your half-life example is technically correct, yet by making a fanciful example of "forty million billion trillion" rhetorically, it makes the statement of harm sound like "oh, 21,000 years of half-life is not at all harmful" .. I think you are aware that spent nuclear fuel is poisonous by any definition.

Second, the amount of material is not at all mentioned in your definition. One half of one gram of material, is not the same as the many kilograms of spent fuels from a working reactor, or the 2000 metric tons of spent fuel generated now in the USA each year. So I ridicule that in an unexpected way, with another extreme example, comparing something tiny, to something massively large that everyone knows.

There is some false confidence in my opinion, about "know the facts" of radiactive materials.. when bio-accumulation in the food chain, uncontained dispersion, poisoning fresh or salt waters where life exists, and human exposure over time, and repeated exposure, are serious things.