6 ms·

One of those safety improvements -- a freeze plug -- passively halts the reaction in the event of a power cut. The reactor sits on top of a vault that has a lar

One of those safety improvements -- a freeze plug -- passively halts the reaction in the event of a power cut. The reactor sits on top of a vault that has a larger volume separated by a narrow tube containing molten salt that has been frozen into a plug by cryocoolers powered by the turbines themselves. If the pumps stop for any reason, then the plug quickly melts and the molten fluid from the reactor drains into the larger vault via gravity at which point it cools and freezes into a solid.

throwaway894345 4y ago
Freeze plugs sound super cool, but this part breaks my brain: > containing molten salt that has been frozen into a plug Presumably salt can't be both molten and frozen at once, or is there something about this domain that I don't understand?
- phire 4y ago
  I believe they use active cooling to keep the plug frozen. If the power fails, the cooling fails and the plug melts.
  - ortusdux 4y ago
    It is literally just a tube with a fan blowing over it. Most designs just barely solidify it, so any over-temperature events also cause a passive shutdown.
  - throwaway894345 4y ago
    Yeah, I got that (and it seems like a really elegant solution! very cool, pun intended, etc). My question was about how salt (or any other matter, really) can simultaneously be molten and frozen.
    - ortusdux 4y ago
      It can't. Well, maybe at its triple-point, but that is another story. It is just a quirk of the language due to it being know as molten salt. For example, my drink is full of molten water that has been frozen into cubes.
    - el_nahual 4y ago
      Part of it is molten (above the plug), part of it is frozen (the plug). Think pipes in a house in winter, where one little part of the pipe gets frozen while the rest of the pipes have liquid water in them.
      - xorbax 4y ago
        And in Spring, when the broken pipe plugged with solid molten ice thaws, your house's molten ice circuit performs an emergency evacuation into the yard, saving you from the convenience of adequate molten ice pressure
        politician 4y ago
        Hah! Well, we usually think of salt in the solid state (i.e. frozen) and not in the liquid state, so it's not so strange to be explicit in this case. For H2O, we do have pretty common encounters with three of its phase states, so there's less need to be explicit (or, rather, we have separate words for each state: steam, water, ice).
        jabl 4y ago
        Water is actually a very unusual substance in that it expands upon freezing. (Plutonium being another) Most materials instead contract when freezing.
    - robotresearcher 4y ago
      Think of a lake in the winter. Just the top layer of water is frozen, exposed to the cold air above the lake. Some of the lake water is frozen and some is liquid, depending on its position in the lake.
- andrewflnr 4y ago
  I think it was just a clumsy phrasing, with the point being that it's a solid plug of whatever salt is molten and circulating above.
- politician 4y ago
  I wrote it that way to aid the reader in understanding what the plug material was composed of. I didn't want the reader to think that the material was water ice. Yes, the material is the frozen state of the once liquid reactor fluid. I hope that clears it up, and would welcome a better way of explaining it!
  - throwaway894345 4y ago
    Yes, that makes a lot of sense. I wasn't trying to nit-pick your phrasing, I was just curious if I was misunderstanding something as a lay person. :)
JumpCrisscross 4y ago
Are freeze plug failures recoverable? Or is is this a final failsafe that toasts the reactor?
- ortusdux 4y ago
  Easily recoverable. The tube drains into a collection tank filled with control rods, so any reactions are halted. You can just reheat the salt and pump it back into the reactor. Reportedly, one of the first test reactors in the 50's was shut off every Friday and restarted on Monday. A full power loss, what would be catastrophic for any other reactor, was tested weekly for a year without issue.
  - panick21_ 4y ago
    I love the image of some guy before going home hitting the red button and some guy on Monday just hitting the green button.
ortusdux 4y ago
One of my favorite safety elements is often over-looked: they operate at 1 atmosphere. So much of the cost and bulk of a traditional reactor is the shielding needed to protect from an over-pressure event.
- panick21_ 4y ago
  And even better, most of the things that are dangerous in a traditional reactor is chemically bound to the salt. So even if there is some kind of explosion, these materials would not be vaporized and transported in the air with wind. So no crazy misinformed graphics all over the news. So even if you drop a hand-grande into that reactor, it just gone be like throwing a rock into a bucket full of toothpaste. Its gone be a mess but its not gone result in a lot of airborne materials.
deleted 4y ago
[deleted]
elihu 4y ago
It seems like that might suggest a potential option in the search for appropriate materials to build the containment vessel and piping to hold in the salt: just make the whole thing out of salt. Anything that needs to be solid can have built-in channels with coolant piped through. The rest can maintain a sort of steady state. I'm sure there's all sorts of practical reasons why that wouldn't work, but it's an interesting thing to think about.
- lolc 4y ago
  Nice hack! That sounds so easy that I wonder what the catch is. Maybe it would need more energy for cooling than what it generates? So it would have to be scaled up to a size where the surface-to-content ratio becomes favourable. There might not be enough salt for that :-)
  - elihu 4y ago
    I suppose one problem might be selecting an appropriate coolant that doesn't dissolve the salt on contact. Presumably water would, but google says that salt doesn't dissolve in oil so maybe that's an option. Ideally you wouldn't need to expend energy to keep the coolant cold enough; rather, you'd use the coolant to boil water to run your steam generator.
    - usrusr 4y ago
      “Vessel built out of nothing but heat transfer interface and freeze plug". Do some of the materials problems get easier while it's only touching the salt in frozen state?
  - jasonwatkinspdx 4y ago
    Ok so I recently watched a video that went into some of this, and I had know idea how crazy some of these nuclear fuels are in terms of physical requirements. If I recall the video correctly, solid metal oxide fuel produces offgas at a 50:1 ratio, while being insanely dense. This produces pretty insane internal pressures if the goal of the fuel design is to keep everything contained. Like strong enough to eventually cause mechanical failure of the fuel. I think there's a lot of really difficult constraints intersecting in these fuel assembly designs, and something that seems obviously simple probably isn't.

throwaway894345 4y ago

Freeze plugs sound super cool, but this part breaks my brain: > containing molten salt that has been frozen into a plug Presumably salt can't be both molten and frozen at once, or is there something about this domain that I don't understand?

phire 4y ago
I believe they use active cooling to keep the plug frozen. If the power fails, the cooling fails and the plug melts.
- ortusdux 4y ago
  It is literally just a tube with a fan blowing over it. Most designs just barely solidify it, so any over-temperature events also cause a passive shutdown.
- throwaway894345 4y ago
  Yeah, I got that (and it seems like a really elegant solution! very cool, pun intended, etc). My question was about how salt (or any other matter, really) can simultaneously be molten and frozen.
  - ortusdux 4y ago
    It can't. Well, maybe at its triple-point, but that is another story. It is just a quirk of the language due to it being know as molten salt. For example, my drink is full of molten water that has been frozen into cubes.
  - el_nahual 4y ago
    Part of it is molten (above the plug), part of it is frozen (the plug). Think pipes in a house in winter, where one little part of the pipe gets frozen while the rest of the pipes have liquid water in them.
    - xorbax 4y ago
      And in Spring, when the broken pipe plugged with solid molten ice thaws, your house's molten ice circuit performs an emergency evacuation into the yard, saving you from the convenience of adequate molten ice pressure
      - politician 4y ago
        Hah! Well, we usually think of salt in the solid state (i.e. frozen) and not in the liquid state, so it's not so strange to be explicit in this case. For H2O, we do have pretty common encounters with three of its phase states, so there's less need to be explicit (or, rather, we have separate words for each state: steam, water, ice).
      - jabl 4y ago
        Water is actually a very unusual substance in that it expands upon freezing. (Plutonium being another) Most materials instead contract when freezing.
  - robotresearcher 4y ago
    Think of a lake in the winter. Just the top layer of water is frozen, exposed to the cold air above the lake. Some of the lake water is frozen and some is liquid, depending on its position in the lake.
andrewflnr 4y ago
I think it was just a clumsy phrasing, with the point being that it's a solid plug of whatever salt is molten and circulating above.
politician 4y ago
I wrote it that way to aid the reader in understanding what the plug material was composed of. I didn't want the reader to think that the material was water ice. Yes, the material is the frozen state of the once liquid reactor fluid. I hope that clears it up, and would welcome a better way of explaining it!
- throwaway894345 4y ago
  Yes, that makes a lot of sense. I wasn't trying to nit-pick your phrasing, I was just curious if I was misunderstanding something as a lay person. :)

JumpCrisscross 4y ago

Are freeze plug failures recoverable? Or is is this a final failsafe that toasts the reactor?

ortusdux 4y ago
Easily recoverable. The tube drains into a collection tank filled with control rods, so any reactions are halted. You can just reheat the salt and pump it back into the reactor. Reportedly, one of the first test reactors in the 50's was shut off every Friday and restarted on Monday. A full power loss, what would be catastrophic for any other reactor, was tested weekly for a year without issue.
- panick21_ 4y ago
  I love the image of some guy before going home hitting the red button and some guy on Monday just hitting the green button.

ortusdux 4y ago

One of my favorite safety elements is often over-looked: they operate at 1 atmosphere. So much of the cost and bulk of a traditional reactor is the shielding needed to protect from an over-pressure event.

panick21_ 4y ago
And even better, most of the things that are dangerous in a traditional reactor is chemically bound to the salt. So even if there is some kind of explosion, these materials would not be vaporized and transported in the air with wind. So no crazy misinformed graphics all over the news. So even if you drop a hand-grande into that reactor, it just gone be like throwing a rock into a bucket full of toothpaste. Its gone be a mess but its not gone result in a lot of airborne materials.

deleted 4y ago

[deleted]

elihu 4y ago

It seems like that might suggest a potential option in the search for appropriate materials to build the containment vessel and piping to hold in the salt: just make the whole thing out of salt. Anything that needs to be solid can have built-in channels with coolant piped through. The rest can maintain a sort of steady state. I'm sure there's all sorts of practical reasons why that wouldn't work, but it's an interesting thing to think about.

lolc 4y ago
Nice hack! That sounds so easy that I wonder what the catch is. Maybe it would need more energy for cooling than what it generates? So it would have to be scaled up to a size where the surface-to-content ratio becomes favourable. There might not be enough salt for that :-)
- elihu 4y ago
  I suppose one problem might be selecting an appropriate coolant that doesn't dissolve the salt on contact. Presumably water would, but google says that salt doesn't dissolve in oil so maybe that's an option. Ideally you wouldn't need to expend energy to keep the coolant cold enough; rather, you'd use the coolant to boil water to run your steam generator.
  - usrusr 4y ago
    “Vessel built out of nothing but heat transfer interface and freeze plug". Do some of the materials problems get easier while it's only touching the salt in frozen state?
- jasonwatkinspdx 4y ago
  Ok so I recently watched a video that went into some of this, and I had know idea how crazy some of these nuclear fuels are in terms of physical requirements. If I recall the video correctly, solid metal oxide fuel produces offgas at a 50:1 ratio, while being insanely dense. This produces pretty insane internal pressures if the goal of the fuel design is to keep everything contained. Like strong enough to eventually cause mechanical failure of the fuel. I think there's a lot of really difficult constraints intersecting in these fuel assembly designs, and something that seems obviously simple probably isn't.