Hi, new member here.. with a quick nuke blast question...

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  • #1
Hi all, thanks for allowing me on to your forum. I'm not a physicist, but I am trying to write a piece of fiction and I'm aiming to get my physics at least some what plausible, so I thought I'd ask the experts.

It goes like this: A character in a novel I'm slaving on (set in the early 80s) is imagining a many megaton nuclear bomb blast. He is pretty far away -- a mile or so -- and at the very perimeter of the effect, to where he can watch the destruction race towards his position.

My physics question is: if one is far enough away from the epicenter of the initial blast does the air move for a brief second towards the center of the explosion due to the massive consumption of oxygen? Or does the blast-wave just outpace any vacuum effect at all? The way I've described it is the character can feel a slight breeze pulling him towards the epicenter of the explosion an instant prior to the blast-wave. I don't think this is right at all. I want the description to be fairly accurate and realistic.

Anyways, thanks for taking the time to consider my question! Any help would be greatly appreciated.
 
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  • #2
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It goes like this: A character in a novel I'm slaving on (set in the early 80s) is imagining a many megaton nuclear bomb blast. He is pretty far away -- a mile or so -- and at the very perimeter of the effect, to where he can watch the destruction race towards his position.

A quick calculation suggests that if your character was standing two kilometres (a bit more than a mile) from a 10-megaton explosion, he'd be incinerated on the spot. The immediate effect would be an intense blast of heat, followed by an actual blast wave that would smash everything within a considerable distance that wasn't already seared. Afterwards there would be a backrush of air towards the explosion--not because oxygen had been used up (a negligible effect at this stage) but because superheated air was streaming up into the sky, creating the characteristic mushroom cloud and pulling in air from around it.

Wikipedia estimates that all the explosives used in WW2 (including the two nuclear bombs) amounted to about 3 megatons. This page https://en.wikipedia.org/wiki/Effects_of_nuclear_explosions implies that your character would need to be 20 or 30 km away to have any chance of watching the detonation in safety (which would leave him exposed to an air burst but would put him over the horizon from what was happening at ground level).
 
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  • #3
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I would suggest you watch some blast videos to see what actually happens, then work with what you learn from that. Youtube has tons of videos. Tests Able and Baker were the first post-war shots filmed and Baker is iconic.
 
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  • #5
chasrob
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Looks like initially there's a blast wave out from ground zero followed by the opposite effect-
 
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  • #6
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Shock wave followed by air being drawn up into the mushroom cloud. But is that true for all blasts?
 
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  • #7
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I'd imagine that behaviour would be true for any that did form mushroom clouds, and they seem a characteristic feature of big blasts.
 
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  • #8
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I'd imagine that behaviour would be true for any that did form mushroom clouds, and they seem a characteristic feature of big blasts.
Yep, within the limitations of the material detonating, of course. They used 100 tons of explosives to calibrate their instruments before the Trinity test, and got a mushroom cloud from that. The Texas City explosion produced reports of a mushroom-like cloud.
 
  • #9
chasrob
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Around ground zero there must be a lot of vacuum/suction in multi-kiloton blasts. Look at the stem rise to meet the mushroom-
 
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  • #10
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OP, do you need a ground burst or air burst?
 
  • #11
A quick calculation suggests that if your character was standing two kilometres (a bit more than a mile) from a 10-megaton explosion, he'd be incinerated on the spot. The immediate effect would be an intense blast of heat, followed by an actual blast wave that would smash everything within a considerable distance that wasn't already seared. Afterwards there would be a backrush of air towards the explosion--not because oxygen had been used up (a negligible effect at this stage) but because superheated air was streaming up into the sky, creating the characteristic mushroom cloud and pulling in air from around it.

Wikipedia estimates that all the explosives used in WW2 (including the two nuclear bombs) amounted to about 3 megatons. This page https://en.wikipedia.org/wiki/Effects_of_nuclear_explosions implies that your character would need to be 20 or 30 km away to have any chance of watching the detonation in safety (which would leave him exposed to an air burst but would put him over the horizon from what was happening at ground level).


Thanks for the response. Apologies for the confusion! If I move my character to an elevated position, say mountain overlook 30km away, would he feel the effect of air moving towards the mushroom cloud?
 
  • #12
Looks like initially there's a blast wave out from ground zero followed by the opposite effect-


Thank you! This is very helpful to the scene!
 
  • #13
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The blast radius of the bomb dropped on Hiroshima was about 1.7 miles, and the bomb left a crater that was 2 miles deep. The largest bomb as of August 2014, the 100 megaton Tsar Bomba, had a blast radius of 7.7 miles. The fireball resulting from the detonation of the Tsar Bomba is estimated at 1.8 miles, with a thermal radiation radius of 47.8 miles.
 
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  • #15
OP, do you need a ground burst or air burst?

Ehh, not sure.. I think for minimal accuracy I need to place the vacuum effect after the shock-wave. Which, come to think of it, makes a lot more sense.
 
  • #17
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Ehh, not sure.. I think for minimal accuracy I need to place the vacuum effect after the shock-wave. Which, come to think of it, makes a lot more sense.
The shock wave will travel at the speed of sound in the medium it is traveling through. The "inhalation" or "draw-back" will be much slower than that.
 
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  • #18
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Around ground zero there must be a lot of vacuum/suction in multi-kiloton blasts. Look at the stem rise to meet the mushroom-

Reference https://www.physicsforums.com/threa...uick-nuke-blast-question.908075/#post-5719645

I assume that what's causing the mushroom cloud is a body of extremely hot air rising very fast, and pulling air in behind it. I don't know if the altitude of detonation would make much difference, within reason. An air burst would be more plausible, though; as I understand it, that's standard practice to maximise the damage.

There's an account somewhere on You-Tube (?) that when they did the first test explosion at Los Alamos, once of the scientists at an observation site scattered bits of paper in the air and estimated the yield of the bomb from how much the blast moved them.

Edit: I think there may be a video about it, but this is the key part (Wikipedia: "Trinity (nuclear test)" ). The words are Enrico Fermi's:
About 40 seconds after the explosion the air blast reached me. I tried to estimate its strength by dropping from about six feet small pieces of paper before, during, and after the passage of the blast wave. Since, at the time, there was no wind I could observe very distinctly and actually measure the displacement of the pieces of paper that were in the process of falling while the blast was passing. The shift was about 2 1/2 meters, which, at the time, I estimated to correspond to the blast that would be produced by ten thousand tons of T.N.T.

Assuming the blast travelled mostly at the speed of sound (I imagine it started as a supersonic shock wave), forty seconds represents about 13 km or 8 miles*. The official estimate of the yield was about 22 kiloton.

* Fermi himself estimated 10 miles.

Also this is some of what Wikipedia says about "Blast Waves":
The flow field can be approximated as a lead shock wave, followed by a 'self-similar' subsonic flow field. In simpler terms, a blast wave is an area of pressure expanding supersonically outward from an explosive core. It has a leading shock front of compressed gases. The blast wave is followed by a blast wind of negative pressure, which sucks items back in towards the center.
 
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  • #19
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The blast radius of the bomb dropped on Hiroshima was about 1.7 miles, and the bomb left a crater that was 2 miles deep.

Are you sure?

Wikipedia "Little Boy":
Because Little Boy was an air burst 580 metres (1,900 ft) above the ground, there was no bomb crater and no local radioactive fallout.

That makes sense to me.
 
  • #20
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Yeah, there was no crater at either Japanese site. I got a bad citation. Apologies to all.
 
  • #21
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Is 15 "many" megatons for you?
That's Castle Bravo, on Bikini, 1954.
And it was unexpected - the expected yield had been 5...6 MT.
How far were the surviving witnesses of Castle Bravo?
 

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