What Happens When An Atomic Bomb Goes Off?

in #explanation7 years ago

WHAT HAPPENS WHEN AN ATOMIC BOMB GOES OFF?

In this essay, I thought I would describe what happens when an atomic bomb is detonated. By 'what happens' I mean the sequence of events that the nuclear device itself goes through and not so much the horrific consequences of a successful detonation. The sequences I will be describing refer to the fission bomb of the kind dropped on Hiroshima at 8:16 am on the 6th of August 1945

The first atomic bomb to be used aggressively was carried by a B-29 bomber called Enola Gay. This bomb, which was christened 'Littleboy', somewhat resembled an elongated trash can with wings. When it was released, it fell for forty-three seconds before detonating.

image.jpeg

(Image from wikimedia commons)

Upon its release, wires were tugged out of small holes that ran around the bomb's midpoint, and this started the clock switches of its first arming system. Further back on its steel casing were more small holes, and these allowed air samples to be taken in as the bomb continued to free fall. Inside the bomb there was a barometric switch that sampled the air and primed the second arming device once the bomb had fallen to 7000 feet above ground level.

Below the bomb was Hiroshima's Shina Hospital. Little Boy sent out weak radio signals to that building, and while most of them would have been absorbed by the hospital's walls, some were reflected skyward. Toward the rear of the bomb, near its fins, there were thin radio antennae. These used the time lag each returning radio signal took to travel there and back to measure how far away from the ground the bomb was.

The reason for knowing how far away the ground was had to do with reasons of destructive power. Had the bomb been detonated too high up its energy would have been mostly dissipated in the air. Had it been detonated too close to the ground, it would have dug a huge crater. Brilliant minds like that of John von Neumann had calculated that, by detonating the device at 1,900 feet above the ground, the resulting explosion would cause the most devastation.

Inside Little Boy there was what looked like a conventional gun barrel. Unlike most gun barrels, this one was intended to be fired only once. As such, it weighed barely a fifth of the U.S Navy weapons it had been copied from (in early designs it was a direct copy, until somebody figured out that those guns were so heavy because they had to withstand recoil after recoil as the weapon was fired over and over again).

The 'bullet' this gun fired was a small segment of purified uranium 235. It travelled about four feet before impacting with the rest of the uranium. Now, why had this particular element been used? The explanation can be found by looking at the nucleus of Uranium 235. It's really big and in a pretty precarious state. In contrast to the nucleus of an element like carbon (which is small and has a far stronger gluing force counteracting the electrical repulsion of its protons) the nucleus of uranium 235 is somewhat like a water balloon that is almost full to bursting.

When that segment of uranium 235 was fired at the remaining bulk of uranium, stray neutrons sprayed everywhere, not impeded by the clouds of electrons because neutrons have no electrical charge. While most of those neutrons would have flown straight through an atom without hitting the nucleus at its centre, a few inevitably were on a collision course.

image.jpeg

(Image from wikimedia commons)

Normally, the nucleus of an atom has protons surrounding it that prevent anything from entering. But that is no barrier to a neutron, for the same reason that the cloud of electrons could not impede the neutrons: Neutrons have no electric charge. And so unstable is the nucleus of uranium 235 that one extra neutron is all it takes to tip its instability over the edge.

That extra nucleus breaks the strong force glue and once that happens the ordinary electricity of the protons forces them apart. Those speeding fragments set off a chain reaction as released neutrons enter the nucleus of other atoms. Every time this happens, the mass within the atoms is transformed into the energy of speeding nuclei fragments.

Without doubt the most famous equation in physics is Einstein's E=MC squared, which in plain English means "energy equals mass multiplied by the square of the speed of light". Now, a fragment of an atom has only a tiny amount of mass, obviously, but the speed of light multiplied by itself is such a huge number ( in miles per hour, it's 448,900,000,000,000,000) that huge amounts of energy is being produced every time mass is undergoing this conversion process.

This chain reaction lasts for just a few millionths of a second, during which time the bomb will have fallen just a fraction of an inch closer to the ground. Once the conversion of mass into energy is over, the energy of those speeding nuclei fragments battering against the inside of the bomb's casing gets converted into heat energy. This is the same principle by which you can warm your hands up by rubbing them together, but where those speeding uranium fragments are concerned they are rubbing against the metal at speeds close to that of light. This creates an enormous amount of heat energy, going from near body temperature of 37 degrees Celsius to several million degrees Celsius (hotter than the core of our Sun) in mere moments.

As you might well expect all that tremendous heat energy goes through the bomb's metal casing like it isn't there. But at this instant you wouldn't see it, because the explosion has energy that must first be released in the form of X-rays that it pushes away from itself. The explosion hovers in space for as long as the fragments are trying to cool themselves off by spraying out X-rays. But, given that this event lasts just 1/10,000th of a second it would seem instantaneous were you foolish enough to look at something burning brighter than the core of the sun.

After the release of X-rays the heat ball can resume its spread outwards. Whereas before ordinary light photons could not push through the X-rays, now the full flash of the explosion becomes visible. If you could look at it, it would resemble a star filling several hundred times the patch of sky that our sun fills. But you could not look at it without suffering instant blindness as the intense light and heat burned your optic nerves.

The newborn star burns at full power for about half a second, and then begins to fade away over the duration of about three seconds. During this process energy is converted into heat that sprays outward in all directions. This is the moment when the first tens of thousands lives are lost.

image.jpeg

(Image from wikimedia commons)

The heat pushes on the air around it, accelerating it to speeds far exceeding that of any hurricane. In fact, it's so fast it's silent, because it outruns any sound it makes. Following this air pulse is a second one, travelling a little slower. According to David Bodanis, "after that the atmosphere sloshes backward, to fill up the gap pushed out. This briefly lowers the air density to virtually zero. Far enough from the blast, life-forms that have survived will now begin to explode outward, having been exposed-briefly-to the vacuum of outer space".

Not all of the amount of heat can move forward. Some of it remains behind, hovering at the point where the bomb had once been. It stays close to the point of detonation for only a few seconds before it begins to rise, swelling as it goes and spreading out once it reaches sufficient height. Dust and debris gets sucked up with it, and to anyone still alive at this point, the result would look like a towering cloud shaped just like a mushroom.

REFERENCES

Wikipedia

"E=MC^2: Biography Of An Equation" by David Bodanis

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Very cool post! And highly relevant with the whole North Korea situation. Thanks for sharing, just followed.

@exite-dasliva When An Atomic Bomb Goes Off? Than A nuclear bomb explosion starts with an intensely bright flash, followed by a fireball, which burns everything it touches. The heat from the explosion is so powerful that it can be felt for miles away, in a "heat blast." it's really harrowing view:))

WOW. Very interesting stuff. I never knew all of that.

Thanks for the comments, everyone!

What is the different between atomic bomb going off and a nuclear bomb going off, i still cant place the different btw the two if they are different

An atomic bomb relies on nuclear fission aka 'splitting the atom'. A nuclear bomb relies on nuclear fusion, transforming an element into another element and releasing tremendous energy in the process.

Wow . I wonder what the first scientist was trying out when they found these out, thank you so much for explaining it to me

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