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#1 2021-07-23 10:30:52

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Hybrid nuclear fission fusion technologies

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.

(th)

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#2 2021-07-23 10:37:52

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 1,154

Re: Hybrid nuclear fission fusion technologies

Previous post pasted below from Large Ship thread.
*******************************************************

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.


Interested in space science, engineering and technology.

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#3 2021-07-24 12:26:02

GW Johnson
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From: McGregor, Texas USA
Registered: 2011-12-04
Posts: 4,576
Website

Re: Hybrid nuclear fission fusion technologies

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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#4 2021-07-24 12:52:10

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Hybrid nuclear fission fusion technologies

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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#5 2021-08-05 04:10:28

Calliban
Member
From: Northern England, UK
Registered: 2019-08-18
Posts: 1,154

Re: Hybrid nuclear fission fusion technologies

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)


Interested in space science, engineering and technology.

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#6 2021-08-05 08:04:44

tahanson43206
Moderator
Registered: 2018-04-27
Posts: 7,326

Re: Hybrid nuclear fission fusion technologies

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.

(th)

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