Termination of a nuclear reaction

Phil App · Feb 8, 2024

I have long pondered the question of whether it is possible to extinguish an ongoing atomic explosion. From a practical standpoint, it is certainly possible that currently it is a supernatural, if not impossible, task. However, from a theoretical standpoint, I cannot stop thinking about all possible alternatives. Unfortunately, my knowledge of physics and mathematics is not rich enough to give meaning to my theories or to know whether they are truly possible or even worth contemplating.
All my assumptions stem from the idea of creating a device capable of halting an already ongoing reaction.

1. Theory
We know that in an uncontrolled chain reaction, unstable neutrons are released, which rapidly collide with atoms and break them into further neutrons. But the question is whether it would be possible to slow down or, ideally, stabilize these neutrons in such a short time and under such immense energy impact.

2. Theory
A perhaps slightly more conceivable idea would be the possibility of absorbing such an unbelievable amount of neutrons. But how strong would the absorbing materials have to be? Is it possible to absorb a sufficient number of neutrons to halt the reaction? The overall question still remains: is it all possible?

3. Theory
The most sinister idea that has stuck in my mind is whether the gravitational field of black holes is so strong that it could swallow a nuclear explosion. If so, I still believe that it is an almost impossible task to deliberately create a singularity for such purposes.

I would very appreciate every response or opinion which relate to this issue.

Drakkith · Feb 8, 2024

Phil App said:

A perhaps slightly more conceivable idea would be the possibility of absorbing such an unbelievable amount of neutrons. But how strong would the absorbing materials have to be? Is it possible to absorb a sufficient number of neutrons to halt the reaction? The overall question still remains: is it all possible?

Where would you place the absorbing material? The neutrons are inside the core. You can certainly surround the core with some material in which case it becomes it becomes an exercise in determining how thick your material needs to be to contain the explosion. The easiest way is to just bury the warhead inside a very deep hole and have the surrounding earth absorb it.

Phil App said:

The most sinister idea that has stuck in my mind is whether the gravitational field of black holes is so strong that it could swallow a nuclear explosion.

Absolutely. Once the warhead passes the event horizon, none of the explosion can pass back out.

snorkack · Feb 8, 2024

Drakkith said:

Where would you place the absorbing material? The neutrons are inside the core. You can certainly surround the core with some material

Empty space absorbs neutrons best, though.
Therefore the best way to stop a chain reaction in progress is to insert into core something that moves faster than the core parts and has the neutron capture cross-section and density to push aside parts of core.
A table of cross-sections may be found at
https://www.oecd-nea.org/dbdata/nds_jefreports/jefreport-3.pdf
For thermal only:
https://www.ncnr.nist.gov/resources/n-lengths/ - more complete, jef misses out many isotopes
Note that for the explosive chain reaction, the relevant cross-sections are the fast ones.
Looking at natural isotopes with big thermal cross-section (mainly over 100 barn absorption)...

p - 330 mb but 80 b scattering; 40 μb fission average, 4 b scattering
He-3 - 5300 b (but around 1,4 ppm in air, so He-0 is just 7,4 mb); 830 mb fission average
Li-6 - 940 b (7,5%, so Li-0 is 70 b); 320 mb fission average
B-10 - 3800 b (20%, so B-0 is 760 b); 420 mb fission average
Kr-83 - 210 b (11,5% but Kr-82 also has 29 b so Kr-0 is 25 b; note that Kr-83 is a middle isotope); 40 mb fission average
Rh(103) - 145 b; 83 mb fission average
Cd-113 - 20600 b (12,2% so Cd-0 is 2520 b; Cd-113 is middle); mere 57 mb fission average
In-115 - 202 b (96% so In-0 is 194 b); 133 mb fission average
Te-123 - 420 b (0,91% so Te-0 is 4,7 b; Te-123 is middle); 85 mb fission average
Xe-124 - 165 b (0,1% and Xe-131 also has 85 b so Xe-0 is 24 b); 105 mb fission average
Nd-143 - 340 b (12% and others so Nd-0 is 50 b; Nd-143 is middle); 80 mb fission average
Sm-149 - 42000 b (14% and more ahead; middle isotope); 270 mb fission average
Sm-150 - 104 b (7,4% and more ahead; middle isotope); 140 mb fission average
Sm-152 - 205 b (27% so Sm-0 is 5900 b; Sm-152 is middle isotope); 90 mb fission average
Eu-151 - 9100 b (48% and more ahead); 450 mb fission average
Eu-153 - 310 b (Sm-0 thus 4500 b); 260 mb fission average
Gd-152 - 735 b (0,2% and more ahead); missing in JEF
Gd-155 - 61000 b (15% and more ahead; middle isotope); 370 mb fission average
Gd-157 - 259000 b (16% so Gd-0 is 50000 b; Gd-157 is middle isotope); 200 mb fission average
Dy-161 - 600 b (19% and more ahead); 120 mb fission average
Dy-162 - 200 b (25% and more ahead); 280 mb fission average
Dy-163 - 130 b (25% and more ahead); 79 mb fission average
Dy-164 - 2800 b (28% so Gd-0 is 1000 b); 98 mb fission average
Er-167 - 660 b (23% so Er-0 is 160 b); 110 mb fission average
Tm(169) - 100 b; missing in JEF
Yb-168 - 2230 b (0,14% so the 35 b of Yb-0 is mainly due to others; lightest isotope); missing in JEF
Lu-176 - 2065 b (2,6% so Lu-0 is 74 b); 125 mb fission average
Hf-174 - 560 b (0,2% and more ahead); 365 mb fission average
Hf-177 - 370 b (18% so Hf-0 is 104 b; Hf-177 is middle isotope); 345 mb fission average
Ta-180 - 560 b (120 ppm, so the 20 b of Ta-0 is mainly of Ta-181); missing in JEF
Re-185 - 112 b (37% but Re-187 also has 76 b so Re-0 is 90 b); 140 mb fission average
Os-184 - 3000 b (200 ppm and more ahead); missing in JEF
Os-187 - 320 b (1,6%; Os-0 is 16 b but Os-187 is middle isotope); missing in JEF
Ir-191 - 950 b (37% and more ahead); missing in JEF
Ir-193 - 110 b (so Ir-0 425 b); missing in JEF
Pt-190 - 150 b (100 ppm; Pt-0 10 b is mainly due to others); missing in JEF
Hg-196 - 3080 b (0,2% and more ahead); missing in JEF
Hg-199 - 2150 b (17%; Hg-0 is 372 b; Hg-199 is middle isotope); missing in JEF

That´s it. p was under 100 b, so I get just 37 natural isotopes (besides the fissionable actinides) that have over 100 b thermal absorption. This exercise helps spotting patterns, rules of thumb, exceptions...
Note that the fission average is not well correlated to thermal absorption (example - Cd-113), but JEF data are less complete.

Vanadium 50 · Feb 8, 2024

snorkack said:

Empty space absorbs neutrons best, though.

snorkack · Feb 8, 2024

Vanadium 50 said:

In a limited sense. If you want to get neutrons out of a core, it would be good to make empty space next to the core, for the neutrons to escape into, and for the core material to escape into. A dense obstacle would backscatter at least a few neutrons - and prevent the core from expanding and letting neutrons escape.

Bystander · Feb 8, 2024

Phil App said:

possible to extinguish an ongoing atomic explosion.

https://www.google.com/search?q=eff...Q0MTg4ajFqN6gCALACAA&sourceid=chrome&ie=UTF-8

Yes. Next question: can we agree this is a purely speculative, verboten, inquiry and tie it off/move it to GD/WW II...?

russ_watters · Feb 8, 2024

I'd recommend Tom Clancy's "The Sum of all Fears" for a good literary treatment of an exploding atomic bomb. It has an entire chapter describing the explosion nanosecond by nanosecond (full scientific accuracy not required here). At the end of the chapter it says something to the effect of; 'at this point the reaction has finished, yet the bomb casing remains intact.' So there's nothing you can do to stop it once it is triggered. It's too fast and everything that makes the explosion happen happens inside the bomb.

berkeman · Feb 8, 2024

russ_watters said:

I'd recommend Tom Clancy's "The Sum of all Fears" for a good literary treatment of an exploding atomic bomb

I like how Tom was careful to insert some misinformation in the parts of that book that describe how the bomb was made, to keep it from being a cookbook for terrorists. Tom is a good guy.

Vanadium 50 · Feb 8, 2024

For years, articles on atomic weapons had a diagram with one seemingly innocuous mistake. This mistake would ensure that it would not work. (And no, I won't say what it was) Today, a few sources get it right, but many still have drawings of devices that can't possibly work.

The idea that empty space absorbs neutrons at all, much less "the best" is just plain wrong.

mfb · Feb 9, 2024

The chain reaction in a nuclear weapon takes about 1 microsecond.

A light gas gun can accelerate a projectile to ~10 km/s. In that 1 microsecond it'll move 1 centimeter. The gun is much longer than 1 cm, so if you trigger it after the chain reaction started it's not going to do anything. Even if you trigger it well before the bomb is triggered, so it reaches the core right as the chain reaction starts, it's not going to do much. And why would you include a light gas gun inside a nuclear weapon anyway?

You could use a particle accelerator to shoot neutron absorbers into the core. It would need to be part of the bomb design, too (again, why?), but at least the nuclei would still arrive within that microsecond. The amount you can shoot in is very limited, so it's probably not going to stop your reaction.

Slowing down neutrons would be counterproductive, it makes fission more likely.

@snorkack: Bombs use fast neutrons, thermal cross sections are not relevant.

Rive · Feb 9, 2024

Phil App said:

I have long pondered the question of whether it is possible to extinguish an ongoing atomic explosion.

As it was already explained, once it's started it'll be over too fast for any intervention without some time-reversal like sci-fi technobabble.

Before it started there may be some means to disarm or make it a fizzle, with just classic bomb disposal methods: though the risks involved are at a quite different scale than usual, so no 'classical' expert would try anything in sane mind except in absolute necessity ...

kered rettop · Feb 9, 2024

Phil App said:

I have long pondered the question of whether it is possible to extinguish an ongoing atomic explosion. From a practical standpoint, it is certainly possible that currently it is a supernatural, if not impossible, task. However, from a theoretical standpoint, I cannot stop thinking about all possible alternatives. Unfortunately, my knowledge of physics and mathematics is not rich enough to give meaning to my theories or to know whether they are truly possible or even worth contemplating.
All my assumptions stem from the idea of creating a device capable of halting an already ongoing reaction.

1. Theory
We know that in an uncontrolled chain reaction, unstable neutrons are released, which rapidly collide with atoms and break them into further neutrons. But the question is whether it would be possible to slow down or, ideally, stabilize these neutrons in such a short time and under such immense energy impact.

2. Theory
A perhaps slightly more conceivable idea would be the possibility of absorbing such an unbelievable amount of neutrons. But how strong would the absorbing materials have to be? Is it possible to absorb a sufficient number of neutrons to halt the reaction? The overall question still remains: is it all possible?

3. Theory
The most sinister idea that has stuck in my mind is whether the gravitational field of black holes is so strong that it could swallow a nuclear explosion. If so, I still believe that it is an almost impossible task to deliberately create a singularity for such purposes.

I would very appreciate every response or opinion which relate to this issue.

I'm going to assume that you mean an intervention from outside rather than a built in scram device.

So the first thing to do is detect that the reaction has started. If you simply detect that the bomb has been delivered, you can obviously zap it with your laser beam. I imagine that for a well-defended city it would be possible to detect some kind of gamma ray signature, or maybe some escaping neutrons.

Then, of course, you need to get your reaction-quenching device/material into the bomb before the released energy is enough to break the shell. I'm going to guess that would require a total response time of the order of 500ns. There's a calculation on Reddit about this. 500ns corresponds to a round trip of 150 metres at the speed of light, so there would have to be launchers every 50 metres or so in the protected zone.

Interception before the explosion seems a bit more attractive now doesn't it?

Still, let's look at the third step. How to get the quencher there in time. It's got to be sent at relativistic speeds. Hence the kinetic energy will be of the same order as the mass-equivalent energy. So anything with a mass comparable with the bomb would need another nuclear bomb to launch it - or, of course, an energy storage device with the same capacity. The constraints are tight. Safely stored antimatter would not be much use, the annihilation reaction would still be another explosion. You need a cold source of energy. Some sort of electrical capacitor might work, but it would still have to store a bomb's worth of energy, and at current energy densities that means a 100m cube. Which is comparable to spacing of the defence stations. So it's feasible if the whole place sits on 100m of capacitor -or, to be generous, let's assume the discovery of a super-dielectic which gets it down to around a meter - everywhere, of course not just as a metre cube.

Another possibiity might be a ready-pumped laser medium, just waiting for a trigger. Not sure whether that's workable. But it has the advantage of allowing some fancy trigger that is part of the steering mechanism -remember this all has to happen in a few 200ns max.

But all that assumes a rather brute-force quencher that weighs about the same as the bomb, or its core. It might be possible to quench the reaction with a tiny fraction of the mass using an efficient poison. Someone might be able to say how much boron you'd need, for example. (ChatGPT has just informed me that even talking about this subject is illegal, make what you will of that.) So let's assume, that you only need a gram. That makes the whole plan a bit more realistic, although you would still be dumping a small nuke's worth of kinetic energy right in the reaction you're trying to stop.

The more practical approach might be to smash the containment vessel so the reaction fizzles out. You could do that with a big laser pulse. But aiming it would have to be pretty instantaneous. Of course bombs don't just appear in the middle of cities, so tracking them as they arrive and intercepting them before they go off sounds a lot more sensible. That's what my nanobots do in a story I'm writing anyway.

As for more exotic schemes, the idea of swallowing the bomb with a tame black-hole... well, yes. Or maybe no. I'll ask my nanobots. Right, they say that's what they'd do. But they do know how to manipulate spacetime directly, so I imagine their opinion isn't valid in your world where people are still dropping nuclear weapons on each other.

Termination of a nuclear reaction

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