Don't forget, there have been 2.500+ nuclear tests on earth so far + 2 (and a half) power plant meltdowns with hardly any effect on the global population.
Since 1963, international treaty has banned any nuclear testing above ground. Underground testing contains most, if not all of the radiation, as the main source of radioactive fallout is the loose matter being drawn into the explosion, irradiated, and spewed high into the atmosphere as part of the mushroom cloud, and most of these events were spread out over time, giving humanity a chance to clean up and contain any ill effects. There were 240 estimated cases of thyroid cancer caused by the Windscale fire, which was the result of an isotope that has an 8 day half-life, and was minor enough that I'd be surprised if anyone outside of the UK had even heard of it, despite it being a class 5 disaster. Most of the truly big disasters were relatively harmless due to redundant safety measures, remote location, and quick evacuation from the source of the disaster.
A nuclear weapon, although devastating, can be relatively clean when you consider the radioactive side of it. It depends largely on what fissionable material is used for the core of the bomb, and how the bomb is detonated. An aerial explosion, which is where they detonate the bomb some 1000 feet above ground and let the shockwave pulverize the target below, is the most destructive method if you want to level a city, but it is also one of the least radioactive.
The main faults that I can see with my idea are:
1) Interception of the delivery method - some of that can be mitigated by using a manned delivery system, but certainly not all.
2) How much material would we actually need? Your average nuclear warhead only uses 60 grams of material per kiloton of yield, and depending on the material, the half-life can be very short indeed.
My idea was to attempt to introduce large amounts of radioactive materials with a half-life on the order of years, as opposed to days. Some googling shows me that the UK has about a thousand tonnes of high level nuclear waste sitting around, which accounts for 95% of the radioactivity of all their nuclear waste. Dispersing that into an atmosphere as a powder wouldn't be very healthy, but some preliminary math with estimated figures shows that it would probably have less radioactivity than the chernobyl disaster, which would indicate a need for more material. How much to be effective, I don't know, and that I think is really the real weak spot in the plan. Can we even come up with enough material, and if so could we manage to effectively introduce enough of it? After a bit more research, I'm beginning to suspect not.
