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This new topic is a follow up to a suggestion by Calliban, in the Large Ship topic.
In the spirit of Oldfart1939, this topic is created according to the belief that it is (sometimes) better to ask for forgiveness than for permission.
Thus, permission is requested of SpaceNut, to create this new topic, or (in this case) forgiveness is requested of SpaceNut for creating this new topic.
For Calliban, please add detail to your vision of how a young person, working in a basement or in a garage, might be able to achieve measurable fission/fusion at the scale you've described in your introduction of this (relatively new) technology.
A complete description would include how to create the required magnetic force, how to prepare the sample to be operated upon, and how to measure the results from both a performance and from a safety point of view.
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Previous post pasted below from Large Ship thread.
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I suspect we have discussed fission-fusion hybrid propulsion in the past, but I can't remember where it was. It is definitely a good candidate for large ship propulsion, as it combines high ISP with relatively high thrust in a way that may obviate the need for aerobraking in a very large ship. This paper describes a NASA propulsion concept very similar to what I had in mind for an inertial confinement fusion-fission hybrid approach.
https://ntrs.nasa.gov/api/citations/201 … 000723.pdf
The idea is to incorporate small amounts of fissile material into (or in this case around) a lithium-deuteride fuel pellet. Critical mass is inversely proportional to the square of fissile density. So with sufficient compression, critical mass can be reduced by several orders of magnitude, maybe even to milligrams. That increase in density is provided by a z-pinch in this case, which capable of developing gigabars of pressure without the sort of temperature rises that would accompany x-ray ablasion compression in conventional IC concepts.
The key to the functioning of this concept is the close atomic coupling of the fissile and fusion fuels. The range of fission products in matter is ~1E-3 mm. Coupling starts long before critical conditions are reached. A single spontaneous fission releases fission products into fusion fuel. The fission products, each carry average energy of 80MeV. Those that escape the fissile material, enter the fusion fuel, leaving a cone of ionised and high energy lithium and deuterium ions in their wake. A substantial portion of these then undergo fusion, releasing 13MeV neutrons, that lead to more fission in the liner or core. As fission rate accelerates, the escaping fission products heat surrounding ions to temperatures of 10s to 100s KeV. These stream into the surrounding compressed lithium deuteride pellet and act as a detonation wave.
The concept greatly reduces the driver energy needed to trigger fusion. In my previous concept, the fissile fuel was a milligram raisin at the centre of a gram mass fuel pellet. I chose this arrangement so that fusion fuel would serve as a reflector and because neutron flux will be highest at the centre of a spherical fusing system. The NASA concept varies only in the fact that fissile material is a thin foil around the outside of the pellet.
This concept could be used as a near term practical Earth based nuclear fusion power supply, as well as a propulsion scheme. Ideally, fission serves as a trigger, but contributes negligible net energy gain. That way, fission product release will contribute negligible radioactivity in the exhaust. As a power supply concept, the limited driving energy lends itself to more compact fusion reactor concepts. However, the required thickness of the neutron absorbing blankets and the critical mass at achievable compression, suggests a minimal achievable size.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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If you modernize the nuclear explosion drive from 1955-vintage fission device technology, then you have high Isp at very high thrust. We already know it will work. Why not do it?
GW
GW Johnson
McGregor, Texas
"There is nothing as expensive as a dead crew, especially one dead from a bad management decision"
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For Calliban re the excellent question by GW Johnson ...
My guess is that this question will require some time to work through ...
If I understand what you are presenting, it is significantly different from nuclear thermal technology, which is a one-and-done concept.
I'm discounting restart capability here. That doesn't count (in my understanding of the situation).
The nuclear thermal rocket is loaded with fissionable material and a supply of hydrogen (or equivalent) for working fluid, and it fires until it is done, and that's the end of it.
If I understand your idea, it is to gain the many advantages of fusion in a long distance constant thrust device which uses fission to facilitate fusion, and not as the primary source of energy.
Furthermore, (again if I understand correctly), this engine design can be refueled as many times as the operator can afford, which could be for hundreds of thrust operations.
This engine would (if I understand it correctly) require a reliable power supply external to the engine, much as would be the case with VASIMR.
While you're working on the answer, could you differentiate the concept you're describing from VSSIMR as well?
I ** think ** VASIMR is a fancy ion thrower, but it may be more than that.
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I think the best way to carry out initial testing of this concept would be using a monte carlo type model, maybe MCNP or something similar. It wouldn't be an easy project, because there are numerous interactions taking place simultaneously:
Spontaneous fission in the fissile core. This will result in fission products entering the lithium deuteride shell surrounding the core, causing fusion in their ion trails, which would release neutrons. Some of these will reenter the core causing more fission. Some will be absorbed by lithium-6, creating tritium, which will cause even more fusion events when it interacts with deuterium. As the whole assembly compresses, density increases and escape probability from the fissile core decreases and the LiDe shell around it becomes a more efficient reflector of neutrons. Fission rate in the core will increase sending more fission products into the shell, triggering fusion, which increases neutron bombardment of the core, etc.
This sort of coupling between the two systems should reduce critical mass to milligrams and under sufficient compression, fission will inject sufficient energy into the centre of the pellet to produce a detonation wave, which would rapidly fuse the LiDe shell. But the situation is complex and difficult to model, because of the number of different interactions taking place. One thing that would reduce critical mass and required compression would be to surround the shell with a dense tamper material like lead. This would constrain the rate of expansion of the shell as it heats up, due to its high inertia. This allows more time for the two interacting systems to undergo fission and fusion, before the entire system disperses due to thermal expansion. Any neutron and charge particles entering the lead tamper, would generate x-rays, which would further heat the fusing shell material. If lead is used as a tamper material, then it provides a useful means of triggering fission in the core when sufficient density is reached. When density approaches its maximum value, the lead tamper would be targeted with 1GeV protons. This will result in a small number of lead nuclei undergoing fast fission. Each fission event would release around 100 fast neutrons, which would shower the fissile core, pushing fission rate to the point where fissile-fusion coupling results in a chain reaction.
The project would probably make a good PhD dissertation. Maybe I can talk my company into funding it for me? Or maybe a fresher and younger mind could pick it up?
Ultimately, the goal is to reduce the amount of driver energy needed to initiate fusion, to improve net energy gain and to make the entire system sufficiently compact to be suitable for a space drive, naval ship power supply or Mars surface power supply. Present day inertial confinement facilities are huge and lasers consume TJ of electrical energy per pulse. The laser driver efficiency is extremely poor and heating introduces plasma instabilities that disperse the system before sufficient fusion is induced. In a fission triggered system, heating of the plasma is not necessary and is undesirable, as the goal is simply to increase density to the point where criticality takes place. Ion beams that result in ablasion of the surface may be a more effective trigger than laser beams. Electrostatic ion acceleration has the potential to be far more efficient in converting electric driver energy into shell kinetic and pressure energy.
Ultimately, we want the fissile core to be as small as possible and to act as a thermal trigger rather than providing any big contribution to total energy yield. So the goal is to achieve the highest net energy yield per fissile atom, as this will minimise the amount of radioactive products in the exhaust stream. If fission can be reduced to something like 1E-6 of total energy yield and the system has sufficient power-weight ratio, we would have a propulsion system with high thrust and high ISP, that is clean enough to use in Earth atmosphere. These are the sorts of propulsion systems needed to reduce the cost of space travel to levels that ordinary people can afford and transit times within the inner solar system to weeks. Colonisation can then proceed rapidly and it is possible that a sizeable proportion of Earth population could leave the planet within a few decades.
As GW notes, we already know that a pulse propulsion scheme of this type will work. The question is how well it will work? How small can pulse units be and to what extent can we reduce the size of the fissile charge? At some point, charges need to become clean enough to allow the pulse drive to operate within Earth atmosphere. The EMP issue is unlikely to be a problem, as the charges are small, explode inside the engine nozzle and fission presents a small proportion of total energy. The gamma ray burst associated with fission weapons will not therefore occur.
Last edited by Calliban (2021-08-05 07:38:24)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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For Calliban re #5
Nicely Done! PhD Dissertation subject noted!
SearchTerm:Hybrid fission fusion optimized space propulsion system
If anyone not already a member read's Calliban's post and would like to pursue this, read Post #2 of Recruiting.
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The paper at the link below is from June of 2020 ... there may have been additional progress since then, but if there is, Google did not find it.
If a member runs across an update, please post a link in this topic ...
The paper includes graphics that explain the process under investigation ...
https://www.osti.gov/pages/servlets/pur … %20present.
Progress Toward a Compact Fusion
Reactor Using the
Sheared-Flow-Stabilized Z-Pinch
E. G. Forbes, U. Shumlak, H. S. McLean, B. A. Nelson,
E. L. Claveau, R. P. Golingo, D. P. Higginson, J. M.
Mitrani, A. D. Stepanov, K. K. Tummel, T. R. Weber, Y.
Zhang
June 4, 2020
FUSION SCIENCE AND TECHNOLOGY
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tahanson43206,
With that kind of breeding ratio, I hope their process for extracting and consuming the Tritium produced has superb efficiency. I assume that is sufficient, but this is one of several major stumbling blocks to achieving a practical design. The newest superconducting materials can generate electromagnetic fields sufficient to stabilize the plasma and get more power output than required input, but there's a difference between slightly better than break-even output (which we have already achieved using the laser inertial confinement approach) and enough output for a practical commercial power plant.
1. Plasma Stability
2. Tritium Breeding Ratio
3. Thermal Power Conversion
4. Mechanical Fatigue of Containment Vessel and Support Structures
5. Computerized Control - because it's not possible for a human operator to respond quickly enough
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For kbd512 re #8
Thank you for your helpful contribution to this topic!
Because the paper cited in #7 is a couple of years old, I am hoping that forum members may find something more recent and add it to the topic.
This topic was created as a place to collect work done by Calliban and others in this specific focus.
Z-Pinch was suggested by Calliban as potentially capable of providing the plasma conditions needed to promote the hybrid process.
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A chemical rocket engine using liquids as inputs maintains a constant ignition promoting environment, after initial ignition.
As new input liquids enter the combustion chamber, they find themselves inside an environment that promotes their chemical combination with suitable partners.
In thinking about the fusion rocket concept of this topic, I'm wondering if the use of fission elements to start a reaction might be tapered off, if the conditions inside the magnetic containment field are sufficient to promote fusion of Deuterium (in particular, because of it's abundance).
The fusion process for a rocket needs to deliver a constant flow of charged particles to the outlet port, while (somehow) delivering power back into the process to keep the magnetic field strong and the input flowing.
The ideal situation would be to achieve a steady state in which Deuterium is fed into the fusion chamber, and thrust is delivered to the output at a constant rate.
The situation would be a little bit like a jet engine, in which inputs are prepared ahead of the combustion chamber, combustion occurs, and energy is drawn off to sustain the intake process, and the balance of the energy released is directed to the rear.
A sustained fusion propulsion would be similar.
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A chemical rocket engine using liquids as inputs maintains a constant ignition promoting environment, after initial ignition.
As new input liquids enter the combustion chamber, they find themselves inside an environment that promotes their chemical combination with suitable partners.
In thinking about the fusion rocket concept of this topic, I'm wondering if the use of fission elements to start a reaction might be tapered off, if the conditions inside the magnetic containment field are sufficient to promote fusion of Deuterium (in particular, because of it's abundance).
The fusion process for a rocket needs to deliver a constant flow of charged particles to the outlet port, while (somehow) delivering power back into the process to keep the magnetic field strong and the input flowing.
The ideal situation would be to achieve a steady state in which Deuterium is fed into the fusion chamber, and thrust is delivered to the output at a constant rate.
The situation would be a little bit like a jet engine, in which inputs are prepared ahead of the combustion chamber, combustion occurs, and energy is drawn off to sustain the intake process, and the balance of the energy released is directed to the rear.
A sustained fusion propulsion would be similar.
(th)
This is unlikely to work, unless the engine chamber is extremely large. The problem is that for the plasma to ignite, the energy lost by the escaping ions, must at least balance the energy addition from fusion. This is the famous Lawson Criterion.
https://en.m.wikipedia.org/wiki/Lawson_criterion
The smaller the reaction chamber is, the lower the confinement time of the average ion. To achieve ignition, the plasma must be relatively dense (which increases chamber pressure) or very hot (which increases chamber pressure and radiant heat loss by bremstraalung). The achievable strength of magnetic fields limits our ability to meet the Lawson criterion for all possible combinations of density, temperature and confinement time. Density and temperature are both limited due to limitations on stable plasma pressure with the magnetic fields that we can achieve. The only other variable that we can control is chamber diameter. By increasing chamber diameter, we increase confinement time, as it takes more time for the average ion to traverse the chamber. This is why experimental fusion reactors are trending bigger. The problem is that a large tokamak like ITER weighs tens of thousands of tonnes. Not much use for our purposes. We need something that weighs no more than 100 tonnes. Preferably less than that.
It is the inherently poor power density of magnetic confinement fusion that is its achiles heel in my opinion. Even if plasma stability problem are solved and we successfully build a powerplant that can capture fast neutron energy as heat, convert heat into electric power and breed tritium fuel, power density for a tokamak is going to be at least an order of magnitude poorer than a bog standard PWR. It was this, more than anything, that prompted by interest in inertial confinement fusion. At least in this case, the plasma density is high enough to provide a respectable power density. But for this to be the case, the compression driver needs to be more efficient and compact. Hence my interest in ion beam and z-pinch methods and the use of fissile materials as trigger materials. The use of ion beams created by electrostatic acceleration, allows for both efficient and compact driver assemblies. However, the achievable compression will be reduced compared to lasers, because ions will tend to thermalise at the very high flux density at the surface of a fuel pellet. The fissile trigger plays a key role in working around this limitation, as fission can deliver the injection of energy needed to create a hot spot, at the centre of a pellet, in the region of greatest density and coinciding with exact time that peak density is reached. Fission is therefore the essential trigger if we wish to produce the sort of compact fusion reactors that are useful for space drives, Mars surface power and indeed, Earth based powerplants.
Whether this is a problem or not, depends on just how daft people are prepared to get. For each unit of energy yielded, we need thousands to millions of times smaller quantities of fissile fuel than a nuclear fission reactor. This is because fissile fuel is being used used as a spark plug. We will probably generate more radioactivity from fast neutron interactions than we would from fission. All of the fission projects released by all of the nuclear testing between the 1940s and 1970s, managed to push up background radiation levels by 5% at peak. A surface launch fission fusion pulse rocket would release the energy equivalent of a medium yield fission bomb. But only a thousandth to a millionth as much radioactivity. So it is unlikely to make a big impact on background radiation levels. A bigger issue is the x-rays and thermal energy release from the exhaust plume. It would be enough to cause burns and start fires many miles away from its take off point. And in Space, it will pump the magnetosphere with charged ions, which would be a hazard to satellites and Space stations. Any decent performing space drive is a weapon of mass destruction because of the energy density it represents.
Last edited by Calliban (2022-01-05 08:54:11)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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For Calliban re #11
First, thank you for giving the continuous flow idea a careful reading, and courteous (helpful) reply!
The role I am attempting to play here is to (try to) encourage your creativity, and to (occasionally) trigger a line of thought. The forum archive is slowly filling up with posts worth serious study, amidst all the lighter banter that keeps things flowing.
With that in mind, I'd like to invite your consideration of a continuous flow of a cylinder of multiple layers of material into the fusion chamber.
Your original series on this topic seemed (as I remember it) to have taken inspiration from the US laser experiments, which are packaging fusion inputs into little packets that are struck by laser beams.
The Lawrence Labs facility is cathedral-like in size and scope, but you've given a figure of 100 tons as a target for your fusion drive.
Meanwhile, GW Johnson has proposed a mass of 5000 tones for the Large Ship of RobertDyck.
I like the ratio of drive to vessel those two figures provide.
Let's take the unfolding of your vision a step at a time....
For a first step ... can you imagine an alternative future in which a cylinder of inputs to your proposed device might be fabricated?
The diameter of the cylinder would surely be small. Perhaps it might be on the order of a human hair?
Such a cylinder could be coiled up before flight, and unrolled as needed during propulsion events.
That presumes that the needed materials can be contained within the form factor of a cylinder.
The lasers of your ignition system would then deliver energy to the little tip of the cylinder as it pokes it's head into the fire zone.
Some clever engineering would be involved.
Fortunately, this forum is blessed with just the right mix of people so that (if this concept is possible at all) the parameters of success may be discovered.
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For Calliban about plasma output from hybrid-fission-fusion space drive....
First ... if case you missed it, (and that is certainly possible), kbd512 has opened a line of discussion of use of plasma to provide momentum transfer to a space craft.
Second, your recent post ended with a reminder that a civilian application of a technology may have military counterparts.
In his recent posts, kbd512 has written about the impact of Newton's Third Law upon the space craft that creates a stream of plasma that is directed at another space craft for the purpose of delivering momentum.
I would like to point out that a military vessel, intending to direct a beam of plasma at an enemy space craft or at a fixed target on a celestial body (such as a moon or asteroid) would need to provide an exactly equal and opposite force at the time of the discharge.
If the captain of the military vessel chooses not to create an exactly equal force in the opposite direction, then (of course) the military vessel will be moved in the opposite direction in proportion to the force applied and the mass of the vessel.
***
In his thinking about using plasma streams to impart momentum to a space craft, kbd512 appears to have been thinking about delivering grams of force. In the post to which I am replying, I gather that the force of plasma coming out of a hybrid-fission-fusion space drive would be measured in tons. Presumably all that kinetic energy could be harvested by a suitably equipped space craft.
However,. and this is the end point of my post here, whatever system kbd512 comes up with to harvest kinetic energy from tons of plasma is ** also ** going to be ** very ** useful in dealing with solar flare events from our own Sun.
To the best of my knowledge, kbd512 has not yet arrived at a design for the receiving vessel capable of safely absorbing tons of energetic particles flowing from a source, but when he ** does ** design such a system, he will have simultaneously solved the perplexing problem of high power particle flow events.
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It appears that the idea of hybrid fission fusion has been around for a while ...
However, Calliban appears to have flipped the old concept on it's head ...
Google came up with a list of citations of the original set of ideas ...
About 1,330,000 results (0.56 seconds)
Nuclear fusion–fission hybrid - Wikipediahttps://en.wikipedia.org › wiki › Nuclear_fusion–fission...
Hybrid nuclear fusion–fission (hybrid nuclear power) is a proposed means of generating power by use of a combination of nuclear fusion and fission processes ...
Fission basics · Fusion basics · Hybrid concepts · Overall economy
Fusion-fission hybrids: nuclear shortcut or pipe dream?https://www.power-technology.com › features › feature...
Aug 6, 2017 — The fusion-fission hybrid concept is envisaged as a system that balances the advantages and disadvantages of the two nuclear generation ...
People also ask
What is the concept behind hybrid fusion fission reactors?
There may be nothing available on Calliban's version of the idea, because (it is remotely possible) he may be the first (or one of the first) to think of it.
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A discussion of use of a linear accelerator to accelerate charged particles to "ignite" one of Calliban's (imagined) fission/fusion packages might be enhanced if the work of James Maxwell were brought forward:
Google came up with these suggestions for study:
Maxwell's Equations: Electromagnetic Waves Predicted and ...https://courses.lumenlearning.com › physics › chapter
The waves predicted by Maxwell would consist of oscillating electric and magnetic fields—defined to be an electromagnetic wave (EM wave).Scientists and Electromagnetic Waves: Maxwell and Hertzhttps://www.univie.ac.at › site › ems › consider
Hertz proved the existence of radio waves in the late 1880s. He used two rods to serve as a receiver and a spark gap as the receiving antennae. Where the waves ...The Electromagnetic Field - James Clerk Maxwell - MacTutor ...https://mathshistory.st-andrews.ac.uk › chapter-7
Maxwell's theory of electromagnetic radiation was without doubt his greatest piece of work. It revolutionized (edit) the way we view the world, changing our ...Maxwell's Equations and Electromagnetic Waves - UVa Physicshttp://www.phys.virginia.edu › more_stuff › Maxwell_Eq
Ampere discovered that two parallel wires carrying electric currents in the same direction attract each other magnetically, the force in newtons per unit ...Maxwell's equations - Wikipediahttps://en.wikipedia.org › wiki › Maxwell's_equations
Known as electromagnetic radiation, these waves may occur at various wavelengths to produce a spectrum of radiation from radio waves to gamma rays. The ...Maxwell's equations | Institute of Physicshttps://www.iop.org › explore-physics › maxwells-equations
In our Explore Physics series, we examine how Maxwell's four equations ... Maxwell's death, German physicist Heinrich Rudolph Hertz discovered radio waves.Maxwell's equations and lighthttps://web.pa.msu.edu › lectures › emwaves › maxwell
The third equation is Gauss's law, which expresses the fact that electric field ... Radio signals are also electromagnetic waves; after the discovery of ...16.1 Maxwell's Equations and Electromagnetic Waveshttps://openstax.org › books › pages › 16-1-maxwells-e...
Oct 6, 2016 — The four basic laws of electricity and magnetism had been discovered experimentally through the work of physicists such as Oersted, Coulomb, ...
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The idea is to put a small quantity of fissile material at the centre of a fuel pellet. Fission then provides the energy to heat a central hotspot. I have rerun the calculations for how much Uranium is needed to heat a 0.02mg hot spot to a minimum of 5KeV. It works out at 0.1 microgram.
Am241 is fissile and there is approximately 0.3 microgram in a single smoke detector. This should be just sufficient to provide the energy needed to heat the hotspot. Unfortunately, this is likely to be far beneath any realistic critical mass even under the highest achievable compression. One solution might be to bombard the Fusion fuel with ions. This will produce fusion reactions that will bombard the fissile material with neutrons. As these fission, they will send fission fragments into the fusion fuel surrounding the core. Each fission will deposit 160MeV of thermal energy into this region. As the fragments deposit their energy, the result will be numerous fusion events along their tail. This will feed additional neutrons into the core, hopefully causing it to go critical. It is the close coupling between fission and Fusion that is key to making this work using quantities of fissile materials far beneath any realistic critical mass.
Within a 1mg pellet of fusion fuel, the 0.0001mg of fissile fuel will generate only 1 in 100,000 of the final energy. This is probably clean enough to use in Earth's atmosphere. Certainly it is small enough to remove any of the usual radiological hazards associated with nuclear power.
I do not yet know how to take this fwd to experimentation. Yet that is clearly the next step.
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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For Calliban re #16 ... thank you for continuing development of this interesting line of thought ...
"Dayton Engineer" has contributed to this forum in the past, and he has agreed to offer his thoughts on your ideas ...
He is skeptical of fusion as a source of mechanical power, as you will see in the quotation below.
If you would like to engage with "Dayton Engineer" I will provide correspondence by email.
You are asking a good question, because even though it has great potential, the way fission does, one still must convert the energy released by fusion reactions into a form in which it can be used.
Fission reactors obviously just replace the furnaces under boilers that burned coal, oil, or gas, but the fission reaction creates a lot of heat that is easy to carry away in the form of steam to the heat engines that will convert a small portion of the heat energy into mechanical power, which can then be used to run generators or other rotating machinery (like the screws of a nuclear Naval vessel).
One of the big problems with fusion is the fact that a lot of the energy released in the reaction is in the form of high energy neutrons, and as their name suggests, they are neutrally charged. Being so makes it difficult to absorb their energy and use it to boil water to drive the heat engines that will convert the released energy into a useful form.
Even thermonuclear bombs have a problem with the energy released in the fusion reaction. Up until the Soviet Union collapsed, no one in the public world really understood how they worked, but ex-Soviet scientists "spilled the beans" on their construction and operation after they fled to Israel, Europe, and the US. Not much the DOE could do about their divulging some juicy details.
It turns out that the casing surrounding the fusion fuel, light atoms with excess neutrons, is made of Uranium 238, the isotope that is used in reactors. It likes fast neutrons to induce fission, but in a weapon, you want slow neutron fission to occur, hence the preference for Uranium 235 or Plutonium 239 as the fissile material in the first stage of the bomb. Once the fission reaction gets rolling, the immense pressure and heat from the reaction starts to compress the fusion fuel, and the X-ray flux from the fission "lights off" the fusion reaction. This releases the unimaginable amount of energy available from the fusion reaction, but it is mostly in the form of high energy neutrons. These then blast into the U238 casing, and their high energy forces the U238 to fission, releasing the heavy nuclei and other charged particles that wreak havoc on the target with immense blast effects.
If you remember the mysterious "Neutron Bomb" of 45 to 50 years ago, it was a thermonuclear device that didn't have the fissile casing to add to the bang, hence it would irradiate and kill the people with intense neutron bombardment, but it would the buildings standing and not create a lot of radioactive debris that would poison the target area.
Maybe the addition of fissile material to the fuel pellets will have the same function as the U238 casing on a thermonuclear device. Its presence will add some oomph to the bang when the fusion fuel lights off. Now you might have some radioactive species inside the fusion reactor, but they might be on such a small scale as to be negligible or able to be handled safely. If the idea is to use such a fusion/fission "duplex" reaction as an Orion-like drive out of the atmosphere, then the radioactivity may not be as much of a problem as it would have been if Orion had flown. The neutron flux might be a problem in a crewed vehicle, but water needed for the journey can be used as shielding to slow down and absorb the neutrons.
As I mentioned at the beginning of this dissertation, you are on the right track when considering the Snow engine as a conversion mechanism to get mechanical power out of heat energy. The 600 HP Snow at Coolspring is a Nuremburg type of engine, so called because its design is based on steam engines of the same pattern (double-acting cylinders in a tandem configuration) that were manufactured by the predecessor company to today's MAN, "Maschinenfabrik Augsburg-Nuremburg". This pattern of steam engine gained popularity in the 1870s, and by the end of the 19th century were well established as a common industrial form. When companies started to investigate internal combustion, it was natural to convert what you know worked instead of heading out into the unknown of engine design. The predecessor companies of MAN pioneered the form of the tandem double-acting gas engine, and companies like the Snow Steam Pump Works bought licenses from the Nuremburg firm to build the same style of engine. Around the turn of the 20th century, many engine companies licensed the design, or went out on their own and copied the form.
Unfortunately, the large bore and stroke, tandem double-acting gas engine is a very inefficient design. As engine speeds increased, the form of engine changed to more what we see today, and the big tandems were relegated to the scrap heap. The Snow at Coolspring and its sister at Rollag survived into the 90s because of the unimaginable inefficiency of the oil and gas industry. There is little competition in the industry, and if something works, the oil and gas industry would rather just run it than upgrade it. As the 20th century wore on, it became tougher and tougher to find men who were interested in running such engines, and the environmental regulations finally caught up with them, too. Once common in many industries, including city electrical power generation, their last hurrah was compressing gas in pipeline service. The final death knell was their inefficiency. As gas compression companies split their organizations into different divisions, the fuel gas for the compressor engines was no longer "free" and had to be accounted for in balance sheets.. It didn't take long for the tandems to be replaced by much smaller modern engines that were much cleaner running and more fuel efficient.
The higher speed of the modern engine, its use of turbo-supercharging and the reduced losses in their machinery led them to completely replace the tandems. One of the biggest problems with the tandems is the mechanical drag that must be overcome by burning fuel. Since they are double acting, the piston rod runs through the combustion space of the cylinders, which means the rod must be sealed where it enters and exits the cylinders. This adds lots of friction drag, and the rod going through the cylinder displaces volume and it absorbs heat from combustion, leading to losses. The piston rods are carried by crossheads that support their weight and that of the pistons. These crossheads run in large bearing surfaces, that also add drag, leading to rather large friction losses. The last nail in the coffin lid is the fact that the big tandems hark from the very early days of gas engine development, and many have very primitive and inefficient combustion spaces designed into their cylinders. Poor combustion leads to reduced thermal efficiency, and rather bad emission of gasses now regulated by government agencies.
If fusion was to be used in a reciprocating heat engine, which it could in the pulsed form (possibly! see below), I really doubt if the double-acting tandem form would be chosen for the engine layout.
Usually, fusion occurs in a reaction vessel here on the earth, and they are usually evacuated to a rather high vacuum. This also makes it hard to get ahold of the energy released in the fusion reaction. In the gas engine, the fuel burns with some of the oxygen in the cylinder, which heats the inert gasses that constitute our atmosphere. The heated gas experiences a pressure rise which pushes the piston down, and in doing so, about a third of the energy released by the combustion can be converted to a mechanical form to turn a shaft. If the fusion reaction must occur in vaccuo, then I think a typical piston (or turbine) engine format would not be useful.
In the vacuum of space, the Orion style drive might be the best way to go, especially if a lot of the energy comes from fusion so you don't have the fissile daughter products hanging around after fission reaction occurs.
The jury is still out! Progress on fusion has been painfully slow over the last 75 years.
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Your friend is correct - the bulk of energy produced from the fusion reaction is in the form of neutron energy. This can either be captured in some other material, like water or steel, and converted to heat, or used to fast-fission a heavy Uranium or Thorium blanket. In the second approach, you get a much higher net energy gain, because each 14MeV neutron will cause at least one fission, which will liberate 200MeV of energy. That fission will also produce roughly three more neutrons, which will breed plutonium or 233U in the blanket material. We have lots of depleted Uranium tailings that could be used in this way. The fissile materials that are bred within the blanket region could also be used to fuel several additional nuclear fission reactors. So in addition to the power produced by the hybrid reactor, you benefit from a ready supply of nuclear fuel for more conventional reactors.
The hybrid DU blanket reactor, provides a solution to a problem that would otherwise limit the ability of fast breeder reactors to supply a rapidly increasing flow of energy - long doubling time. One of the problems with fast neutron reactors is that they require a large initial fissile fuel load, which is expensive and around 30 years is needed to breed enough fuel to start a second reactor. So in reality, it would take a long time to build up enough plutonium to power an economy like the United States. But Fusion reactions generate so many 14MeV neutrons, that doubling time can be reduced to just a few years. If we needed to build up nuclear fission generating capacity very quickly, but we're limited by fuel supply (which we would be) then a hybrid reactor is a neat solution. This would use fission triggered Fusion micro explosions in a reaction chamber as we have described previously. But we would surround the reaction chamber with metallic Uranium fuel rods in a bath of liquid sodium. Depleted Uranium rods would gradually be shuffled inwards until about 10% of the initial Uranium atoms had been consumed. After fuel removal, they would undergo cooling, before fission products are removed by electrorefining. The actinide metal would then be blended to the correct fissile concentration for fast reactor fuel. Given that fast reactors are also capable of breeding, we might need only a handful of hybrid reactors to provide starter cores for a fleet of fast reactors that could generate all US electricity in just a couple of decades.
Alternatively, with more hybrid reactors we could do away with the need for sodium cooled fast breeder reactors completely. Just build a few dozen hybrid reactors and use them to produce MOX fuel for a larger fleet of pressurised water reactors. The actinide wastes from the PWRs can be cycled back into specialised burner hybrid reactors, which will generate power through fission of those activities in the 14MeV neutron stream and obviate the need for long term nuclear waste storage.
However, this assumes that we are content with powering the world using fission. If we wanted something a little less toxic, then we can use tiny amounts of fissile material to initiate inertial confinement Fusion and capture neutron energy directly as heat to drive turbines. The S-CO2 cycle that Kbd512 describes would be ideal in this case. The neutrons would deposit their energy in steel baffle plates which would contain stainless steel tubes containing hot CO2 gas. The steel would be made radioactive. But this is a much easier disposal problem than fission products.
For a space propulsion system, which is what we are most interested in here, we would need to find a way of capturing neutron energy within water or hydrogen which is then used as propellant. The easiest way would be to surround the pellets with a shell of neutron absorbing material like plastic, which would be compressed along with the Fusion charge. This dense low-Z shell will then scatter fast neutrons streaming out of the core and would atomise giving rise to a low molecular weight exhaust.
Last edited by Calliban (2022-03-28 04:50:11)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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Kbd512's article discussing self healing nickel-chromium alloys got me thinking. The fissile fuel surrounding the Fusion reaction chamber could be in the form of a molten chloride or fluoride salt, containing Uranium, Thorium and plutonium. The advantages of keeping the fuel in liquid form are much greater heat capacity, which is beneficial in a pulsed power regime and the ability to remove spent fuel and add fresh fuel without shut down for refuelling. High capacity factor is important to the economics of a power reactor. Ideally, we wouldn't shut down unless there were a fault.
If hybrid reactors can produce lots of cheap nuclear fuel, then we have options for nuclear fission that might not be practical otherwise. We can build pressure tube reactors using a light water moderator tank with fuel contained in steam tubes running through it. These are much cheaper to build because we avoid the need for thick walled pressure vessels. Decay heat removal is via natural circulation cooling loops in the moderator tank.
Or aqueous homogenous reactors. These would operate at low temperatures and have poor thermal efficiency. But they are self regulating and cheap to build. If fuel is cheap and available in abundance thanks to hybrid reactors, then fuel efficiency won't be much of a cost driver. The AHR is a particularly neat solution, because the fuel supply can be generated by dissolving the hybrid reactor metallic blanket materials in nitric acid. The result is actinide nitrates that can directly input to the AHR as fuel. The remaining liquor is fissium waste stream which can be vitrified. The waste from the AHR can be pure fission products. The actinides would remain in the AHR until they eventually fission, with the hybrid providing a constant supply of fresh fissile actinides. This ensures that there is very little long lived radioactive waste as nothing leaves the AHR until it has undergone fission. We can do that with the AHR, because there is essentially no burn up limit for the fuel. I can see a hybrid-AHR fuel cycle as having a lot of benefit on Mars, because we have abundant uses for the low grade heat produced. The fuel cycle is also simple. Metal fuel coming out of the hybrid would be dissolved in a nitric acid bath. The actinide nitrates in their entirety would then feed the AHRs. No need for actinide separation.
Last edited by Calliban (2022-03-28 05:28:58)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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For Calliban ... thank you for your reply to Dayton Engineer ... I have forwarded a link to your reply, and have (again) invited Dayton Engineer to take the risk of accepting a membership in this somewhat rambunctious forum.
At least under the benign Administration of SpaceNut, we seem to have found a way to interact without too much disruption, most of the time.
As I understand the issue at hand, the output of fusion (of Deuterium and Tritium) is primarily neutrons, and those are difficult to harness to deliver useful power.
Boron 11 and Hydrogen do much better (I am told) but (so far) no one has built a successful reactor using them either.
***
Question for you ...
Is there a reason why the packet has to be a sphere? I understand that the National Ignition Facility is working with spheres.
Your text appears (as I read it anyway) to be leaning toward ions delivered by a linear accelerator to provide ignition energy to a packet.
So my question is ... why not use the tried and true practice of chemical bullet acceleration? A ram strikes a cap, which provides the vigorous energy needed to ignite the slower burning chemical mix beyond it, so that the payload is given the acceleration it needs to perform a useful function.
A linear accelerator needs to operate with a vacuum.
The cap would be something that can respond to the arrival of a small crowd of ions (not sure how many, or of what mass, or velocity).
The cap would then (presumably) deliver useful energy to whatever is beyond it.
The "bullet" handling mechanism would need to remove the spent cartridge from the breech and replace it with a new one before the vacuum is lost.
In space that is not a problem, but I'm looking for a solution for Earth.
The mechanical power takeoff system shown by the "ancient" Snow machine is inefficient, but so what?
It is the cost of the energy delivered that matters, and at this point I don't think we (or at least ** I ** ) have any idea what the actual cost of operation of a system would be.
In any case, any energy not harnessed directly is distributed as heat, and in winter that would be just fine with the neighbors.
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Pellets need to be spherical, as this is the only shape that allows symmetrical compression without introducing instabilities. The compression of spherical pellets allows peak implosion velocities of 200km/s. Kinetic impact compression has been discussed before by Winterberg. The problem is projectile velocity must be extremely high - 100s km/s. There are limits to practical acceleration of solid projectiles, due to friction, bulk modulus and magnetic heating. Whilst it is possible in principle, the accelerators would be kilometres long. With ion beams, there is no effective limit to acceleration. Particle beam accelerators can be made relatively compact.
More here on the hybrid molten salt reactor. In this concept, the IC Fusion chamber is surrounded by molten salt blankets containing depleted Uranium or Thorium salts.
https://www.osti.gov/biblio/1618294
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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For Calliban re #21
This is important work, and you are the only person here (and there are very few anywhere) qualified to do it.
I am going to attempt to provide a question or two that I hope will stimulate your creative thinking.
Re spherical shape ... that sounds to me like an error made by someone else, and not your idea.
If you fire an ion beam at a sphere, the ions arrive from only one direction, so what's the point of going to all the trouble of making a tiny sphere, when a tiny cylinder would do just as well?
And! If activity is taking place in nanoseconds, what is the meaning of the word "instabilities"
In the context of a Stellarator or Tokamak, it might me sense to talk about "instabilities".
In the case of laser illumination of a pellet at the Lawrence Livermore labs, it makes NO sense (at least to me).
So what's the difference if the ignition comes from an ion beam?
Again, these are just questions designed to stimulate your creativity. I make no claims of prior knowledge whatsoever.
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th,
I believe the plan is for the ion beams to come from multiple directions? Otherwise you won't really get compression. The designs I've seen tend to be spherical (the Farnsworth-Hirsch fusor isn't though).
Use what is abundant and build to last
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For Terraformer re #23
Thank you for helping to move this (to me important) topic along!
Your capabilities are (understandably) unknown to me, but whatever they are, thanks for pitching in!
This is a global think tank, mediated by software and the generosity of the Mars Society which pays for the hardware.
This topic is led by Calliban, and all existing members who can help are encouraged to do so.
We (the human race) need actual working systems to show up in the not too distant future.
You may well be right, that multiple ion beams are needed, but at this point, I don't think anyone ** really ** knows. If one ion beam (ie, linear accelerator) can do the job, that is much better for both space propulsion and on-Earth power production.
If the packages of material (whatever they are) can be packaged as cylinders, the human race has hundreds of years experience packing explosive cylinders into suitable receptacles.
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Compression works by evaporating the target surface using lasers or ion beams to heat the outer surface to high temperatures. In this way, compression of the pellet works in much the same way as a rocket. Pressure is induced through momentum change of the outer layers. For compression to be stable, every part of the surface must be heated at the same rate, otherwise there are pressure discontinuities on the surface that will introduce turbulence into the plasma.
One of the problems that led to the use of indirect drive was variations of laser light intensity on the surface. Imagine trying to compress a wet clay ball with your fingers. If there are any gaps, the wet clay would spray out between your fingers. In reality, a laser shining onto any surface applies pressure in the same way as your fingers. Intensity is highest at the very centre of the beam but spreads out across the surface as you get further from the centre of impact, just like gaps in your fingers. To apply uniform compression, would require an infinite number of lasers, which is obviously impractical. So we have to live with variations in pressure across the surface of the pellet. This is an important source of instabilities.
A sphere is the easiest shape to compress, as compressing any other shape would aggravate problems of variable heating rate across the surface. For a cylinder, we would need to deliberately introduce changes in heating rate across a discontinuous surface during early stages of compression, in order to force the irregular surface into a sphere. It is easiest just to start with a sphere and provide as close as possible to uniform heating rate across its surface to achieve a smooth implosion without any surface discontinuities. Otherwise we face the additional complication of having to correct for an irregular surface in the early stages of compression.
An important source of instabilities in plasma compression is the so called Raleigh-Taylor instabilities. This occurs whenever a hotter less dense plasma is used to compress a colder denser plasma. There is turbulent mixing between the two layers, giving rise to splatted cross section. This problem is aggrevated by turbulence and variations in surface heating rate will worsen the problem. Hotter plasma has a greater atomic speed and effectively has a higher diffusion coefficient than colder and denser plasma. If the heated surface has temperature variations, it will obviously aggravate this problem. And any variation in shape will tend to introduce temperature variations.
One of the strengths of the Farnworth electrostatic accelerator is that it consists of two concentric spherical electrodes. The inner cathode is a thin mesh. Most of the ions accelerating towards the cathode pass through it and arrive at the centre of the accelerator. This will produce a uniform ion pressure on any spherical surface at the centre of the accelerator. To achieve compression will require delivering a current pulse of amplitude of a few MeV over a time period measured in picoseconds. Total confinement time is on the order of 200ps. Ideally, the ion energy must be delivered to the surface in the shortest time duration possible. About 10KJ must be delivered to the surface to achieve compression. If compression time is about 10% of confinement time (20ps) then current would be 500MA. Power delivered to the surface must be on the order of 1E15 Watts. Power density on the surface would be 3E21W/m2. One of the reasons that heavy ions and high individual ion energy is preferred for compression, is that at such high current density, charged particles may begin to deflect each other. This may begin to introduce inefficiencies in the energy transfer to the surface.
Last edited by Calliban (2022-03-31 16:50:13)
"Plan and prepare for every possibility, and you will never act. It is nobler to have courage as we stumble into half the things we fear than to analyse every possible obstacle and begin nothing. Great things are achieved by embracing great dangers."
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