The problem is that radiation is deadly long beyond the time we can foresee being able to control its disposal or storage.

a quote:
"Because some radioactive species have half-lives longer than one million years, even very low container leakage and radionuclide migration rates must be taken into account.[31] Moreover, it may require more than one half-life until some nuclear materials lose enough radioactivity to no longer be lethal to living things. A 1983 review of the Swedish radioactive waste disposal program by the National Academy of Sciences found that country's estimate of several hundred thousand years—perhaps up to one million years—being necessary for waste isolation "fully justified."

http://en.wikipedia.org/wiki/Radioactive_waste