[petelamana (OP)]

While watching the movie "Fat Man and Little Boy", a 1992 film about the Manhattan Project, I started thinking back to my basic undergrad courses.  I know that U decays, eventually, to Pb.  However, I seem to remember that an isotope of U occasionally decays "down" to Hg.  Am I correct?  Which isotope of U?  Why mercury and not lead?  What is the probability of uranium to mercury decay?

[chiralSPO]

According to this decay chain presented on wikipedia, a very small fraction of nuclei from the ²³⁸U decay chain become mercury nuclei, but this is only transient (half life of minutes), and ultimately lead is the only stable product.

https://upload.wikimedia.org/wikipedia/commons/4/4e/Uranium_series.gif

[Bored chemist]

Mercury is just about possible as a fission product- though the yield would be very small.