Talk:Radioactive Rarity on Mars

Thanks for starting this!

What is the real energy available form Thorium? Reference 10 mentions 11 MWh/g. Or 11 000 MWh/ kg. Now this seems to be an individual personal page.

On the other hand, https://www-pub.iaea.org/mtcd/publications/pdf/te_1450_web.pdf , the International Atomic Energy Agency puts the value of energy from thorium at 15-60 MWd/kg

When I convert the first value of 11 MWh/kg to MWd/kg, I get 458 MWd/kg or 40 times more. This seems to be an error. Almost 10 times more than the best result to date. (There may be simply a confusion between day and hour)

I also have my doubts since for some reason the author of (10) puts uranium at 0,035 MWh/kg. I understand the burn up fraction is higher for thorium, but it's not 300 times higher. After all if uranium just burns 5% then we have a 1500% burn up fraction.

If we divide reference (10) by 40, we get instead of 11 000 $/m3, 275$/m3. If we use the Martian surface average that is six times less than the Earth average we get about 265/6 = 45$ of energy per m3 of Martian dirt, or 45$ for 2 tons, or about 23$ per ton of materials processing, where we manage to remove the entirety of the Thorium in a perfect separation process.

I fear we may be spending more $ to extract the energy that the revenue from the energy. It is just too diffuse.

This does not make thorium from Mars impossible, it just makes it less likely and dependent and a natural enrichment process that may not exist.

And if Thorium must come from Earth, this means the Martian colony is not energy independent and open to all kinds of pressures from Earth.

BTW the cost of fuel for nuclear reactor is not entirely negligible. It is about 1/2c per kWh. this seems like very little until we see that the price for solar in new installation can be as low as 2c per kWh in the US, and has been sold for less elsewhere in the world. Can more reasonable attitudes reduce the cost of reactor by 5? Perhaps. are they likely? And if the processing costs are high because mineral concentration is low, then the cost of nuclear just cannot keep up.

https://www.world-nuclear.org/uploadedfiles/org/info/pdf/economicsnp.pdf https://cleantechnica.com/2021/11/17/utility-scale-solar-reaches-lcoe-range-between-2-4%C2%A2-per-kwh-in-the-usa-record-low/

--- Most modern reactors burn 0.5% of their fuel, not 5%. They are WILDLY inefficient. Busy right now, will spend more time later. Rick.

-Are there any Thorium reactor that reprocess their fuel or is this a theoretical proposition? When I see thorium reactors at 40 MWd/ton, is it because they are being viewed as single cycle, and exclude fuel reprocessing? -Is a traveling wave reactor (25 to 40% burnup fraction) a viable alternative to thorium? -Can a thorium reactor be configured as a travelling wave and therefore do away with the reprocessing?

-Thorium in Martian thermal vents, from meteoritic data. Also good source for Rare Earth minerals. https://www.hou.usra.edu/meetings/lpsc2015/pdf/1287.pdf

--- Rick here. I've caught Covid, and don't have much energy for research, or much of anything. :-/

Fuel rods in traditional reactors crack from fission products, (especially Xenon), and the cladding is damaged from neutron bombardment. So they are removed when a trivial amount of fuel is 'burnt'. In a molten salt Thorium reactor, the reactor core is a FLUID. The Th232 is bred into U233, which remains in the reactor indefinitely. (1) Some fission waste products (e.g. Xenon), do not dissolve in the salt and are removed at once. (2) Others are easy to remove chemically and likely will be removed continuously. (3) Some will remain, slightly lowering the efficiency to be batched processed at a later time (say 5 years later). The idea is to keep the Actinides INSIDE the reactor until they have all burnt. As the U233 is burnt, more is slowly added continuously.

1, and 2 have been done in experimental reactors. 3 is theoretical. Removing the Xenon by bubbling He thru the salt is a big deal, since Xe is a powerful neutron poison.

Chemical processing is simpler in a 2 fluid design. (Where the Th232 breeding material is kept separated from the reactor core.)

I have not read of a traveling wave reactor with Thorium.

--- Rick here. Nov 21, I added a LOT to the discussion on the economics of Thorium, added a link to that page, and added a few references to this page. Tomorrow I'll go over the this page in more detail.

Note: The planetary nebulae that formed the Earth may have been density separated already. I've seen models of planetary nebulae that put denser materials neat the star and less dense materials further away. Such an arrangement would reduce the amont of dense radioactive material the further you get from the sun. Need to look if this model is still current, or if it is outdated. Another way of putting this is if Mars is close to the ice line, it may have accreted more water ice than Earth, and therefore may be less dense overall. Good article here: https://bigthink.com/starts-with-a-bang/earth-densest-planet/

--- Rick: Hi Michel, the early proto-planetary disk, didn't sort atoms by their macroscopic density. Everything is in free fall. Dust particles existed in the inner and outer solar systems, and Jupiter has plenty of iron under all that gas.

What happened, is the volatiles evaporated near the sun. You would expect to find less of the metal mercury near the sun than around Earth or Mars since mercury has a low boiling temperature. The planet Mercury, will have less potassium than Earth, because potassium has a low vaporization temperature. Now elements that have high melting points tend to have higher density so in fact, planets near the sun have a higher density. But this is not because orbiting dust particles sorted themselves by density. (How could they know in free-fall?) The light stuff just evaporated.

Water ice is made in massive quantities, (since O2 and H2 are VERY common in the solar nebula). Once you get far enough from the sun that you pass the snow line, (or frost line, or ice line), bodies can grow larger much easier.

Luna was formed when a massive collision blasted much of Earth's mantle into orbit around Earth. Luna is very short of all volatiles since any that were there boiled away. Don't look to the moon for the gases, ices, iodine, sodium, sulphur, phosphorus, mercury, potassium, zinc, etc. etc. They all boiled off, while the refectory elements combined into liquid drops, then into dust, and then formed Luna.

The article you suggested I read, also mentioned the 'soot' line. Inside that, big carbon chains don't exist, so the carbonaceous chrondites meteors will evaporate. Bits of tar are all mass to growing proto-planets.

But in my discussion of early Mars I said that Mars was richer in volatiles (mostly ice and gases), but it then LOST those gases and water. That brings it back to close to the mix that Earth had.

Note that the temperature gradient between early Earth and Venus is greater than that between early Earth and Mars. That would suggest Earth and Mars would be more similar chemically than Earth and Venus. (Except Mars was so small that it lost much of its volatiles.)

Now if uranium, thorium or potassium (U/Th/K) were very easy to evaporate, you would expect more difference between the gross chemical composition of Earth and Mars. But thorium and uranium are silicon loving elements, they readily combine into rocks. And nothing was evaporating sand and gravel near early Earth or Mars. Potassium has a relatively low boiling temperature. It evaporates at 759 C. (Most rocks evaporate at around ~2,600 C.) This suggests that Mars would be MORE radioactive, because it would get a _slightly_ richer mix of potassium.

I'm over simplifying here. Elements are not existing in the solar nebula in pure form. If an element combines into something with a high temperature of vaporization, it can hang around near the sun. Look at sodium and chlorine. The boiling temperatures are: Na = 883 C, Cl = –34 C. But salt evaporates at 1,465 C. So much of Earth's budget of chlorine came from table salt in the dust.

This brings up an interesting point. 99.98% of the mass of living creatures are made up of these elements C, H, N, O, P, S + Table Salt (NaCl). The rarest of these needed elements is phosphorus, and its evaporation temperature is 280 C. This is the element in shortest supply for life, and it is likely to be _slightly_ more common on Mars which is good news.

SUMMARY: Earth and Mars were close to the same temperature in the early solar system. Mars was closer to the ice line, so we would expect it to get a bit (maybe 3% or 5%?) more water ice, and other gases than the Earth. But Mars then lost more of those elements (especially water) than the Earth did, bringing its composition back closer to Earth. Simply, I don't see any reason why Mars rocks would be grossly deficient in U/Th/K than Earth rocks. The early sun was not hot enough to evaporate uranium oxide near the Earth, but not evaporate uranium oxide near Mars. Uranium oxide dust particles would exist just fine in both locations. (UO2 evaporates at around 2,600 C.)

Hi Rick, seems clear enough. I had a very vague souvenir that there was some segregation by density but you are right and I've seen papers that agree with you. Uniform distribution. Looking good for Thorium!