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One of the weapons I'm planning on having a character use in a sci-fi world is a rapid fire nuclear autocannon. While the strategic use of nuclear weapons is common on inter-galactic battlefields, an automatic version certainly isn't. Basically, imagine a 105mm caliber autocannon firing 15 kiloton nuclear artillery shells at 150 rounds per minute (essentially a rapid fire version of the M65 automatic cannon). Obviously, the destruction would be immense, but what would be the effects on the immediate environment? If the cannon fired for a minute, how big would the resulting firestorm be? What would its effect be if it were raining?

I'm not asking about the feasibility of the weapon itself; it's meant to be a single use thing that was liberated from essentially a mad scientist.

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  • $\begingroup$ Every 10 to 20 shots would be equivalent to a single bomblet within a MIRV warheads. $\endgroup$
    – DKNguyen
    Jan 14, 2021 at 6:00
  • $\begingroup$ Might as well sabotage many nuclear reactor plants, this way the damage is lasting compared to the relative clean bombardment ;D $\endgroup$
    – user6760
    Jan 14, 2021 at 6:34
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    $\begingroup$ You've got "strategic" mixed up with "tactical" for nuclear devices. A strategic nuke is one meant to destroy a major city - hundreds of kilotons or megaton-range thermonuclear weapons. Small warheads intended for battlefield use are tactical warheads - as was that used in the nuclear cannon. $\endgroup$
    – jdunlop
    Jan 14, 2021 at 8:15
  • $\begingroup$ (It's also worth noting that even assuming all the shells detonate - not a safe assumption - your combined explosive power is a fifth that of Ivy Mike. By nuclear weapon standards, pretty tame.) $\endgroup$
    – jdunlop
    Jan 14, 2021 at 8:18

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I will start from the same premise of PcMan's answer

Your cannon has a muzzle velocity of 625m/s It is firing a continuous stream of 15 kiloton nuclear bombs, at a rate of 150 per minute, for a minute duration.

Your bombs are spaced apart by only 250m.

The first detonation occurs before you stop firing.

Now, consider that a nuke is designed to explode only when a set of carefully designed circumstances is met. As again correctly stated in PcMan's answer, no nuke is designed to functionally survive inside the primary fireball of a nuke, because if it was it won't explode.

This means that all the nukes after the first which happen to be in the primary fireball of the first explosion would simply fizzle and not explode. There is no sympathetic explosion for nukes.

But how big is the fireball of a 15 kTon nuke? Believe it or not, somebody has made a calculator for that!

For 15 kTon that calculator gives a fireball of 130 meters and a duration of 0.7 seconds. Thus, if we assume perfect firing (no spread due to recoil etc. and aim always at the same point) every other nuke will be inside the fireball (you are firing one ever 0.4 seconds). Everything fine then?

Well, consider that they would still be subjected to the thermal radiation and to the EMP. Now, I assume that whoever designs the controlling circuits for nukes knows that they need to protect them from EMP, else MAD would turn into a joke. And the thermal radiation should be manageable, too. When nukes were exploded above ships, the ships were wrecked but not vaporized.

You seem to have then about 75 Hiroshima bombs in close succession. What happens to the target? Since a single bomb on Hiroshima caused a firestorm, tore the city down and made a carnage of the inhabitants, and considering that more bombs won't bring more combustible for the fire nor more buildings to destroy nor more people to kill, the only serious consequence of using this weapon would be an increase of the fallout and of the long term and long distance contamination.

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  • $\begingroup$ I don't know that it follows that the shells will not be within the fireball, since they're still travelling and the fireball isn't instantaneous. I suspect a large number of the shells would have their explosives cooked off by their immediate predecessors and not detonate properly, even if they weren't within the fireball at the moment of detonation. (Still, this answer is otherwise great; it doesn't matter if you hit a city with one nuke or fifty, the result is about the same.) $\endgroup$
    – jdunlop
    Jan 14, 2021 at 8:12
  • $\begingroup$ Surely if the nukes are all meant to detonate the same distance from the weapon they will by definition have to travel into the previous shot’s fireball? It’s either that or a chain of nuclear explosions creeping increasingly close to the weapon... $\endgroup$
    – Joe Bloggs
    Jan 14, 2021 at 8:26
  • $\begingroup$ @JoeBloggs, fair point, however consider that already firing an automatic weapon will spread the bullets around due to the effect of recoil on the shooter, and nukes weight way more than a standard round $\endgroup$
    – L.Dutch ♦
    Jan 14, 2021 at 8:30
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    $\begingroup$ @JoeBloggs, on second thought I have reworked my answer including the evaluation on the fireball duration. $\endgroup$
    – L.Dutch ♦
    Jan 14, 2021 at 8:38
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SUICIDE

Your cannon has a muzzle velocity of 625m/s
It is firing a continuous stream of 15 kiloton nuclear bombs, at a rate of 150 per minute, for a minute duration.

Your bombs are spaced apart by only 250m.

The first detonation occurs before you stop firing.

Each projectile is just within the fireball distance of the preceding one, and promptly undergoes sympathetic detonation. (don't you try to tell me you can design a nuke that can functionally survive being inside the primary fireball of another nuke!!)

The chain detonation propagates back along your projectile stream, and vaporizes your whole remaining magazine.

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    $\begingroup$ I disagree you'll get a chain detonation. Detonating a nuke properly requires precise timing of the primary charges - You'll get damage to the second nuke, that may result in a radioactive explosion, but it's likely to be more of a fizzle than a chain reaction of full yield explosions. $\endgroup$
    – Ash
    Jan 14, 2021 at 7:37
  • $\begingroup$ It's called "nuclear fratricide", and occasionally affects MIRVed warheads. Nuclear weapons caught within the detonation of another nuclear weapon do not themselves go nuclear, but rather just scatter their radioactive contents with the rest of the fallout. $\endgroup$
    – jdunlop
    Jan 14, 2021 at 8:10
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    $\begingroup$ I believe the above comments to be correct. In implosion-based warheads achieving nuclear detonation requires precise timing of directed explosive charge. Blast of another nuke would likely damage the casing, causing the... disassembly of the warhead, and "maybe" detonate the conventional explosive, but no nearly in the precise fashion needed to achieve criticality through implosion. Scattering of radioactive material is the most plausible outcome. In case of gun-type fission warhead the blast "might" push the two masses together, but it is highly unlikely and the structure would sooner fail. $\endgroup$
    – JANXOL
    Jan 14, 2021 at 9:21
  • $\begingroup$ What's the chance of a very-close-to-critical-mass nuke going boom, or at least big fizzle, when hit by a blast of neutrons from the exploding nuke just a few meters ahead of it? $\endgroup$
    – PcMan
    Jan 14, 2021 at 9:30
  • $\begingroup$ (+1) I don't discard the possibility of the projectiles "fizzling back" to the cannon. Specially since there's no need that every projectile in the sequence fizzles. Since the nuclear explosion has a duration, and projectiles are travelling towards it, several of them can fizzle at a time, with different degrees of efficiency. Even if they don't fully explode, you could end with a 0.001kton fizzle inside your cannon. Or, in other words, an explosion equivalent to 1000 kg of TNT. Your weapon is not going to survive this. $\endgroup$
    – Rekesoft
    Jan 14, 2021 at 11:06

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