> On the off chance that my earlier reference to the half-life of U-235 was mis-taken as cheap hystericizing or dopey moralizing, let me elaborate and sharpen the point. I submit that there is no honest way to "calculate" risks associated with time frames of even thousands of years, let alone 10s or 100s of millions; to suppose that one even knows how to coherently formulate the question with any concrete content, let alone answer it, seems delusional.
Sandy may be technically correct, but I suspect that misses your point. Roughly speaking, long half-lives imply low rates of radiation, much in the manner of combustion.
But the problem of planned and unplanned long-term storage requiring projections millenia into the future, to the point where planners feel compelled to consider how to communicate the hazard after current major languages have passed from recongizability, certainly outstrips any sane confidence in anybody's competence.
http://en.wikipedia.org/wiki/Nuclear_semiotics
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Problem Statement
Due to the detonation of atomic or fusion bombs in a supposable war as well as the usage of nuclear power plants in times of peace an unnaturally high amount of radioactive material is produced. This material will continue being a threat to health and even life for thousands of years. Consequently, the nuclear technology forces the responsible ones to assure a secure terminal storage of such materials for an unusually long time period.
Unfortunately, there is no method at disposition which could continuously provide the necessary knowledge over thousands of years. The culture of earlier centuries becomes incomprehensible when it is not translated into new languages every few generations. National institutions do not exist longer than a couple of hundred years. Even religions are not older than a few millennia and hand down not scientific knowledge but stories.
Furthermore, the length of storage is disputed between specialists. In Germany the conclusion of a work group was that the nuclear waste must be separated from the biosphere up to one million years which equals to about 30,000 human generations. Earlier assumptions selected 10,000 years which seems to be too short in respect of the half-life of certain radioactive isotopes (e.g. Plutonium-239 at 24,000 years).
However, the written historical tradition of humanity is only in existence for about 5000 years. Potential warnings in cuneiform script could be interpreted by some specialists, such in the writing of the Indus Valley civilization are already illegible at this point in time.
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More than any overall or specific risk, it's the unwillingness to admit when one is in over one's head that disquiets me.
-- Andy