To most people an explosion is just a big BOOM. Most people have heard the term ‘detonation’, but few have ever heard the term ‘deflagration’. In terms of actual experiences, most people are way more familiar with deflagrations and have little familiarity with detonations. Of course this begs the question, why should anyone really care how the explosion occurred at Fukushima Daiichi #3?
From a practical perspective it doesn’t matter how the Uranium and Plutonium blasted into the air and ended up in EPA RADNET air filters in California and possibly into our lungs. The primary value in understanding the explosion at Fukushima is in redesigning future nuclear reactors and spent fuel storage pools so that the same does not happen again. However , if you have a desire to understand what happened at Fukushima, or if you would just like to be able to figure out if that house that exploded in your neighborhood was from dynamite or natural gas, read on.
The scientific difference between a detonation and a deflagration is the speed at which the flame front moves and the pressure waves expand. In a detonation the flame and pressure fronts move faster than the speed of sound. In a deflagration the flame and pressure fronts move at the speed of sound or slower. In practical every day experience, pressure waves move at the speed of sound. Since a deflagration does not move faster than a normal pressure wave, the pressures generated by a deflagration can be “pulled” towards areas of lower pressures like open windows and such. On the other hand, since the pressure waves generated by a detonation move faster than the speed of sound, those pressures cannot be “pulled” towards areas of lower pressure.
And that leads us to the practical differences between a detonation and a deflagration. A detonation will spread out in all directions equally at a rate FASTER than the speed of sound, and the resultant damage to structures will tend to be uniform in all directions. On the other hand, a deflagration will start to move in all directions uniformly but will be quickly pulled towards areas of lower pressure, the resultant damage will tend to concentrate towards the exterior of buildings. Common examples of deflagrations are, firearms and car engines; in each, a pressure wave builds and then is released towards the area of lower pressure. In a firearm the area of lowest pressure is down the barrel, hence the bullet flies out of the barrel. In a car engine’s combustion chamber the area of lowest pressure is the piston, hence the pressure pushes the piston down. If a detonation occurs in a car engine, the pressure slams the entire combustion chamber equally and causes a knock sound. Repeated engine detonations can destroy the engine. In a firearm, a detonation often destroys the gun; the pressure never redirects down the barrel and the chamber takes the brunt of it.
In terms of buildings, the most common example is a natural gas explosion. When a spark hits a house filled with natural gas and explodes, the deflagration causes the windows and exterior walls to be blown out. The interior walls will show lesser signs of damage. The damage will be greatest in areas towards the points of lowest pressures, door, windows, and exterior walls. The debris from a deflagration will be larger in size; lengths of 2x4’s and large pieces of drywall and plywood. On the other hand, a house that has exploded as the result of dynamite will have the same amount of damage on the interior walls as the exterior walls (since the pressure is moving faster than the speed of sound it cannot be “pulled” towards the exterior walls, windows, etc). The debris one finds from a detonation will be small pieces, often splinters instead of lengths or big chunks. With that knowledge in hand, we are ready to tackle the question of the explosion of Fukushima Daiichi #3.
One of the first things people notice about the Fukushima Daiichi #3 explosion was the large straight up almost mushroom cloud type blast that occurred; the first thing that crossed many people’s minds was an atomic explosion. However, that straight up ward blast is strongly indicative of a large hydrogen explosion. Hydrogen is much lighter than air and will shoot straight up when released, the effect is magnified when the hydrogen is also burning. That same blast will tend to suck up debris with it. The video of the mushroom cloud at Fukushima is primarily indicative of a hydrogen deflagration . The majority of debris photos/video I have seen from Fukushima 3 indicated greater damage to exterior walls than interior walls; the debris also appears to be larger rather than smaller. Again, these are all signs of deflagration not detonation. The question has been raised by Arnie Gundersen how the fuel rods could have shot out of Fukushima 3. The fact that larger sections of fuel rods have been found outside the building would tend to precluded a detonation. A large detonation would have tended to tear those fuel rods into small bits.
Moreover, there are several possibilities that can explain the fuel rods blasting out of the building- none of which need involve a nuclear detonation. The vacuum from a large hydrogen explosion could have sucked up the fuel rods and expelled them from the building. It is also likely that the water in the fuel cooling pond was supersaturated with hydrogen. When the explosion occurred, the hydrogen in solution in the cooling pond water would have frothed up, burned, and deflagrated, sending the fuel rods out of the top of the holding pond. Visualize a bottle of Coke dropping; the resultant the carbon dioxide coming out of solution and shooting out of the end; now instead of carbon dioxide picture it as burning hydrogen shooting out. The fuel rods would have shot out of the top of the holding pond much like a bullet shoots out of a gun when the gun powder deflagrates.
Here is a video that will let you see and hear the differences between a deflagration and detonation. Notice the during the detonation the apparatus and flag receive a greater impact, than during the deflagration. The difference is that the expanding gases from the deflagration are pulled towards the areas of lower pressure (the open air), where as the supersonic expanding gases from the detonation go in all directions uniformly .
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