ok, ok, call me a kgb buzz kill but...
man if you're going to get all hela specialized in super secret international espionage poisons and then use them in the western world for assasinations, would you really want to use something radioactive with a half life of well over 4 months? that's called extremely unusual and inherently traceable with a geiger counter off the back of the cereal box. i can see the appeal of a tasteless, odorless, colorless poison with horrific potential. and maybe the symptoms would be missed in many situations- written off as bad gi with sepsis yada yada. but people's hair falls out for christsakes. it's bad bad bad. bad to use it. and bad to have thought it'd be all clandestine superspy in the first place.