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I've been looking for how much ionising radiation is produced during an atomic blast (as a percentage).

I have seen a claim of 5% of the energy in an atomic blast is ionising energy (Wikipedia), but if it's so little why do we put so much emphasis on radioactivity protection when discussing a nuclear blast. It just doesn't add up to me.

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    $\begingroup$ 5% of a nuclear explosion is most definitely a lot of energy. $\endgroup$ – Pritt Balagopal May 26 '18 at 6:33
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    $\begingroup$ We don't. If you are close enough to the blast site for radiation to be an immediate issue, you won't survive the blast anyway. It is only in the long run that the radiation becomes important, when all other effects of the blast are long gone. $\endgroup$ – Ivan Neretin May 26 '18 at 7:48
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The amount of energy in an atomic blast releases as radiation (gamma rays, beta particles, alpha particles and neutrons) depends on how the bond is designed but isn't usually a big proportion of the total energy released. But that's not why we worry about the radiation from a bomb.

The reason why we worry about radiation is the radioactive products created in the blast (eg from fission of the large nuclei in a fission bomb or from the potentially radioactive nuclei created when the initial radiation blast interacts with other materials). Typically many dangerous isotopes are generated that continue to release radiation for days or years. Worse the explosive blast scatters many of these over a large area.

But not all bombs are the same. Neutron bombs are designed to release far more of their energy as radiation (in this case a large blast of high energy neutrons) with the express purpose of killing people in the immediate area while having a much smaller blast. The original idea was to be able to use them on invading Russians in Western Europe without doing too much damage to the infrastructure. Neutron bombs can release ~40% of their energy as neutron radiation which is far more than the 5% released in a bomb tuned to create a large physical explosion.

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I would treat Wikiepdia with a lot of caution. The relative yields of energy as heat, blast, initial radiation and delayed radiation to a person on the ground will vary a lot depending on the situation.

At the moment of detonation a major energy transfer mode from the inside of the bomb to the surrounding environment is the emission of gamma and X-ray photons. These photons give up their energy in the air, water or rock around a bomb.

While many people regard infra-red as "heat rays" all photons are "heat rays" as they are able to increase the internal energy of the things which they strike.

If we were to put a bomb in a deep hole in the ground then the amount of heat (IR) radiation which would reach a person on the surface would be zero. Also almost not initial ionizing radiation will reach the person. If the hole stays sealed than very little radioactivity will reach the person. However this blast will be optimised for giving a large semsic shock.

If we were to put the bomb in the sea then it will greatly reduce the initial radiation, the IR light. But we will get greater mechanical effects against the hull of a ship. Also the explosion will churn up a lot of very nasty radioactive fallout.

If we were to burst the bomb high in the air, then we would optimise the burst for the delivery of heat (IR) to the ground and also the air blast will be enhanced. The delivery of initial radiation will be lower and also the delivery of delayed radiation (fallout) to the person will be lowered. While much of this question and answer is physics, the behaviour of fallout is controlled greatly by chemistry. The chemical form of the nuclides at about 20 seconds post detonation will control their behaviour. The involatiles at that point will go onto large particles while the volatiles will end up in fine particles.

The burst on the surface of the ground will deliver less heat to a person at a moderate distance and also more delayed radiation in the radioactive fallout. What will happen is more of the fission products will fall quickly to ground near the blast. The air burst will inject much of the radioactivity into the upper atmosphere where it will have plenty of time to decay before it falls back to earth.

If you burst the bomb in space then the blast effect will be minimal or even zero. Thus I think that it is impossible to give a fraction of how much energy in blast, "heat", fallout and initial radiation for bombs in general.

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