The half-life of Uranium is 93 years. Supposedly it is ‘safe’ after 93 years.
The half life is just 50% of the original radioactive breakdown rate. If it started at 1000% over safe levels, after 93 years it would only be 500% of safe levels.
Rule of thumb...any radioactive element is considered ‘safe’ after 5 half lives of decay.
Which isotope is that?
The most common isotope, U-238, has a half-life of 4.468 billion years, comparable to the age of the earth, a fact that makes it useful for geologic dating. U-235, the isotope useful for fueling reactors and making bombs, has a half-life of 704 million years.
Supposedly it is ‘safe’ after 93 years.
More like, half as dangerous. Saying something has a half-life of 93 years simply means that half of it is gone after 93 years, 75% gone after 186 years, 87.5% gone after 279 years, etc. (only an eighth as dangerous).
The half life of uraniums shortest lived isotope is about a quarter million years. 235 and 238 are close to a billion and 4.5 billion years respectively. Half of any given isotope decays within its half life period.
U235-—700 million?