Pulsed Fission Fusion Propulsion

tahanson43206 · 2024-06-02 13:09:12

Calliban has been writing about this concept for some time.

This topic is offered as a venue for NewMars members who would like to concentrate on this particular variation of nuclear powered propulsion. The field has advanced significantly beyond the heady early days when small fission bombs were imagined for propulsion.

A search for pulse* and fusion and Author: Calliban provides a list of more than 25 posts ...

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tahanson43206 · 2024-06-02 13:09:50

This post is reserved for an index to posts that may be contributed by NewMars members over time.

(th)

Calliban · 2024-06-02 18:03:42

The current NASA pulsed-plasma concept uses a z-pinch device to generate the neutrons needed for fast-fission. The ISP is listed as 5000s, with a 100KN thrust. That would certainly meet the needs of the large interplanetary ship that Robert is developing.

Maybe some version of this device could be used for interstellar travel as well. Fission fragments travel at 3-5% C when first created. If some 10% of atoms fission in an enhanced design, that equates to an exhaust velocity of 1.25% C. Building a ship that can accelerate to 2.5% C would appear plausible. That is over twice as fast as the Trisolarian fleet. Take that alien scum!

But I think this is a stretch case. Due to the quantity of actinides needed, I think the choice of fuel will be low enriched uranium, DU or thorium. Thorium is most difficult, because it has the lowest fission cross section even under 14MeV neutrons. To fission a large fraction of the fuel atoms, I would suggest a three stage device. The driver stage generates an initial burst of fast neutrons. These will fast-fission nuclei in the fuel slug. To multiply the effect, we would put a lithium deuteride core within the slug. As fission takes place within the slug, the heavy atoms are heated to several KeV and bath the lithium deuteride capsule in soft x-rays, heating it and compressing it to Mbar pressures. Fusion then takes place in the LiD, which showers the surrounding uranium or thorium with fast neutrons. These create even more fission, leading to even higher x-ray pressure, and so on, in a positive feedback. By the time the fuel disassembles, a large fraction of the fissionable nuclei will have fissioned.

A 2% C final velocity would allow a ship to reach proxima centauri in 212 years. Making some allowance for acceleration time, a 250 year journey time seems possible. That assumes we don't need to slow down of course. That wouldn't necessarily be a requirement for a probe. But it would be for a manned colony ship. Barnard's star is 5.96ly away. Our probe could get there in 336 years. That is a long time by human standards. But it is a blink in cosmic history.

Last edited by Calliban (2024-06-02 18:17:01)

Calliban · 2024-06-06 13:16:29

Thorium concentration in the regolith of Vesta.
https://core.ac.uk/download/pdf/82593361.pdf

The average appears to be about 0.7 gram per tonne. Assuming that the bulk rock has density typical of stony asteroids (2670kg/m3), 1 litre of Vesta rock will contain some 1.87mg of thorium. That amounts to 4.85E18 atoms. Fission of a single thorium atom releases 3E-11J of recoverable energy. Fission of all of the thorium in 1 litre of Vesta rock will therefore release some 145.5MJ of heat. Thorium is 2.5x more abundant than uranium. Adding the uranium as well, increases the total harvestable energy to 203.7MJ/litre. Even at the meagre concentrations present in Vesta regolith, the bulk rock of Vesta contains enough uranium and thorium to produce about 5x the energy per unit volume as anthracite coal.

The question is, could we extract it, at an affordable energy cost? I am guessing we would need to mill the material into a fine dust and then leech it in acid. The uranium and thorium would then dissolve into solution. We then need a method to seperate it from other ions in solution.

The abundance of thorium in Martian soils appears to vary to a maximum of 1ppm, so concentration is comparable to what is found on Vesta. Concentrations in lunar soils vary from 0.1 - 10ppm. Mercury appears to be highly depleted. It is not known if this is the case for regolith only or the bulk rock.
https://agupubs.onlinelibrary.wiley.com … 12JE004141

If we find thorium present at concentrations of 1ppm on Mars, with about 0.4ppm uranium, then each litre of rock will contain some 290MJ of potential fissionable energy. That is equivelent to 6.7x its volume in anthracite coal. I'm inclined to think that 1ppm represents a lower limit at which fissionable material is worth mining for its energy content.

Last edited by Calliban (2024-06-06 13:50:23)

tahanson43206 · 2024-06-06 13:55:17

For Calliban re #3 and #4

Thank you for continuing to explore this promising topic!

Please note that in another topic (or perhaps more than one) there is reporting on research (and possibly field trials) of promising use of microwave energy to liberate individual atoms from a mass for purposes of vertical drilling to reach thermal energy resources. I bring this up because when atoms are liberated in this way, they are briefly (potentially) available for sorting, before the clump together again in a new arrangement.

I understand (or more accurately, ** think ** I understand) that the drilling method produces gas containing vaporized regolith that can be pumped to the surface as an alternative to the traditional bulk transport method used by most existing drills (ie,,fluid mud transport).
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Calliban · 2024-06-09 12:16:35

I think a multi-stage process is key to making this concept work well. The NASA concept design uses a z-pinch device to generate a shower of neutrons that fast-fission a low enriched uranium slug. This slug is heated to ~100,000K, turning into a superheated plasma. The exhaust velocity will be proportional to the square root of energy yield per unit mass of propellant, in this case, uranium. Increasing thrust and ISP depends upon increasing the proportion of uranium nuclei that fission.

I would propose modifying the process in two ways. The first, makes use of lattice confinement fusion. Firstly, small quanties of Li-6 deuteride will be blended into the uranium metal lattice. As the fuel slug is initially bombarded by fast neutrons from the triggering z-pinch reaction, secondary fission neutrons and gamma rays, will collide with the LiD. This will heat it to high energy and will transmute Li-6 into tritium. Tritium and deuterium in the uranium metal lattice then fuse, releasing fast neutrons that cause more fission. At the centre of the uranium metal slug, there will be a thin rod of lithium-6 deuteride. As the uranium slug heats to millions of Kelvin, x-ray pressure compresses this. Neutrons transmute the Li-6 into tritium. This ignites a tertiary fusion reaction, which showers the surrounding uranium plasma with fast neutrons.

By using this fusion boosting, a large proportion of the uranium atoms will fission. It may indeed be possible to make use of non-fissile 238U and 232Th, as the fusion produces enough fast neutrons to fast fission these heavy atoms.

Last edited by Calliban (2024-06-09 12:27:08)

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#1 2024-06-02 13:09:12

Pulsed Fission Fusion Propulsion

#2 2024-06-02 13:09:50

Re: Pulsed Fission Fusion Propulsion

#3 2024-06-02 18:03:42

Re: Pulsed Fission Fusion Propulsion

#4 2024-06-06 13:16:29

Re: Pulsed Fission Fusion Propulsion

#5 2024-06-06 13:55:17

Re: Pulsed Fission Fusion Propulsion

#6 2024-06-09 12:16:35

Re: Pulsed Fission Fusion Propulsion

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