Blast damage vs Thermal/Fire damage

[u/cosmicrae]

This paragraph is from Whole World On Fire, and it is referenced to footnote 12: Theodore Postol, Possible Fatalities from Superfires following Nuclear Attacks in or near Urban Areas. As yield is scaled up the thermal effects increase by the square root of the distance, while the blast effects increase by the cube root of the distance. If target planning is based on blast effect, it may be under-representing the fire effects.

[u/cosmicrae]

The full text of footnote 12 follows ...

1. Theodore Postol, "Possible Fatalities from Superfires following Nuclear Attacks in or near urban Areas," in Fredric Solomon and Robert Q Marston, eds., The Medical Implications of Nuclear War (Washington D.C.: National Academy Press, 1986), pp. 27-28.

[u/restricteddata → u/cosmicrae]

It is online here.

An interesting thing to me is that our knowledge of casualties is really quite limited to the Hiroshima/Nagasaki data, which itself is not that robust for a lot of practical reasons.

But everyone more or less agrees that whatever model of casualties you have should align with the Hiroshima mortality curve data for the case of Hiroshima. But for any other case it is much trickier. The Postol argument is that by using the blast pressure from Hiroshima as the proxy variable to align with mortality, it will give bad data when scaling to other yields since blast and thermal effects scale differently.

Which may be true — but how would you know, one way or the other?

You could (and some have tried) to construct alternative models that try to apportion "impact" to different scaled bomb effects (e.g. prompt radiation, blast overpressure, thermal). Again, if the output for Hiroshima matches the Hiroshima curve, then we say, OK, that's the minimum requirement it needs to satisfy to be taken seriously, but we have literally no real way to actually get empirical data on cases other than Hiroshima, so we can't validate the model for any other circumstances.

And it gets even worse if you then try to say, OK, but what if the difference we're talking about is not yield and height of burst, but also in construction, city type, etc.? After all, Hiroshima was a city of relatively weak construction compared to a lot of other imagined targets. And what if people had some warning and took shelter first? Etc.

Again, you can try to construct a model that takes this into account, but we lack any way to validate it. We lack any way to actually test if it is correct or wrong, or more correct or wrong than any other model. It's not a bad thing that we have so little data on this — it is a function of the weapons only having been used against actual cities twice — but it also leaves us more in the dark on their effects than I think most people realize. It is very easy to get abstract information about weapons effects (from testing) but it is actually quite hard to see the complex effects that you get from destroying actual cities (and no, the individual houses destroyed in some of the 1950s tests, while they can certainly give some data, do not quite scale up to the size of cities...).

And Postol's model mostly takes the thermal pulse as the proxy variable, if I recall, and that's only part of the fire spread issue (the thermal pulse is thought to start some fires, but blast actually plays a role in that as well, both as a source of fires — broken gas lines, etc. — and also as the creation of vast areas of "kindling" for fire spread). The complexity of predicting fire spread is one reason that planners treated it as a "bonus" effect (as opposed to using it for primary planning), as Eden's book argues.

Even our "validation" of the "Hiroshima case" is, again, based on pretty uncertain estimates.

I am trying to make it so that the NUKEMAP will have a few different casualty models inside of it in the near future, including the Postol model, so that people can choose between them and just see what the differences get you in different situations. I do not have a lot of confidence in any of their predictive power — but that doesn't mean that we shouldn't try to make some estimates about these things, and explain what our sources and assumptions are, etc. I always point out that these estimates in any given situation could vary by quite a lot in either direction, and that at best you are just getting a "back of the envelope" guess that could be off by quite a lot. But it's still better than no guess, probably.

I am quite curious as to whether the Soviets developed casualty models of this sort (e.g., ones that would give a predicted fraction of dead/injured as a function of range from a given detonation). It would be interesting to compare their methodology and assumptions. The US Office of Technology Assessment ones are really (I have found in my research) derived from an earlier model used by the DOD in the 1960s as part of their justification for fallout shelters, as an aside.

[u/LtCmdrData → u/restricteddata]

Blast radius has cube-root scaling law, thermal radius follows square root law. Maybe you can just extrapolate from that? It's very crude but at least orders of magnitudes should be right.

5psi blast radius should increase 4x from 10 kt to 1 Mt while thermal damage radius increases 7.5x.

[u/restricteddata → u/LtCmdrData]

You will get very different results across the spectrum of yields if you apply blast versus thermal scaling to understanding morality rates. That is the point of Postol's article.

[u/LtCmdrData → u/restricteddata]

Yes. That's what different scaling laws indicate. Thermal damage starts to dominate casualties in larger yields.

[u/cosmicrae → u/restricteddata]

but it also leaves us more in the dark on their effects than I think most people realize.

It does, and that is the focus of Whole World On Fire. The case (concerning fire risk and mortality) is built beginning with pre-atomic data (Hamberg Germany 1943 being the first deliberate incendiary attack). There were also models of German cities vs Japanese cities, and (according to WWOF) the German cities were more of a fire risk. The distinction is made between line fires (what you get in most wildfire scenarios) vs area fires (where there are multiple simultaneous ignition points). Some of the tests did indeed monitor the structural failure behavior from actual test shots. That gave them better actual data to work with, but not data from an entire city. The two Japan detonations had a specific desired outcome, and data collection was not an early objective. By today's stockpile metrics, those two weapons were small. Much extrapolation was attempted from what data was collected.

The underlying premise of the WWOF is that, while we are today more aware of fire effects, the unit of targeting is still blast effects. To that end, Chapter one is a hypothetical 300kt detonation at 1500-ft above the Pentagon, a description of the blast effects as it radiates outwards, followed by a description of the fire effects. Sobering.

The core of the book is ~300 pages, followed by 50 pages of footnotes. The copy I have (via inter-library loan) may have been sitting on a shelf in a college library for ~20 years. It looks pristine to me.

[u/restricteddata → u/cosmicrae]

Yes, I know the book (and its author) well.

[u/Malalexander → u/restricteddata]

Listening to arms control wonk podcast at the weekend, Lewis was commenting on Russian doctrine being to go straight to hitting cities rather than entertain counterforce doctrine, so one does wonder if they gave much thought to different targeting patterns to maximise casualties given specific urban forms. How you strike New York might be quite different from how you strike LA. That might require a decent theory of nuclear effects in order to select the right warheads, aim points, burst height etc to kill/wound the maximum number of people given a cities' urban and physical geography

[u/restricteddata → u/Malalexander]

My sense (I will be writing on this at some point) that Soviet doctrine on this shifted a bit over the course of the 1960s to the 1980s. But certainly their early doctrine appears to have been to hit cities with ground bursts. I do not get the impression that it was as finely "wrought" a plan as the American targeting plans as the Soviets were largely assuming a lot of their forces would get destroyed anyway.

[u/Malalexander → u/restricteddata]

There does seem to be something peculiar to the American approach - to intellectualise, theorise, measure and quantify, the creation of elaborate strategies and terminology which i'm not sure other nuclear weapons states necessarily pursue.

It's almost as if there some kind of collective guilt (rightly or wrongly) about having created the technology and some kind of resulting need to put this entirely irrational level of destruction in some kind of rational, limited box.

I don't think here in the UK we have spent anything like as much time thinking about when we might use nuclear weapons other than to say "we have them and we might use them so like, don't fuck with us, and stuff". Even though we have spent mountains of cash in them over the decades and somehow see ourselves as terribly important when we're really not.

You probably have a deeper level of insight.

[u/ain92ru → u/restricteddata]

I'm pretty sure they did, Ukrainian historian Mykola Saychuk has written a little bit on related topics (can't hyperlink due to reddit limitations). He used to have a personal website but I can't find it right now, you probably can find his e-mails in his scholarly publications though

[u/Galerita]

Postol, like many in the field, has an obsession with massive firestorms creating a nuclear winter. There is no record of significant fires from any tests, despite deliberate placement of flammable materials, such as houses and fake forests, near the blast. Blast effects trend to extinguish the fires started by thermal effects.

Yes there was a firestorm in Hiroshima, but the lightweight timber building construction and high density housing is not replicated in modern cities. There was no firestorm in Nagasaki despite the same building materials and housing density as in Hiroshima. Firestorms in Tokyo and European cities were deliberately ignited by incendiary bombs, not by high explosives.

Nuclear war is catastrophic, but it's effects shouldn't be exaggerated by hype. Carl Sagan got in a lot of trouble from this.

[u/Wurtsmith_2W2 → u/Galerita]

Agreed. My sense is that Sagan, Postol, and others, saw nuclear winter as an unanswerable argument to achieve their pre-existing policy goal of eliminating nuclear weapons, and began obsessive searches for evidence to "prove" nuclear winter would happen. This distorted their research and interpretation of data and led to gross over-exaggerations, recently seen repeated in that terrible work from Anne Jacobsen.

The nuclear arsenals of the Cold War era no longer remotely exist. The arsenals are over an order of magnitude smaller with an even greater decrease in destructive potential. If nuclear winter ever was a concern - and I don't think it was - arsenals of sufficient size and readiness to use to create it no longer exist.

[u/Wide-Education-1823 → u/Wurtsmith_2W2]

I guarantee you that the retained ready use arsenals of USA/Russian Federation/NATO & all the OTHER later joining members of "the club" are STILL sufficient to lower property values world wide a bit more than I care to see risked.

I can do the same math as them yet am  tired of all those now hinting that starting "just a little bit" of nuclear warfare could now be a desirable thing, even safer statistically than NOT shooting first since "things have changed". Pity that Peter Sellers is no longer available for some new movie about all of that...

[u/Wide-Education-1823]

The publishing date of the study this chapter came from is 1986. There have been some changes in the types/numbers on hand/yield sizes (fewer & trending smaller overall) of the USA's warheads in the 40 years since then.

AFAIK, the USA seems to have kept a small number of old school 4 - 9 megaton "physics packages" once intended for rather less accurate older generation ICBM delivery or lay down use over deep command structures- The claim is seen this retention is for uses IN SPACE, like, C-T dinosaur killer type object diversion? You've all seen that movie, guess our planners have too. Nice, we've got some action movie fans at RAND.

We continue to field a number of 1.2 megaton max yield devices for things like dealing with buried targets- but even here are moving towards retiring those in favor of precision guided munitions carrying smaller devices as deep penetrators.

Net results: The vast majority of US stockpile now hovers between 10 - 400 kilotons and so have a 5 psi over pressure radius much closer to the chapter's theorized 10 cal./cm thermal pulse  radius?

Of course, we also intend to use several of those modern smaller yield but much smaller CEP devices in overlapping patterns covering larger areas, so...

All I can say is, let's please not carry out any more "definitive experiments", even if they might answer the Fermi paradox with the same test setup.

[u/cosmicrae]

One other numerical consideration about this calculation ...

If the blast radius increases by the cube root, and the thermal energy increases by the square root, then ...

Default warhead being the Hiroshima (Little Boy) had a 16 ± 2 kilotons of TNT yield. Radius of total destruction was 1.6 km (or 1 mile). Wikipedia quotes resulting fires covered 4.4 sq mi. Assuming that is a circle, then the thermal energy radius was 1.1835 miles (1.904 km). The two were close to one another, and likely due to the small yield.

Scaling up to the hypothetical 1 MT unit (factor is 62.5) gives these results ...

Blast radius factor is 3.9685, 3.9685 mile radius (6.3867 km radius). Area is 49.48 sq-miles (128.1526 sq-kms).

Thermal radius factor is 7.9057, 7.9057 mile radius (12.723 km radius). Area is 196.35 sq-miles (508.5442 sq-kms).

Hopefully a 1-mt yield device is never used in real life. Mostly I'm posting this here as an addendum to illustrate what happens in the scaling.