Proponents who are now calling Nuclear Power clean are "cherry-picking the data" by conspicuously omitting any mention of plutonium as one of the waste products, produced in quantity, by nuclear power plant reactors.  Prior to 1940, plutonium occurred naturally only in trace amounts in the immediate vicinity of naturally decaying uranium.  Within 20 years of Nuclear Power Proliferation, plutonium existed in quantities of hundreds of tons on the planet.

According to the "Handbook of Chemistry and Physics", a massive compendium of much of the known data for the elements and chemical compounds, "The maximum permissible body burden, or the amount that can be maintained indefinitely in an adult without producing significant body injury, is 0.06 micrograms (.0000021 oz).  Plutonium (Pu), therefore, is one of the most dangerous poisons known."  Also worth noting is that the half-life of Pu (time for half the mass to disintegrate) is 24,360 years.

And all this refers only to chemical and radiological toxicity.  This does not include the significant fact that Plutonium can also be made to reach a critical mass as demonstrated by the Nagasaki bomb which was Plutonium based (Hiroshima was a Uranium based bomb).

No containment method, including burying in mountains, even if we assume no breech, such as earthquakes or other explosions, can be expected to last for more than a few centuries before leeching out into the environment.

"Contrary to the claims of its proponents, nuclear power is not only tremendously expensive, but also very dirty and highly dangerous -- producing thousands of tons of long-lived radioactive waste each year, for which there exists no permanent storage facility". ~ Physicians for Social Responsibility (http://www.psr.org)

Nuclear Power is most assuredly not a clean energy alternative.