Just so you can appreciate precisely how stupid your notion that a dirty bomb could kill millions is:
“A dirty bomb (RDD) is not a weapon of mass destruction.” — Center for Disease Control.
“In 1987, in Brazil, thieves stole Caesium and distributed it throughout a local population. The effect was similar to a dirty bomb explosion. Despite the International Atomic Energy Agency calling it one of the worlds worst radiological incidents, only four lives were lost.”
“The primary threat of dirty bombs are not direct loss of life, but of potentially life-threatening mass panic and economic dislocation.”
“A similarly sized conventional bomb would likely be deadlier than a dirty bomb (RDD), causing death primarily through a shock wave. The chief threat of a dirty bomb would from the ensuing panic.”