4 ms·

To begin with you might start with a geometry which by design is already close to maximally compact, so that a geometry change would tend to go in one direction

To begin with you might start with a geometry which by design is already close to maximally compact, so that a geometry change would tend to go in one direction. > For example, how do you prevent melted fuel from flowing into the cooling channels that go through the core? Expect that to happen and use a geometry that doesn't cause the reaction to increase in speed if it does, e.g. because fuel flowing into the cooling channels would make the fuel less rather than more compact.

gpm 2y ago
Slightly hesitant to jump in since pfdietz definitely knows more about this than I do... but... Cooling typically means things like maximizing surface area, minimizing the thickness of the object being cool, etc. Maximum neutron density presumably happens in a sphere, which is coincidentally the shape that minimizes surface area. The whole point of a nuclear reactor is that it heats up, and you can convert that heat into useful fuel. Presumably that means you need to carry quite a lot of heat away per volume. Presumably that means putting the fuel into a spherical shape really doesn't work that well.
- AnthonyMouse 2y ago
  To cool something, you need a material which is a decent conductor of heat. The reaction materials are mostly uranium, plutonium and thorium, which are metals. They conduct heat pretty well all on their own.
  - pfdietz 2y ago
    Metal fuels also melt at considerably lower temperature than oxide fuels. Uranium oxide melts at 2865 C; uranium metal at 1132 C, plutonium metal at just 639 C. In contact with iron, plutonium forms a eutectic with a melting point of just 410 C, below the melting point of zinc. There was a crazy reactor at Los Alamos, LAMPRE, that used molten eutectic Pu-Fe in tantalum tubes as the fuel.
    - AnthonyMouse 2y ago
      1132 C shouldn't be an infeasibly low temperature when the reactor is only expected to heat the coolant to 300-350 C. Meanwhile thorium metal is 1750 C. Or use uranium carbide. High melting point with still decent thermal conductivity.

gpm 2y ago

Slightly hesitant to jump in since pfdietz definitely knows more about this than I do... but... Cooling typically means things like maximizing surface area, minimizing the thickness of the object being cool, etc. Maximum neutron density presumably happens in a sphere, which is coincidentally the shape that minimizes surface area. The whole point of a nuclear reactor is that it heats up, and you can convert that heat into useful fuel. Presumably that means you need to carry quite a lot of heat away per volume. Presumably that means putting the fuel into a spherical shape really doesn't work that well.

AnthonyMouse 2y ago
To cool something, you need a material which is a decent conductor of heat. The reaction materials are mostly uranium, plutonium and thorium, which are metals. They conduct heat pretty well all on their own.
- pfdietz 2y ago
  Metal fuels also melt at considerably lower temperature than oxide fuels. Uranium oxide melts at 2865 C; uranium metal at 1132 C, plutonium metal at just 639 C. In contact with iron, plutonium forms a eutectic with a melting point of just 410 C, below the melting point of zinc. There was a crazy reactor at Los Alamos, LAMPRE, that used molten eutectic Pu-Fe in tantalum tubes as the fuel.
  - AnthonyMouse 2y ago
    1132 C shouldn't be an infeasibly low temperature when the reactor is only expected to heat the coolant to 300-350 C. Meanwhile thorium metal is 1750 C. Or use uranium carbide. High melting point with still decent thermal conductivity.