Accidents with releases from power stations do not compare to what would come from full-scale nuclear warfare.
Mainly because nuclear war, with the detonation of nuclear bombs, involves the release of massive amounts of three kinds of radionucleides:
1) the sort of "fallout" a lot of the dismissers of the extreme danger of this aspect focus on exclusively--the secondary transmuted external materials, neutron activated by close proximity to the release of neutrons from the fireball. This varies a lot, with airbursts generating relatively little and groundbursts churning up a lot of dust and generating a thousand times more. This is a broad spectrum of isotopes, most of them activated in the sense of being radioactive isotopes. Optimists tend to argue airbursts would be preferred over groundburts.
But the other two kinds of radionuclides form just the same regardless of where the bomb is detonated:
2) decayed daughter products of fission of uranium or plutonium. Since the mass that is released as energy by fission is about 1/1000 of the parent mass, the mass of the daughter products is essentially the same as the portion of fissile metal one has managed to make react. It is a bell curve of nuclei with atomic weights about 1/2 that of the parent nucleus, in this case the curve is centered on atomic weight 120 or so. But it has considerable breadth too. Again the majority of these nuclei are radioactive to one degree or another.
3) unfissioned parent fissile material, uranium 235 or plutonium. Plutonium is very poisonous, mainly because chemically speaking it tends to be taken up and deposited in bone marrow, and small quantities put out enough radiation there to make one very very sick.
All of these materials are vaporized and become dust of various grades, shading down to individual molecules. These materials are gradually dispersed, large numbers remaining near the release site, but others being blown with winds great distances away, portions remaining in the stratosphere and being blown all around the world.
The Japanese reactor is not at all a valid comparison because it was a matter of secondary materials being released, not the primary mix of uranium and its daughter isotopes. Chernobyl was admittedly more comparable because what was released there was a combination of combustion products of the graphite moderator with air, where the graphite had itself been neutron activated and to an extent permeated with stray daughter products and the occasional spalled uranium nucleus, and then the hot combustion going on to a certain extent set the fuel elements themselves burning and contributing to the soot cloud.
So, how valid a comparison the neighborhood of Chernobyl might be to a site hit by a nuclear warhead strike depends on whether the fire, before it was finally contained with layers of boron-laced sand dropped on it, managed to burn and vaporize a mass of core uranium comparable to the tonnage of fissile material installed in each bomb. A much greater quantity of material needs to be incorporated than undergoes an actual fission reaction, and the vast majority of the energy a so-called "hydrogen bomb" releases is also from secondary fission triggered by the fusion event. So if one knows the ratio of effectively fissioned nuclei to those present to enable that, one can estimate the total mass of fissionable material built into the bomb if one also knows the energy release. That mass is the minimum gauge of how much each bomb releases, based on its own materials and internal fission reactions; to this fallout of the first type might or might not be added, and this junk might or might not far outmass the fissionables initially loaded. The degree of hazard each of hundreds of different types of radioisotope produced will vary, with its chemical properties, half-life and mode of radioactive decay. On the average, the initial danger is from intense radiation release that is dangerous from outside the body and this is mostly due to short half-life species that because of their rapid decay time are putting out radiation quite intensel. Over time, these will decay, generally into less intense, longer half-life species, all tending toward the atomic weight of iron, but the hazard then shifts from exposure to an external radiation field to ingesting isotopes which can take far longer to decay, but because they are radiating inside the body, even the weakest forms of decay are sure to do damage.
Now if the release of uranium--fissionable U-235 that is--at Chernobyl amounted to many tonnes, bearing in mind that most of what was released ascended in the smoke column and was distributed very widely over Europe and around the world--then we might have a fair sample of what war would do to an ecosystem, but it might be that the fire was put under control before any region would show a fraction of what doses of fallout would do to a targeted region, and clearly the effects of Chernobyl on the world as a whole was about equivalent at most to a few bombs being blown up, if that. We'd have to imagine thousands or tens of thousands of Chernobyl events to compare to what the 1984 arsenals of the Eastern and Western bloc would release if they were all expended in war.
