# Does the size of a nuclear explosion make a difference at zero range?

That is, supposing advanced aliens with force field technology appeared and we brought out the nukes as the last resort. At point blank range (i.e. the nuke detonates on impact with the force field) would a 10 megaton blast be any more intense (temperature, blast overpressure) than a 100 kiloton blast? Assuming the duration and total energy released made no difference to the force field, only peak intensity?

My WAG is yes. 10 megatons would be 100 times the intensity of 100kt. That’s the whole point of the TNT equivalency rating, right?

From a practical perspective, anything over what is needed to bust the alien force shield thing would be wasted, though.

It reads to me as though you have confused “equally ineffective” with “equal exertion of force.”

To get serious about your question, though, you need to get more detailed about this alien force field. So far, you are defining it as impervious to anything we can hit it with. Essentially, you are defining the alien force field as being outside the laws of physics, if its reaction to a 100 kiloton blast is identical to its reaction to a 10 Megaton blast.

Perhaps think of it in more mundane terms. If you have a two inch thick armor plate between you and someone who is attacking you, and they first throw a sledge hammer at the plate, and then they throw a piece of foam plastic, the amount of force that hits the plate will be different, even though the plate will remain intact. But since the plate has its limits, being only metal, you might discover that it has a tiny scratch on it from the sledge hammer, while the foam plastic leaves no trace at all.

There is a direct linear relationship from energy released to mass of TNT used to define bomb yield.
Note that this is energy. Translating this to mechanical force is another matter. At point blank to the nuke there will be not so much mechanical force, but a huge flux of x-rays. Lots of very fast neutrons, and gamma rays too. But the x-rays carry the lion’s share of the energy. The x-rays are the result of black-body radiation from the bomb material. You might argue that that material, being ultra-super-duper heated plasma, is creating a blast, but I have no idea what the fluid dynamics of such a material would be, and whether you can sensibly talk about its mechanical coupling to anything else.

The intensity of those x-rays, gamma rays and neutrons will scale linearly with the bomb tonnage. Your alien technology shield will need to do something with that energy. Maybe it is capable of reflecting (some of) them. Otherwise it is going to have to absorb that energy. No matter, the problem it has does indeed scale with the equivalent TNT.

If you stand off the bomb some distance the x-ray energy is adsorbed by the air, and you get a mechanical shock wave to carry the energy. Which is when things become much more TNT bomb like. Point blank to a nuke is going to involve very weird extreme physics.

No, I meant the force field is very very tough but not infinitely so; so maybe it would resist a plasma/radiation blast of 1,000,000 tonnes per cm² but 2,000,000 would breach it. To put my question another way, are all nuclear explosions equally hot at their core or are more powerful ones hotter?

One of the obstacles to the development of the H-bomb was making an A-bomb that was “hot” enough to compress the D-T material sufficiently to cause fusion. When Teller first started working on his “Super,” the existing A-bombs were too inefficient to use as a starter for an H-bomb. One development that increased the efficiency and temperature of the explosion was the use of levitated cores. So, yes, there are differences in the energy density of different bombs.

What is “some distance”, roughly.

Is it 10 feet, 100 feet, or more like a football field or so? Or more?

That is, maybe there are problems if Yosemite Sam sets up his thermonuclear device right next to Marvin the Martian’s shield and lights the long ssparly fuse, there may be problems.

But if Bugs Bunny tosses his bomb at the shield, it would probably go off near the shield, not exactly touching. Would that be “some distance”?

I guess some distance would be proportional to the yield of the device?

Here’s a link to a good paper on the physics of nuclear fireballs: The Effects of Nuclear Weapons, 1977: Scientific Aspects of Nuclear Explosion Phenomena

Here are some salient quotes:
" Immediately after the explosion time, the temperature of the weapon material is several tens of million degrees and the pressures are estimated to be many million atmospheres."

“The system then immediately emits electromagnetic (thermal) radiation, the nature of which is determined by the temperature. Since this is of the order of several times 10^7 degrees, most of the energy emitted within a microsecond or so is in the soft X-ray region”

“If the burst occurs in the lower part of the atmosphere where the air density is appreciable, the X rays are absorbed in the immediate vicinity of the burst, and they heat the air to high temperatures. This sphere of hot air is sometimes referred to as the ‘X-ray fireball’.”

“the X rays are absorbed within some yards of the burst point, and the relatively small volume of air involved is heated to a very high temperature.”

“As a result of interaction with the atmospheric molecules, the X rays so alter the chemistry and radiation absorption properties of the air that, in the air burst at low and moderate altitudes, a veil of opaque air is generated that obscures the early growth of the fireball. Several microseconds elapse before the fireball front emerges from the opaque X-ray veil.”

“When the average temperature of the isothermal sphere has dropped to about 300,000°C, the expansion velocity will have decreased to a value comparable to the local acoustic (sound) velocity. At this point, a shock wave develops at the fireball front and the subsequent growth of the fireball is dominated by the shock and associated hydrodynamic expansion. The phenomenon of shock formation is sometimes called “hydrodynamic separation.” For a 20-kiloton burst it occurs at about a tenth of a millisecond after the explosion when the fireball radius is roughly 40 feet.”

IANA nuclear physicist, but it sounds like at a very short range- <1 meter, or comparable to the physical size of the bomb- an object would be subjected to the direct kinetic energy of the fission/fusion particles generated, as opposed to “merely” the x-rays produced by heating of the surrounding medium. Doesn’t answer my original question but definitely indicates that point blank is going to have much higher energy densities that even a few meters away.

Given enough force, a piece of foam plastic can indeed be disastrous.

Yes of course, and I remember that incident all too well. However, that wasn’t the result of someone THROWING a piece of foam at the shuttle.