You guys have got it all wrong.
You're giving the people at GW who came up with the cool sounding names waaaaaay too much credit.
They weren't thinking about a radioactive substance.
The fact that it even exists is pure coincidence.
I call to attention the pidgin latin (yeah right, it isn't even that!) that is used in WHFB as well as in 40K. Don't think the guys that came up with any and all names had anything to do with actual sience.
Just look at the plethora of stolen names from many different mythologies and you'll see what I mean. Horus? Abaddon? That primarch of the raven guard who's name just means raven in some dead language? Ferrus Manus (I mean, come on!)?
GW, when they wrote the groundwork for their universe weren't the brightest bulbs in the box.
Not to say that it didn't turn out cool but that's beside the point
The name comes from the Greek titan Prometheus
Promethium is nothing more than Napalm. Gasoline mixed with detergent. Maybe some high tech equivalent or some such but basically just napalm.