Author Topic: Clean lithium fission rocket  (Read 61911 times)

Offline gospacex

  • Senior Member
  • *****
  • Posts: 3024
  • Liked: 543
  • Likes Given: 604
Re: Clean lithium fission rocket
« Reply #60 on: 03/29/2016 09:06 pm »
I WANT to thermalize the fission products. I DON'T want to try and collimate relativistic ions; it is far far easier to dissolve the lithium salt in water and let the water serve both as a thermalization medium and as the pressurized reaction mass.

The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.

If you thermalize neutrons and fission products, you get a NTR and/or NSWR. If you don't thermalize them, you get a fission fragment engine (which is a good engine... if you can overcome engineering/materials challenges).

Offline sevenperforce

  • Full Member
  • ****
  • Posts: 1474
  • Liked: 969
  • Likes Given: 599
Re: Clean lithium fission rocket
« Reply #61 on: 03/29/2016 09:38 pm »
I WANT to thermalize the fission products. I DON'T want to try and collimate relativistic ions; it is far far easier to dissolve the lithium salt in water and let the water serve both as a thermalization medium and as the pressurized reaction mass.

The fuel is dissolved in water; the fuel is used to heat the water; the water and the fuel's reaction products are expelled together. Pretty simple.

If you thermalize neutrons and fission products, you get a NTR and/or NSWR. If you don't thermalize them, you get a fission fragment engine (which is a good engine... if you can overcome engineering/materials challenges).
It's a NSWR. Just a neutron-pumped one.

Still concerned about the low number of neutrons. Are there any fissionable isotopes which produce a chain reaction without nasty nucleotide products? Preferably it wouldn't even be radioactive on its own. If such an element could be found, then it could be mixed into the saltwater and used as an intermediary to fission the lithium.

Offline Impaler

  • Full Member
  • ****
  • Posts: 1283
  • South Hill, Virgina
  • Liked: 372
  • Likes Given: 0
Re: Clean lithium fission rocket
« Reply #62 on: 03/30/2016 06:04 am »
A neutron multiplier without a radioactive byproduct, if it exists it would certainly be desirable.  From what I can find Beryllium dose multiply neutrons by splitting into 2 alpha particles + 2 neutrons and by transmuting into carbon + 1 neutron upon absorbing alpha particles so it seems the High Flux reactor noted earlier is likely making significant use of such multiplication.

The only other elements I can identify that are good multipliers are Lead, Bismuth and at a much reduced rate Zirconium which has the advantage in a much higher melting point.  All of these can expel 2 or 3 neutrons when absorbing a thermal neutron WITHOUT fission though they do form radioactive isotopes like Polonium.  But all are considered inferior to Beryllium on Earth before mass has even become a concern as it would in an engine, only the toxicity of Beryllium and it's cost are an issue, and these are likely to be surmountable in a rocket engine.

In addition I'm reading that carbon is considered a good neutron reflector and it's low density would be desirable in keeping the engines power to weight ratio up.  I'm envisioning an engine with a conventional combustion chamber throat and funnel with one or more hollow central channel reactors injecting lithium salt into the combustion chamber.

Offline sevenperforce

  • Full Member
  • ****
  • Posts: 1474
  • Liked: 969
  • Likes Given: 599
Re: Clean lithium fission rocket
« Reply #63 on: 03/30/2016 01:24 pm »
A neutron multiplier without a radioactive byproduct, if it exists it would certainly be desirable.  From what I can find Beryllium dose multiply neutrons by splitting into 2 alpha particles + 2 neutrons and by transmuting into carbon + 1 neutron upon absorbing alpha particles so it seems the High Flux reactor noted earlier is likely making significant use of such multiplication.

The only other elements I can identify that are good multipliers are Lead, Bismuth and at a much reduced rate Zirconium which has the advantage in a much higher melting point.  All of these can expel 2 or 3 neutrons when absorbing a thermal neutron WITHOUT fission though they do form radioactive isotopes like Polonium.  But all are considered inferior to Beryllium on Earth before mass has even become a concern as it would in an engine, only the toxicity of Beryllium and it's cost are an issue, and these are likely to be surmountable in a rocket engine.

In addition I'm reading that carbon is considered a good neutron reflector and it's low density would be desirable in keeping the engines power to weight ratio up.  I'm envisioning an engine with a conventional combustion chamber throat and funnel with one or more hollow central channel reactors injecting lithium salt into the combustion chamber.
https://www-nds.iaea.org/sgnucdat/a6.htmHere's a nice table of neutron yield for various isotopes...of course everything here is wildly radioactive.

If there's no way to get a high enough external neutron count, it still might be worth it to boost internal neutron count using fissile isotopes mixed with the lithium. It's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium. This would make it hopeless for in-atmo use but could still be promising as a transfer engine. I guess that would make it a subcritical lithium-boosted NSWR? Wonder how much lithium-6 you could add before it would stop working, and what kind of specific impulse you could get. Could be feasible for a Mars Transfer Tug.

Another option with a pulsed accelerator design would be to use lithium-6 deuteride as the fuel, then chemically separate the two ions and smash them together in an accelerator, fusing into 8Be which decays to 2 Helium-4 atoms with 22 MeV of energy. That's even cleaner than lithium fission (and releases no harmful radiation whatsoever), but I'm not sure how much energy it would take to get them to fuse, nor whether high enough particle fluxes could be attained.

Offline gospacex

  • Senior Member
  • *****
  • Posts: 3024
  • Liked: 543
  • Likes Given: 604
Re: Clean lithium fission rocket
« Reply #64 on: 03/30/2016 03:03 pm »
Are there any fissionable isotopes which produce a chain reaction without nasty nucleotide products?

There are not. Otherwise military would be all over it: ideal bomb material!

Offline gospacex

  • Senior Member
  • *****
  • Posts: 3024
  • Liked: 543
  • Likes Given: 604
Re: Clean lithium fission rocket
« Reply #65 on: 03/30/2016 03:13 pm »
It's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium.

You are still fixated on lithium.

What "high energy of lithium"? It gives you 4.78 MeV from one Z=6 nucleus fissioning, a bit below 0.8 MeV per nucleon.
Fission of usual suspects U/Np/Pu gives ~200 MeV per Z~=235 nucleus, which is about the same power density, actually a bit better: 0.86 MeV per nucleon.

Offline Hanelyp

  • Full Member
  • ***
  • Posts: 368
  • Liked: 65
  • Likes Given: 252
Re: Clean lithium fission rocket
« Reply #66 on: 03/30/2016 04:29 pm »
It's getting away from the "clean" exhaust, but the reactor chamber under discussion seems promising with a fissile salt at a concentration which would not go critical in a simple bulk tank.

Offline sevenperforce

  • Full Member
  • ****
  • Posts: 1474
  • Liked: 969
  • Likes Given: 599
Re: Clean lithium fission rocket
« Reply #67 on: 03/30/2016 05:11 pm »
It's not as clean or as high-energy as pure lithium fission, but it would still be far better than a conventional NSWR, because the fuel would not be able to achieve critical mass on its own (due to the presence of lithium-6), making it safer. Plus, it would have a much higher specific energy than a NSWR due to the low mass and high energy of lithium.

You are still fixated on lithium.

What "high energy of lithium"? It gives you 4.78 MeV from one Z=6 nucleus fissioning, a bit below 0.8 MeV per nucleon.
Fission of usual suspects U/Np/Pu gives ~200 MeV per Z~=235 nucleus, which is about the same power density, actually a bit better: 0.86 MeV per nucleon.
Hmm, I was under the impression that lithium-6 had a markedly higher specific energy. Where did that come from?

Ah, here. I misread something -- they were factoring in fusion yields. No wonder I was overestimating.

Anyway, it would still be useful, as it would allow a subcritical fuel to still have roughly the same specific energy.

6Li + 2H fusion-fission would yield 2.75 MeV per nucleon, which would be much nicer, if it could be done. That's one of the major reactions in brown dwarf stars and requires significantly lower energies than hydrogen-burning processes...a bit lower than 3e6 K. This corresponds to a deuteron energy of around 300 eV, which is quite low. I'm not sure how large of a linac you'd need to get that, nor what kind of flux could be possible.

Is deuteron decay a thing? Alpha decay is, of course, but I'm wondering whether there are any decay reactions or fissions which knock a bound proton-neutron pair off of an existing molecule. If so, this could prove quite useful.

Offline mlorrey

  • Member
  • Senior Member
  • *****
  • Posts: 2175
  • Director, International Spaceflight Museum
  • Grantham, NH
  • Liked: 25
  • Likes Given: 6
Re: Clean lithium fission rocket
« Reply #68 on: 10/14/2019 09:49 am »
You guys must be missing something about Lithium being such a nice fission fuel, since none of the existing fission reactors or bombs use lithium in any way. If it would be usable, surely it would be used by military instead of, or as an additive to the heavy and more expensive Uranium.
Uranium has a high atomic mass, therefore it is going to have much lower Isp as a propellant. This design uses low  enriched uranium as a neutron source to fission the lithium. Lithium IS used in nuclear weapons, lithium deuteride and tritium is bombarded with neutrons from a supercritical uranium or plutonium  fission reaction to fuse the deuterium and tritium, and lithium fission happens in there as well.  The only real question in this rocket design is whether you'll get deuterium and tritium fusion happening in the middle of this whole firestorm and I would  say probably not, since a thermonuclear weapon relies on the pressures of the implosion to drive the  deuterium and tritium to solar levels of pressure and temperature along with the neutron bombardment, to trigger fusion. This sort of thruster MIGHT be used in clusters to focus their exhaust streams at each other like particle accelerators to create fusion between exhaust streams, but that would take  a whole lotta calculations and work I'm obviously not gonna do here.
Anyways, yeah you are not gonna have a constricting neck on this engine. I'd have, say, three or six of these as cylindrical thrusters in an equilateral orientation pointed toward the focus of a parabolic nozzle. Any chance at fusion is  going to happen at the focus of the parabola, exhaust that doesn't fuse will reach the wall at the far side and be turned aft as it expands and cools like a normal bell nozzle should. You can introduce additional mass (LH2, water, or, potentially, ram-compressed and cooled air) axially into this nozzle to cool the exhaust and produce greater thrust for lower Isp.


Director of International Spaceflight Museum - http://ismuseum.org
Founder, Lorrey Aerospace, B&T Holdings, and Open Metaverse Research Group (omrg.org). Advisor to various blockchain startups.

Offline mlorrey

  • Member
  • Senior Member
  • *****
  • Posts: 2175
  • Director, International Spaceflight Museum
  • Grantham, NH
  • Liked: 25
  • Likes Given: 6
Re: Clean lithium fission rocket
« Reply #69 on: 10/14/2019 10:34 am »
I think I see the flaw with the proposal. To fission Lithium, you need a neutron source to generate *one neutron for every Lithium atom*:

Li-6 + n -> T + He-4 + 4.7829 MeV

Then you need to avoid thermalization of fission fragments and you need to collimate them into a directional beam. How far away form 100% efficient that will be?

You can save yourself a lot of trouble if you just use your neutron source as a rocket engine. Must be about the same ballpark wrt trust....

Which hints that the showstopper here is that it's hard to generate that many neutrons. What do you propose to be your neutron source? What's its luminosity?
The problem is you can't use neutrons as a rocket engine propellant, because neutrons simply do not like to be shaped into an exhaust. They either go through things in all directions, or are absorbed by those things, heating them up and making them radioactive. Neutrons don't flow like a rocket propellant. Even a beryllium "neutron mirror" has limited reflecting abilities that are eliminated by the meltdown of the neutron source.
In this case the neutron source is subcritical, uranium low enriched fuel rods arranged in a cylinder embedded in the nozzle "combustion chamber" producing a focus of neutrons along  the axis in the center of the chamber, through which your LiOD / D2O fluid passes, absorbs neutrons, and has its own fission reaction, the results of which are largely non-neutron emitting, ionized particles you can shape as you wish with a nice magnetic nozzle bell for as big as you wish in a vacuum to maximize vacuum Isp.
As for the environmental impact of releasing tritium in the environment, the risks of tritium are largely overblown.
The quantity of tritium present in the biosphere considerably increased due to thermonuclear weapons testing in the atmosphere and with the development of the nuclear power industry. At global scale, a return to concentration levels similar to those that preceded weapons testing is underway. Thanks to radioactive decay, almost 90% of the tritium introduced into the environment between 1945 and the late 1960s has disappeared. Tritium has a half life of around 12.32 years. The nuclear  industry releases  tritium into the environment regularly. A single plant releases between 8-30 grams of the stuff each year, usually into the ocean. Inland reactors do the  same into rivers. Dilution is the point: tritium as a pure substance ingested can mess you up by slowing metabolic reactions, but diluted, as 0.000001% concentration, poses no health risks. You get more radioactive exposure by eating a banana or a cup of brazil nuts.
Tritium is naturally  produced in the upper atmosphere by impact of atmospheric nitrogen and oxygen with  incoming cosmic rays.
Current tritium levels in rainwater are only slightly above pre-atomic natural levels (natural levels are  less than 0.6 Bc/L of  water, during the age of nuclear testing, levels reached as high as 470 Bc/L of rainwater. Current levels are 1-4 Bc/L. Pure tritium is about 3.5x10^14 Bequerels of radiation per gram of T. Dilution of launch  exhaust products thus obviously mean mixing with quite a lot of water if done locally, but given the launch vehicle spreads its exhaust across thousands of miles of atmosphere, and which  oxidizes with atmospheric oxygen into T20, the launch self-dilutes the tritium in the exhaust into the environment where, given the volume of water in the environment, there is roughly 12,600,000 liters of water in the world for each Bc of tritium exhaust or rather, you produce approximately 0.00000126 Bc per liter of environmental water for each gram of tritium released into the environment, thus, each launch, producing 30G of tritium emissions, will raise global tritium radiation levels by 0.000037~ Bc. You would need thus 100,000  launches of this sort of rocket to raise global tritium radiation by 1 Bc/liter of rainwater. To reach the levels we saw at the height of atmospheric testing of thermoneuclear weapons, of 470 Bc per liter, we'd need 4,700,000 launches of this rocket, which would be a problem, I imagine, if these were rolling off the assembly line like automobiles, but I sorta doubt we're ever gonna reach that level of popularity.
« Last Edit: 10/14/2019 10:35 am by mlorrey »
Director of International Spaceflight Museum - http://ismuseum.org
Founder, Lorrey Aerospace, B&T Holdings, and Open Metaverse Research Group (omrg.org). Advisor to various blockchain startups.

Offline edzieba

  • Virtual Realist
  • Senior Member
  • *****
  • Posts: 6884
  • United Kingdom
  • Liked: 10518
  • Likes Given: 48
Re: Clean lithium fission rocket
« Reply #70 on: 10/14/2019 12:48 pm »
As well as the coated-nozzle layout and the liquid flow-through layout, one could also produce an 'augmented Fission sail' layout: Coat the inner surface of a large sphere-section sail in Lithium-6 Hydride, then mount the Neutron source at centre of radius of the sail (ideally at the nose of your craft also mounted there) 'hanging' from the sail by tethers. Activate the source, and the neutrons impact the inside of the sail producing fast reaction products that propel the sail forwards. The downside is the front your craft is dusted with Tritium, but the front already needs protection from the Neutron source anyway so that shielding can pull double-duty.

Another much older concept using Neutron-riggered Lithium fission is the far more exiting Fizzer.

Re: Clean lithium fission rocket
« Reply #71 on: 10/14/2019 02:51 pm »
Neutron Induced Lithium Fission Reaction

The showstopper is, of course, the need to produce neutrons.

If you have a cheap source of neutrons, you could also consider Calcium-48. If turned into Calcium-49, I think the decay chain Calcium -> Scandium -> Titanium gives a total of 12 MeV, via beta minus decay (I looked it up years ago, this is from memory). Not so good for an engine, but good as a powerplant.

Offline mlorrey

  • Member
  • Senior Member
  • *****
  • Posts: 2175
  • Director, International Spaceflight Museum
  • Grantham, NH
  • Liked: 25
  • Likes Given: 6
Re: Clean lithium fission rocket
« Reply #72 on: 10/14/2019 04:10 pm »
Neutron Induced Lithium Fission Reaction

The showstopper is, of course, the need to produce neutrons.

If you have a cheap source of neutrons, you could also consider Calcium-48. If turned into Calcium-49, I think the decay chain Calcium -> Scandium -> Titanium gives a total of 12 MeV, via beta minus decay (I looked it up years ago, this is from memory). Not so good for an engine, but good as a powerplant.
the OP suggested unrefined natural Uranium as a sufficient neutron source. I'd suggest going to low or mid-enriched uranium to improve T/W and operating  it  at high-sub-critical. I'm working up a possible 3d model using 12 such thrusters, each 15 cm dia, with 8 mid-enriched uranium rods surrounding the Lithium Deuteroxide injection chamber, all thrusters arranged radially, injecting into a 2 m dia parabolic nozzle chamber that has its own tritium injector in the center axis that the 12 lithium deuteroxide fission plasma streams impact against from all sides.
« Last Edit: 10/14/2019 04:11 pm by mlorrey »
Director of International Spaceflight Museum - http://ismuseum.org
Founder, Lorrey Aerospace, B&T Holdings, and Open Metaverse Research Group (omrg.org). Advisor to various blockchain startups.

Offline mlorrey

  • Member
  • Senior Member
  • *****
  • Posts: 2175
  • Director, International Spaceflight Museum
  • Grantham, NH
  • Liked: 25
  • Likes Given: 6
Re: Clean lithium fission rocket
« Reply #73 on: 10/15/2019 01:29 am »
Ok so here's conceptually how this could work for both atmospheric and vacuum operation,with vacuum magnetic containment nozzle of the ionized fission (and hopefully fusion) products extending outward  upon reaching vacuum, looking sorta like the engines on a Y Wing in star wars, or the A Wing.
Director of International Spaceflight Museum - http://ismuseum.org
Founder, Lorrey Aerospace, B&T Holdings, and Open Metaverse Research Group (omrg.org). Advisor to various blockchain startups.

Offline mlorrey

  • Member
  • Senior Member
  • *****
  • Posts: 2175
  • Director, International Spaceflight Museum
  • Grantham, NH
  • Liked: 25
  • Likes Given: 6
Re: Clean lithium fission rocket
« Reply #74 on: 10/25/2019 07:22 pm »
The problem of adequate neutron supply would be solved if you treated this LiDO fission reaction as an injector/accelerator to impact a stream of tritium flowing in the axis of a central core chamber to trigger DT fusion there. The fusion reaction would supply the neutrons needed to boost and maintain the Li6 fission reaction long term. You can then inject LH2 along the skin  of the chamber for cooling, to absorb more neurons and thermal energy, to boost thrust and even have LOX/Air injection in the exhaust nozzle to boost thrust even higher.
Director of International Spaceflight Museum - http://ismuseum.org
Founder, Lorrey Aerospace, B&T Holdings, and Open Metaverse Research Group (omrg.org). Advisor to various blockchain startups.

Offline Pete

  • Full Member
  • ****
  • Posts: 767
  • Cubicle
  • Liked: 1029
  • Likes Given: 395
Re: Clean lithium fission rocket
« Reply #75 on: 11/01/2019 11:19 am »
I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?

Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.
So, where exactly does this "clean" come in?

Offline Bubbacub

  • Member
  • Posts: 12
  • uk
  • Liked: 18
  • Likes Given: 23
Re: Clean lithium fission rocket
« Reply #76 on: 11/05/2019 06:56 pm »
Hi,
Really interesting idea.
To my knowledge you would need fast neutrons to get lithium to undergo fission - unfortunately using heavy water as a moderator with u238 will create a neutron flux of thermal neutrons which won't be helpful in creating lithium fission.
Perhaps you could rejig your concept on the lines of a fast neutron reactor by using a cylinder of plutonium or highly enriched uranium to create a high energy neutron flux that would be able to induce fission in your lithium. I've attached a PDF which characterises the neutron flux of plutonium dioxide as used in a rtg. My knowledge of this subject isn't enough to do the maths but perhaps someone cleverer could tell us if plutonium dioxide, for example, as used in the paper could create enough of a neutron flux to power our rocket engine?

Offline Twark_Main

  • Senior Member
  • *****
  • Posts: 4345
  • Technically we ALL live in space
  • Liked: 2328
  • Likes Given: 1369
Re: Clean lithium fission rocket
« Reply #77 on: 11/06/2019 11:59 pm »
I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?

Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.
So, where exactly does this "clean" come in?

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket#Limitations

It's "clean" only in that it's cleaner than Zubrin's NSWR, which exhausts actively fissioning uranium/plutonium salts.

Offline Lemurion

Re: Clean lithium fission rocket
« Reply #78 on: 11/09/2019 03:57 am »
I'm a little boggled at how a system that expels masses of Tritium is considered "clean"?

Ok, no surplus Neutrons... BUT you need a ridiculously intense neutron source to activate it.
So, where exactly does this "clean" come in?

https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket#Limitations

It's "clean" only in that it's cleaner than Zubrin's NSWR, which exhausts actively fissioning uranium/plutonium salts.

It can be argued that even Orion is cleaner than NSWR. Talk about the very definition of a low bar.

Offline Michel Van

  • Full Member
  • ***
  • Posts: 340
  • Aachen, Germany
  • Liked: 180
  • Likes Given: 187
Re: Clean lithium fission rocket
« Reply #79 on: 06/24/2020 08:23 pm »
i found this NASA paper, hope this helpful

FISSION NEUTRON ATTENUATION IN LITHIUM-6, NATURAL LITHIUM HYDRIDE AND TUNGSTEN

https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19680020529.pdf
Rocket Science Rule

Tags: Nuclear lithium rocket 
 

Advertisement NovaTech
Advertisement
Advertisement Margaritaville Beach Resort South Padre Island
Advertisement Brady Kenniston
Advertisement NextSpaceflight
Advertisement Nathan Barker Photography
1