Mat catastrophe's node claims that if Challenger had carried plutonium on board, "it would have been spread throughout the entire upper atmosphere, and possibly cause upwards of 90% of the planet to develop cancer". Sorry, wrong. No. Nope. Not even close. First, Challenger made a pretty big bang, but Challenger's lander was not destroyed, but in fact impacted the sea a few minutes after the accident. The astronauts on board were not all killed by the explosion. In fact there was a clear signs that one or more of the astronauts was alive as someone had activated air bottles, presumably due to depressurisation. However, it is likely that they passed out quickly due to the extreme altitude- although it is not known exactly how fast the air leaked from the damaged vehicle. Postmortem was unable to pinpoint the moment of death after recovering the bodies after 2 weeks in the ocean.
But in that accident; the lander lost one wing and the tail due to aerodynamic forces - the crew section was fairly intact upon impact at about 200 mph with the ocean. Nuclear materials in any plausible container would have survived any such an accident without any dispersal at all. None. No deaths. 72 pounds? Still none.
Even an accident somewhat like Columbia that led to its dispersion of any nuclear material in the upper atmosphere would not have killed many people on the ground. We more or less know this from experience, unfortunately. In 1968; a satellite, Transit 5 burnt up, as it was designed to in certain situations, depositing 1 kg of plutonium in the upper atmosphere. No peak of deaths in the early 70s was noted, even though many sources still maintain that 1kg of plutonium is 'enough to kill every man, woman and child on the planet'. You may draw your own conclusions from your and our continued existence and try to explain why 72 pounds is supposed to wipe everyone out, but 2.2 pounds didn't even noticeably change the death rate.
Incidentally, modern containment systems would be designed to reenter successfully from any altitude, for example by encasing the object in copper. This is the technique used by some ICBM reentry capsules. So even Columbia wouldn't have killed anyone.