What is the feasability of a Neutron Piston?

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    Neutron Piston
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Discussion Overview

The discussion revolves around the feasibility of a mechanical piston driven by a cylinder filled with Uranium Hexafluoride (UF6) and an inert gas, exploring the potential for a nuclear engine concept. Participants examine various aspects including theoretical mechanics, neutron behavior, and practical challenges associated with such a design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant proposes a piston mechanism that utilizes UF6 and nitrogen, suggesting it could run continuously with proper cooling.
  • Another participant questions the need for neutron moderation, suggesting that neutrons must be slowed for effective absorption by UF6.
  • A different approach is suggested, involving uranium metal alloy cylinders filled to critical volume, potentially simplifying the design by eliminating gaseous fuel concerns.
  • Concerns are raised about the stability of the reaction, with one participant noting that a stable fission reaction could lead to the piston hovering rather than oscillating as intended.
  • Fission products are discussed as a challenge, as they absorb neutrons and produce heat, complicating the system's efficiency and operation.
  • Some participants express skepticism about the practicality of using gaseous UF6 due to its low density and the need for high enrichment to achieve critical mass.
  • Historical references are made to similar concepts, suggesting that there may be precedents for such nuclear engine designs.
  • One participant expresses doubt about the necessity of a nuclear piston engine, questioning its practicality and purpose.

Areas of Agreement / Disagreement

Participants exhibit a range of views, with no consensus on the feasibility or practicality of the proposed neutron piston concept. Concerns about neutron moderation, fission product behavior, and the overall efficiency of the design highlight ongoing disagreements and uncertainties.

Contextual Notes

Participants note limitations related to neutron behavior, the need for moderators, and the challenges of managing fission products, which remain unresolved within the discussion.

Mortagnan
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Hello all, this is my first post so hopefully its not overly foolish.

I propose a mechanical piston driven by a cylinder that is filled with Uranium Hexafluoride and an inert gas (Nitrogen?). As the piston descends, the the UF6 is compressed and the neutron flux spikes. The increased temperature would expand the nitrogen and push the piston, allowing the UF6 to expand and decrease the flux again.

Assuming a proper cooling system for equalizing the temperature after each piston stroke, and ignoring wear and tear, it seems to me that the piston could run continuously. Is there any merit to this idea or am I missing an important piece of the puzzle?

Honestly it seems too simple. I can't find the hole :)
 
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I'm not nuclear savvy, but I think that the UF6 neutrons need to be slowed by something, such as water or graphite, to be effectively absorbed by UF6 molecules.
Separately, the reaction speed of exponential fission is a lot greater than mechanical inertia driven motion. The design, if implemented using some suitable radioactive fissile gas, might achieve an excursion on the first compression, rather than the desired expansion..
 
An easier method for creating a reciprocating nuclear engine is to just make the cylinders out of uranium metal alloy and fill the cylinders so that the maximum volume is critical. You could make the working fluid the moderator or make the pistons neutron absorbers and configure the system accordingly. Then you don't have to worry about storing, shielding, and containing gaseous fuel and fission products.
 
You could make the working fluid the moderator

it's interesting to tinker with a near-critical arrangement surrounded by steam.

Increasing pressure compacts steam increasing moderation...

read up on " undermoderated " you could get yourself setup for a runaway i think.
 
jim hardy said:
it's interesting to tinker with a near-critical arrangement surrounded by steam.

Increasing pressure compacts steam increasing moderation...

read up on " undermoderated " you could get yourself setup for a runaway i think.

No, runaway reaction occurs when you are overmoderated, not undermoderated. You want it to be undermoderated - that is what makes it stable. When the water flashes to steam causing the piston to expand, the "core" becomes subcritical.
 
Thinking about it further, you have moving parts with tight tolerances under high neutron flux. Creep would be a significant problem. The pistons, seals and block would have to be very robust or limited to low burnup.
 
Mortagnan said:
Hello all, this is my first post so hopefully its not overly foolish.

I propose a mechanical piston driven by a cylinder that is filled with Uranium Hexafluoride and an inert gas (Nitrogen?). As the piston descends, the the UF6 is compressed and the neutron flux spikes. The increased temperature would expand the nitrogen and push the piston, allowing the UF6 to expand and decrease the flux again.

Assuming a proper cooling system for equalizing the temperature after each piston stroke, and ignoring wear and tear, it seems to me that the piston could run continuously. Is there any merit to this idea or am I missing an important piece of the puzzle?

Honestly it seems too simple. I can't find the hole :)

Naive question: where did the neutrons come from?
 
It seems like a primitive nuclear stirling engine. The thermal efficiency is probably lower than that of an ordinary pressurized/boiling water reactor.

One problem is that the reaction might be stable and the piston will just hover halfway. There have been a number of criticality accidents where a stable fission reaction developed because thermal expansion made the reaction subcritical again.

Fission products that absorb neutrons will make creating a stable oscillation very hard. some of them will precipitate on the walls of the cylinder.

Without a modereator, you'll need a really large volume of UF6 to get a critical mass with low-enriched uranium

Fission products will continue to produce heat when the cylinder should be cooling, this heat will have removed by cooling and won't help with energy production.
 
I think the biggest "hole" in the idea was just mentioned, that of fission products absorbing neutrons and also continuing to produce heat afterwards. I hadn't considered that.

I also hadn't considered the possibility that it would be less efficient than a standard boiler setup, although it does make sense.
 
  • #10
It would be less efficient but there are advantages to a reciprocating engine - compactness, and the ability to change load on demand. Such an engine might be well suited for a large land vehicle like a tank, mechanized artillery, or possibly airplane.
 
  • #11
QuantumPion said:
It would be less efficient but there are advantages to a reciprocating engine - compactness, and the ability to change load on demand. Such an engine might be well suited for a large land vehicle like a tank, mechanized artillery, or possibly airplane.

A really big problem is the low density of the UF6 gas. You might need a very big cylinder, that's as large as a tank, and you'll need a moderator, or highly enriched uranium.
 
  • #12
willem2 said:
A really big problem is the low density of the UF6 gas. You might need a very big cylinder, that's as large as a tank, and you'll need a moderator, or highly enriched uranium.

As I mentioned previously it would be impractical to try to use a gaseous fuel for multiple reasons. For one, the UF6 would have to be very high enrichment. This leads to many issues in preventing criticality where you don't want it. Then you would need a system for recycling and cooling the highly radioactive exhaust to reuse, because you aren't going to be ejecting a million dollars worth of fuel on every stroke.
 
  • #13
Your idea has precedents.
There was a suggestion from satellite observations during the waning days of the Soviet Union that they had built a couple of enormous pressure spheres, connected with a duct festooned with cabling.
The suggestion was a nuclear pulse device such as you suggest, with the expansion on one side compressing the other, with the plasma power tapped off in the middle.
I have no idea if this was the actual purpose or what happened to that installation, but it is still in the records in the Aviation Week issues of the period.
 
  • #14
Why would you even have a desire to do this? A nuclear piston seems utterly ridiculous and unneeded.
 
  • #15
Have you filed for a patent for your idea yet? If not, you should search espacenet.com to check whether others have filed for it.
 

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