6 ms·

You are correct about the corrosion issue. I've done some work developing molten salt resistant claddings. The main culprit is chromium leaching, which de-all

You are correct about the corrosion issue. I've done some work developing molten salt resistant claddings. The main culprit is chromium leaching, which de-alloys most of the metals approved for reactor design. The leeching happens at the grain boundaries, so you will hear 'intergranular attack' as a research focus. A close second problem is the radiation itself. Elements in both the containment vessel and salt transmute. One study I read estimated that pure tungsten (a viable salt resistant material) would transmute to rhenium at a rate of 1% a year. The radiation also causes void-swelling in both the metals and pure graphite. The standard way to test material's resistance to molten salt is to put a coupon in a crucible full of salt for a few hundred hours. A paper from 2015 showed that the material the testing crucible is made of greatly effects the rate of chromium leeching. They found that both graphite and nickel act as chromium sinks. Many designs call for graphite or nickel parts to be used alongside chromium containing steels. This reactor appears to be stainless steel with graphite moderators. Another paper strongly suggested that radiation induced void-swelling can squeeze together the grain boundaries, greatly slowing down intergranular attack. Very little corrosion testing has been done under exposure to radiation as it is logistically difficult. Basically, the next best step is test reactors. You can only get so far testing things in isolation.

HPsquared 4y ago
Does it even need to be a metal? (Since the pressure is so low strength requirements are lower)... How about ceramic or glass (or quartz), or something else non-metal?
- kenned3 4y ago
  non-metals have their own problems and glass tends to have some really weird properties. I would think a major one would be their failure mode. Metals flex and expand before they eventually fail. Glass/ceramic is fine until suddenly it isn't and has a total failure. Think of a window being hit. If it were metal it would probably deform but if it is glass it shatters. Next would be joining them on-site. If needed, metal piping can be bent and welded in-place. what do you do with a glass pipe that needs a join? what do you do if there is a small variation in the plans and the pipe needs an adjustment? I think there are a host of reasons why glass is not used for pipes.
  - Turbots 4y ago
    what about glass pipes encased in said metals :-)
    - deleted 4y ago
      [deleted]
    - HPsquared 4y ago
      Glass-lined vessels and pipes are already used in the chemical industry so it's a somewhat proven technology. Not sure if it's suited to the chemistry and temperatures (and radiation) of a molten salt reactor, but it seems like an interesting technology.
  - systemvoltage 4y ago
    Composites? Such as the stuff used in Aviation?
    - deleted 4y ago
      [deleted]
    - PaulHoule 4y ago
      Most of those contain plastics which are usually not good for high temperatures but also have long chain molecules which get broken by neutrons and other particles and cannot heal defects the way metals can.
  - mulmen 4y ago
    Glass is already basically welded, or maybe it's more like brazing. But an oxyacetylene torch can be used to work with glass just as well as to cut or weld metal. Metals also have weird properties. Like tempering and hardening based on temperature. In an industrial setting you need expert welders with deep knowledge of the materials or a weld is going to fail and ruin your day. So it doesn't seem like a huge leap to me actually, assuming ceramics or glass actually have desirable properties.
    - 4y ago
- ortusdux 4y ago
  It doesn't, but any material used needs to withstand 4 extremes - high temperatures (650-850 C), corrosion (dissolves uranium), very high neutron flux, and US govt. regulation. Graphite is the current favorite non-metal option. It is already widely used in traditional reactors, so it is approved, and its interaction with radiation is well understood. On the other hand, its interaction with radiation and molten salt is not as well understood, but hopefully this test reactor and others like it will solve that. One of my favorite solutions seals the reactor in with a crane and 8 graphite containment vessels. The best estimate of the lifespan of structural graphite in this environment is ~7 years, so the plan would be to monitor the system and move the reactor to a new vessel as needed. The goal is 50 year life-span for these reactors. Anyway, there is research into alternate materials, but to really test them you need to expose them to both molten salts and radiation. And if they pass, you need to get government approval. The last material to get govt approval was Hastelloy N, and I heard the process was a slog.
  - chris_wot 4y ago
    Ah yes, the most extreme thing one withstands is U.S. regulation.
    - Retric 4y ago
      Nothing requires this research to to be carried out in the US specifically beyond funding being available here. It’s really the inherent difficulties which is holding back progress.
      - panick21_ 4y ago
        My understanding is that you are not allowed to build a research reactor that is bigger then a university reactor but isn't a full scale energy producing reactor. My understanding is that for such a reactor you would need the full operating license just as a grid connected PWR. And since in the US its essentially impossible to get a license for anything but a PWR, that isn't rally possible. Technology independent regulatory framework is one of the main reasons Canada has so many reactor startup, even those that started in other countries. https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-licensing-vendor-design-review/ https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-li...
andy_ppp 4y ago
What about using molten lead or something that can act like a lubricant/insulation at that temperature away from the corrosion in the same way airflow is used over jet turbine blades to stop them melting? It there any high temperature liquid that is molten salt phobic like oil and water?
jxf 4y ago
> The standard way to test material's resistance to molten salt is to put a coupon in a crucible full of salt for a few hundred hours. Q: What is a coupon in this context?
- iAm25626 4y ago
  Great Q: maybe? https://www.researchgate.net/figure/a-MAR-M247Nb-coupon-b-Al-2-O-3-crucible-c-coupon-crucible-used-for-the-isothermal_fig1_270394732 https://www.researchgate.net/figure/a-MAR-M247Nb-coupon-b-Al...
- djcapelis 4y ago
  A coupon is an informal term for a small sample of material, most often metal. It’s perhaps most commonly used in welding where practice welds for people first training to weld are done on pieces of metal often referred to as coupons. :) It is usually an artifact of materials like metal which are hard to cut, and can’t efficiently just be cut as needed during a task, but are prepared and pre-cut into small pieces you can grab easily. Each of those pieces is called a coupon. I used to store mine in a Chinese takeout container box.
  - greyhair 4y ago
    In the mid 1970s, in my welding class, the school was located down the street from a small manufacturer that formed various metal alloys. They would provide the school with bins of scrap from their manufacturing free of charge. Those were our practice coupons in class. We had to be careful to return the welded scraps back to the proper bin so the manufacturer could recycle them. Our class was just a very small part of their recycling flow. It was a great relationship, we needed practice materials, and they hired welders.
- ortusdux 4y ago
  It's used 2 ways in metallurgy. Mainly it is a term for test pieces cut from a batch of metal and used for proof testing. Tensile, elongation, fatigue, weldability, and many other tests require destructive testing coupons. When you take welding certification tests, you weld two coupons together, and then submit your coupon for testing. It is also a term for strips of metal that you mount in a corrosive environment as a way to monitor corrosion of other susceptible components. So if you were worried about your bronze bushings, you would buy some matching bronze coupons, put them in an area of similar flow, and measure their thickness periodically. https://www.alspi.com/coupons.htm https://www.alspi.com/coupons.htm https://en.wikipedia.org/wiki/Fatigue_testing#Coupon_tests https://en.wikipedia.org/wiki/Fatigue_testing#Coupon_tests
pfdietz 4y ago
The corrosion issue is part of what drove Moltex to their rather interesting design (sterile coolant salt is a fluoride; fuel salt in tubes is chloride; this is a fast reactor.) The absence of uranium in the fluoride allows it to be operated at a redox potential where chromium does not dissolve.
- panick21_ 4y ago
  Yes, one of the reason I like that reactor design. They basically put something in the fluoride that is basically designed to corrode so that the actual reactor vessel doesn't. If I understand correctly. Their design doesn't require the 7 year swap cycle most MSR do.
  - pfdietz 4y ago
    Right! They've got the same thing going that ordinary LWRs do: the neutrons lose their energy in a surround liquid rather than in a solid moderator or solid structural materials other than the fuel rods (which are designed to be replaced often anyway). The design even adds some hafnium to the coolant salt (substituting for zirconium, which is the sacrificial metal you refer to there); hafnium serves to shield the reactor walls from thermalized neutrons. One additional advantage of separating the fuel salt from the coolant salt is that the coolant salt volume can be increased as desired, making the thermal inertia of the reactor as large as one likes independent of reactor power or size of the fuel load.
    - panick21_ 4y ago
      > One additional advantage The problem with that is that you would likely require a different design and go threw full licensing. So in practice this will likely not be used.
      - pfdietz 4y ago
        Presumably it would happen at design time and a large, high thermal inertia design would be the one licensed.

HPsquared 4y ago

Does it even need to be a metal? (Since the pressure is so low strength requirements are lower)... How about ceramic or glass (or quartz), or something else non-metal?

kenned3 4y ago
non-metals have their own problems and glass tends to have some really weird properties. I would think a major one would be their failure mode. Metals flex and expand before they eventually fail. Glass/ceramic is fine until suddenly it isn't and has a total failure. Think of a window being hit. If it were metal it would probably deform but if it is glass it shatters. Next would be joining them on-site. If needed, metal piping can be bent and welded in-place. what do you do with a glass pipe that needs a join? what do you do if there is a small variation in the plans and the pipe needs an adjustment? I think there are a host of reasons why glass is not used for pipes.
- Turbots 4y ago
  what about glass pipes encased in said metals :-)
  - deleted 4y ago
    [deleted]
  - HPsquared 4y ago
    Glass-lined vessels and pipes are already used in the chemical industry so it's a somewhat proven technology. Not sure if it's suited to the chemistry and temperatures (and radiation) of a molten salt reactor, but it seems like an interesting technology.
- systemvoltage 4y ago
  Composites? Such as the stuff used in Aviation?
  - deleted 4y ago
    [deleted]
  - PaulHoule 4y ago
    Most of those contain plastics which are usually not good for high temperatures but also have long chain molecules which get broken by neutrons and other particles and cannot heal defects the way metals can.
- mulmen 4y ago
  Glass is already basically welded, or maybe it's more like brazing. But an oxyacetylene torch can be used to work with glass just as well as to cut or weld metal. Metals also have weird properties. Like tempering and hardening based on temperature. In an industrial setting you need expert welders with deep knowledge of the materials or a weld is going to fail and ruin your day. So it doesn't seem like a huge leap to me actually, assuming ceramics or glass actually have desirable properties.
  - 4y ago
ortusdux 4y ago
It doesn't, but any material used needs to withstand 4 extremes - high temperatures (650-850 C), corrosion (dissolves uranium), very high neutron flux, and US govt. regulation. Graphite is the current favorite non-metal option. It is already widely used in traditional reactors, so it is approved, and its interaction with radiation is well understood. On the other hand, its interaction with radiation and molten salt is not as well understood, but hopefully this test reactor and others like it will solve that. One of my favorite solutions seals the reactor in with a crane and 8 graphite containment vessels. The best estimate of the lifespan of structural graphite in this environment is ~7 years, so the plan would be to monitor the system and move the reactor to a new vessel as needed. The goal is 50 year life-span for these reactors. Anyway, there is research into alternate materials, but to really test them you need to expose them to both molten salts and radiation. And if they pass, you need to get government approval. The last material to get govt approval was Hastelloy N, and I heard the process was a slog.
- chris_wot 4y ago
  Ah yes, the most extreme thing one withstands is U.S. regulation.
  - Retric 4y ago
    Nothing requires this research to to be carried out in the US specifically beyond funding being available here. It’s really the inherent difficulties which is holding back progress.
    - panick21_ 4y ago
      My understanding is that you are not allowed to build a research reactor that is bigger then a university reactor but isn't a full scale energy producing reactor. My understanding is that for such a reactor you would need the full operating license just as a grid connected PWR. And since in the US its essentially impossible to get a license for anything but a PWR, that isn't rally possible. Technology independent regulatory framework is one of the main reasons Canada has so many reactor startup, even those that started in other countries. https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-licensing-vendor-design-review/ https://nuclearsafety.gc.ca/eng/reactors/power-plants/pre-li...

andy_ppp 4y ago

What about using molten lead or something that can act like a lubricant/insulation at that temperature away from the corrosion in the same way airflow is used over jet turbine blades to stop them melting? It there any high temperature liquid that is molten salt phobic like oil and water?

jxf 4y ago

> The standard way to test material's resistance to molten salt is to put a coupon in a crucible full of salt for a few hundred hours. Q: What is a coupon in this context?

iAm25626 4y ago
Great Q: maybe? https://www.researchgate.net/figure/a-MAR-M247Nb-coupon-b-Al-2-O-3-crucible-c-coupon-crucible-used-for-the-isothermal_fig1_270394732 https://www.researchgate.net/figure/a-MAR-M247Nb-coupon-b-Al...
djcapelis 4y ago
A coupon is an informal term for a small sample of material, most often metal. It’s perhaps most commonly used in welding where practice welds for people first training to weld are done on pieces of metal often referred to as coupons. :) It is usually an artifact of materials like metal which are hard to cut, and can’t efficiently just be cut as needed during a task, but are prepared and pre-cut into small pieces you can grab easily. Each of those pieces is called a coupon. I used to store mine in a Chinese takeout container box.
- greyhair 4y ago
  In the mid 1970s, in my welding class, the school was located down the street from a small manufacturer that formed various metal alloys. They would provide the school with bins of scrap from their manufacturing free of charge. Those were our practice coupons in class. We had to be careful to return the welded scraps back to the proper bin so the manufacturer could recycle them. Our class was just a very small part of their recycling flow. It was a great relationship, we needed practice materials, and they hired welders.
ortusdux 4y ago
It's used 2 ways in metallurgy. Mainly it is a term for test pieces cut from a batch of metal and used for proof testing. Tensile, elongation, fatigue, weldability, and many other tests require destructive testing coupons. When you take welding certification tests, you weld two coupons together, and then submit your coupon for testing. It is also a term for strips of metal that you mount in a corrosive environment as a way to monitor corrosion of other susceptible components. So if you were worried about your bronze bushings, you would buy some matching bronze coupons, put them in an area of similar flow, and measure their thickness periodically. https://www.alspi.com/coupons.htm https://www.alspi.com/coupons.htm https://en.wikipedia.org/wiki/Fatigue_testing#Coupon_tests https://en.wikipedia.org/wiki/Fatigue_testing#Coupon_tests

pfdietz 4y ago

The corrosion issue is part of what drove Moltex to their rather interesting design (sterile coolant salt is a fluoride; fuel salt in tubes is chloride; this is a fast reactor.) The absence of uranium in the fluoride allows it to be operated at a redox potential where chromium does not dissolve.

panick21_ 4y ago
Yes, one of the reason I like that reactor design. They basically put something in the fluoride that is basically designed to corrode so that the actual reactor vessel doesn't. If I understand correctly. Their design doesn't require the 7 year swap cycle most MSR do.
- pfdietz 4y ago
  Right! They've got the same thing going that ordinary LWRs do: the neutrons lose their energy in a surround liquid rather than in a solid moderator or solid structural materials other than the fuel rods (which are designed to be replaced often anyway). The design even adds some hafnium to the coolant salt (substituting for zirconium, which is the sacrificial metal you refer to there); hafnium serves to shield the reactor walls from thermalized neutrons. One additional advantage of separating the fuel salt from the coolant salt is that the coolant salt volume can be increased as desired, making the thermal inertia of the reactor as large as one likes independent of reactor power or size of the fuel load.
  - panick21_ 4y ago
    > One additional advantage The problem with that is that you would likely require a different design and go threw full licensing. So in practice this will likely not be used.
    - pfdietz 4y ago
      Presumably it would happen at design time and a large, high thermal inertia design would be the one licensed.