Re: What would happen if 20 kilograms of francium were thrown into a river

 I arbitrarily picked F223 with a half life of about 22 minutes with beta and 
alpha decay modes, because F223 is the most stable of the three synthetic 
isotopes of Francium.  Francium does not occur in nature.  With a half life 
of about 22 minutes, 20 kilos of Francium, aside from being appreciably 
radioactive, would be nearly impossible to maintain, since it is decaying 
away so rapidly.

I don’t actually have the figures for Francium right here in front of me, 
with respect to its activity in Curies/mol, but my rough estimate puts it 
in the 100 kiloCurie/mol range, by looking at its average half life compared 
to that of tritium.  Twenty kilos would be roughly 100 mols and would 
deliver, in about 3 hours (roughly nine half lives of Francium) 
1 0mega Curies of beta and alpha, at least that’s probably on the correct 
order of magnitude.  This means that for every 1 million gallons of river, 
each gallon would be 10 kiloCuries, or 2,220,000,000,000,000 decays per 
minute beta and alpha radiation.  F223 beta and alpha are fairly energetic, 
meaning that for each radioactive decay, each decay can do appreciable 
damage to both living and non-living tissue.  

Because of the intensity of 
the radiation, dissipation into the environment would take quite long, 
meaning that although the F223 had decayed away in just a few hours, a lot 
of radioactivity would linger in various forms for an indefinate period of 
time.  Of course, rain, aquifers, etc., would spread the environmental 
catastrophe over an indefinate surface area of the Earth.  The whole thing 
would be quite nasty indeed.

Fortunately, it’s not possible to produce 20 kilos of Francium using 
present technology.  The limiting factor is production rate, which would 
have to produce and purify (isolate from decay products) 20 kilos of 
Francium in 20 minutes or less.  

Can not do.