What's the point of nuclear weapons with variable yield?

If decision makers are at the point where they’re using nuclear weapons, are they really going to dial it down and risk not destroying the target to save civilians or are they just going to go for the biggest punch possible? I just don’t see a general saying an aircraft should use a B61 nuclear bomb to destroy an enemy sensor, command center or launcher but instead of using its full 80 kilotons, we’ll make it 10, 5 or 0.3.

I suppose the only real utility would be for the nuclear equivalent of a creeping barrage or close-air support where you have friendly units with NBC protection close-by to immediately rush into the shelled/nuked zone right after the munitions hit. Pretty nightmarish and it probably discounts the risks to friendly troops by figuring that they’ll die soon of something anyway but I can see that making sense.

But maybe other people can think of uses for it which would be both beneficial and actually likely to be used in such circumstances.

During the Cold War there was a concept that nuclear weapons could be used at a tactical level. They might be in a situation where they want to destroy JUST what was inside the Fulda Gap (or Sariwon Heights, or whatever) but not the surrounding areas due to presence of friendly troops, civilians, etc. They actually did studies of various targets and produced (grossly inaccurate) matrices estimating how much yield would be required to destroy a city of a certain size. During the 1950’s they even used Marines as human guinea pigs and had them occupy fighting positions near nuclear bombs during testing (with predictably terrible side effects).

So if this is how you are going to use a weapon, then it actually does make sense to have a dial-a-bomb. There are plenty of situations in which you want to use the correct yield for a given target, and collateral damage to civilians, friendlies, and infrastructure does remain a concern. You don’t want to be a situation where you need to use a bomb but the only bomb you have available is too big for the job.

But here’s the thing. OP says, “If decision makers are at the point where they’re using nuclear weapons…” This reflects a very modern and very binary mode of thinking. Our modern perspective is that ANY use of nuclear weapons would be an absolutely catastrophic, last resort type of thing that could very well initiate a global apocalypse. The entire point of using a dial-a-bomb would be to avoid that logic. They wanted to have some kind of option in between “no use whatsoever,” and “total nuclear armageddon.”

Wasn’t that very modern and binary mode of thinking brought about in part because the US military’s own wargaming suggested that a policy of tactical level nukes would very likely escalate to full-on strategic Hail Mary?

Beyond that, it’s simpler than it sounds. Basically in modern-day fusion weapons, you have two main components- the fission primary and the fusion secondary. What this means is that the primary is a very small fission bomb in its own right, and its energy is used to compress the secondary to the point where fusion reactions are able to really get going. These days, all fission primaries also use external neutron initiators- external devices that fire a burst of neutrons into the core to help the fission reaction really get going.

In general, the fission primaries can also be “boosted”, meaning that a little bit of tritium is injected into the core to fuse and give off a lot of neutrons, thereby making a fission bomb much more powerful (more neutrons = more of the Pu/U gets fissioned).

So “dial a yield” basically means that there’s a way to do the following:

[li]Fire the unboosted primary alone- this is the 0.3 kiloton yield.[/li][li]Fire the boosted primary alone. (different amounts of tritium can be used, affecting yield.[/li][li]Fire the primary with the ENIs (when and how long the ENIs are used affects yield.[/li][li]Fire the primary and secondary - this is usually full yield.[/li][/ul]

There are situations where you want low yield instead of high yield.

  1. Maybe you are going against a non-nuclear opponent (and hence have no nuclear retaliation to fear) but still need to take out some deep bunker ASAP that can’t be hit conventionally.

  2. Maybe you hope to keep tactical nuclear warfare tactical only but don’t want to hit civilians or your own troops.

  3. Maybe you just want to limit fallout and there’s no need to go big when small will do.

The nuclear material is the priciest part of a bomb. So instead of one big bomb you get 5 (say) small bombs. You get to hit 5 spots spaced miles apart. The one big bomb could take out one of them but not all of them.

OTOH, sometimes you just want to flatten a large city. Nuke them all, let God sort them out.

[li] Nuclear weapons make poor bunker busters as surface detonations tend to produce wide, shallow craters and it is very difficult to make a ground penetrating warhead where the internal “physics package” is protected against damage during impact.[/li][li]Limiting the range of effect is not only important for protecting friendly troops in a tactical strike but also limiting collateral damage, e.g. stopping a troop advance without destroying the city you are trying to protect.[/li][li]The best was to limit fallout is an airburst detonation. Small surface blasts—particularly those with a lot of unused nuclear material—will still produce aignificant amounts of radioactive fallout.[/ol][/li]

That isn’t what a variable weapon like a B61 Mod 4 or Mod 11 does. A variable yield works by injecting varying amounts of tritium to get partial or complete fusion and activating external neutron emitters to control yield. There is the same amount of nuclear material in the weapon regardless of what yield it is set to achieve.

Variable yield weapons were conceived at a time that nuclear weapons were considered for use in limited (tactical) warfare, rather than the strategic deterrence role they eventually grew into in which the entire purpose of the weapon was never to be used in practice but to deter any opposition use. Although wargaming scenarios have pretty conclusively shown that any unprovoked use of nuclear weapons between combatants with strategic-size arsenals (e.g. the United States and Soviet Union) results in total war due to uncertainty, confusion, and fear of losing retaliatory capability if the weapons aren’t used immediately, there has never been a consistent policy of either abandoning limited war or adopting a wholly strategic deterrence posture (i.e. “no first use”). As purely tactical weapons in a battlefield environment, however, nuclear weapons are basically a theatre commander’s nightmare, and their use would be a tacit admission that tactical control of the situation is completely lost. Short of a “Seven Days to the River Rhine” scenario it is difficult to see how any rational theatre commander would every welcome the use of nuclear weapons on the battlefield.


I think either you or I have missed the point. My understanding is the discussion is about a single bomb with configurable (aka variable) yield, and under what circumstances one might want to use anything but the max setting. You seem to be contrasting single large bombs vs multiple smaller bombs.
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And what were these terrible side effects?

I work in the nuclear industry and know about the effects of radiation. They are bad enough that exxageration is not needed.

I know the Straight Dope is a liberal echo chamber. And you probably feel free to come in here and just make up bad stuff about nuclear but its propaganda.

The science behind radiation is very well understood. It was wrong to expose those Marines but I doubt there were any observable side effects. They probably had a slight increase in their risk for cancer.

Consider a B61 nuclear gravity bomb. The Model 3, for instance, can be set to 0.3, 1.5, 60, or 170 kilotonnes. So the same bomb, off the same assembly line, can be used to blow up a bridge, or the entire city, depending on how the ground crew sets it up. Air burst, ground burst, laydown detonation- this handy gadget can do it all!

I think that last phrase is very important.

Developing plans for a nuclear bomb is difficult, and then producing them reliably and efficiently adds additional difficulties. If you can design one good variable-yield bomb, and develop an assembly like that can produce them rapidly & efficiently, you have the benefits of ‘mass production’ – the bombs become cheaper & more reliable. So even if they mostly don’t need the variable-yield feature, there the cheapest one to produce. After all, most nuclear weapons are never used – they just sit around in storage until they decay and are replaced.

How much and in what ways would it affect operations and soldiers? I remember a sergeant in bootcamp talking about NBC exercises in Germany where he wore the suit for several days. He didn’t enjoy it.
Has the rationale for tactical nukes decreased with the advent of precision, cluster or fuel-air explosive munitions?

Yeah I’m trying to look up the results but besides some government payouts and vague reports of people getting cancer it doesn’t look like it was an epidemic. It seems like this is the same style as that rumor that all those people got cancer on that John Wayne film that was filmed downwind of a nuclear test range. There were rumors that most died of cancer but their cancer rates were literally the exact same as the nation at large.

True… although in this day and age, the real value of dial-a-yield weapons is in the concept of proportionality. There may well be times that a 300 ton yield might be just the thing, and 340 kilotons might be ridiculous overkill.

NBC gear is really uncomfortable, and more to the point, it really won’t save you in an attack unless you can rapidly evacuate the danger zone and decontaminate; it just gives you enough time to fight back. During the Cold War, the use of ‘tactical’ nuclear weapons was considered in a Soviet invasion of Western Europe where a conventional response could not stop multiple armor and artillery battalions from flooding through the Fulda Gap, or against Soviet Naval battle groups in the Med coming from the Black Sea or out of the Barents into the North Atlantic. The reality is that nuclear weapons are so destructive to conventional forces that once you start using them the opposition has little recourse but to respond in kind, and escalation ensures. No conventional weapon system begins to compare to the destruction that a large tactical nuclear weapon can do, and even the smallest of them is so compact and easily deployed compared to the fleet of bombers required to deliver comparable destructive power with conventional explosives that there just is no parity. Precision bombs and cluster submunitions have very different purposes other than the kind of mass destruction that nuclear weapons do, and while thermobaric (FAE) bombs are essentially intended for widescale destruction, at their largest their effects are a couple of order of magnitudes less than even a small nuclear weapon and of course leave no attendant radioactive fallout. The idea of proportionality in warfare between major powers in terms of using a nuclear weapon but limiting casualties is really kind of nonsensical given that the use of a nuclear weapon against an opponent with nuclear parity will likely result in escalation of nuclear response.

That really goes into target selection and the desired result (destruction of a unprotected or hardened facility versus mass destruction of a populated area). Proportionality in response is more a question of justifying the use of nuclear weapons versus a conventional response. The only plausible reason for using a nuclear weapon preemptively or in response to a conventional or even a biological/chemical attack would be to eliminate a threat or verified store of additional weapons which could result in another mass casualty attack.


This is probably useful information but I don’t know enough of the terminology to interpret it: [https://www.ncbi.nlm.nih.gov/books/NBK224995/](The Five Series Study: Mortality of Military Participants in U.S. Nuclear Weapons Tests); more

Broken link.

Ionizing radiation usually either does enough damage to make you really sick or kill you within a few days, or it just produces an increase in lifetime incidence in various cancers. For the cast and crew of the The Conqueror, who were not only exposed to radioactive fallout in their on location shooting but also from contaminated dirt that was excavated and moved on set, the fact that the major cast members were all heavy smokers (Wayne smoked three to four packs a day), which exposes them to the incidental radiation in tobacco as well as the other non-ionizing health effects of tobacco smoking makes it really impossible to isolate their exposure during filming to any of the other sources of radioactivity. And of course, many people downwind of the Nevada Test Site were exposed to multiple sources of radioactive fallout until aboveground testing of nuclear weapons was suspended in 1963 with no known immediate illnesses, although there is a slightly statistically higher incidence of chronic illness in the general region vice the population at large, albeit not one that can be conclusively isolated to fallout exposure.

Although we still have a lot to learn about the long term effects of radiation exposure, and especially heavy ion exposure from cosmic radiation during interplanetary space travel on the human body, short term light particle ionizing radiation that is common from terrestrial sources is pretty well understood and there are statistically validated thresholds of exposure below which immediate health effects are known and can be readily mitigated. The effects of above-threshold radiation are less well understood just because relatively few people have suffered near-lethal exposures (thankfully) and there is virtually no work being done today on high dose exposures on mammals outside of the Marvel universe, but significant efforts are made to prevent incidental occupational exposure to radiation levels that even approach an order of magnitude of acceptable thresholds. The “liberal echo chamber” comment aside, the biggest threat of radioactive contamination has always come from materials not understood to cause harm, e.g. radium dials or uranium glazing in tiles and dishware, or unintended/negligent releases of radioactive material from extraction, processing, and enrichment facilities rather than from nuclear reactors themselves.


The return of great power competition has made the rationale return. There is no feasible way to defend against the Russians in the Baltics without them.
Plus modern tactical nukes are customers of the new fangled accuracy, the US had integrated JDAM style kits on B-61 and B-83.

Hiroshima and Nagasaki not enough for an idea?
I recall reading that the Manhattan project scientists during pre-attack damage discussions presumed that there would be few deaths due to acute radiation poisoning since if anyone was close enough to get such an immediately fatal dose, would already be in the 100% killed zone for blast and heat.

In the actual attack, about 15% of the fatalities were due to acute radiation poisoning, since there were people who were shielded from the blast and heat due to circumstances but who got the full blast (!) of radiation.
I’ll admit I find it difficult to conceive exactly how something could protect a person from blast and heat but still leave them susceptible to radiation.


But I think a lot of the reason for the dial-a-yield bombs is that there are, if I’m not mistaken, only two types of nuclear gravity bombs in the US arsenal- the 1.2 megaton B83 and the dial-a-yield B-61.

That limits the nuclear options the President has to play with- you have strategic systems- ICBMs and SLBMs that have multiple high yield (335-475 kt) warheads per missile and the 5-150 kt ALCM. These are all based in the continental US, and with the exception of the cruise missiles, are typically positioned and oriented to hit strategic targets.

So it’s important for the gravity bombs to have flexible yields- an aircraft carrier or air base may only have a small handful of the weapons at most, so it makes sense to have ones that can be sized appropriately to their targets, if and when the choice is made to go nuclear.

You can get X-ray spallation (hence, why when you take a medical X-ray that have you remove all jewelry) and of course absorption of short lived but energetic radionuclides produced by fast neutron emission and distributed by the post-blast thermal uplift that produces the mushroom cloud.

But yes, most people exposed to enough radiation from the immediate nuclear reaction are probably going to be killed by attendant blast effects. People exposed to intense ionizing radiation but no blast usually occurs in a laboratory situation like the various criticality accidents at Los Alamos, and most exposures are at much lower levels that do not show immediate effects. Marie Curie was directly exposed to radium and highly radioactive pitchblende (uranium with a significant amount of [SUP]235[/SUP]U) for over two decades, and while she suffered numerous health effects and eventually died at age 66 from aplastic anemia and other effects of acute radiation syndrome, it didn’t prevent her from having children (including a daughter, Irène Joliot-Curie, who was also a Nobel laureate) and having a full career.