is <5 radioactive particles, then the amount of time to wait would be approximately 8 times the half-life of that element.

The situation at Fukushima is much more complicated. There are multiple radioactive elements, combined with cleanup efforts. They estimate that the combination of the natural decays of the radioactive elements combined with cleanup efforts will take approximately 40 years.