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I'm wondering, they say using sodium, and having tested heat pipes up to 800°C... but their schema shows sodium vapor... but sodium only boils at 900°C ?? So

I'm wondering, they say using sodium, and having tested heat pipes up to 800°C... but their schema shows sodium vapor... but sodium only boils at 900°C ?? So I guess they underpressurize these pipes ? That would make sense, since in a traditional liquid metal reactor, the boiling of the working fluid is instead a failure mode to be avoided at all costs (since boiling dramatically increases the pressure resulting in burst pipes), especially with sodium that burns on contact with the air, explodes on contact with water, oh and also is highly radioactive at that point (for a short time). They seem very sure of themselves, but I still wonder what kind of stresses are involved, and what does this mean for heat pipe longevity ? (and calling heat pipes with a phase changing working fluid "passive" still seems kind of wrong ?) EDIT : The relatively small amount of sodium probably matters a lot here ?

thinkmcfly 4y ago
Static vs passive cooling, I think is what you're looking for here.
jonstewart 4y ago
Heat pipes are used in CPU coolers and work on the same principle. A small amount of liquid is in the pipe, at reduced pressure, which makes heat transfer very efficient. I suck at chemistry, but wiki clearly discusses advantages of sodium for heat transfer in nuclear reactors: https://en.wikipedia.org/wiki/Sodium-cooled_fast_reactor https://en.wikipedia.org/wiki/Sodium-cooled_fast_reactor (tl;dr: safety margin from the large range of temps at which sodium is liquid; doesn't like to absorb neutrons, and isotopes aren't that problematic when it does).
- BlueTemplar 4y ago
  Yes, but they use water. (Also reduced pressure means reduced density, which should in theory make heat transfer less efficient... except when phase changes are involved !) That reactor is a radically different design, operating at a much lower temperature, where sodium doesn't evaporate.
  - jonstewart 4y ago
    But the whole point of a heat pipe is that phase changes -are- involved! So, water’s fine for temperatures in a “normal” range, like a CPU, but perhaps not great for higher temperature ranges. And then, this is a nuclear reactor. Whatever’s in the heat pipe in a reactor will be bombarded with neutrons. Water is a neutron moderator, so you wind up with heavy water in the heat pipes and not-very-nice fission products in the reactor due to slower neutrons.
    - BlueTemplar 4y ago
      I never said they should use water, I'm just wondering what kinds of stresses (including chemical ones ?) are involved here...

thinkmcfly 4y ago

Static vs passive cooling, I think is what you're looking for here.

jonstewart 4y ago

Heat pipes are used in CPU coolers and work on the same principle. A small amount of liquid is in the pipe, at reduced pressure, which makes heat transfer very efficient. I suck at chemistry, but wiki clearly discusses advantages of sodium for heat transfer in nuclear reactors: https://en.wikipedia.org/wiki/Sodium-cooled_fast_reactor https://en.wikipedia.org/wiki/Sodium-cooled_fast_reactor (tl;dr: safety margin from the large range of temps at which sodium is liquid; doesn't like to absorb neutrons, and isotopes aren't that problematic when it does).

BlueTemplar 4y ago
Yes, but they use water. (Also reduced pressure means reduced density, which should in theory make heat transfer less efficient... except when phase changes are involved !) That reactor is a radically different design, operating at a much lower temperature, where sodium doesn't evaporate.
- jonstewart 4y ago
  But the whole point of a heat pipe is that phase changes -are- involved! So, water’s fine for temperatures in a “normal” range, like a CPU, but perhaps not great for higher temperature ranges. And then, this is a nuclear reactor. Whatever’s in the heat pipe in a reactor will be bombarded with neutrons. Water is a neutron moderator, so you wind up with heavy water in the heat pipes and not-very-nice fission products in the reactor due to slower neutrons.
  - BlueTemplar 4y ago
    I never said they should use water, I'm just wondering what kinds of stresses (including chemical ones ?) are involved here...