The idea is that you would have pieces of plutonium or U233 close enough that if left to their own devices they should go supercritical in a fraction of a second. However, in between the pieces you have a layer of magical darkness which can absorb any radiation without generating any waste heat.

Importantly magical darkness can respond to a variety of stimuli and be given complex preprogrammed instructions, which it will then carry out at light speed. This is crucial as it allows the magical darkness to modulate neutrons on an otherwise impossibly short timescale. Magical darkness is physically intangible and can occupy the same space as any material which is fully or semi-transparent.

So say you have 30 lbs worth of plutonium pieces closely packed together, or embedded in a semi-transparent matrix which can carry magical darkness within it. This setup should go supercritical rapidly: So you have the darkness pull back to briefly allow a portion to go supercritical, before moving so that the neutrons can force another now unshielded section to go supercritical, while tamping down the first portion so it goes back to merely critical.

Another much simpler, but I suspect lower output idea is having the magical darkness repeatedly pull back so the reactor goes supercritical for a split second, then tamp things down and repeat the process in a cyclical manner such that the average output is a bit below whatever temperature your reactor materials can handle.

The idea being that you could turn an arrangement of plutonium and neutron reflectors sufficient to go supercritical into a scalable power source. Modulating the neutrons to stop the feedback loop once you reach your desired level of power output.

Is this notion of moving a region of super-criticality around extremely quickly within a reactor to increase its effective output actually plausible?

Since any radiation absorbed by the magical darkness is wasted, the fuel efficiency of such a reactor would be terrible, but that's not a concern in this scenario. Similarly don't be too worried about nuclear contamination in your answers, since the people of this world are about as resilient as those fungi that live in the remains of Chernobyl's reactor.

Such a reactor has very obvious applications in that it lets you make reactors that weigh considerably less than 22 tons. Similarly if you used the hot fuel to directly generate thrust like Project Pluto then ought to allow you to make a 50 lb W54 warhead sized nuclear thruster. Given magical darkness can totally shield you from radiation this seems like it could even allow for flying nuclear powered exosuits. Also people would obviously use the magical darkness for radiation shielding and not just to absorb neutrons within these reactors.

I also find this idea fascinating because it seems like in theory you should be able to come up with a way to do this in real life in order to get super tiny reactors.

For instance by having some neutron reflectors and some neutron poisons in a vacuum, being moved around magnetically extremely quickly. So as to repeatedly get the reactor to go super-critical for brief periods, making its average output nearly as hot as you can handle. Though making such a super-critical reactor safe seems inherently very difficult.

  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    May 23, 2023 at 1:33
  • $\begingroup$ Why is this tagged science-based when you're handwaving away all objections with magic? $\endgroup$
    – Michael W.
    May 23, 2023 at 15:46
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    $\begingroup$ @MichaelW. the science-based tag does not preclude magic, and the OP is not handwaving away all objections $\endgroup$
    – M S
    May 24, 2023 at 11:41

6 Answers 6


Theoretically you could, but it seems rather pointless. Going supercritical makes your fuel degrade quickly and unevenly. It's like trying to make rocket engine working on hydrogen and oxygen "more efficient" by throwing C4 into combustion chamber - if the rocket survives the experiment you get a kick, but it is not the point of the engine.

Insufficiently hot fuel rods is not a problem that limits efficiency of nuclear reactors. Critical state provides even sustained level of thermal energy high enough for conversion of this energy into other forms to be a significant technical challenge. Going supercritical would only make it more difficult.

"should be able to come up with a way to do this in real life" - yeah, you just need to solve a minor problem of real human DNA being destroyed by radiation and convince them that it's cool to be fraction of second from nuclear explosion 24x7 for no tangible benefits. Fascinating indeed.

  • $\begingroup$ I didn't say a real version of this reactor would be practical, but surely you can see how an advanced civilization could manage this sort of hard technical problem, and how they might want a nuclear engine that massively beats out any similarly sized alternatives except maybe antimatter (which is way less practical to make of use of than a this reactor, but people don't respond to every question about antimatter tech by just saying the premise is dumb). $\endgroup$ May 22, 2023 at 17:42
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    $\begingroup$ @Vakus Drake If you ignore technical problems, namely - a) supercriticality makes reactors worse without adding any benefits, b) deadly levels of radiation and c) magic is required to make it work you are not really talking about real nuclear technology anymore but about some weird unicorn farts. So, any fascination about it usability in real life becomes questionable at best. "nuclear engine that massively beats out any similarly sized alternatives" - the engine you described doesn't beat anything because you don't "manage hard technical problems", you ignore them. $\endgroup$
    – D'Monlord
    May 22, 2023 at 18:15
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    $\begingroup$ Those criticisms are mutually incompatible, if you are using the magic described then radiation isn't a concern and you don't care about the longevity of the core per say for many of the imagined applications (though magical radiation shielding and fuel reprocessing helps a lot). It's also strange that you keep saying a supercritical reactor wouldn't have any advantages, when you seem to just mean that it would have deal breaking disadvantages which is very different. $\endgroup$ May 22, 2023 at 19:23
  • $\begingroup$ I don't know why you don't consider that such a reactor might be operated from a distance. Saying I'm ignoring hard technical problems seems odd since the whole question is about whether this is actually possible not whether it is easy or in your opinion reasonable, I'm asking whether such hard problems are in principle solvable. $\endgroup$ May 22, 2023 at 19:23
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    $\begingroup$ @VakusDrake, what you describe sounds similar to pulsed reactors like the Godiva device. These have a distressing habit of blowing themselves up from time to time. (Godiva II, notably, "was constructed inside a concrete building with 20-inch-thick walls and 8-inch-thick roof in a canyon a quarter-mile away from the control room".) $\endgroup$
    – Mark
    May 23, 2023 at 0:50

This is vaguely similar to traveling wave reactors, where a small part of the fuel is critical at any given time, breeding fissile material from fertile fuels such as U-238 or Th-232 in a wave that burns slowly through the latter. This is not particularly helpful in miniaturizing the reactor, however. Rather the opposite: the objective is to burn through a mass of non-fissile but fertile material far larger than the mass of the fissile isotopes necessary to maintain criticality.


Given your other question, this is implausible. It's just way more difficult and complicated than your world has any need for.

In How Effectively Could You Use Magical Darkness As A Compact Heatsink?, you say the magical darkness can

...physically separate non-solid atomic matter by desired qualities. It can easily physically separate the components of a gas or liquid.

You already have a superpowered Maxwell's Demon ready to go. Why on earth would you even mess with atomics? If you can arbitrarily reverse entropy, you can trivially have all the energy you want, in whatever form you want.

For example: Take H2O, split it to hydrogen and water, burn it to H2O for energy, repeat. Use the same thing to power a steam loop to get refrigeration. Put the darkness in parallel with a battery membrane for arbitrary amounts of electricity. Etc.

  • $\begingroup$ Directly generating energy like you're proposing would require you put way too much strain on the enchantments for it to plausibly be economical because wards aren't cheap or free to maintain, just like I already said trying to absorb too much mass or energy is prohibitive. Also this setting is very explicitly about removing every possible limitation like safety concerns preventing nuclear technology from being used to its full potential. $\endgroup$ May 22, 2023 at 17:31
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    $\begingroup$ "just like I already said trying to absorb too much mass or energy is prohibitive..." This part is confusing because in our world, we'd expect absorbing/shielding the energy from supercritical nuclear fuel to be far more difficult than separating water into hydrogen and oxygen. If your answer is "well, that's not how this magic works," that's totally reasonable for world-building, but you're pretty far outside the realm of science. The magic just does whatever you say it does. $\endgroup$ May 23, 2023 at 18:43
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    $\begingroup$ Indeed, this magic is inexplicably specific in being useful for making nuclear power cheap and easy while being too expensive or complicated for any other use. $\endgroup$ May 23, 2023 at 23:36

There are a lot of drawbacks, and not much upside.

The "not much upside" is, if you could do it and control it, you have a small source of heat. You could do with that some of the things you could do with a source of heat. For example, you could run a boiler to run a steam engine.

You could not use it without a huge amount of radiation shielding. It still produces the requisite amount of radioactivity. The easy form of shielding is a pool of water. Typically such pool reactors are built at the bottom of a pool in the neighborhood of ten meters deep. This tends to reduce the convenience factor.

There is also the fact that when you operate this device, you get radioactive waste which must be disposed of. And the structure around it becomes activated. And the cooling water needs to be monitored. And so on. All the usual things that nuclear stations have to do in order to keep the radioactivity under control.

But the main stumbling block is controllability. A critical device of this type is a challenge.

A critical reaction is one where the number of neutrons produced is constant over time. Criticality is based on two significant categories of neutrons: delayed and prompt. Depending on the specific design, the delayed neutrons will be approximately 1 percent of the total.

One way to express the situation when the reactor is above critical is based on a number called k-effective. When keff is greater than 1, reactor power will be rising. When less than 1, it will be falling. The delayed neutrons will typically provide about one percent of the value of keff when keff is 1.0. A little more or a little less, depending on the design.

If the value of keff is slightly larger than 1.0, such that it would only be larger than 1.0 including the delayed neutrons, then the rate power rises is relatively slow. This is the controllable range. If the control devices can react in seconds, then the power can be controlled.

When the value of keff is large enough such that it will be greater than 1 even without the delayed neutrons, then the power will rise extremely quickly. This is called "prompt critical." Under this condition the power becomes extremely difficult to control. The time it takes the power output to double depends on the specific design, but can be from 0.1 millisecond to a few microseconds. Even with the slower case, it is basically not controllable. If it were at the 0.1 millisecond conditions, and your control process required 1 millisecond to respond, then there will be ten doublings before the control system responds. That is a factor of 1000.

So if you have your 30 kg of Pu at a power that is convenient, but at prompt critical, it reaches 1000 times that within 1 millisecond. If your control system took 2 milliseconds to respond, it reaches 1 million times your desired power.

This, by the way, is the usual analysis of Chernobyl. The explosion happened when incorrect operation of the reactor pushed it into prompt critical.

Small devices of this type are notorious for being difficult to control. The Triga reactor is specifically designed to be able to deal with short pulses of the type that can result.

The TRIGA reactor uses uranium zirconium hydride (UZrH) fuel, which has a large, prompt negative fuel temperature coefficient of reactivity, meaning that as the temperature of the core increases, the reactivity rapidly decreases. Because of this unique feature, it has been safely pulsed at a power of up to 22,000 megawatts.

That is, the reactor is designed such that when it gets hot, it tends to produce less neutrons. And the hotter it gets the less neutrons it produces. That is, it shuts down a power pulse on its own.

But the Triga reactor is not convenient for extracting heat. It's great for research and training and related. But it's not great for running a boiler.

So, for research programs, your pulsed reactor might be interesting. But probably for any other purpose, you want a more conventional reactor.

  • 2
    $\begingroup$ Radiation shielding isn't really a concern because of the same magical darkness that's used to absorb radiation in the reactors. Plus responding quickly enough to not let things melt down isn't likely to be an issue when the magical darkness can respond in preprogrammed ways to stimuli like radiation flux at light speed $\endgroup$ May 22, 2023 at 0:42
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    $\begingroup$ @VakusDrake No, shielding is important. The radiation comes out with the heat. When you look at the sucker you get dosed. Also, during the pulses, only about 2% of the energy comes out as photons. Does magic darkness stop neutrons? $\endgroup$
    – Boba Fit
    May 23, 2023 at 12:15
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    $\begingroup$ @BobaFit: The question says yes, it can stop neutrons if you want it to. It can absorb "any radiation", and the question also says "... it allows the magical darkness to modulate neutrons on an otherwise impossibly short timescale." But if most of the power output is in the form of high-energy neutrons, then yeah you need shielding to absorb them and get hot. The magical darkness does not get hot. $\endgroup$ May 23, 2023 at 15:32

So there is a real one of these. (no magic needed) https://en.wikipedia.org/wiki/Godiva_device

There is a proposed solution which works as a rocket.

The "Nuclear salt-water rocket" (NSWR) https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket


The NSWR has a fuel tank of enriched radioactive salts dissolved in water. The tank is filled with neutron absorbing materials, so it does not go critical!

The fuel is pumped into the reaction chamber/ nozzle where it can go (prompt!) critical. This heats the water, which acts a propellent.

Versions of the design have an IPS of 6,730 seconds (the best chemical rockets top out at about 450 seconds).

The rocket is more efficient/powerful, with more highly enriched fuel. So for the sake of your story, you could have an after-market 'boost tank` of, lets say, 90% enriched Uranium. Which could be bled into the main supply when you really need to be going places.

I strongly encourage you to watch Scott Manley's video on this:


See also: https://en.wikipedia.org/wiki/Aqueous_homogeneous_reactor


No need for magic. You are just detonating nuclear bombs.

Starting, maintaining and stopping supercritical conditions is hard because nuclear reactions are very fast and very energetic and when you approach criticallity the whole set tends to explode. What you are trying to do is like make a nuclear bomb that you could switch on and off, but if you want energy you don't actually need to stop the bomb if you can harvest the energy released.

That was the goal of Project PACER. They tried to develop a power plant where steam in a cavern was to be heated by exploding there a nuclear bomb twice a day. In spite of being technically feasible, the project was cancelled in 1975 because it was uneconomical when compared with a nuclear reactor.


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