I don't think anyone has built a Cobalt bomb (maybe secretly, so who knows?) because nobody really feels the need to make sure that a nuked area is uninhabitable 100 years after it was nuked, and it makes fallout much worse. There's no real reason to use a cobalt bomb because creating that much fallout after a nuclear attack doesn't really serve a military purpose and will make it much worse for you should the irradiated cobalt isotope blow your way.

Please excuse my ignorance. Why is cobalt so dangerous when added to a nuclear device? What size of weapon have people considered lacing with cobalt? Few kT or Few MT? Would a 100kT cobalt laced bomb be more dangerous (to all <1 year old humans over 100 years, for instance ) than a sub-surface (say 30m, 100ft) thermonuclear 100kT blast which sends fallout laced steam and crater contents into the atmosphere?

A cobalt laced bomb is dangerous for the following reason. Cobalt naturally comes in the form of Cobalt-59, after a nuclear explosion some of the neutrons will bond into the Cobalt and form Cobalt-60. Cobalt-60 is really bad stuff, it has a half-life of 5 years which means it is very radioactive over a long period of time.

Here in the UK its all over the media that POTUS is nuts and wants to vapourise the bunkers holding WMD in Iran (though to be "fair" it is mention it is just his inital negotiating position - so we should not worry our little minds about it!) . Also there is all sorts of noise about senior mil staff being mighty pissed about this policy. So in order to start my own counter MADness campain I need some help. How does one go about doing accurate fallout death calcs that stands up to muster by those in the know? Does the rock type vapourised make a difference to the deaths due to fallout? Does the nature of the warhead make a significant difference? Or it it just the yield that matters. Will the lives of innocents around the world be traded for those in Israel by the US/UK/AUS alliance?