| Lunar Nuclear Fuel, At Last?? by Dave Dietzler From the excellent book "Megawatts and Megatons" by Richard L. Garwin and Georges Charpak, we find that French and Japanese scientists have successfully experimented with the extraction of uranium from seawater with plastic filters. From one kilogram of plastic filter they obtained 3g uranium, 2g titanium, 6g vanadium and 6g of cobalt. The metals are extracted and the plastic filter is reused many times. Not only does this mean that we can have a massive supply of uranium for nuclear power on Earth obtained more cleanly than by mining which causes the release of radon gas from mine tailings, it means great news for us moon miners. You see, KREEP, although it is richer in uranium and thorium than other forms of moondust, it is still a very low grade ore. See the table below (info from Van Nostrand's): Constituent Anorthositic Rocks KREEP Al2O3 >25% 15-20% FeO 0-5% 8-10% MgO 2-8% 7-13% P2O5 0-0.06% 0.3-2% K2O 0.01-0.2% 0.2-2% Uranium <0.4 ppm 2-6 ppm Lanthanum 0.1-4.5 ppm 40-80 ppm Hafnium <0.01-5 ppm 10-30 ppm Only 2-6 ppm of uranium is not very much, but seawater only has 3.3 milligrams per cubic meter or ton. That's just 3.3 parts per billion. If we can get that uranium out of seawater we can certainly get it out of KREEP which has a thousand times as much(that's 3.3 ppm/ 3.3 ppb = 1000)! Useful Elements in KREEP The KREEP will consist of complex minerals made mostly of silicon, oxygen, aluminum, iron and magnesium with some other good stuff like phosphorus, potasssium, uranium and rare earth metals like lanthanum (used to increase the refractive index of glass) and hafnium (used for reactor control rods). Other sources indicate that KREEP contains 10 ppm thorium, another source of nuclear energy. Extracting Uranium What we must do is break down the crystal matrix like we do with other minerals by melting, quenching and grinding. Iron bearing material can be removed with magnets. Then we can carbochlorinate the stuff by mixing it with carbon dust, exposing it to a stream of chlorine gas (both C and Cl will be carefully recycled and replenished by volatile mining) and heating it with solar reflectors or lenses. This will convert the stuff to chloride salts like that which we find in seawater. The silicon tetrachloride will boil off at only 56.9 C. It will be decomposed with solar heat to get pure silicon for solar panels and recover chlorine gas. Aluminum chloride will sublimate at 178 C. It can be recovered and electrolyzed to get aluminum. Carbon monoxide will also form and vaporize off to be recycled by reaction with hydrogen for conversion to methane and water which can be pyrolized and electrolyzed respectively to recover hydrogen, carbon and get some oxygen. The chloride salts that remain will be dissolved in water and pumped through plastic filters to get the uranium. We can imagine other plastic filters that will absorb phosphorus, potassium, rare earths, thorium and other trace metals perhaps. After uranium filtration, the salt laden water will be boiled down, condensed, and the metallic chloride salts will be decomposed with extreme solar heat in a ceramic retort to recover chlorine, or they will be subjected to electrolysis. Note that the nuclear scientists also filtered titanium, vanadium and cobalt out of seawater, so we should capture these in the process of filtering out uranium if they are present. Separating those metals from each other could be problematical. Uranium can be fluorinated to make UF6 which has a low boiling point, so we could just do that to roast it out of the mix of metals we filter out of our salt solution. Uranium hexafluoride can than be run through gas centrifuges which use only 10% as much energy as gaseous diffusion to enrich the uranium for use in nuclear power reactors and nuclear rocket engines. However..... Reality Check KREEP has only 4 ppm U and 10 ppm Th. Is this a really worthwhile resource to exploit? Might we just broil the phosphorus out with solar heat and extract the potassium with acids? We need phosphorus for n-type solar cell dopants and potassium has agricultural uses. Could there be a better source of uranium, if we choose to use fission in an age when fusion has probably been commercialized, in 30 to 50 years, when we mine the Moon for He3 to make money? Fission reactors might be lighter and cheaper than fusion reactors, so we might still use them in outer space. There is another potential source of uranium and thorium-The Planet Mars. Sedimentary processes on Mars could have created deposits of pitchblende like minerals, easier to mine and refine than KREEP. Could uranium and thorium be part of the Trade Triangle between Earth, Mars and the Moon?? The Moon may have helium 3, but this must be fused with deuterium, another resource available on Mars, unless we achieve he3-he3 fusion. Lunar Nuclear Industry, Not So simple We will need gas centrifuges or laser isotope separators to enrich uranium. Natural uranium in our solar system is about 0.7% U235 and the rest U238. For light water reactors an enrichment of about 3% to 5% U235 is necessary. For breeder reactors and enrichment of about 10-12% is required to reach criticality (1). To make the most of the puny uranium resources we have on the Moon we will need to use breeder reactors to convert the 99.3% U238 to fissionable Plutonium. We will need chemical processing to recycle nuclear waste to extract plutonium and other fissionable elements formed and extract plutonium from the U238 jackets surrounding our breeder reactor cores. We will need special alloying elements upported from Earth for the steel or titanium alloys we use to build the reactors. We will need hafnium for control rods. We might extract hafnium and other rare earth elements from KREEP by sulfuric acid leaching, carbochlorination and fused salt electrolysis as is done with REE laden monazite sands on Earth. Breeder reactors are cooled with liquid sodium because it doesn't moderate neutrons down as much as water and fast neutrons are needed to transmute U238 to Pu 239. Sodium can be mined on the Moon. We might get sodium from magma electrolysis or during aluminum extraction from anorthosite by the melt-quench-leach-carbochlorination and aluminum subchloride electrolysis process. We will need turbines and alloying ingredients for turbine blade steels. Cobalt is often used for turbine blades and cobalt can be produced on the Moon. We will also need Sun shielded space radiators covering many acres of land to get rid of waste heat from turbines. It seems that on the lifeless, waterless, airless Moon we can dispense with containment buildings and thick concrete bases; however, should there be a complete melt down that even ruptures the core pressure vessel radioactive vapors could diffuse all over the Moon and spacesuited workers who contact those vapors would need a full washdown before going inside habitat. We might be wise to bulid a heavy containment building and thick stone or concrete base for reactors. Thorium 232 from KREEP is not fissionable but it can be converted in a heavy water reactor like the CANDU reactor to fissionable U233. About one out of 6000 H2O molecules on Earth are D2O. Let's hope the polar ice contains some D2O too. Even though we might obtain uranium and thorium from KREEP or even the planet Mars, processing those nuclear fuels and building reactors with turbogenerators or thermoelectric or thermionic elements with no moving parts will be a difficult, time consuming, expensive and complex task. Lunar nuclear power might come from upported RTGs and even small reactors in the early decades of lunar exploration but nuclear powerplants supplying hundreds of megawatts will require many decades of industrial development on the Moon. 1) "Megawatts and Megatons" by Richard L. Garwin and Georges Charpak |
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