One of the problems with CO2 capture is that it requires either time for diffusion, like when growing crops, or processing tremendous quantities of moving air for very little CO2. If you got wind then it's easy, but in places with low winds, building large chimneys that go very high to where the atmosphere is cooler, can generate natural convection, such as even laying a giant tube flat up the side of a steep mountain, where it's cooler up there than at the foot of the mountain, that could generate cheap reliable airflow.

I've thought about this some more. First of all there are better technologies that require less energy to release the CO2 than lime. As far as I know burnt lime is burnt at 8-900 C to release the CO2, but zinc, lead and copper carbonates, though partial hydrates, they require much lower temperatures, and they do form naturally from their oxides when exposed to air. Or there is research for low tempearature amines - submarines capture their CO2 via ethanolamine, and there are many articles for other types, such as

pubs.acs.org/doi/10.1021/ja2100005

though they used high pressure to do the capturing, and vacuum to do the release, which may or may not be cheaper than simple heat.

Second, if you successfully capture CO2 from the atmosphere at a low cost, then you probably would not want to bury it, but instead recycle it into gasoline. I'm a fan of making liquid anhydrous ammonia for energy storage instead of hydrogen that has horrible storage properties, or batteries that are very expensive on capital per stored energy amount, and their storage energy density is low, about 20x less than ammonia and 45x less than gasoline. But if you have CO2 you captured from the air, then you can make gasoline, that has even higher energy density than anhydrous ammonia, and can be stored in a can, unlike ammonia that would need storage similar to 1 lb propane canisters you can buy at Walmart in the outdoor section, to power things like lawnmowers, or 20lb cans to power barbecues and such. To make ammonia combustible it needs to be mixed with a small quantity of hydrocarbon like propane or gasoline.

Oh further down someone clarified, under the heading "that's clever". They burn lime to release the CO2 which they pump underground, then use the burnt lime to capture atmospheric CO2 and repeat the process. The question is will that CO2 stay underground or will it eventually escape. When. plants absorb CO2 that stays underground in the form of coal, oil and natural gas, hopefully the CO2 will act like natural gas and stay underground. A plant when it fossilizes does not fully convert to natural gas, but a combination of coal + natural gas or oil + natural gas, and even if the geological formation releases the natural gas, some of the carbon is guaranteed to stay under as coal or oil.

Limestone is already a calcium carbonate. Burnt lime where the carbon dioxide content has been released will reabsorb the equivalent amount of carbon dioxide with no net gain. Calcium bicarbonate is not stable in high concentrations. Plants are great carbon sinks but limestone in nature is already saturated with CO2. This smells like a scam. Technically you could get the calcium oxide content out of silicate rocks and make it absorb carbon dioxide with net gain, but the energy expense would be tremendous. But limestone is already a carbonate and is not a carbon sink as mined in nature.

This is a somewhat offtopic continuation of my previous posts.

Well, in the fast neutron energy region there are very few heavy elements whose neutron absorption cross section is small, relative to the fissionable elements cross sections. I found a site that has all the data:

https://www.nndc.bnl.gov/endf/...

in the basic retrieval section we put
target: K*,Na*,Hg*,Cd*,Zn*,Cl*,Br*
reaction: n,g
quantity: sig

and after submitting we pick the naturally occurring isotopes looked up from wikipedia isotopes of zinc, etc. pages. A maximum of 16 isotopes can be graphed simultaneously. Then we zoom in into the 1 to 10 MeV region where fast neutron reactions occur. You can uncheck some of the isotopes and select repaint to see them more clearly.

One thing to note is that nothing beats sodium 23 in this group of elements. Potassium is also good, low neutron absorbing but it has some peaks near 1 MeV and is not as good as sodium. Chlorine may be decent, and Zn 70 may be acceptable, next best is Hg 196 then Cd 116. None of the cadmium isotopes seem ok, nor any of the two bromine isotopes.

Unfortunately uranium is not soluble in molten sodium, or just infinitesimally so, so you could not create a boiling sodium liquid pool reactor that has dissolved uranium. Locking up the uranium in floating balls does not really simulate a solution, such as they would not go toward the edge in a funnel shaped reactor on increased boiling, to increase neutron losses to the environment. So pretty much the only solution with sodium is a solid uranium or uranium oxide fuel that can melt down, surrounded by a liquid pool of sodium, as practiced in todays fast reactors, and using control rods, and control of fast reactors has been historically a difficulty.

So instead of liquid cadmium metal above which floats cadmium bromide, the least worst option is liquid zinc 70 metal above which floats a pool of boiling molten zinc chloride. Zinc dissolves uranium decently from what I've seen, and zinc chloride I have yet to find data on, but should dissolve uranium trichloride decently too. The densities are 6.58 g/mL for molten zinc, 2.91 for solid zinc chloride, and 5.50 for UCl3. Boiling point for zinc is 907 C and zinc chloride 732 C. The uranium should be more active than zinc and should concentrate in the zinc chloride. Increased boiling in a funnel shaped reactor should spread the dissolved fuel out sideways increasing neutron losses to the environment, and the height in the center not proportionally increasing to the void fraction, but less, and thus be self regulating even without control rods. This is the least worst option from the above list of isotopes, considering the solubilities of fuel in molten medium and boiling points. Unfortunately the neutron absorptions for zinc and chlorine are not as low as with sodium, or oxygen.

I've been also meditating about the bomb based Geocore. Hopefully the shockwave absorption ability of vapor-liquid equilibrium steam near 350C 160bar is very high, functioning as a good shock absorber, allowing for small chambers. I mentioned a 2km height chamber to allow room for the fast moving mushroom cloud to cool by radiation, which can be a blessing in the sense that it spreads out the pressure increase along the height somewhat, as opposed to concentrating all at the bottom as it would be with a complicated waterfall-pulse with a bomb inside it, that would generate no mushroom clouds. The fast rising mushroom cloud is both a curse as a blessing, it requires a tall chamber that's more expensive, but it distributes the pressure out more evenly, more gentle on the granite. Even a non-shockwave low pressure large area wave would flex the granite over and over, and the less the amplitude the better.

Well well, I was looking for fast neutron capture cross sections for Cd 114 and I 127. Unfortunately I found one that says iodine 127 is near 0.1 barn for capture in the 1 MeV region. That's very high, much higher than indium 115.

www.researchgate.net/publication/273095227_Medium-Energy_Evaluation_of_Iodine-127_Iodine-129_for_JEFF-31
page 11 of the full text.

That's a shame, because the bromide, chloride and fluoride boil at 844, 960, 1748C, respectively, quite high, vs 742 for the iodide.

While I was looking for bromine and could not find any fast spectrum other than 14 MeV fusion conditions, I came across another Iodine 127 reference, this one says 0.04 barns near 1MeV compared with a ton of prior data that's higher. But 0.04 barns is still high.

www.researchgate.net/figure/Experimentally-measured-cross-sections-result-of-127-In-g-128-I-reaction-and-its_fig3_353792722

Also
en.wikipedia.org/wiki/Neutron_cross_section
table at the bottom has some fast neutron absorption and fission cross sections, to put everything into perspective. Without this comparison one does not know what's high and what's low. I still can't find a Cd 114 fast spectrum, but it seems that is not the problem, iodine is for fast neutrons. It's hard to find another volatile compound of cadmium, or another metal that boils in that range, there is mercury lower, zinc higher, and sodium and potassium near, but it's hard to find a compound of theirs that boils low, so that both the metal and the metal compound could be distilled and purified. I just looked into it and, though zinc boils high, its bromide and chloride boil low. Now I have to look for fast neutron absorption cross sections for zinc, chlorine and bromine isotopes, perhaps there is a combo that's resistant to fast neutrons.

Cadmium metal has like the ideal boiling point. If it does dissolve uranium and plutonium, perhaps you could just boil cadmium metal without any iodide present, and distill the cadmium metal to purify it. Cadmium vapor should not react with the iron in granite walls, unlike zinc, though with zinc the metal would have low vapor pressure and only the chloride or bromide would boil. Both are very corrosive. Because cadmium boils at higher temperature it has a better chance to dissolve the fuel metals than mercury, though it really comes down to phase diagrams.

There are other fast neutron poisons that are not as easy to deal with as xenon 135 and have higher barns in the fast spectrum, such as some samarium 149 and perhaps indium 115. They would be burnt up slowly by neutrons or some would come out during distillation.

I had the idea if you ran the reactor at higher pressure than the distillations, then you could use the reactor contents as the source of distillation heat, as they would be at a higher boiling temperature. Most likely the hot liquid metal cadmium from the bottom. I don't know if it's worth pressurizing though, depends on at what rate you want to distill, if, say, 1% every 24 hours, then simple electric heat powered by the generators might be easier even with the 70% energy efficiency loss. But it could be an option, something to consider during design. Depends on what the poison accumulation rate is, or how fast the waste is generated.

Everyone dealing with nuclear reactors should be aware of iodine 135 and xenon 135. In our case the iodine 135 would intermix with the cadmium iodide 127, and escape through distillation, and only the half life would remove it. It's about 6% fraction of decay products of uranium. However its fast neutron behavior is not that bad, so it should be much less of an issue for a fast reactor compared to a moderated reactor.

Here is how it caused the Chernobyl incident by unaware operators:
hyperphysics.phy-astr.gsu.edu/hbase/NucEne/xenon.html

and this page has a graph showing that fast neutron (1 to 10 MeV) cross sections are quite low (near 0.001 barns) compared to moderated thermal neutron spectrum where it's 2 or 3 million barns:
www.nuclear-power.com/nuclear-power/reactor-physics/reactor-operation/xenon-135/

Void coefficient in Chernobyl was positive because reactivity increased with bubbling, instead of decrease, because of less neutron absorption by liquid water. In our case the fuel itself is in the liquid, and bubbling decreases the fuel concentration in any localized region, decreasing reactivity, as opposed to where you have solid fuel bundles. This requires that uranium and plutonium iodides be soluble in boiling cadmium iodide, something that needs to be checked. Also the xenon would distill out in the Geocore chamber and concentrate in the vapor phase until it decayed, it would be less of an issue than when it is stuck in a solid fuel bundle.

I'm continuing my previous posts, about a liquid cadmium/boiling cadmium iodide fast reactor. Previously I said if you stacked 100 critical masses side by side the boiling negative void coefficient would only increase neutron losses to the environment near the perimeter of a cylindrical shape reactor, in the center you would need boron control rods immersed into the iodide. A little further thinking brings up a simple solution, have the reactor conical shaped, funnel shaped. If the uranium and plutonium iodides are soluble in the boiling cadmium iodide, then increased bubbling even in the center region makes neutron losses to the environment greater, because the material flows away sideways, and the height does not increase proportionally to the void fraction, but less. What angle to use for the cone, 15 degrees or 45 or 75 degrees, I don't know which one is optimum.

Also, if the reactivity gets out of hand locally, one may generate van Arkel de Boer style free iodine - it would be the cadmium that decomposed before the plutonium and uranium iodides - but if iodine gets into the vapor phase it may be corrosive near 750C even to relatively noble molybdenum heat exchangers. In that case you would need a large pool instead of shell and tube heat exchangers, of enormous surface area for 300GW thermal, where the molybdenum tubes are covered and protected by a lake of liquid cadmium, that reduces any free iodine, and the cadmium iodide cools and liquefies above this pool, both in a relatively thin layer for reduced heat transfer losses. But I think even the cadmium metal would have enough vapor pressure in cadmium iodide gas, to capture and reduce any free iodine, and the shell and tube heat exchanger may be fine.

Also muography, the xraying of rocks for kilometer depths based on muons can be an invaluable tool for selecting sites for a Geocore, that have a contiguous bulk granite structure, and lack water leak sites or pressure release sites. I don't know how fast it is, whether it's good at monitoring a running Geocore, compared to things like radar, but at least for site selection it should be an invaluable tool.

making ammonia via lithium video:

www.youtube.com/watch?v=dFcaEUj43OY

I've tried to hunt down that patent I read 15 years ago, about nitride formation on wires, I would recognize it from the wording. I searched 1940 to 1994. After 1994 a whole lot of lithium related noise shows up because of lithium based batteries. I found another interesting patent though, sometimes I get an eerie feeling that an AI may compose a patent in 2023 to get inserted into the patent database showing up as 1961, but I'm being paranoid. Here it is, US3,208,882. In it it teaches all the essentials of manufacturing nitride from metal and unseparated air at room temperature, though it self heats which increases the reaction rate, also pressure increases reaction rate. Basically you start with moist air, about 50% relative humidity is fine, then you can continue with dry or moist air. The ultimate energy storage efficiency may be low, but at least you would have a product you could ship from the desert to anywhere in the world, liquid anhydrous ammonia. Else you need a very extensive HVDC power network, such as from Saudi Arabia all the way to the Netherlands or Japan, to transfer solar power, what would the capital costs and energy losses be involved there, in comparison?

There are of course lots of questions. Such as what is the exact boiling point of cadmium iodide, how soluble is plutonium and uranium in liquid cadmium, and why they would be soluble but tungsten-molybdenum is not. Also as pressure increases, do the boiling points of cadmium metal and cadmium iodide diverge or converge? Also, what is the actual equilibrium temperature of a meltdown, it's most likely not 4000C vapor, but just molten metal, I would guess closer to 2000 Celsius is where the liquid density is low, the thermal radiation is high, all in equilibrium with the rate of internal energy production. I'm thinking here what temperature did the meltdown at Chernobyl and Fukushima attain. So even at 2000C a drop of liquid metal would fall through and boil the cadmium metal as it was falling, and create an insulating Leidenfrost layer as it falls, greatly reducing heat exchange cooling and ability to blend into the melt and dissolve away in the cadmium. There may be a need of a tungsten plate at the bottom, which is both dense in case it melts, but it does not melt at 2000C, and it would allow to cool the meltdown metal by boiling cadmium until it is able to dissolve in cadmium, if it does dissolve.

Also a steam explosion of cadmium iodide is not welcome, as explosions in a gas can be tolerated, but in a liquid they are devastating. That's how the Nazi dams were brought down, by underwater explosions. The only solution to it I can think of is a jacuzzi of cadmium iodide vapor being bubbled through the molten cadmium metal, which would be able to compress and liquefy and absorb shockwaves through the liquid, but creating and maintaining such a bubbling jacuzzi may be too complex, too much to ask for, via numerous tuyeres.

Oxides are a problem, as uranium and plutonium oxides have higher density than liquid cadmium near 11 g/cm3 vs 8 for cadmium metal. The cadmium amalgam of these metals cannot contact silicate as it may lead to oxide formation. So you would probably need a tungsten-molybdenum lining. The cadmium iodide vapor should be ok to contact polished granite walls of the chamber.

The leftovers after boiling cadmium iodide or cadmium metal should be soluble in HI/H2O, except lead, copper, silver, etc. Might need periodic nitric acid to dissolve the crud left over after distillation, and what vessels material to use for the distillation is a good material science problem.

Boron 10 control rods could be hanged from above if you are avoiding explosions anyway, and if the reactivity gets too high and the boiling too intense the liquid rises into the control rods, when the boiling quiesces it would drop below them. In case of an explosion the control rods would fall and float on top of the iodide, with specific gravity of 2.3, something that could be adjusted though by combining with a heavier metal as a boride, to sink inside the iodide but float on top of liquid cadmium. The boride would have to be of a metal more noble than cadmium, yet not dissolve out and be corroded by cadmium. In absence of control rods, the boiling itself is a negative void coefficient and should self regulate the reactivity somewhat, by increasing the fraction of neutrons lost to the environment due to lower density, and higher surface area, with some caveats, as follows:

A usual nuclear reactor is on the order of 1GW electric or 3GW thermal or so, with Chernobyl jumping to 30 GW power during the accident. Is there a way to have a large fast reactor, on the order of 100 GW continuous power? I believe in economies of scale, instead of spending billions per 3 GW thermal, it should be cheaper to build a large one, per GW. Part of the problem is that a critical mass is a critical mass, and you can only go so big on it. You could imagine a floating pool of metal cadmium, above which floats cadmium iodide dissolved uranium 235, 238 and plutonium 239 iodides, reacting. If you put many critical masses side by side, as a, say, 10x10 grid, or 100 total, if each individual one produces 3 GW of power by boiling cadmium iodide, 100 of them would be 300 GW. The neutron economy is improved too, because the losses to the environment are reduced, as one neutron lost from one critical mass section lands in the other, still useful, not lost, though here is the above caveat, that reactivity control becomes more difficult, as the void coefficient does not increase surface area and neutron loss to the environment, from increased boiling-bubbling, except around the perimeter of 100 grid sections. Then you would absolutely need the hanging boron control rods, as bubbling increases they get covered more. Also there is a danger of boiling off the coolant iodide completely, then the reacting mass would start boiling the metal cadmium beneath, possibly sitting on an insulating Leidenfrost cushion and reaching 2000C meltdown temperatures, decomposing the iodides van Arkel de Boer style, and sinking through the cadmium until it hits the tungsten plate, and sit there continuing to boil cadmium metal. There would need to be enough liquid cadmium to be able to absorb all the meltdown heat, yet not too much to over pressurize the chamber with 2000C cadmium vapor, though the pressure limits of a Geocore are extremely high. Under usual operation the 742C iodide may be under very low pressure and still generate an efficient steam pressure, because of its high temperature.

Yeah, lots of questions and issues, but if you design around the mishaps to begin with, and especially take everything underground with containment chambers, if both Chernobyl and Fukushima were underground with containment chambers carved into granite instead of containment buildings, we probably would not have heard about them much.

Also en.wikipedia.org/wiki/Uranium-233 under weapon material mentions a critical mass of 4-5 kg, less than the 8 to 10 kg I previously mentioned. This would make a total burnup be equivalent to 80-100 kiloton for a bomb based Geocore, but I'm afraid by the time you include enough fusion to generate enough energetic neutrons to fully destroy any leftover fissile material, you would most likely end up back at 200 kilotons minimum explosion size. One of these every 200 seconds would generate 4 terawatts thermal average with probably 1 terawatt electric obtainable from it. Dealing with borates to stop criticality of the leftovers is risky, but if it works that would require lots of recycling effort and cost of the leftovers, but a much smaller chamber size, probably a 10-20 kiloton bomb every 10-20 seconds, which would still be a 4 terawatt thermal reactor.

Also cadmium is a strong absorber of thermal neutrons, in particular Cd 113, but for fast neutrons it's not that bad, but still the Cd 113 is like 10x worse than the others, so an isotopic purification might be in order to remove the 113.

I've been meditating about what happens when a lot of critical mass accumulates at the bottom of a Geocore chamber. In particular could we design around this reactivity and make it something useful instead of being a problem. As I've said above the global problem with nuclear power is lack of sufficient fuel for a significant portion of the global energy budget. This is because mostly the 0.7% U235 is burned, and very little of the U238, or depleted uranium, is burned up to provide energy. Fast reactors can burn U238, though with some caveats, such as pure U238 cannot be made to go critical, it still needs a lot of U235, and as much as 25% enrichment is used in some sodium cooled reactors.

https://en.wikipedia.org/wiki/...

So a lot of nuclear issues are related around meltdowns, and overpressure caused by them breeching reactor containment buildings.(Sodium-water reaction is also a big issue at fast reactors) A meltdown is also the issue in the Geocore from leftover fissile material from a usual explosion pooling at the bottom of the chamber, and becoming a hot, dense, glowing melted mass that starts melting the rock at the bottom of the chamber, and keeps sinking through it because of its higher density, until it gets diluted enough by the molten rock to become subcritical, when everything stops. It's like a "slow hole to China" except it stops within a relatively short distance. So how can we deal with this? Is there a way to have a continuous meltdown fast reactor, without the fuel sinking and being lost to rock below? Basically what you need is a denser material than the fuel itself. Unfortunately metallic uranium and metallic plutonium are both extremely dense, at 19.1 g/cm3 for U and 19.8 for Pu. This is much more dense than 2.7 g/cm3 granite, and even tungsten is only 19.3 g/cm3, gold is 19.3, platinum is 21.4, mercury liquid is 13.5, lead is 11.3, and most usual metals like steels, nickel, chromium, copper are in the range of 7 to 9. But luckily both uranium and plutonium are highly electropositive and reactive, and their salts are much less dense. In particular I came up with this system: Cadmium liquid metal with a density around 8 g/cc, with cadmium iodide liquid floating on top of it, with density of 5.6, and in between these two layers uranium(III) 6.8 and plutonium(IV) iodides 6.9 g/cc. The other oxidation states I could not find. The boiling point of liquid cadmium and cadmium iodide are 766C and 742C respectively, though some of the boiling point data is conflicting. Plutonium and uranium iodides melt at slightly higher temperature, but they may be soluble enough in cadmium iodide. If you have a localized meltdown from a fast reaction going out of control, then according to the van Arkel de Boer process the iodides convert back into the elements. This would be the case with bromides and chlorides too, though at higher temperatures, though, suppose we get a 4000C "meltdown" vapor of uranium or plutonium locally, this would boil off the iodine and then the metal would cool into liquid and fall back down into and start sinking through the liquid cadmium metal, but hopefully dissolve in it, giving a cadmium-uranium and cadmium-plutonium amalgam, which would then react back to the iodides by displacing cadmium from cadmium iodide above. The whole system would operate at the boiling point of cadmium iodide, though this would bring some instability as you boil off the CdI2 you concentrate up the fuel which increases reactivity which boils off faster, it's not a self regulating process, though neither is a control rod situation, and you could regulate the reactivity by dropping in pure distilled CdI2 to dilute things back down. But even if things get locally out of control, and you have localized "steam explosions" of CdI2, as long as it's in a somewhat large Geocore chamber the pressure should be easy to absorb. You would have a continuous draw-off of Cd metal and CdI2 to distill off in a separate vessel and the radioactive waste be left over from distillation can be reprocessed, thus keeping both at reasonable purity levels. Molybdenum-tungsten alloys may be able to withstand corrosion by liquid cadmium, and there is probably good materials that would work as heat exchangers to steam. So this continuous meltdown fast reactor Geocore chamber would be tiny compared to one dealing with nuke explosions, but the rest of the design would stand, containment chamber, etc.

Converting electricity to ammonia may be especially worth during incidents where there is such an oversupply of renewables that the price goes negative. Also for a nuke chamber carved into granite a 2km tall 4 cubic km chamber should be manageable, it should fit 20 kiloton explosions and give room for their mushroom clouds to cool as they rise before they hit the water showering water cooled ceiling, but 200 kiloton, I don't know about that, it may need too enormous a chamber. Question is the shockwave absorbing ability of VLE steam, the maximum remnant shockwaves that granite can handle, which would allow you to figure out chamber size. As far as x-rays go they get absorbed in the steam over very short distances and only UV and visible rays need to be dealt with, which is possible with dark rain or slightly murky rain and a sheath of water flowing down cooling the chamber walls from the showerheads above. Cylindrical chamber with a cone hat roof holding showerheads, the wall on a slight incline and smooth to sheathe the water properly.