Considering that the main destructive effects of nuclear weapons are the non-nuclear ones, I don’t think that non-nuclear WMD-sized detonations would be accepted. I mean, if you erased a city using a non-nuclear WMD, do you think people would really care that you didn’t cause any fallout?
The radiation’s not the really destructive part of most nuclear weapons.
There are two types: prompt radiation and fallout. Prompt radiation is the initial burst of gamma rays, x-rays and neutrons given off immediately on detonation. Fallout is when radioactive particles settle out of the air after a nuclear detonation.
Despite what ** keeper0 ** says, the prompt radiation zone of most normal-sized nuclear weapons (~10 kilotons and up) is smaller than the blast/heat zones of the same bomb. In other words, if you’re close enough to worry about dying from prompt radiation, you are more than likely to be killed by the blast and heat. Only extremely small bombs such as the Davy Crockett have prompt radiation zones larger than the lethal heat/blast zones.
Fallout is the other main radiation mechanism from nuclear weapons. The fallout levels and pattern is heavily dependent on the height of the detonation, the yield of the weapon, and the type of weapon.
Airbursts tend to be relatively fallout free, compared to ground bursts, because with an airburst, the only material that comprises the fallout is that which made up the original bomb. Groundbursts have the bomb, and whatever dirt/rock/concrete/etc… that got consumed in the fireball. Obviously this other stuff will be highly radioactive. This stuff settles out relatively rapidly as well, creating zones of intense radiation somewhat nearby. Airbursts on the other hand, are thousands of feet in the air, so the fallout tends to be more dispersed.
Larger weapons tend to lift a lot of their fallout into the stratosphere because of the extremely rapid rise of the fireball. Smaller ones tend to not raise the stuff as high or as fast.
Finally, pure fission weapons of a given yield will produce more fallout than a fusion weapon of the same yield. Most US, Russian, British and French weapons are fusion weapons, although the casings of the secondary may be U238, which increases fallout (and yield) when compared to a lead or tungsten secondary casing, due to the fissioning of U238 by fast neutrons generated by secondary’s fusion.
So, a relatively large, clean fusion bomb detonated in a high airburst would have very little fallout relative to the blast/heat damage, while a small, pure fusion bomb detonated at ground level would have a great deal of fallout relative to its yield.