10 ms·
To the best of my understanding, one of the main reason that MSRs have not gone to market is that the salts are so corrosive that containment over the long term
by ortusdux 6y ago
To the best of my understanding, one of the main reason that MSRs have not gone to market is that the salts are so corrosive that containment over the long term is not currently possible. Note the questions about materials, alloys, and flow sensors in the video.
ORNL had fully functional reactors in the 60's, but those reactors were only safe to operate for a few years, and they were at lower temperatures than what we are targeting today. Liquid fluoride thorium salts at 700+ C will readily dissolve chromium, which makes working with stainless very difficult. Other common alloying agents are susceptible to radiation (Co & Ni transmute when irradiated) which further shortens the lifespan. There is also the issue of tritium, which can permeate stainless steels, cause embrittlement, and escape into the environment.
ORNL developed Hastelloy N to help address these issues, and there is an effort to certify other structural steels to for use in reactors (316H, 800H, inco 617). None of the studies that I have seen indicate that any of these metals will survive for more that 5 years or so.
- jbverschoor 6y agoWhy not use a stone type of material. Like a furnace?
- fennecfoxen 6y agoFurnaces use stone because it is not very sensitive to heat. Stone generally remains sensitive to acid.
- echelon 6y agoPlastics would be great to withstand the acid, but they're sensitive to heat.
- mywittyname 6y agoUltra-high-temperature polymers might get there. There are polymers used in specialized industrial applications that can handle ~500° C.
- ortusdux 6y agoThere are a few reasons, but the biggest one is that ceramics and composites also react with the molten salts. One study from 2015 (1) showed that even testing the corrosion resistance of metals was difficult because the type of crucible used had a large effect on the results. These systems need pumps at a minimum. Sensors (temp, ph, flow rate, chemical composition, etc.) are also very important for prolonged, low maintenance use. These cannot be made of ceramics (that I know of) The container will also need to safely hold a dense pool of molten salt. The linked $88k test reactor has a capacity of 350 liters, which at 1.94 g/cc is ~680kg/1500lbs of Li2BeF4. Another important reason is that heat transfer is the whole point of the reactor, and most metal alternatives have poor thermal properties. 1. https://www.researchgate.net/publication/282433901_Impact_of_Corrosion_Test_Container_Material_in_Molten_Fluorides https://www.researchgate.net/publication/282433901_Impact_of...
- dukeofdoom 6y agoI know on boats they use a piece of sacrificial metal (Anodes) that corrodes more readily than what the boat is made of. This prolongs the life of the boat. Maybe they can use something like that in this situation too.
- corty 6y agono, because you would have to let the molten salt react with the sacrificial metal, introducing a lot of complicated impurities into your salt bath that may influence your reactor in numerous ways.
- 08-15 6y ago...unless the sacrificial material is a component of the salt anyway, like Beryllium. ORNL proposed to have a Beryllium rod in contact with the fuel salt. It isn't needed to prevent corrosion of Hastelloy N in clean fuel salt, but it was intended to scavenge the fission product Tellurium, which was found to otherwise cause corrosion.
- petre 6y ago
- HexagonalKitten 6y agoGranite, for instance, has a 1200c melting point which sounds good. Its large grainy structure might cause porosity issues. And it's main constituents are silicon and aluminum, both of which do transmute under neutron bombardment. (An expert could say if they transmute to anything you really don't want around, and how long it'll be there.) I think ceramics are an answer to the porosity and similar issues. Consistent structure, no inclusions or other weaknesses. But they would face the same transmutation and induced radioactivity problems as stone of the same material.
- petre 6y agoCeramics used in water boilers crack. You don't want that in a nuclear reactor.
- 08-15 6y agoYou mean ceramics? There are so many to chose from, and most are unusable for one reason or another. The leading candidate being investigated is carborundum (silicon carbide). It doesn't corrode in molten salt, it low appetite for neutrons, is reasonably easy to manufacture, conducts heat fairly well. It's even mentioned in the video.
- tomp 6y agoWhy use stainless steel? I thought "stainless" is mainly useful because it doesn't rust (is there water & oxygen in a MSR?). Also, couldn't the steel have another layer inside (e.g. just iron or something else non-reactive) so that it never comes into contact with the salts?
- flyingfences 6y ago"Rust" is just oxidation and there are a lot of substances that will cause oxidation.
- deleted 6y ago[deleted]
- mywittyname 6y agoTo add to this, temperature plays a really critical role in oxidation/corrosion of stainless steel (and other metals). The scale that forms at high temperatures has a different composition than what develops at lower temperatures. In certain environment, the scale remains on the surface, inhibiting further oxidation, but in other environments, the scale is continuously removed, eventually destroying it.
- ortusdux 6y agoFor many of the same reasons that Spacex is using it for their starship. It's cheap and readily available, relatively easy to work with, and very well studied. There also really are not that many materials that have passed certification for use in reactors. I believe that "BPVC Section III-Rules for Construction of Nuclear Facility Components-Division 5-High Temperature Reactors" only covers half a dozen stainless steels. As for cladding, the DOE's 2021 budget includes money for testing novel claddings. They are also looking for robust redox reference electrodes and additive manufacturing methods.
- Sharlin 6y agoIron is vastly more reactive than stainless steel, that's exactly why we have stainless steel.
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- sergio102305 6y agoI think you have to have oxigen present for it to be corrosive. Can't they vacuum it?
- ortusdux 6y ago"Corrosion is a natural process that converts a refined metal into a more chemically stable form". Oxidation is the most well known form of corrosion, but there are many other corrosive reactions out there. https://en.wikipedia.org/wiki/Sulfide#Corrosion_induced_by_sulfide https://en.wikipedia.org/wiki/Sulfide#Corrosion_induced_by_s...
- sandworm101 6y agoFrom an engineering standpoint any chemical reaction that changes the properties of a metal in an unwanted way will be called corrosion. It is something to prevent and/or monitor that will inevitably shorten the lifespan of the metal part. (Non-metals like fiberglass can also "corrode".)
- ta988 6y agoYou need an oxydizer whatever it is. Oxygen is only one oxidizer. Chlorine, fluorine and the other halogens are oxidizers too.
- andreareina 6y agoIIRC fluorine is/can be a more potent oxidiser than oxygen.
- 08-15 6y agoFluoride (with a 'd') is not fluorine (with a 'n'). The difference is as big as that between sodium and sodium chloride. You can eat one, but not the other. The most powerful oxidizer in MSR fuel salt is the U(IV) ion.
- christkv 6y agoWould diamond do better?
- hajile 6y agoDiamond will burn like super-expensive coal. Probably not ideal for those temperatures.
- kurthr 6y agoUmm, that's doubtful. This isn't oxygen that generates a gas on reaction. You would need to look at the actual conditions at moderate temperature (800-1000F is only ~400-500C) for metal fluorides in contact with diamond films and carbon solubility. Even fluorocarbons tend to actually be moderately stable in these temperature conditions. Diamondlike film coatings should last longer than carbon or graphite crucibles (since those materials are far softer). Those have been tested, although it would seem that high nickel (rather than floridizing Cr) steel is probably the prefered material. SiC also works and they discuss this, but there are concerns about Tritium absorption for the larger crystal structure and porous sintered components. For some comments and measurements, see this thesis on metal fluoride corrosion control for reactors: http://fhr.nuc.berkeley.edu/wp-content/uploads/2014/10/06-002_B.Laurenty-MS-report.pdf http://fhr.nuc.berkeley.edu/wp-content/uploads/2014/10/06-00... research on SiC also continues: https://www.osti.gov/servlets/purl/1531263 https://www.osti.gov/servlets/purl/1531263
- hajile 6y ago> To the best of my understanding, one of the main reason that MSRs have not gone to market is that the salts are so corrosive that containment over the long term is not currently possible. I've wondered why "long-term" is such a huge criteria. One of the biggest nuclear issues today is using them way past their lifespan because they are so expensive to build. Let's assume molten-salt is cheaper to build and less dangerous (from a radioactivity point of view). Why not make expendable designs? Make them smaller so the building requirements are easier. Make them easy to remove and replace. Ease up on the restrictions a bit so they can be replaced every 5-10 years instead of decades. Long-term mass production has tons of advantages. Costs per unit decrease. Defects per unit decrease (this is part of cost per unit usually, but not if a defect gets through inspection). Recycling used units should radically decrease material costs when getting close to peak reactor count (chain of custody paperwork from mine to installation in the nuclear plant is why an otherwise $0.15 screw winds up costing $50-100). Constant employment will build experience over many years and further decrease errors. Once the site design is finalized and enforced, the reactor design can be gradually improved and given the short lifespan, efficiency will increase a minimum of every decade. Likewise, design mistakes (once caught) will only be around a decade at most instead of a half-century like we see today.
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- escape_goat 6y agoKibitzing outside of any expertise that I have, here, but thorium reactors are generally regarded as a significant nuclear weapons proliferation risk. This has been an inhibiting factor with regards to investment in research. This design — cheap and transportable in a standard shipping container — significantly ups the ante on that risk, to put it mildly. Deployed singly, small, low-power, short-lifespan reactors would increase the burden of maintaining regulatory control over those reactors — destined, by design, for catastrophic failure unless they are retired within a safety period — which in turn would the risk of a nuclear contamination incident. This could be mitigated by operating the reactors in banks at a containment/maintenance site. I'm not sure what parts of the reactor could be usefully recycled. All the metals would be embrittled by neutron bombardment.
- baybal2 6y agoYes, exactly. Making a working reactor is as easy as disoling a critical mass of uranyl nitrate in a bucket of vodka. Making a lasting reactor that will work non-stop for years is not so at all. This is why I am sceptical about this. Solid fuel is by far more troble free, even if it doesn't last that long.
- ortusdux 6y agoMakes me think of Oklo: https://en.wikipedia.org/wiki/Natural_nuclear_fission_reactor https://en.wikipedia.org/wiki/Natural_nuclear_fission_reacto...
- yholio 6y agoA similar issue exists with metal coolant reactors, which conceptually would be very simple, and would use proven technology, fuel rods and cladding and have similar or better operating characteristics to molten salt: low internal pressure, high temperature, inherent safety (fuel is physically separated from colant and never touches internal piping, pumps etc.) It's very hard to formulate a metal coolant that has low melting point (pure lead is already too high), does not react with air or water (Sodium cooled reactors are notorious), has low cross-section (Hg fails), does not activate under the neutron flux to long lived chains (Bismuth and potassium in eutectic alloys) and does not dissolve the steel structure of the reactor (Tin). If this problem could be fixed and have a safe, inert metal coolant with good thermal characteristics, fast breeders would become common place.
- BlueTemplar 6y agoAren't they already used in Russian submarines?
- Tuna-Fish 6y agoThey are, but this is because the Soviet designers thought that the improvement in power density were worth the serious disadvantage of having coolant that freezes well above room temperature.
- p_l 6y agoThey were used and the design was modified into small portable nuclear reactor, the SVBR-100, a 100MW(e) reactor that can be sold containerised and delivered on a rail flatcar. The problem is that a) the project owners have issues getting money to finish certification b) we don't mine enough bismuth. The second problem is pretty major issue, because as far as I know, we don't actually mine bismuth - all the bismuth available worldwide is from processing of tailings in other mines, and isn't under any kind of high production rate. Making just a dozen or so reactors for submarines was easy. Making mass-production line would actually mean a noticeable drain on world-wide supply of bismuth! Then there are political problems involving buying russian tech.. :(
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- carapace 6y agoTo contain molten salt use solid salt.
- 08-15 6y agoThe corrosion problem has been blown way out of corrosion, and I suspect, you're confusing it with the graphite swelling problem. Pure nickel won't corrode in fuel salt, so to a first approximation, Hastelloy N works fine and lasts forever. Chromium is dissolved superficially, but this process is self-limiting. Two more corrosion mechanism were identified: some nickel transmutes and releases helium, which migrates to grain boundaries, and the fission product tellurium reacts with the metal. Solutions for both problems were identified by 1974, but never tested, because the MSR program was terminated. What is more likely to limit core life, is the graphite. Graphite, when irradiated, first shrinks, then swells. MSBR had a very complicated core made of precisely manufactured graphite pipes. These would crack and deform, and then they need to be replaced. Everyone who is considering a graphite moderated core seems to plan for a long life of the pot and a four year life of the graphite inside it. Copenhagen Atomics plans to use heavy water for moderation, so there is no graphite to worry about.
- legulere 6y agoIs there a reason except price that titanium isn’t used? It has a higher melting point than steel and resists acids.
- achalhp 6y agoYou're correct. The vessel & primary heat exchanger of a MSR which are in contact with liquid-fuel is equivalent to fuel assemblies of many solid-fuel reactors. In solid-fuel reactors the clad which is designed to contain fuel doesn't last long. Similarly, in MSRs the materials in contact with fuel doesn't last long. So, we need to dispose the vessel and primary heat exchangers similar to Zr-clad million dollar fuel assemblies.(PWR/BWR reload costs $60-100 million per 12-18 months.) Material wise Zr clad is way more expensive than stainless 3XX & graphite combined. Even if Ni-alloy is used in disposable MSRs Ni-alloy+graphite will be slightly cheaper. (Base metal costs: Zr metal: ~$22,700/ton Ni metal: ~$13,100/ton)