[oleg]

Originally posted by Suicide Junkie:
[QB...]Almost any element you pick won't undergo fission or fusion, even if you manage to nail it with subatomic particles or other atoms.

All the neutrons flying around will hit other atoms in the atmosphere, but they won't fission. They'll just become radioactive, and spit out beta (electrons, positrons), and alpha particles (He4 nuclei) a bunch of times until they become stable. Relatively little energy is released that way, and it isn't released fast enough to help the shockwave.
The shockwave isn't radioactive (its simply the pressure front - sonic boom style), its the neutron irradiated garbage thrown up in the air by the bLast, and the direct byproducts of the uranium scattered out.

Notes:
- Don't worry about the elements with short half-lives. They'll be at undetectable levels in mere minutes.
- Don't worry about the elements with long half-lives. They'll emit hardly any radiation. U235 for example (700 million years).
- Do worry about the stuff that Lasts for countable numbers of years, and the stuff that collects inside you. It's radioactive enough to be of concern, and remains dangerous for your lifetime.
- Don't worry about low dosages. Denver, Colorado gets 1000 mRem per year from natural sources. Radiation sickness dosen't show up until about 50,000 mRem in a single dose. Even your coffee is radioactive and puts out over 400 Bq/kg, which would make it nuclear
- The NRC's annual public dosage limit for artificial sources is 1/4 of the average naturally occuring dose.
- Coffee has a natural radioactivity of over 400 Bq/Kg. Some regulations
And, most important:
- Do not start nuclear wars; its bad for your health.[/QB]

Technically, you are incorrect about fission: if atom absorbs neutron and then emits alpha-particle, it IS a fission, the products would be He4 and another stable nucleus.