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I am looking at the possibility of future space habitats being powered by Thorium. Main question is where to get Thorium? Could there be a metallic asteroid (like 16 Psyche) that contains Thorium and we could capture it? Unfortunately I am not aware of any known asteroid that is rich in Thorium.

Alternatively, I came to know that meteorite SAU 169 contains a very high concentration of Thorium AND SAU 169 is a lunar meteorite that was blasted off from the moon from a the Lalande Crater .

Hence the Lalande Crater on the moon could be a source of Thorium. Would this assumption be correct? Or am I being too optimistic?

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    $\begingroup$ Is there a worldwide moratorium on launching a breeder reactor into space? "After five years, the core was removed and found to contain nearly 1.4% more fissile material than when it was installed, demonstrating that breeding from thorium had occurred." $\endgroup$
    – Mazura
    Oct 16 at 19:41
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The "very high" thorium concentration of SaU 169 is around 30 parts per million for the highest measurements (see table on page 6). That's only high in comparison to the bulk abundance of thorium in Earth's crust: thorium is typically extracted from monazite sands containing a couple percent to a couple tens of percent of thorium oxide.

Monazite sands themselves were weathered out of other rocks and concentrated by the action of flowing water sorting grains by density and producing placer deposits. You are not likely to find similar concentrated ores on the moon or asteroids, though you may find them on Mars. Without them, extracting the thorium will be a very complex and energy-intensive process, likely as a byproduct of the extraction of other materials.

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  • $\begingroup$ speaking of monazite: Why would India have so much thorium on its beaches? $\endgroup$
    – uhoh
    Oct 16 at 14:14
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    $\begingroup$ I'm very glad you linked to that paper. I'm going to take its figures on element concentrations as the basis for our assertions as to mining resources in Lalande Crater. More than good enough for a fictional depiction of industrial development. Thank you. $\endgroup$
    – kim holder
    Oct 16 at 15:11
  • $\begingroup$ Out of this one may conjecture, that "space mining" is a non starter, and even advanced space faring civilizations will want to stick to "habitable" worlds for their resource needs. :-) $\endgroup$
    – oakad
    Oct 18 at 6:30
  • $\begingroup$ medium.com/swlh/… $\endgroup$ Oct 20 at 21:15
  • $\begingroup$ @ChrisB.Behrens the argument for PGEs is better, but a typical terrestrial PGE ore is enriched thousands of times over. A rich asteroid might be 10 times as rich, but it'll be a lot more than 10 times as hard to access. $\endgroup$ Oct 20 at 21:59
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Partial answer by a non-expert.

The thorium as 232Th itself will be a very, very small part of the mass of a safe, reliable thorium reactor. I assume that it will pelletized and pretty safe if you don't ingest/inhale it. It's not actually radioactive, and used to be given to people to eat as a contrast agent for X-rays, so it really could be sent from Earth as a few kg carry-on along with one of the passengers (again, as long as they don't eat it).

Thorium 232 is fertile, meaning in the reactor you'll convert it to Uranium 233 with a thermal neutron source, and whatever that is is a different challenge and you might not want that in a carry-on bit of luggage.

According to World-nuclear.org's Thorium(Updated November 2020) those

fissile driver options are U-233, U-235 or Pu-239

so back to square one with the danger of launching. Mining in space is a way around that. You could launch everything else as non-radioactive payload but find your fissile neutron source (required to use thorium) in space somewhere.

enter image description here source

Now you've still got to build a big nuclear reactor, and it's going to have a lot of exotic high temperature materials in it that are difficult to process as raw materials and then manufacture in to components.

Bottom line

Don't worry about the Thorium! It's the least of your worries. Worry about the U-233, U-235 or Pu-239 neutron sources.

"But I asked where!"

Thorium 232 is stable and primordial, and you'll likely find it mixed with Uranium.

The 2010 Geophysical Research Letter Uranium on the Moon: Global distribution and U/Th ratio presents an analysis of gamma-ray maps of the Moon from JAXA's Kaguya gamma ray spectrometer. They don't measure Thorium directly, but instead they pick up a 2614.5 keV line from the beta decay of Thalium 208 in the decay chain of Thorium 232.

Yes I said that it is stable, but it alpha-decays with a half-life longer than the age of the universe.

They detected Uranium 238 from a 1764.5 keV gamma ray line emitted as a result of β decay of Bismuth 214 which is in its decay chain.

Figure 3. Distribution map of U on the lunar surface measured by Kaguya GRS. The abundances were determined by peak-fitting analysis of 238U 1764.5 keV peaks....

Figure 3. Distribution map of U on the lunar surface measured by Kaguya GRS. The abundances were determined by peak-fitting analysis of 238U 1764.5 keV peaks. Labels on the map indicate the following lunar topographies: A, the Apennine Bench; C, Copernicus; I, Mare Imbrium; J, Montes Jura; S, South Pole-Aitken Terrane; and T, Mare Tranquillitatis. Dotted squares labeled as E and W indicate specific highland regions defined as East and West Highlands, respectively (see text). The shaded relief in Figure 3 and 4 was drawn using topographic data by Kaguya Laser Altimeter (Araki et al., 2009)

Figure 4. Distribution map of Th on the lunar surface measured by Kaguya GRS. The abundances were determined by peak-fitting analysis of 232Th 2614.5 keV peaks.

Figure 4. Distribution map of Th on the lunar surface measured by Kaguya GRS. The abundances were determined by peak-fitting analysis of 232Th 2614.5 keV peaks. Labels on the map are the same as those in Figure 3 (see also text).


What your reactor might look like, from Use of Thorium in the Generation IV Molten Salt Reactors and Perspectives for Brazil

enter image description here

Figure 1. Scheme of Molten Salt Reactor (MSR) (US DOE, 2002).

But you might instead see if a variant of the Kilopower reactor built for space applications can be rigged up to work with thorium 232 somehow.

In your reactor your neutron source first converts thorium 232 to uranium 233, then your neutron source helps to induce fission in your real fuel, uranium 233.

enter image description here

Figure 3. Nuclear reactions involved in the transmutation of 232Th to 233U


update: Here's a map of Thorium in higher resolution from the Lunar Prospector spacecraft that orbited a few years later. Looks delicious!

enter image description here

source

Map of the Moon (left: nearside; right: farside), generated from gamma-spectrometric data collected by the Lunar Prospector vessel. The map shows the global distribution of the element Thorium in surface rocks, with high Thorium concentrations indicating the occurence of the so-called KREEP rocks, which have high contents in Potassium, Rare Earth Elements and Phosphorus. The heterogeneous distribution of KREEP rocks at the moon’s surface implies fundamentally different geological histories of the individual lunar regions (terranes). For further information see Jolliff et al. (2000)1

1Bradley Jolliff, Jeffrey Gillis, Larry Haskin, Randy Korotev, and Mark Wieczorek (2000): Major lunar crustal terranes: Surface expressions and crust-mantle origins. Journal of Geophysical Research. 105(E2): 4197–4216, doi:10.1029/1999JE001103.

The paper is long and excellent and goes into great detail, it may be particularly helpful for someone wanting to figure out where to start looking first to find Thorium and Uranium necessary to power their reactor after a heck of a lot of refinement.

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    $\begingroup$ "Don't worry about the Thorium! It's the least of your worries. Worry about the U-233, U-235 or Pu-239 neutron sources."... nah, no problem. All you need is a bit of deuterium, which is available in water. Use that in a fusor, which can be a literally desktop device. Feed your Thorium some of neutrons, make some U233, and then use that as you primary Neutron source. It will take some time at the typical neutron fluxes fusors can generate, but is definitely doable. $\endgroup$ Oct 16 at 12:25
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    $\begingroup$ Surely you know about Fusors? en.wikipedia.org/wiki/Fusor Its a simple way to source neutrons using electricity and deuterium. (its easier if you have tritium too, but not needed). The amount of fusion is small, biggest one I've heard of emits less than 3e11 neutrons/second flux, but that is enough to kickstart the Thorium chain. And it is a lot simpler and easier to build than an actual particle accelerator as a neutron source. These guys build and sell commercial variants: nsd-fusion.com/Neutron%20Generators/… $\endgroup$ Oct 16 at 13:58
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    $\begingroup$ The D + D → T + n reaction has a high cross-section, but it's still a nuclear rather than an atomic cross-section. You have to accelerate thousands or millions of deuterons to 100's of keV to MeV of energy to get one neutron out, and then you've got to get that neutron to the nuclear fuel. I'm quite sure it's not practical from an energy perspective. $\endgroup$
    – uhoh
    Oct 16 at 14:11
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    $\begingroup$ It's not supposed to be practical from an energy perspective. It is not supposed to run a reactor. it.is.not.a.way;.to.make.energy! It is a way to change a small amount of Thorium into Uranium233, which then serves as source for more neutrons, which then serves to give you a working thorium-fuelled reactor. Which was a silly exercise in the first place, because it is a lot easier to just start the setup with a few hundred grams of the real stuff to begin with, but you objected to that with the comment of "Don't worry about the Thorium... Worry about the ... neutron sources" $\endgroup$ Oct 16 at 16:39
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    $\begingroup$ There probably aren't any such sources, since this is something nobody on Earth will have ever needed to do. However, the existence of fusion-based neutron sources and breeding of U-233 from Th-232 are hardly exceptional claims, and your objections show you didn't understand what was being proposed. $\endgroup$ Oct 16 at 17:50
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Given the data in the other answers it would be easier to source thorium from Earth and send it into space.

The current resources of thorium on Earth are large,

Country           Tonnes
India             846,000
Brazil            632,000
Australia         595,000
USA               595,000
Egypt             380,000
Turkey            374,000
Venezuela         300,000
Canada            172,000
Russia            155,000
South Africa      148,000
China             100,000
Norway             87,000
Greenland          86,000
Finland            60,000
Sweden             50,000
Kazakhstan         50,000
Other countries 1,725,000
World total     6,355,000

The estimated grade of thorium resources in Australia is 7 percent.

The grades for lunar deposits, as per the answer by @uhoh, are given in parts per million, as is the grade of the SAU 169 meteorite. This is 10,000 times more dilute than what can be obtained from Earth.

Extracting thorium from such deposits would be possible, but it would be very expensive. Getting it from Earth would be much easier and cheaper.

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Both Uranium and Thorium deposits on Earth are concentrated AFTER the emergence of the oxygen-producing photosyntesis. In general, it is the free oxygen in the water that makes these elements soluble, e.g. selectively transportable.

On a celestial body with no liquid water, water cycle and a free oxygen source, one cannot expect these elements to be concentrated to mineable amounts. Well, this is more or less true for almost any chemical element, except the most abundant ones.

What's more, most of the heavy elements (and U and Th are among the heaviest) are concentrated in the core of any body large enough to be molten at some point in the past (our Moon qualifies pretty much). You will have do dig a lot.

The only somewhat practical source of heavy elements are "iron" asteroids. Looks like they are the above mentioned cores, exposed by collisions in the past. There, you will get easilly-accessible metals and (probably) Thorium as a by-product of metal purification.

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While I cannot improve on the previous answers in respect to the abundance of thorium. I would like to add that I find it doubtful that any inner solar system space habitat (perhaps baring the moon or L2 LaGrange points) would use any type of nuclear reactor. This is because solar panels are simply a far better source of power for these applications; they are far lower maintenance, have fewer issues with heat and don't have to deal with the transportation of fuel or disposal of waste.

Additionally, while space travel is going to necessitate the adoption of some form of nuclear propulsion in the long term, I suspect that a nuclear powered space station in orbit of earth (especially LEO) would be a tough sell politically.

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