Fox said:
"There would be no chain reaction in the atmosphere. That requires both particularly high-energy free neutrons (from the decay process) and what's called critical mass (which, when you think about it, is actually "critical density") in order to be self-sustaining, which is why they're called "chain" reactions."

You are right of course about the actual chain reaction as described for bombs or power plants happening in air. By saying "chain reaction" effect, I was hoping to emphasize the fact that a chain of actions or events happen when a radioactive nucleus decays even if it is in the atmosphere. According to my understanding every time a nucleus decays there is a release of gamma radiation. The released gamma radiation can be very energetic and must be absorbed by the atoms in the surrounding air molecules, the absorption excites the atoms to a higher energy level, the same type of absorption reaction must happen for the alpha and beta particles. These exited atoms then excite other atoms surrounding them when they release the excess energy. When any atom absorbs and emits any kind of energy, the said atom usually gains momentum, which is the same as kinetic energy, temperature is related to kinetic energy of the molecules.

IF and that is a big if this type of activity is increased in the upper atmosphere (thermosphere) then the kinetic energy that the atoms gain will increase their chances of escaping the earth's gravitational field.

I know that several atomic bombs have been exploded in the upper atmosphere and we are still breathing, which simply means that it takes quite a bit of energy, exerted for a long time period, before any noticeable effects will be happening. Once the effects are noticeable, then it will be too late to do anything about it.

That is the premise behind the heating atmosphere controversy.
IF the numbers that RL is so fond of, were actual measurements of the thermosphere temperature before and after the atomic explosions that were conducted there, then I would pay very close attention to them.

Thanks for the advise and links.

Each fission produces approximately 200 MeV in the form of gamma radiation and the kinetic energy of two fission fragments that are produced. This energy is dissipated in the fuel material as the fission fragments interact with surrounding atoms and slow down.

Gamma radiation frequently accompanies alpha and beta emissions and always accompanies fission. Gamma rays are very penetrating and are best stopped or shielded by dense materials, such as lead or depleted uranium. Gamma rays are similar to x-rays.