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Post by David B. Benson on Aug 7, 2013 13:28:07 GMT 9.5
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Post by grlcowan on Aug 7, 2013 20:46:26 GMT 9.5
According to the USGS, At 100 bar, that 52 cubic kilometres would be reduced to ~0.52 km^3. Nuclear power is important enough, I think, to deserve at least one percent of it, 0.0052 km^3. If it were equally divided between, let's say, 10000 reactors, each one's allotment of 100-bar helium would be 520 m^3, a sphere of diameter 10 m. Unlike carbon dioxide, it doesn't become radioactive at all, and as fuel and helium temperature rise they never become mutually chemically active at all. You "suspect"?
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Post by cyrilr on Aug 7, 2013 23:37:46 GMT 9.5
Even if you can get enough helium, theres a big cost to using it. Bearings and seals are very difficult (expensive) because of the tight tolerances that must be held. Helium is very light so difficult to engineer for a turbine.
CO2 has negligible neutron activation. If you wish to pick nits, helium produces some tritium and contaminants in the helium produce activated products as well. But neither CO2 nor helium have troublesome activation in the sense that it would require costly engineering or added waste disposal.
Unfortunately, graphite reacts with CO2 to an equilibrium with lots of CO. This degrades the graphite fuel.
Silicon carbide coating in stead of graphite could solve the problem, and has many other mechanical advantages (silicon carbide is strong and hard so makes for better coating than graphite which is soft and weak).
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Post by David B. Benson on Aug 8, 2013 9:29:02 GMT 9.5
[cyrilr/b] --- Why is there any coating at all?
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Post by RogerClifton on Aug 8, 2013 11:03:32 GMT 9.5
One of advantage of helium is its conductivity, sqrt(44/4)~3.3x CO2. Apart from superior cooling of the core, it also allows for a smaller heat exchanger after the working gas leaves the turbine.
The website (and wiki entry) for EM2 seems to be unaware of the need for that cooling stage. Other serious questions seem to be fumbled too. The start-up description seems to promise no-reprocessing initially, then goes on to promise reprocessing (how?) for its own used fuel. Beryllium is a moderator and neutron multiplier, but I would have thought a reflector should be steel. If the proposal really is serious, they had better tidy up their website.
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Post by cyrilr on Aug 8, 2013 16:28:37 GMT 9.5
[cyrilr/b] --- Why is there any coating at all? To prevent the fuel particles from getting dislodged. This is especially important for pebble beds with moving fuel. But even with solid fuel and offline refuelling like GT-MHR, there's always a possibility of thermal stress cracking, water ingress matrix oxidation, or other fuel matrix failure. If that happens you want the fuel particles to stay in the damaged fuel so that you can replace the fuel element and start up the reactor again.
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Post by cyrilr on Aug 8, 2013 16:38:03 GMT 9.5
One of advantage of helium is its conductivity, sqrt(44/4)~3.3x CO2. Apart from superior cooling of the core, it also allows for a smaller heat exchanger after the working gas leaves the turbine. The website (and wiki entry) for EM2 seems to be unaware of the need for that cooling stage. Other serious questions seem to be fumbled too. The start-up description seems to promise no-reprocessing initially, then goes on to promise reprocessing (how?) for its own used fuel. Beryllium is a moderator and neutron multiplier, but I would have thought a reflector should be steel. If the proposal really is serious, they had better tidy up their website. Actually the conductivity of helium is not much use. Its density is too low. Supercritical CO2 is very dense, that more than makes up for the lower thermal conductivity. S-CO2 also has excellent density change that gives better reactivity feedback and better natural circulation. Natural circulation drops the pants off of conduction. Conduction as a heat removal path is quite a poor one. It is one of the reasons why helium cooled reactors have low power density. Compare to say a fluoride salt cooled version: same TRISO fuel, higher power density, much lower peak accident temperatures. The S-CO2 turbomachinery and heat exchangers are far more compact than helium versions. It's liquid-like density versus flimsy gas. Reflectors can be most materials. Beryllium is good because it scatters well and moderates well. It also has a neutron production from (n,2n) reaction. It makes for very high neutron efficiency. Steel is a non-moderating reflector. This has it's uses, in some cases reflectors must be non-moderating to prevent thermalizing the spectrum (as in fast reactors) and in other cases the neutron losses can actually increase with a moderating reflector (because it increases the power and thus neutron production near the edge of the reactor where neutrons are more likely to be lost).
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Post by David B. Benson on Aug 9, 2013 10:54:18 GMT 9.5
RogerClifton --- The reprocessing is pyroprocessing just as for the S-PRISM.
cyrilr --- Thank you.
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Post by edireland on Aug 10, 2013 6:53:08 GMT 9.5
AGR overcame the carbon monoxide reaction problem by using a re-entrant system to cool th graphite to the reactor inlet temperature rather than the outlet temperature.
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Post by David B. Benson on Aug 10, 2013 11:33:48 GMT 9.5
I had dinner with Dr. George Hinman who used to be employed by General Atomics. He states that the General Atomics gas reactor concept is about 50 years old. It is a graphite moderated thermal reactor design using actinide oxides as the energy source. So the reprocessing is not pyroprocessing but I suppose then must be a MOX plant.
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Post by cyrilr on Aug 10, 2013 20:23:31 GMT 9.5
AGR overcame the carbon monoxide reaction problem by using a re-entrant system to cool th graphite to the reactor inlet temperature rather than the outlet temperature. This also improves graphite lifetime, though you have to be careful not to cool it down too much: you get the Wigner energy buildup, which was the initiator (though not the root cause, which was insufficient design for annealing run temperatures)for the Windscale accident in the UK. I still prefer coating with silicon carbide. Above the Wigner energy temperature, reaction of graphite with CO2 is slow but not zero. If graphite reacts with CO2 to form CO, then one might consider the use of carbon monoxide as a coolant: considerably less oxidising than carbon dioxide, too bad about the relatively high toxicity. Does it have a supercritical transition at reasonable temperatures?
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Post by grlcowan on Aug 10, 2013 22:08:34 GMT 9.5
If graphite reacts with CO2 to form CO, then one might consider the use of carbon monoxide as a coolant: considerably less oxidising than carbon dioxide, too bad about the relatively high toxicity. Does it have a supercritical transition at reasonable temperatures? It is isoelectronic with N2, and so must have about the same critical temperature, around 140 K. Below the temperatures where graphite and CO2 react, the reverse reaction is favoured, i.e. CO is favoured to disproportionate.
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Post by Roger Clifton on Aug 19, 2013 12:59:42 GMT 9.5
In a letter to Physics Today we read a little more of EM2. ' ... by better harnessing the reaction energy through a high-heat-capacity medium and state-of-the-art turbine generators. ... The EM2 is a compact fast reactor about 12 meters high, with 265 megawatts electric (MWe) output. The immediate challenge for the reactor is proving out the fuel element, which consists of novel ceramic cladding and fuel that enable the reactor to operate at high temperatures and high power densities. The company is also developing and testing a compact high-speed turbine generator that can achieve efficiencies of more than 50%. ' It is unclear what the GA author means by "high-heat-capacity medium". Sodium? Or does he mean high heat transfer capability, as in helium? He says the selling point is "small", at least for fast construction. And then asserts that high thermal efficiency is necessary for cost efficiency. But if he intends a helium turbine and cooling cycle, it could be risky innovation.
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Post by cyrilr on Aug 19, 2013 18:10:16 GMT 9.5
In a letter to Physics Today we read a little more of EM2. ' ... by better harnessing the reaction energy through a high-heat-capacity medium and state-of-the-art turbine generators. ... The EM2 is a compact fast reactor about 12 meters high, with 265 megawatts electric (MWe) output. The immediate challenge for the reactor is proving out the fuel element, which consists of novel ceramic cladding and fuel that enable the reactor to operate at high temperatures and high power densities. The company is also developing and testing a compact high-speed turbine generator that can achieve efficiencies of more than 50%. ' It is unclear what the GA author means by "high-heat-capacity medium". Sodium? Or does he mean high heat transfer capability, as in helium? He says the selling point is "small", at least for fast construction. And then asserts that high thermal efficiency is necessary for cost efficiency. But if he intends a helium turbine and cooling cycle, it could be risky innovation. That's silly. Helium has very poor heat capacity, and fast reactors lack the greatest heat capacity in a reactor core - the moderator. As for better harnessing the reactions with high heat capacity media, that's completely clueless sales talk. Compact means it has no great heat capacity. Can't have cake and eat it. I would like to see the transient performance (peak temperatures) during a loss of all forced circulation. I fear the worst.
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Post by cyrilr on Aug 20, 2013 3:11:20 GMT 9.5
By the way, Roger Clifton, where does this square root of the relative molar weight difference come from? Helium has about 10x the thermal conductivity of CO2. Not 3.3x. Roughly 140 mW/K versus 14 mW/K. According to this source, hyperphysics.phy-astr.gsu.edu/hbase/tables/thrcn.htmlH2 has 25% more thermal conductivity than helium. Its molecular weight is 2, helium is 4. So square root of (4-2) = 1.42 = 42% more. That's not right either. It's neither 2x nor 1.42x. It is well known that lighter molecules tend to have higher thermal conductivity. An exact formula weight based prediction of thermal conductivity would be very useful to me. I was under the impression this is not possible as many other properties than molecular weight affect thermal conductivity...
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Post by Roger Clifton on Aug 20, 2013 14:54:07 GMT 9.5
cyrilr asked, where did the square root of the molecular weight come from? I had recalled a distant memory from undergraduate physics. In Reif, "Fundamentals of statistical and thermal physics", page 481, a ballpark formula had been derived from first principles for the thermal conductivity of a dilute gas: K = 2/3*C/sigma0*sqrt(kT/M/pi) which provided a justification for me saying that K varies inversely as the sqrt molecular weight (M). But I was wrong in implying that was the only difference between the two gases. Now that I check my old text, it goes on to say that C is the specific heat per molecule, proportional to the degrees of freedom to which kinetic energy can be transferred. Although (monatomic) helium remains with 3 degrees of freedom at all temperatures, polyatomic CO2 has 3 only at cryogenic temperatures, 5 above that, and more than 7 in the furnace temperatures of our interest, when its internal oscillations contribute to the specific heat. Say, an up-factor of 2.5 for CO2. On top of that, sigma0 is the collision cross-section area, where helium is spherical and has only a weak van der Waals field, while CO2 is long and polarised with negative ends. Say, a down-factor of 5 for CO2. The sqrt of the molecular weights 4 vs 44 gives a down-factor for CO2 of 3.3. In total, the formula predicts He to be 6.6x more conductive than CO2. The measured figures you quoted (He 140.5, CO2 14.4 mW/K/m) have a ratio of 10. So the formula isn't as exact as you hoped for or as I assumed. But it does at least predict that K rises with sqrt(T), which may be of some use to you.
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Post by cyrilr on Aug 20, 2013 17:22:53 GMT 9.5
Ok Roger, so it's as I suspected, it can't be estimated accurately like that. Here's something interesting: www.pirika.com/ENG/ChemEng/SCFThc.htmlThe thermal properties of supercritical CO2 are very favorable around the critical point; a great thermal conductivity of over 250 mW/m.K is available at the critical point. Good for a cooler. Even at higher temperatures and supercritical pressures, it looks like you can easily get 3x the value of helium. So it appears that the premise of higher conductivity in favor of helium over the S-CO2 cycle is not correct.
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Post by cyrilr on Aug 20, 2013 17:30:24 GMT 9.5
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Post by Roger Clifton on Aug 21, 2013 17:06:13 GMT 9.5
cyrilr (graph for He, showing increase of conductivity with temperature) Yes, that is the predicted shape, log(sqrt (T)), so I think you'll find that would be true of CO2 as well. The behaviour of supercritical fluids is certainly fascinating! However they are only "supercritical" within a very narrow band of temperature and pressure. Otherwise they are simply liquid or gas with relatively boring behaviours. CO2 is supercritical in the immediate vicinity of 7.38 MPa (72.8 atm). I think when steam engineers refer to supercritical steam, they are only referring to it being above the critical temperature, but with the wide range of pressure needed for it to work a turbine. PS: as Cyrilr corrects me below - a fluid is called supercritical if it is anywhere above both its critical pressure and critical temperature. The most interesting behaviour lies along and around the extrapolation of the liquid/gas line on the phase diagram.
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Post by cyrilr on Aug 21, 2013 19:58:34 GMT 9.5
That's what "super" stands for - above the critical point. In case of supercritical water turbines, usually only the first high pressure stage is actually supercritical. That is, the exit from the first turbine stage is subcritical and the downstream turbine stages are just "ordinary" steam turbines.
Supercritical transition is also attractive for heat capacity reasons. In the case of water, the transition form pressurized subcritical water to supercritical water sucks up a much greater amount of heat than boiling water at say 300 degree Celsius, and the resulting supercritical water takes up a fraction of the space required for steam. This, combined with very low viscosity, makes it the most efficient coolant available. Coolant average specific volumetric enthalpies of over 15000 J/l/K are achievable. With helium, even at similarly high pressures, you'll never even get 75 J/l/K. 200x lower. How can helium compete with supercritical water cooled coal or nuclear plants?
CO2 is pretty dense even well above the critical point. A significant fraction of water at room temperature, in fact. Not anywhere near as good as supercritical water though, but it could end up a simpler cheaper and more compact power cycle (because of heat rejection under pressure meaning compact equipment).
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