Meditations | Visions

What the NUKEMAP taught me about fallout

Posted August 2nd, 2013 by Alex Wellerstein

One of the most technically difficult aspects of the new NUKEMAP was the fallout generation code. I know that in practice it looks like just a bunch of not-too-complicated ellipses, but finding a fallout code that would provide what I considered to be necessary flexibility proved to be a very long search indeed. I had started working on it sometime in 2012, got frustrated, returned to it periodically, got frustrated again, and finally found the model I eventually used — Carl Miller’s Simplified Fallout Scaling System — only a few months ago.

The sorts of contours the Miller model produces.

The sorts of contours the Miller scaling model produces.

The fallout model used is what is known as a “scaling” model. This is in contrast with what Miller terms a “mathematical” model, which is a much more complicated beast. A scaling model lets you input only a few simple parameters (e.g. warhead yield, fission fraction, and wind speed) and the output are the kinds of idealized contours seen in the NUKEMAP. This model, obviously, doesn’t quite look like the complexities of real life, but as a rough indication of the type of radioactive contamination expected, and over what kind of area, it has its uses. The mathematical model is the sort that requires much more complicated wind parameters (such as the various wind speeds and sheers at different altitudes) and tries to do something that looks more “realistic.”

The mathematical models are harder to get ahold of (the government has a few of them, but they don’t release them to non-government types like me) and require more computational power (so instead of running in less than a second, they require several minutes even on a modern machine). If I had one, I would probably try to implement it, but I don’t totally regret using the scaling model. In terms of communicating both the general technical point about fallout, and in the fact that this is an idealized model, it does very well. I would prefer people to look at a model and have no illusions that it is, indeed, just a model, as opposed to some kind of simulation whose slickness might engender false confidence.

Fallout from a total nuclear exchange, in watercolors. From the Saturday Evening Post, March 23, 1963.

Fallout from a total nuclear exchange, in watercolors. From the Saturday Evening Post, March 23, 1963. Click to zoom.

Working on the fallout model, though, made me realize how little I really understood about nuclear fallout. I mean, my general understanding was still right, but I had a few subtle-but-important revelations that changed the way I thought about nuclear exchanges in general.

The most important one is that fallout is primary a product of surface bursts. That is, the chief determinant as to whether there is local fallout or not is whether the nuclear fireball touches the ground. Airbursts where the fireball doesn’t touch the ground don’t really produce fallout worth talking about — even if they are very large.

I read this in numerous fallout models and effects books and thought, can this be right? What’s the ground got to do with it? A whole lot, apparently. The nuclear fireball is full of highly-radioactive fission products. For airbursts, the cloud goes pretty much straight up and those particles are light enough and hot enough that they pretty much just hang out at the top of the cloud. By the time they start to cool and drag enough to “fall out” of the cloud, they have diffused themselves in the atmosphere and also decayed quite a bit.1 So they are basically not an issue for people on the ground — you end up with exposures in the tenths or hundreds of rads, which isn’t exactly nothing but is pretty low. This is more or less what they found at Hiroshima and Nagasaki — there were a few places where fallout had deposited, but it was extremely limited and very low radiation, as you’d expect with those two airbursts.

I thought this might be simplifying things a bit, so I looked up the fallout patterns for airbursts. And you know what? It seems to be correct. The radiation pattern you get from a “nominal” fission airburst looks more or less like this:

The on-side dose rate contours for the Buster-Jangle "Easy" shot (31 kilotons), in rads per hour. Notice that barely any radiation goes further than 1,100 yards from ground zero, and that even that is very low level (2 rads/hr).

The on-side dose rate contours for the Buster-Jangle “Easy” shot (31 kilotons), in rads per hour. Notice that barely any radiation goes further than 1,100 yards from ground zero, and that even that is very low level (2 rads/hr). Source.

That’s not zero radiation, but as you can see it is very, very local, and relatively limited. The radiation deposited is about the same range as the acute effects of the bomb itself, as opposed to something that affects people miles downwind.2

What about very large nuclear weapons? The only obvious US test that fit the bill here was Redwing Cherokee, from 1956. This was the first thermonuclear airdrop by the USA, and it had a total yield of 3.8 megatons — nothing to sniff at, and a fairly high percentage of it (at least 50%) from fission. But, sure enough, appears to have been basically no fallout pattern as a result. A survey meter some 100 miles from ground-zero picked up a two-hour peak of .25 millirems per hour some 10 hours later — which is really nothing to worry about. The final report on the test series concluded that Cherokee produced “no fallout of military significance” (all the more impressive given how “dirty” many of the other tests in that series were). Again, not truly zero radiation, but pretty close to it, and all the more impressive given the megatonnage involved.3

Redwing Cherokee: big boom, but almost no fallout.

Redwing Cherokee: quite a big boom, but almost no fallout.

The case of the surface burst is really quite different. When the fireball touches the ground, it ends up mixing the fission products with dirt and debris. (Or, in the case of testing in the Marshall Islands, coral.) The dirt and debris breaks into fine chunks, but it is heavy. These heavier particles fall out of the cloud very quickly, starting at about an hour after detonation and then continuing for the next 96 hours or so. And as they fall out, they are both attached to the nasty fission products and have other induced radioactivity as well. This is the fallout we’re used to from the big H-bomb tests in the Pacific (multi-megaton surface bursts on coral atolls was the worst possible combination possible for fallout) and even the smaller surface bursts in Nevada.

The other thing the new model helped me appreciate more is exactly how much the fission fraction matters. The fission fraction is the amount of the total yield that is derived from fission, as opposed to fusion. Fission is the only reaction that produces  highly-radioactive byproducts. Fusion reactions produce neutrons, which are a definite short-term threat, but not so much a long-term concern. Obviously all “atomic” or fission bombs have a fission fraction of 100%, but for thermonuclear weapons it can vary quite a bit. I’ve talked about this in a recent post, so I won’t go into detail here, but just emphasize that it was unintuitive to me that the 50 Mt Tsar Bomba, had it been a surface burst, would have had much less fallout than the 15 Mt Castle Bravo shot, because the latter had some 67% of its energy derived from fission while the former had only 3%. Playing with the NUKEMAP makes this fairly clear:

Fallout comparisons

The darkest orange here corresponds to 1,000 rads/hr (a deadly dose); the slightly darker orange is 100 rads/hr (an unsafe dose); the next lighter orange is 10 rads/hr (ill-advised), the lightest yellow is 1 rad/hr (not such a big deal). So the 50 Mt Tsar Bomba is entirely within the “unsafe” range, as compared to the large “deadly” areas of the other two. Background location chosen only for scale!

The real relevance of all of this for understanding nuclear war is fairly important. Weapons that are designed to flatten cities, perhaps surprisingly, don’t really pose as much of a long-term fallout hazard. The reason for this is that the ideal burst height for such a weapon is usually set to maximize the 10 psi pressure radius, and that is always fairly high above the ground. (The maximum radius for a pressure wave is somewhat unintuitive because it relies on how the wave will be reflected on the ground. So it doesn’t produce a straightforward curve.) Bad for the people in the cities themselves, to be sure, but not such a problem for those downwind.

But weapons that are designed to destroy command bunkers, or missiles in silos, are the worst for the surrounding civilian populations. This is because such weapons are designed to penetrate the ground, and the fireballs necessarily come into contact with the dirt and debris. As a result, they kick up the worst sort of fallout that can stretch many hundreds of miles downwind.

So it’s sort of a damned-if-you-do, damned-if-you-don’t sort of situation when it comes to nuclear targeting. If you try to do the humane thing by only targeting counterforce targets, you end up producing the worst sort of long-range, long-term radioactive hazard. The only way to avoid that is to target cities — which isn’t exactly humane either. (And, of course, the idealized terrorist nuclear weapon manages to combine the worst aspects of both: targeting civilians and kicking up a lot of fallout, for lack of a better delivery vehicle.)

A rather wonderful 1970s fallout exposure diagram. Source.

A rather wonderful 1970s fallout exposure diagram. Source.

And it is worth noting: fallout mitigation is one of those areas were Civil Defense is worth paying attention to. You can’t avoid all contamination by staying in a fallout shelter for a few days, but you can avoid the worst, most acute aspects of it. This is what the Department of Homeland Security has been trying to convince people of, regarding a possible terrorist nuclear weapon. They estimate that hundreds of thousands of lives could be saved in such an event, if people understood fallout better and acted upon it. But the level of actual compliance with such recommendations (stay put, don’t flee immediately) seems like it would be rather low to me.

In some sense, this made me feel even worse about fallout than I had before. Prior to playing around with the details, I’d assumed that fallout was just a regular result of such weapons. But now I see it more as underscoring the damnable irony of the bomb: that all of the choices it offers up to you are bad ones.

  1. Blasts low enough to form a stem do suck up some dirt into the cloud, but it happens later in the detonation when the fission products have cooled and condensed a bit, and so doesn’t matter as much. []
  2. Underwater surface bursts, like Crossroads Baker, have their own characteristics, because the water seems to cause the fallout to come down almost immediately. So the distances are not too different from the airburst pattern here — that is, very local — but the contours are much, much more radioactive. []
  3. Why didn’t they test more of these big bombs as airdrops, then? Because their priority was on the experimentation and instrumentation, not the fallout. Airbursts were more logistically tricky, in other words, and were harder to get data from. Chew on that one a bit… []

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18 Responses to “What the NUKEMAP taught me about fallout”

  1. RLBH says:

    Great work on the new NUKEMAP! I’ve had the discussion about modelling accuracy with folks elsewhere – seems that some people are never satisfied! If/when you get around to releasing the effects library, I think that would be an impressive resource to have available.

    Strangely, it’s fallen over for me lately; the sidebar loads, but the map window remains blank. The Classic version still works, and the new version works on my home computer. I’ve got the latest Java update, so any ideas would be welome…

  2. Whitehall says:

    The US had a fleet of B-52 with 25 MT bunker busters to attack the Soviet command and control systems. I wonder if the US estimated the collateral Russian deaths from such an attack?

  3. Daryl Press says:


    This is interesting work.

    A couple of years ago, after poring over the Glasstone and Dolan book on Nuclear Effects, then working with HPAC, then going back to Glasstone, etc.. I came to the same conclusion as you about the feasibility of minimizing local fallout via HoB.

    But there’s a second “aha” that your initial realization can lead to, which you haven’t yet taken: it seems possible to create *substantial* amounts of overpressure on the ground (ie, static and dynamic) with low-yield weapons set to detonate near or above the fallout threshold. If so, that creates options for low-fallout counterforce targeting, mitigating the “damned if you do, damned if you don’t” conundrum that you identify. The “target” of course is the silo cap or blast doors on the command bunker.

    Have you looked into this?


    • My understanding of this, which is admittedly not deep, is that ground bursts maximize the area of extremely high overpressure (say, 3000 psi or so) which is necessary for counterforce targeting assuming hardened silos. The current NUKEMAP calculations only model up to 200 psi so it is hard to get a sense of how that range is affected by it. Presumably there is an accuracy issue involved as well, given how relatively localized that range of effects must be.

      • Daryl Press says:

        Yes, ground bursts *maximize* areas of very high overpressure, but at the cost of very nasty fallout — as your good post suggests.

        As Figures 3.73(a) in Glasstone shows, even as small as a 1 kiloton bomb generates 10,000 psi of overpressure out to about 70 feet, depending on the HoB. You’re right that dealing in such small areas of effect make accuracy key, but we’re living in an era in which 5-15 meter accuracy is pretty straightforward, on the low side of that range if you face no countermeasures (e.g. GPS jamming among other things), and the higher end of that range if there is jamming.

        What you discovered and posted about regarding fallout is profound. When you combine that observation with these facts about 10,000+ psi being achievable without fallout, the implications are pretty significant.

        • “but we’re living in an era in which 5-15 meter accuracy is pretty straightforward” — is this true for the kinds of weapons that would be used in counterforce scenarios? I just don’t know. The sources I’ve seen (just public ones) imply that the CEP for our long-range warheads (ICBMs, SLBMs) is that they have CEPs ten times this number. The only nuke in the US arsenal that has CEPs that low is the B61-12 concept that has the JDAM-like tail kit. I imagine this is why the USAF started looking into low-CEP, low-yield weapons in the mid-1990s. Presumably the actual use of these sorts of things are not in more balanced counterforce scenarios, though — the one-off bunker buster rather than something that would go against a whole fleet of things. But again, I am probably out of my depth on this particular issue.

          • Daryl Press says:

            1) You’re not out of your depth.

            2) If your original blog post was about “capabilities in the current US nuclear arsenal,” then you’re right that only a few current U.S. delivery systems seem to be accurate enough. Perhaps bombs from either B-2s or tactical aircraft — because 15meters is accurate enough — and perhaps cruise missiles. But your original blog was broader and more important. It was about *nuclear effects*, and you were moving toward some pretty important “ahas’ which might reveal how professionals (not me) who actually think about employment and create war plans (and advise on force structure) might think about them. It turns out that “minimal fallout” counterforce is just as feasible as “minimal fallout” counter value. Don’t take my word for it; look up the fallout threshold formula in Glasstone and see the overpressure you can generate with low-yield weapons at / near those altitudes. Combined with your first realization about fallout, it’s pretty stunning.

            3) Last, we shouldn’t think about the implications of #2 only in terms of *US* choices about what weapons and delivery systems we want in the force. What you’ve stumbled upon has implications for, say, what the Israeli arsenal and capabilities probably look like. And the Chinese arsenal. We’re not the only one who can choose to build high-accuracy / low fallout weapons . The Pershing II ballistic missile reportedly had a 30 meter CEP — and that is 35 year old technology! Israel is said to have modeled Jericho on Pershing. China already reportedly has very accurate theater ballistic missiles.

            My point is that much of the open-source blog / arms control community may be operating under fundamentally wrong assumptions about what a nuclear war would look like. It would probably be terrible, but could be very different from what most commentators are suggesting.

          • Daryl Press says:


            Currently only a few current U.S. delivery systems seem to be accurate enough to deliver NW within 15 meters or so. But the implications of low-fallout counterforce go beyond the U.S.

            1) According to Glasstone and Dolan (Fig 3.73a) you can generate 10,000+ psi with a 1-kt bomb at a HoB that won’t produce much fallout (that’s static overpressure; you can calculate dynamic overpressure too and it’s impressive as well).

            2) Other countries may very well have 15 meter accurate nuclear delivery systems today. The U.S. Pershing II ballistic missile reportedly had 30 m CEP, and that guidance technology is 30 years old. Israel may have modeled aspects of Jericho on Pershing II. China has been — according to DoD — producing very accurate conventional ballistic missiles. Whether the US goes this direction or not, this is the world we’re currently living in.

            Bottom line: your “aha” about low-fallout airbursts means that low-fallout counterforce is possible. The conundrum you identified in your post — that counter value = murder, and counter force = mass fallout — is no longer so clear.


  4. Allen Thomson says:

    You might want to visit the late-1980s NAPB-90 and see how its results compare with NUKEMAP. IIRC, the general attack scenario included 2-on-1 SS-18 Mod 4 RVs ground bursting on US ICBM silos, which led to a *lot* of fallout in the Midwest.


    “Four levels of potential fallout risk were defined:

    “Very High Fallout Risk Counties were defined as those which have the potential to receive a one-week unprotected radiation dose of equal to or greater than 15,000 roentgens (R). The counties which were defined at this risk level have resident populations totaling 9.6 millions (4 percent of the U.S.) and cover approximately 421,669 square miles.

    “High Fallout Risk Counties were defined as those which have the potential to receive a one-week unprotected radiation dose of equal to or greater than 6,000 roentgens but less than 15,000 roentgens. The counties which were defined at this risk level have resident populations totaling 49.2 millions (20 percent of the U.S.) and cover approximately 624,407 square miles.

    “Medium Fallout Risk Counties were defined as those which have the potential to receive a one-week unprotected radiation dose of equal to or greater than 3,000 roentgens but less than 6,000 roentgens. The counties which were defined at this risk level have resident populations totaling 62.6 millions (26 percent of the U.S.) and cover approximately 618,811 square miles.

    Low Fallout Risk Counties were defined as those which have the potential to receive a one-week unprotected radiation dose of less than 3,000 roentgens. The counties which were defined at this risk level have resident populations totaling 120.8 millions (50 percent of the U.S.) and cover approximately 1,886,339 square miles. “

  5. Peter Solem says:

    Given that there are projected to be about 500 nuclear reactors scattered around the world within a decade, each fueled with hundreds of tons of radioactive material and typically storing even larger quantities of fission and transuranic products on-site, it would seem that the worst-case fallout scenario is not a surface detonation above a coral atoll, but rather a surface detonation directly above a reactor and its spent fuel storage pond (which are co-located in Fukushima-type reactor designs). The fire resulting after a nuclear explosion over a nuclear reactor would surely lead to large-scale radioactive fallout of the kind seen after Chernobyl:

    In any modern nuclear exchange (say, India-Pakistan or Israel-Iran) nuclear reactors would surely be on the targeting list. It might not even matter if they were targeted at the surface or high above, since an uncontrolled fire at the reactor site would result in either case. This is a point made by Gorbachev in the aftermath of Chernobyl, by the way:

    “Gorbachev noted with seemingly genuine horror the devastation that would occur if nuclear power plants became targets in a conventional war much less a full nuclear exchange.” – George Schultz, as quoted in Richard Rhodes, Arsenals of Folly

    • There has been a study on this very question. My recollection is that it doesn’t change the contours too much, but the overall length of time of contamination is much longer. I’ll dig that up again. But yeah, one can imagine worse contamination issues — I meant in terms of nuclear testing.

    • Daryl Press says:

      “In any modern nuclear exchange (say, India-Pakistan or Israel-Iran) nuclear reactors would surely be on the targeting list. It might not even matter if they were targeted at the surface or high above, since an uncontrolled fire at the reactor site would result in either case.”

      This is not really true. There are plenty of nuclear exchange scenarios — in my view the most likely ones — in which neither side is trying to maximize civilian damage to the other. Pakistani officials and analysts say, for example, that a major Indian incursion into their territory would trigger Pakistani nuclear strikes on Indian ground forces on Pakistani territory. It wouldn’t make sense for Pakistan to strike Indian cities, let alone Indian reactors.

      Nuclear war could escalate out of control. It could very well get horrific. But if one is interested in understanding the weapons, their strategic rationale, and perhaps affecting policy, we should keep a clear picture about why countries value them, and how they intend to use them.

      • I noticed on the fallout from a total nuclear exchange watercolor map that there is an example of Peter Solems idea. I presume all of the major nuclear weapons sites are targeted in this scenario, along with major bases and major cities.

        The large plume from SE Washington State (all the way down to the Columbia River on the WA/OR border) is the result of ground burst attacks on Hanford (plus a B52 base near Spokane). Plenty of stuff to loft into the air from those targets.

        As Daryl Press says, apart from some apocalyptic terrorist groups or nation states (and even those I think might get rational at that point), “modern” nuclear war is a much more considered option especially when you have a well armed opponent. We’re not in the Strangelovian Fifties anymore.

        Regarding very accurate conventional IRBMs or ICBMs the Chinese are working on them as anti-shipping (specifically US anti-carrier) weapons. That would require accuracy down to a couple of meters and they would almost certainly be terminally guided (as was the Pershing II RV with a active millimeter wave radar/imager, IIRC).

        The US also suggested the Prompt Global Strike which could be an ICBM though a hypersonic glider might be more acceptable given your nuclear opponents can’t tell if you have a nuke in the incoming ICBM warhead. That said “launch on warning” is a poor strategy for everyone especially for a single incoming warhead. After all how do you tell the difference between a conventional cuise missile and a nuclear one?

  6. […] that the Cherokee test was a true airburst (the fireball didn’t touch the ground), and so didn’t generate any significant fallout. The Goldsboro bomb, however, was meant to operate on impact, as a surface burst, and would have […]

  7. […] like Hiroshima and Nagasaki, this doesn’t matter too much, as long as the fireball is above the altitude which produces local fallout, but for a “hard” target, where the goal is to put a lot of pressure in one spot, this […]

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