Nuclear Explosion Effects

[castlebravo92]

Just read this and wanted to share some thoughts.

You're correct that most tactical strikes would be air bursts. With (reactivated) B57 and the (almost ubiquitous) B61 probably delivering most of the strikes during the Twilight War. Nuclear bunker busters, such as the B83 nuclear bomb, which could also be dialed into the small kiloton range, would instead be used in ground burst or even subterranean explosions.

For explosions in these low kiloton and up to mid-double digit kiloton ranges, there are some important differences to keep in mind to the much bigger, strategic device explosions: First and foremost, their fireballs do not reach into the stratosphere, making them irrelevant for any discussions on the "nuclear winter" topic as this phenomenon is caused by black soot reaching the stratosphere in relevant amounts to have long lasting effects. Everything not reaching the stratosphere and thus remaining in the troposphere will fall back down onto the surface quickly, within hours or days, and thus have a negligible effect on climate (but not weather, an important difference!).

Second, even though we're talking tactical devices, these explosions are huge and extremely pinpointed. A 20 kt device will leave a crater 25 m deep, 15 m for a 5 kt device and still 6 m deep for the lowest possible yield a B61 can be dialed upon: 0.3 kt. These are no values conventional devices could attain, maybe with the exception of the Mk. 118 demolition bomb. The massive difference of nuclear bombs stems from the high pressure (psi) values reached, I believe, and not so much purely from the blast yield in tons TNT equivalent.

Third, the mode of why craters are formed is different, I believe. A nuclear device produces enormous levels of pressure (psi) an simulations from this site suggest that crater depth does not depend on whether a burst occurs on the surface or in the air. Conventional, "dumb" bombs however tend to explode after hitting the surface and thus in the ground. This changes crater creation dynamics and the amount of matter ejected out of the crater: I would presume a nuclear airburst crater to have a wider crater lip radius (from the center to the outer boundary of the lip) due to the explosion ejecting matter outward to the sides. A conventional explosion might eject matter more into the air as well.

This latter point, the blast wave going to all sides, is what creates the immense pressure waves that destroy the surrounding area. A 0.3 kt blast, still being ca. 335 times larger than a that of a Mark 118 demolition bomb (with its 896 kg warhead) will result in moderate blast damage out to about 300 m to 340 m (depending on air burst optimization (radii or overpressure). Moderate blast damage here means 5 psi overpressure, most residential buildings collapse, injuries being universal, and fatalities being widespread. Light damage, including shattering glass and probably broken off trees, would go out to 1.32 km.

So, the main differences visible to the naked eye between a nuclear and a conventional explosion would simply be the massive damage to the surrounding environment far beyond the immediate crater. When the Saudis used a Mk. 87 bomb on a Yemeni market in March 2016 (with a 428 kg warhead filling) they killed 97 people. Hitting a market in an urban center with a 0.3 kt device would likely kill close to 6,000 people immediately and wound another 20,500 people, many of them would die from maiming and 3rd to 2nd degree burns, which occur out to about 380 m and 480 m respectively. These burns would be also visible on trees, with charring marks at least out to 350 m, but probably more, and knocked over trees out to beyond 1 km in a concentric pattern around the 6.18 m deep crater (that's easily a two story building).

All good points. One slight quibble, according to the published data, crater formation scales logarithmically with TNT yield equivalent, regardless of explosive type:



Another thing to point out is that for nukes, blast damage follows an inverse cube law - doubling the effect requires 8x the explosive yield. Lethal radiation from x-rays, gamma-rays, and neutrons, however, doesn't scale well at all.

https://nuclearweaponsedproj.mit.edu/nuclear-weapon-effects-simulations-and-models/nuclear-weapons-blast-effects-calculator/radiation

A 1 kt nuclear explosion will delivery 450 rads between neutrons and gamma rays to an unshielded person at 914 meters, falling off to 0 rads at 3.7 km.

100 kt will only deliver 0.19 rads at 3.7 km (but 45,000 rads at 1 km).

A 1 megaton explosion will deliver 1.9 rads at 3.7 km.

This is "important" because as yields go up, radiation ceases to be much of a factor in prompt deaths and injuries because blast and thermal injuries take over (for example, for the 1 megaton explosion, PSI for an airburst at 2280 meters height, 3.7 km away would be ~12.7 and wind speed would be 360 mph, which would be sufficient to flatten all but the most heavily reinforced structures - fatalities in this zone approach 100% unless underground or in a blast shelter).

Conversely, for that 1 kt explosion @ 120 meter detonation height, overpressure would be 1.9 PSI at 914 meters, and wind speed would be 68 mph. That's enough to break windows and damage wood siding in homes, and cause some injuries and maybe a few fatalities from flying debris. Thermal effects are not sufficient to cause even 1st degree burns.

But 914 meters away from GZ, people are still receiving 450 rads of radiation exposure. The LD50 for radiation exposure is 250-500 rads. So half of the people in a 914 meter radius would die, the other half would have serious radiation injuries. These are deaths & injuries from the bone marrow dying off and immune compromise and subsequent infection as a consequence.

[Raellus]

Clues

So, the main differences visible to the naked eye between a nuclear and a conventional explosion would simply be the massive damage to the surrounding environment far beyond the immediate crater. When the Saudis used a Mk. 87 bomb on a Yemeni market in March 2016 (with a 428 kg warhead filling) they killed 97 people. Hitting a market in an urban center with a 0.3 kt device would likely kill close to 6,000 people immediately and wound another 20,500 people, many of them would die from maiming and 3rd to 2nd degree burns, which occur out to about 380 m and 480 m respectively. These burns would be also visible on trees, with charring marks at least out to 350 m, but probably more, and knocked over trees out to beyond 1 km in a concentric pattern around the 6.18 m deep crater (that's easily a two story building).

To be clear, I was not contrasting the effects of a single large conventional bomb and a tactical nuclear weapon, but your points are well-taken.

The war in Ukraine has produced some stunning images of the effect of large volumes of conventional explosives on urban areas- Bakhmut, being an especially good example. Aside from being in color, aerial shots of present-day Bakhmut don't appear all that dissimilar from photos of Hiroshima taken in September, 1945 (it's also worth noting the fact that Bakhmut started with many times more concrete structures than Hiroshima did). To reiterate my point, a couple of years after a battle/campaign like Bakhmut, it would probably be difficult for the average soldier (not in possession of a Geiger counter) to distinguish whether such extensive damage was created by intense conventional warfare or by a tactical nuclear weapon. This would be especially true the further from ground zero one happens to be.

Of course, as you pointed out, there would be other clues, that those more experienced, or with special training perhaps, would likely recognize.

In a T2k campaign, in trying to clue the audience into the fact that the party was entering the outer margins of an old tac-nuke blast zone, I described the damage to a wooded area near a kilometer or two from ground zero as a concentric band of fallen trees all lying pointing roughly in the same direction. I got the idea after seeing photos of some of the forest surrounding Mt. St. Helens (a volcano in upstate Washington) a year or two after the surprise 1980 eruption.

-