The material used for each determination was uranium very highly enriched in the isotope under investigation.
This results in different bond strengths, with 238U preferentially incorporated into the more condensed solid phase.
To be useful for dating, a mineral must be retentive with respect to U, Pb and the intermediate daughters, and it should be widely distributed in a variety of rocks.
There are no reputable reports of cancer or other negative health effects from radiation exposure to ingested or inhaled natural or depleted uranium, despite much study.
In one case, plutonium, one of the byproducts of the reaction, was found less than 10 feet away from where it formed two billion years ago.
Uranium is sold only to countries which are signatories of the Nuclear Non-Proliferation Treaty, and which allow international inspection to verify that it is used only for peaceful purposes.