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While researching unique methods of nuclear energy generation and propulsion, I encountered many things, both in rocketry and aerospace. For instance, the nuclear thermal jet engine. However, I found the thermal intermediary of most nuclear energy applications annoying and lacklustre when it came to small scale applications and power density, and direct conversion methods such as PIDEC or fission fragment to be... lacking outside of space.

I came up a potential solution. In a model akin to a normal internal combustion, it is a NUCLEAR piston driven engine that provides direct shaft power. The cylinder is made of a neutron moderator-reflector, with the 'fuel' being highly enriched uranium or plutonium hexafluoride (gaseous state). You could probably add some helium for better heat transfer. The 'spark plug' is a neutron source. A piston compresses the UF6 gas where it then goes supercritical, rapidly expanding and driving the piston with direct mechanical power. I imagine this system has greater power to weight ratios than nuclear-thermal energy, meaning it has better small-scale applications such as ships.

Some issues that could present themselves is the fluorine that occurs following each stroke, meaning cycles would have to include 'washes' to prevent internal corrosion.

How possible is this system?

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Oct 19, 2022 at 2:55
  • $\begingroup$ Note that just compressing the gas will probably increase its radioactivity enough to cause a lot of heat generation without the need for a separate neutron source. $\endgroup$
    – user4574
    Oct 20, 2022 at 19:55
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    $\begingroup$ You could probably more simply just use nuclear fuel rods to heat a steam engine. $\endgroup$
    – user4574
    Oct 20, 2022 at 19:55

11 Answers 11

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Possible? Sure. Efficient? I can't imagine so.

When you're introducing things like washes, you're adding a lot of thermodynamic overhead in addition to mechanical wear/stress. You'd also need a way to cool the contents of the piston, because otherwise you get one powerstroke and then the system is at equilibrium and doesn't reset, so you're also exhausting gasseous nuclear fuel which isn't a great look for your product.

And after all of that, you're still limited to the same Carnot efficiency that a basic steam piston would be.

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    $\begingroup$ Ceramics instead of metal cylinders? $\endgroup$ Oct 18, 2022 at 13:57
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    $\begingroup$ Doesn't change the thermdynamic nature of piston mechanisms. Any system that must return to its original state to prepare for another power stroke runs afoul of Carnot. Ceramics help with the heat management, perhaps, but you still have to cool the cylinder contents so the piston will return to starting position. $\endgroup$ Oct 18, 2022 at 13:59
  • $\begingroup$ @'you're still limited to the same Carnot efficiency that a basic steam piston would be' And that is different from steam-driven turbines used in conventional nuclear power stations how? $\endgroup$ Oct 18, 2022 at 14:00
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    $\begingroup$ It's not. That's my point. OP asked if this thing was possible. I'm saying "yeah it's possible, but there's no operational advantage to doing so." $\endgroup$ Oct 18, 2022 at 14:02
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    $\begingroup$ As I said: now you're flushing radioactives out your exhaust pipe and introduce extreme temperature swings - melting isn't your only issue now, cracking is too. My answer, however, assumes the materials engineering is solved. Turbines have an advantage, yes, but are still limited by carnot, so my answer remains: the juice is not worth the squeeze when you can just run thermovoltaic or steam turbine systems and avoid the whole "how do we handle nuclear fuel in the 'combustion' chamber" question. $\endgroup$ Oct 18, 2022 at 14:13
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Frame flip: Instead of fission with Uranium, consider fusion.

General Fusion is a company working on a novel fusion reactor design.

They begin with a blob of molten lead. At the top and bottom are injectors that produce "smoke rings" of plasma. The lead is spun by pumping it in at the side at an angle. This opens a vertical hole.

The plasma is injected in a smoke ring so that the ions are rotating such as to produce a magnetic field. It's like a Tokamak without the Tokamak. One puff of plasma from the top and another from the bottom. When they hit they reinforce.

At exactly the correct time, all the pistons around the outside are fired. This produces a shockwave in the lead, collapsing the central hole. The compression brings the plasma to fusion temperature and density, producing a pulse of fusion. The lead absorbs the energy from the pulse and carries it out to the heat exchangers. From there it is a fairly normal power plant.

The reason the lead can compress the plasma is because the magnetic field in the plasma induces a counter current in the lead as it is compressed. This tends to keep the plasma together. This process was discovered in the 1970s, and made to work with explosives. However, research into it was cut off due to nuclear proliferation concerns. After all, if it were pushed in the wrong direction, it might become a nuclear enhanced explosion.

The pistons achieve the accuracy required in a keen way. There is a laser range-finder on each piston that detects exactly where it is. And there is a small electromagnetic brake on each piston, controlled by a computer, to slow the piston to the exact required speed. This degree of accuracy has already been achieved.

They are by no means done with challenges on this design. But if they make it work, the plan is one pulse per second, each pulse producing about 100 mega-Joules of energy. That means a thermal output of about 100 MW.

It isn't exactly an engine since the pistons don't get driven by the fusion. And the smallest vehicle you are likely to mount such a thing in is an aircraft carrier. But hey, it's a steam-punk fusion reactor.

enter image description here

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  • $\begingroup$ So basically magnetic target fusion, with a lead as opposed to copper liner and mechanical compression of the liner instead of magnetic? $\endgroup$
    – Dan Mills
    Oct 19, 2022 at 18:02
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    $\begingroup$ While reading this, my brain kept switching between lead and lead, kinda like the chicken/rabbit visual illusion (toggling between "element Pb" and "wires") $\endgroup$
    – Criggie
    Oct 19, 2022 at 18:43
  • $\begingroup$ @DanMills It's still magnetic confinement. It's just that the pressure is provided by the extremely rapid compression of the metalic walls. The compression of the plasma induces a counter (compressive) electric current in the lead, as I said. And it only lasts for some extreme short time like 100 microseconds or something. I forget the exact speed but it's a supersonic shock imposed by the pistons hammering on the shell. $\endgroup$
    – BillOnne
    Oct 20, 2022 at 4:04
  • $\begingroup$ @BillOnne The classical MTF is much the same, with a conductive liner being compressed to induce current in its inner surface that produces compression of the plasma. I would classify the general fusion machine as MTF, they are just going for mechanical liner compression rather then electromagnetic. I do think lead is an odd choice however. $\endgroup$
    – Dan Mills
    Oct 20, 2022 at 12:29
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It is not only possible but it ran cars ("Volga-Atom") in the Soviet Union.

Volga Atom

The first one describes uranium piston and helium gas engine. The second one describes piston-based engine with UF6 inside.

(automatic translation)

After six months of settings and experiments, the engine installed on the stand worked for three months completely normally, while the conditional mileage was about 70,000 km. It was time to put it to the test. Engineers of a specially created working group of the Gorky Automobile Plant (GAZ) were involved in the design of the chassis. The task set surprised them a lot. The suspension had to be significantly strengthened: the A23 weighed not 200 kg, like the regular GAZ-21 engine, but almost 500. At the same time, the engine had absolutely fantastic characteristics at that time: 320 hp power. and a torque of more than 800 Nm at low RPM (60 rpm). The requirements also stipulated the complete exclusion of access under the hood, the absence of a fuel system and attachments, and especially the presence of a productive cooling system.

In April 1965, the car went to the test site near Seversk. According to the memoirs of Valentin Semenov, who took part in the development of the engine, who managed to drive a car (or an atomobile?), the sensations were very unusual: the car was very heavy, but the engine power compensated for the increased weight. Acceleration was brisk, but braking was worse. And the engine was also very hot, and in the car, despite the Siberian cool spring, it was very hot. The tests carried out showed that the design is quite working, while the real mileage resource was more than 60,000 km.

Volga-Atom hits the road

It never got past prototyping stage, but it got the car around for some mileage.

Update: Looks like the prototype is April Fools. You can still use the concept, though.

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  • $\begingroup$ There's also the Ford Nucleon (although it was just a concept). $\endgroup$
    – Rafael
    Oct 19, 2022 at 14:46
  • $\begingroup$ I don't think Ford Nucleon targeted some specific reactor type, such as piston nuclear engine. $\endgroup$
    – alamar
    Oct 19, 2022 at 14:48
  • $\begingroup$ Correct, the Nucleon was intended to be a steam powered car. I still think it's relevant to OP's goal of nuclear-engines. $\endgroup$
    – Rafael
    Oct 19, 2022 at 14:49
  • $\begingroup$ It is worth pointing out that the sources claiming this was actually built (even only as a prototype) are not well verified. Looking at the designs, I suspect that if any of the claims are true, the nuclear engine was a conceptual design only, and that any prototypes used a standard combustion engine for testing purposes. $\endgroup$ Oct 19, 2022 at 15:35
  • $\begingroup$ OP specifically avoids "nuclear-thermal", which is the steam-powered engines mentioned for the Ford Nucleon and possibly for the Volga Atom as well. $\endgroup$ Oct 19, 2022 at 15:58
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No, this is not going to work.

You've missed the difficultly of reaching critical mass by using compressed UF6 gas. First, you have interspersed a bunch of fluorine in the middle of your reaction mass. This alone will increase the average atomic difference considerably, which is terrible for your purpose.

And you converted your fuel to a gas. Your average atomic distance just went up massively. You will not be able to overcome this by injecting neutrons - there is no way to control enough neutrons sufficient to support a massive injection rate at the time of the spark, and turn it off at other times in the power cycle. In the reaction chamber won't be able to play the neutron reflector card to make a real difference either.

If you want a nuclear bang to power your engine, you will have to use solid fuel nuclear core bombs and feed them into the chamber for each power stroke. I.e., you now have a nuclear pulse ICE instead of a nuclear pulse rocket.


@BillOnne - toolforger is correct, and I should have made that point clearer. It is also a very non-linear relationship. IIRC, the implosion in a PU-239 bomb only makes the fuel about 10% denser, but this is key to its function.

Never considered including a moderator as part of your fuel source in a bomb - and not qualified to do so, but since no bomb uses this approach (as far as unclassified info reveals), I have no reason to believe it is significantly beneficial to the process.

I did a little research to confirm that use of a moderator does not benefit nuclear explosion. This is taken from the notes in the Los Alamos Primer.

Slow neutrons cannot play an essential role in an explosion process since they require about a microsecond to be slowed down in hydrogenic materials and the explosion is all over before they are slowed down.

Slow neutrons, a.k.a. thermal spectrum neutrons, are those that have been slowed down by the moderator. This is useful in a nuclear reactor because it increases the cross-sectional area of the fuel - which makes it significantly more likely to trigger a fission reaction before the free neutron escapes the fuel source.

I mentioned in a comment that critical mass is not linear with fuel density. As a first order approximation, critical mass is proportional the 1 over density squared.

The reason Pu-239 requires an implosion to make a bomb is that all plutonium is poisoned by the presence of Pu-240 - an artifact of making plutonium in a reactor. The Pu-240 fissions more readily and a simple bullet plug bomb will make a fizzle explosion before the supercritical plutonium has a chance to involve the bulk of the Pu-239 in the explosion.

I did not mean to imply that you could not make a critical mass of plutonium without compression, just that the sub-critical mass is converted into a critical mass by the implosion even though the compression effect is relatively minor.

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    $\begingroup$ I don't have my software avaiable just now. But some things. UF6 has a molar mass of 349 with U235. And F is a medium-low moderator, which is why they are planning on using it in molten salt reactors. So to get 10 kg of fuel at 100 atm. would be something like 7 liters. You might need a cylinder such that at maximum compression it was 50 liters. I think you could get this thing critical. The practicality of such a system is poor for other reasons. $\endgroup$
    – BillOnne
    Oct 18, 2022 at 18:35
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    $\begingroup$ @BillOnne The critical mass goes up if density goes down, and even at 100 atm, gas is massively less dense than a solid (you need Gigapascals to compress gas to a solid). $\endgroup$
    – toolforger
    Oct 18, 2022 at 19:50
  • $\begingroup$ Isn't "critical mass" just a shorthand for neutron efficiency? In other words, the ~10 kg is based on a solid sphere, where you have a neutron efficiency approaching 1.0, and thus a chain reaction. Neutron efficiency losses are due to neutrons escaping. That still holds for gases, although the chance is of course bigger. (square/cube effect) $\endgroup$
    – MSalters
    Oct 20, 2022 at 7:15
  • $\begingroup$ @MSalters - critical mass is often an abbreviation of the free sphere critical mass, i.e., without the use of tampers, reflectors, etc. In reality critical this is affected by neutron efficiency as you termed it. It is quite a bit more complex than the free sphere critical mass. $\endgroup$ Oct 20, 2022 at 16:20
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This is kind of a frame challenge, but we have researched ways to turn nuclear energy into motive energy and they generally don't rely on engine-like devices. Pistons, though, that we like.

First off, containing a nuclear explosion is going to be a really hard problem from a materials perspective. Your "combustion chamber" either needs to be gigantic, or it needs to be made out of unobtanium - there's very few materials that can survive repeated nuclear blasts, and none that can do so at close range.

So, we went with making the combustion chamber gigantic. That led to Project Orion - essentially, just create the piston part of your engine, then put an entire spaceship on top of it. Toss nuclear bombs out the back, blow them up, and ride the shockwave. The "combustion chamber", in this case, is "all of space". Sure, you're losing a lot of energy, but it's not like you can feasibly harvest the rest of it anyway.

But what if instead of a nuclear internal combustion engine (pfft that's technology from the 1700's!) we went with a nuclear turbine? (also a technology from the 1700's) Turbines work well because they don't actually need a full on explosion, just a continual burn. This sidesteps a lot of our issues and seems really feasible!

Well, that train of thought gets you nuclear powered aircraft, though there's nothing in principle keeping you from mounting the turbine on a car or whatever - the only reason why they planned on putting these things in the sky is because of the horrible trail of radiation poisoning they leave behind, which nobody wanted at ground level.

The basic idea is you run a reactor exposed to the atmosphere, and then force air into it like a ramjet. The heat from the reactor makes the turbine go without using any other fuel, and also leaves a nice radioactive trail because, again, you are running a reactor exposed to the atmosphere and forcing air into it.

So yeah, those are the two ways our wacky scientists back in the 60's figured out how to turn nuclear power into go juice. These days we just use nuclear power to charge battery driven vehicles, which works better for everyone in the near vicinity.

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  • $\begingroup$ "there's nothing in principle keeping you from mounting the turbine on a car" - Well there is, there's weight. I don't envision that being light, especially if you want to protect the users from radiation, and you want to protect the engine from damage in a crash. $\endgroup$ Oct 19, 2022 at 5:52
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    $\begingroup$ If you're mounting an air-breathing nuclear reactor to a ground vehicle, you don't care about protecting anyone from radiation. And sure it probably won't get mounted to a Civic, but you could probably mount one of those bad boys to a street legal truck with a little bit of R&D and a lot of reckless disregard for the sanctity of life. $\endgroup$
    – Tacroy
    Oct 19, 2022 at 17:53
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In addition to the technical challenges others have mentioned, there's what's known in fusion as the "First Wall Problem". Essentially, you're facing a high-temperature, highly-corrosive, high-pressure gas that has a tendency to ablate and/or chemically remake and/or atomically remake everything it comes in contact with. You're dealing with fission rather than fusion, but many of the issues are going to be closely related:

  • neutrons embedding in the wall material will change the elemental composition of the wall, corroding it.
  • High fluxes of Uranium and Flourine ions (and stray electrons & gamma rays) are going to chemically corrode the wall.
  • Heat is going to tend to melt the wall in places (or spot-weld joints).
  • Material ablated off the wall is going to pollute the gas / plasma you're using as fuel, making the compression and reactivity less efficient.
  • Material ablated off the wall is going to re-condense in the exhaust, possibly blocking and/or corroding downstream systems.

That being said, this is one of the coolest awful ideas I've heard since the Orion nuclear rocket, which consumed nuclear bombs by the hundreds to send aircraft-carrier-scale payloads to Mars and back...

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    $\begingroup$ I think you may be mixing up fission and fusion. UF6 will produce fission. The pressures will be a few 100 atm. at most, which is possible with existing tech. No plasma. The neutron fluxes are similar to what exists in current nuclear reactors, presuming the power could be controlled. Zirc will deal with it. UF6 was chosen by the military specifically because it does not tend to erode the walls. The temperatures would be welll below steel melting. The fuel is a gas at working temps so won't condense and block pipes. Of course, with all that, there are way many other problems. $\endgroup$
    – BillOnne
    Oct 19, 2022 at 0:15
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    $\begingroup$ I specifically acknowledge fission vs fusion, so not really mixing them up. The normal uses of UF6 are for isotopic separation, which does not involve building up critical masses and chain reactions. Once you've got the intended fusion reactions happening, you no longer have UF6 because you no longer have uranium. Existing U-based reactors tightly control the density of neutrons and U, and provide moderating media with a variety of properties so feedback removes the possibility of a meltdown. Existing fission reactors also run at temperatures of 300 degrees Celsius or more... $\endgroup$ Oct 19, 2022 at 15:13
  • $\begingroup$ Phase diagrams of UF6 range up to about 300 degrees, suggesting that if you're actually using a fission chain reaction and power cycle, rather than a steady-state thermal source, you're probably exceeding the dissociation energy of UF6... and stray flourine atoms are highly corrosive. HF, a natural waste product (because of stray neutrons' beta decay to protons and the stray flourine atoms) is one of the most corrosive acids available. If you've got a fission-based power stroke, your peak energy density is higher than for a reactor which boils water. $\endgroup$ Oct 19, 2022 at 15:22
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This answer is about doing it as a fission system. The OP mentions Uranium.

As you describe it, there are huge challenges.

There are two basic time scales to nuclear fission. The slow one is critical but not prompt. And the fast one is prompt.

Prompt has a doubling time of a few microseconds, possibly faster depending on the design. That means you can't usefully control the energy output. Any engine is going to have RPM in the few thousand at most. It cannot respond to the power changes. Your machine explodes on the first stroke. Early in the power part of the first stroke.

Critical but not prompt has a doubling time in the milli-second or slower range, again depending on the design. You might hope to make this useful.

The difference between prompt and not-prompt is not large. Power reactors are carefully designed to stay in the not-prompt range, and for it to be difficult to get across that threshold. Chernobyl was an example of passing over to prompt, and that only for a very short time, some few milliseconds. That was long enough to heat the coolant water enough to produce a steam explosion that destroyed the station.

Controlling power through compressing the fuel would be a finicky thing. An extra 1% compression would be more than enough to push you over. A tiny errror in the input fuel pressure or composition and your engine disappears.

There are other problems. To complete the cycle you either have to cool the fuel in place, or pump it out and cool it outside.

For you to cool it in place is tough. I am unaware of any engine that cools the fuel in place. When the fission stops the decay heat is round about 5% of the full power value. After 1 minute it is down to about 1%. These values depend on the design but they are reasonable approxes. So you need to cool your fuel in a useful time, while dealing with the decay heat. In order to get the heat out in a reasonable time you might well require a cooling system that consumed more energy than produced in the pulse.

If you were prepared to have an hour between strokes you might get something. It seems mighty inefficient.

To pump it out you have to keep in mind that only a minute fraction of the total energy is released in one cycle. So you would need to cycle the fuel through the engine many thousands of times. (100's of thousands? Depends on the design.)

You would need a system that is capable of pumping out the gas and pumping in the previously cooled gas. But note that 5% decay heat. Your pumping system has to deal with 5% of the total power coming out as mostly radiation. And it will be cooking everything near the exhaust port of your engine.

Plus you need to remove the fission products. These are usually quite radioactive. Some of them are gas. And several of them are challenges at handling, such as the Tritium that likes to leak through stuff. These are issues that are being solved for molten salt reactors, but using systems that weigh 100s of tonnes. And that cycle the fuel every few hours, not on a cycle of once-per-engine-stroke. It gives a lot of the nastier isotopes time to decay while still in the reactor.

So your working fuel is going to be this tiny fraction of total fuel. The rest of the system will be a 100's of tonnes station that will tend to leak radioactive isotopes at a prodigious rate. And it will have a very tough time avoiding exploding.

And there are already nuclear power station designs on that scale that work far better in every way.

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  • $\begingroup$ The UF6 should be mixed with He for heat transfer to cool I would assume. I already assumed the fuel would be pumped through the system constantly following the exhaust and passing through a heat exchanger, and then reprocessed when possible or the fission products become too much of an issue. Perhaps since the UF6 is gaseous already you could use a centrifuge already driven by the shaft power of the piston. $\endgroup$ Oct 19, 2022 at 0:21
  • $\begingroup$ I'm not aware of an engine that cools the fuel in place, either. But it's kind of the same as what a Stirling engine does: it heats and cools the inert gas in the cylinder once each stroke. $\endgroup$ Oct 19, 2022 at 15:03
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I believe the problem would be “knocking”. In an engine with a spark plug to ignite the fuel in the chamber there is a condition where the engine could run too hot and the fuel ignites from the heat before the spark plug fires, and that leads to a rough running engine and lost power. A similar condition is likely in a fission powered piston engine where a stray neutron from a spontaneous fission sets off the fission too soon and you lose power.

I can imagine how such an engine might work in theory but in practice I can imagine a spontaneous fission causing timing problems on the “detonation”. I doubt a fission explosion is required to make it work, it just needs to get hot after compression and then have a cooling off cycle somewhere. The “spark plug” could be something like a proton gun or a small fusion device, whatever would be a reliable and controllable neutron source, but the uranium fuel and fission products could trigger the critical condition too early with a spontaneous neutron.

If put in a continuous critical fission state for heat, like some kind of rocket or turbine, then the issue of early (or late) fission is not an issue. With enough fission events going on the statistical timing of these events becomes a smooth curve that can be calculated out and put into the design. There’s no need to know when each critical event happens, there is just one continuous critical state. Just keep the hot ball of gas in the right spot and the engine keeps running smoothly.

Knocking, that’s the problem I see in a piston nuclear fission engine. Maybe with the right neutron absorbers the knocking goes away but then you are throwing away a lot of valuable neutrons you could have used for fission somehow. With neutrons having no charge they can’t be funneled into an adjacent piston easily, or stored in a cyclotron (or whatever they are called).

I like the theory. I’m just thinking knocking will kill the idea in practice.

Edit to add:

I was talking about this hypothetical engine with my brother and he mentioned something to the effect of combustion engines being able to use valves to release pressure, potentially a simple means to prevent knocking in an engine. So the solution to the problem is likely much simpler than I considered initially. This still leaves the issue of fuel being burned and not turned into useful heat, instead the pressure from the heat is vented out the exhaust. Perhaps the lost heat is trivial but over time that would likely count against the idea with other engines that aren’t “burping” excess waste heat out of the exhaust with every premature detonation. Delay things somehow to prevent premature detonation and that’s also lost efficiency because the power stroke was shortened.

I believe this can be made to work, it’s just going to have efficiency problems.

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  • $\begingroup$ And what of a ‘diesel’ method where supercriticality occurs purely from compression? $\endgroup$ Oct 19, 2022 at 3:23
  • $\begingroup$ If injecting already compressed uranium gas then it could potentially already be critical as it is injected. That might not be a bad thing since the critical gas will only get hotter in the cylinder. Then on the power stroke the gas becomes subcritical, then pumped out on the exhaust stroke. If the gas is already critical when it goes in then it looks to me more like a steam engine than a diesel engine. If not critical when the fuel enters then there is still the problem of spontaneous fission being poorly timed. There might not be any knocking, just less power. $\endgroup$
    – MacGuffin
    Oct 19, 2022 at 3:36
  • $\begingroup$ If there’s a container with hot critical gas getting hotter then don’t bother with difficult to time pistons, run it through a turbine. The gasses out the back can be collected, cooled, filtered from some of the fission products, some new fuel added, then fed back into the critical fission chamber for more. I’m seeing that it can work, just not as efficient as other options. $\endgroup$
    – MacGuffin
    Oct 19, 2022 at 3:43
  • $\begingroup$ If fission were induced by just heat, I think you'd be right. Do you expect the inside of the cylinder to become a neutron source? Maybe, but the fuel could be adjusted to account for that. $\endgroup$ Oct 19, 2022 at 15:42
  • $\begingroup$ Heat will usually inhibit fission by causing thermal expansion of the nuclear pile/core/whatever. It is this phenomenon that allows reactors used on Navy ships to be largely self regulating. There was a mention of a neutron source “spark plug” so I ran with that. The fuel cannot be adjusted for spontaneous fission “sparking” the fission because that is an inherent feature of the fuel. This is a problem with fission weapons which can lead to less than optimal yield. They compensate for this by keeping the window for premature detonation very small, and that requires self destructive forces $\endgroup$
    – MacGuffin
    Oct 19, 2022 at 16:00
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I've given this some thought since the other day when I commented. There are several almost insurmountable issues with such a concept.

First, just how large are you willing to allow this IFE (internal fission engine?) to be? Project Orion had variations that could launch from the surface, and these tended towards being huge. The pusher plate that prevents the occupants and their vessel from being vaporized is several feet thick, and made of steel (or perhaps some weird iron alloy designed to be slightly more robust).

Even then, while it protects the occupants from being pulverized by mechanical damage or burnt down to carbon, it's not exactly something it'd be safe for you to lick like in some dumb Tiktok challenge video.

If this is a mechanical engine with pistons, this isn't for an aircraft carrier, it's for those giant mobile cities in that bad Mortal Engines movie. Something on that scale. Seriously... it is difficult to imagine something large enough to bother moving with such a motor.

Then there's the issue of just how often the thing can fire these off. Project Orion had different pulse timings depending on whether we're talking about launching from the surface, or if it was cruising in deep space, but at most you're talking about 1/s. I'm not sure that can work for a piston engine.

Then there's the issue of stroke length. The pistons in your car move only inches, the pistons in a large ocean-going vessel's diesel engine move only feet. Can even a small, sub-kiloton explosion move this fewer than hundreds of feet? If you try for a shorter stroke, doesn't the nuclear fireball just melt the piston and the walls of the cylinder?

And about that... Orion's pusher plate only survives in solid form because mere millionths of a second later it is hurling itself away from the epicenter of hell. The cylinder walls do not have this luxury. The piston? I think it survives much like the pusher plate, but the cylinder itself does not. And while it won't melt all the way through, my limited understanding of automotive mechanics is that you want the cylinders to remain in their solid state (instead of, you know, skipping liquid and gas and going straight for plasma). This part almost requires exotic matter to work... something that can put up with alot of bullshit. Think neutron-degenerate matter, but maybe not that particular sort of exotic matter (you're already talking about playing with neutrons after all).

But, if you can overcome the problems of: having something large enough to move, the absurd engine geometries involved, and the materials science of something that can survive constant nuclear detonations and remain mechanically sound... well, it sounds like it will take at least the entire 180 minutes for the comic book superheroes to save the world from you. Even then it will be tight, they might have to montage it.

Also, thanks for giving the world the concept of tera-horsepower. No one knew that unit existed until yesterday.

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    $\begingroup$ I don’t see why the pistons need to contain a nuclear blast, just getting hot would be enough to make the engine work. We get things hot with nuclear fission all the time without detonations. I agree it is likely impractical, just not for the reasons stated. $\endgroup$
    – MacGuffin
    Oct 19, 2022 at 15:00
  • $\begingroup$ My understanding is that he wants pistons turning a crankshaft for horsepower, rather than a nuclear rocket. If I've misunderstood, then oopsmybad. $\endgroup$
    – John O
    Oct 19, 2022 at 20:46
  • $\begingroup$ @JohnO No you're correct, but it's hardly a 'nuclear detonation.' The supercritical event causes the gas to heat up immensely and rapidly expand, preventing a 'significant' detonation due to runaway fission. Actively cooling the cylinder and gas are an obvious option on each exhaust stroke so that the next cycle can continue. $\endgroup$ Oct 19, 2022 at 20:58
  • $\begingroup$ @JohnO and thanks, it's always interesting to apply old concepts to new ideas. $\endgroup$ Oct 19, 2022 at 21:15
  • $\begingroup$ @JeffereyDawson I'm with the other guys on this... I don't think that gaseous U235 at any enrichment level is going to be dense enough to reach supercriticality. I didn't realize that particular version was important to you. $\endgroup$
    – John O
    Oct 19, 2022 at 21:21
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I recall a free piston Sterling cycle nuke design intended for electrical generation on the moon or similar, the free piston design had the advantage that the whole thing could be hermetic allowing the use of helium as a working fluid.

The elephant in the room for ANY nuclear plant better then simple nuclear thermal is the usual one of the Carnot limit and being able to dump the waste heat, space may be cold, but it is one hell of a good insulator.

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    $\begingroup$ Cooke-Yarborough did that at Harwell. t wasn't powered by a nuclear reaction though; it used the decay heat of a lump of radioactive material to drive a FP Stirling. Quite low power density but very long operational life, I think they were used for marine navigation buoys. Nasa used a rival free piston design too. $\endgroup$ Oct 19, 2022 at 19:28
  • $\begingroup$ @user_1818839 Pretty sure the machine I am thinking of was an actual reactor design and not a Pu240 decay heat job. IIRC the one reactor powered satellite that NASA flew was thermo electric generation rather then mechanical, I don't know what the soviets used. $\endgroup$
    – Dan Mills
    Oct 20, 2022 at 12:36
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A lot of the other answers here are trying to suggest something equivalent with a modern car engine, but there are a lot of problems with this concept, as others have covered in their answers. But you can make this work if you look at older technologies. Think steam engines, like in the old trains. Those would have a big tank full of water, that you heat up and the resulting steam would push the pistons inside the cylinder. Incidentally this is very similar to how the current nuclear power plants work... The nuclear reaction heats up the water used to cool it down and turn it to steam which is then used to spin a turbine that spins an electric generator. Replace the turbine with a piston and you have your nuclear powered piston engine. Obviously this will be a huge engine, so it would not go in your car, but you could conceivably install it in a ship, or a fixed building.

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    $\begingroup$ The others are not coming with something similar to a modern car engine, they are following OP's question, stating "a model akin to a normal internal combustion" $\endgroup$
    – L.Dutch
    Oct 20, 2022 at 14:24

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