[jarvyeip] Bomb of the Long Now

A fissile radioactive substance in solution slowly reaches critical mass as the solvent evaporates.  I think similar nuclear criticality incidents have occurred, often as accidents.

In the context of a dirty bomb using this mechanism as a fuse, the fascinating thing is the lack of a need of any trigger mechanism to finally initiate the nuclear chain reaction, unlike chemical reactions which have activation energies.  The trigger is nuclear physics itself.

One imagines dissolving a critical mass of fissile substance into an endorheic lake, where it is harmless for perhaps millions of years until continental drift moves the lake into a drier climate, and the lake dries up.  One does not need to worry about the trigger becoming damaged or worn away in the intervening years.  This is very unusual for a machine.