How high a dose of radiation would it take to kill you within seconds?

That is a tough question to answer. Around 500R will eventually kill you. But that may take days to weeks. In order to kill you in seconds would be really, really difficult. Most of the effects of radiation take place when a cell divides, so blood forming and the digestive tract are most quickly affected. The nervous system is much less affected since the cells don't divide very fast, if at all. So basically, what you have to do is kill a non-dividing cell.

Just as a WAG (Wild A$$ Guess) I'll say 100,000R  in less than one minute should be pretty quick.  If you want to be sure, use 1,000,000R in a minute. Trying to find that level of radiation might be hard unless you happen to be inside an exploding atomic bomb. Of course, if you were, the radiation would be the least of your concerns. Of course, on the other hand, if you were inside an atomic explosion, you wouldn't have time to be concerned about much of anything before you were dead.

Short answer- A whole body dose of 2,50,000 Sv will kill any human being within a few seconds (Let's say that it's gamma radiation)

Long answer-

Predicting the time it takes to die from a high dose of radiation is difficult. However, I will try to do the same in this answer.

Let's start with median lethal dose.

At a dose of 4.5 Gy you can expect to die in a month, without medical treatment. However, with proper medical treatment and a proper luck, you can survive. That is, you can prevent death from the direct effects of radiation poisoning in that case. What about the indirect effects? You will have a 63% chance of getting cancer in your lifetime after being exposed to this dose. To be continued after a short note...

Note:-Finding the dose required to kill a human as well as the time required to die from radiation exposure is a herculean task because every human being is different. It's like asking how much weight can a human body bear before being crushed to death. For example,

  • Most of the people can survive a whole body dose of 2.5 Sv(instantaneous dose), but some can't.
  • Some very lucky people can completely survive a dose of 7–8 Sv (instantaneous dose) with luck and very, very good as well as quick medical treatment. But most of the people can't. Those who survive are likely to have a very poor quality of life filled with infections, diseases, pain, bleeding, cancers and organ failure.
  • A whole body, instantaneous dose of 10 Sv is likely to be fatal, with death within 2–3 weeks even with medical treatment. But I know a person who survived for 7 months. Can you guess who he is?
  • A whole body, instantaneous dose of 17 Sv is likely to kill you in a day or two without medical treatment and in a week or two with medical treatment. But I know a person who survived for 83 days (can you guess who he is?). He would have died in one day if medical treatment were not provided since he was unable to breathe properly. His white blood cell count was zero and the doctors tried to restore it by transplanting some peripheral cells. Each and every chromosome of his body was shattered. His digestive system was completely destroyed. There was constant haemorrhage, pain, peeling of skin and melting of internal organs. Even his heart stopped a few times during that 83 day period and his muscles started to detach from his bones.
  • A person who receives an instantaneous whole body dose of >50 Gy usually dies within two days even with medical treatment and there's immediate destruction of a large number of neurons at this dose. But I know a person who received 60 Sv but lived for 5 days. Another person who received a dose of 20 Sv lived for two weeks. A dose of 1 Sv can mean radiation sickness/nausea/headache for one person and no symptoms at all for another person. Similarly, a person may have no symptoms at all other than fatigue for several minutes or even hours at a dose of 8–10 Sv (though this is very rare) while most will have immediate symptoms within minutes. At around 15 Sv some will have immediate loss of consciousness while some will have the usual mild symptoms of radiation sickness like headache, nausea and fatigue.
  • Albert Stevens survived a dose of 64 Sv because it was spread over 20 years. Bugorski is still alive after receiving a dose of 3000 Sv because only a very small portion of his face received this dose. That side of his face has not aged at all after the incident.
  • An interesting things about radiation sickness may be a "walking ghost" phase.

Though no one has received a dose of thousands of Gy, I think it should be enough to kill the nervous system of a person and lead to instant death.

However, to be completely on the safe side, I would say that a dose of 2,50,000 Gy is required to kill someone instantly. How?

2,50,000 Gy means that 2,50,000 J of energy is deposited per kg of body mass. Let's say that the specific heat of human body is 4186 J/kg/°C. Then this dose would increase the body temperature to 96.7°C !

Seconds? The question didn't specify the type of radiation - and if one considers any kind of electromagnetic radiation, then consider the intense energy burst in the nuclear flash that incinerated people at Hiroshima or Nagasaki. The USA has already demonstrated the principle and performed the action empirically, not to mention the willingness to do it. I'd be surprised if the specifics for the needed radiation intensity do not exist in a technical report for the military.

Here's a similar Q & A with links.

Dave Scheller's answer to How would you die if exposed to intense radiation (>1000 rad)?

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