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> Plus, if the magnets fail while the fusion reaction is happening, then the superheated plasma will violently explode in all directions, killing anyone nearby,

by XorNot 2y ago

> Plus, if the magnets fail while the fusion reaction is happening, then the superheated plasma will violently explode in all directions, killing anyone nearby, and spreading the radioactive remnants of the vessel all around. No. Just...no. This is entirely wrong. The density of any proposed fusion plasma is 250,000 times less then the earth's atmosphere[1]. Fusion plasma would be crushed the surrounding air rushing in, not "explode". Fusion plasma's are incredibly light, and incredibly thin. In the event of a full magnet quench, the only significant damage would be from magnetic quench boil off of coolant...which is a designed for failure mode, and would vent either liquid nitrogen (in HTS designs) or liquid helium in LTS designs like ITER. [1] https://www.ipp.mpg.de/15144/zuendbedingungen https://www.ipp.mpg.de/15144/zuendbedingungen

sandworm101 2y ago
People always think that the heart of a big machine will be some massive block of dense metal. The heart of these machines is, for failure mode purposes, an empty void. Be more afraid of the associated refrigeration plant. That is the bit more likely to explode.
worik 2y ago
True, very thin but very hot Yes It would initially collapse, but still a lot of heat to dissipate, and I would expect an explosion
- rcxdude 2y ago
  The fact that it's very hot doesn't mean an explosion, especially if it's also very thin. The total amount of heat energy actually stored in a theoretical miniature fusion reaction needn't be much.
- gravescale 2y ago
  Not that much heat, because the heat comes from an ongoing process, rather then being extracted from a dense thermal mass of something under pressure. Intuition about mass, flow and inertia can be misleading. If there's even a tiny leak or magnetic wobble, the plasma just goes out instantly. The cooling blanket is at a steady state with the reaction so once the plasma goes out, excess heat from the walls is very quickly absorbed. The thermal flux in the wall is high when the reaction is happening (multiple megawatts per square metre), but the temperature where the beryllium face is bonded to the copper-steel backing structure is below 300 Celsis, so there's not a huge heat storage of immense temperature in the structure. The plasma itself is 1 gram of mass at 150 million degrees. The tokamak full of sea-level air would be 1 million grams of air, more or less. So it won't superheat 800 cubic metres of air into an explosion, even if there was a huge leak and pressure equalised very fast. The bigger explosion hazard is the 800 cubic metre vacuum chamber imploding (unlikely, it's only 1 atmosphere difference, it's not a submersible), a magnet quench (which does store a lot of energy that can be released at once, just ask CERN, though the result isn't exactly catastrophic because it's part of the design, but it might not be good for the magnet) or some mundane but dramatic electrical, hydraulic, pneumatic or steam system failure that could happen in a coal power station. Future reactors may have a cooling blanket that could leak liquid lithium.

by XorNot 2y ago

sandworm101 2y ago

People always think that the heart of a big machine will be some massive block of dense metal. The heart of these machines is, for failure mode purposes, an empty void. Be more afraid of the associated refrigeration plant. That is the bit more likely to explode.

worik 2y ago

True, very thin but very hot Yes It would initially collapse, but still a lot of heat to dissipate, and I would expect an explosion

rcxdude 2y ago
The fact that it's very hot doesn't mean an explosion, especially if it's also very thin. The total amount of heat energy actually stored in a theoretical miniature fusion reaction needn't be much.
gravescale 2y ago
Not that much heat, because the heat comes from an ongoing process, rather then being extracted from a dense thermal mass of something under pressure. Intuition about mass, flow and inertia can be misleading. If there's even a tiny leak or magnetic wobble, the plasma just goes out instantly. The cooling blanket is at a steady state with the reaction so once the plasma goes out, excess heat from the walls is very quickly absorbed. The thermal flux in the wall is high when the reaction is happening (multiple megawatts per square metre), but the temperature where the beryllium face is bonded to the copper-steel backing structure is below 300 Celsis, so there's not a huge heat storage of immense temperature in the structure. The plasma itself is 1 gram of mass at 150 million degrees. The tokamak full of sea-level air would be 1 million grams of air, more or less. So it won't superheat 800 cubic metres of air into an explosion, even if there was a huge leak and pressure equalised very fast. The bigger explosion hazard is the 800 cubic metre vacuum chamber imploding (unlikely, it's only 1 atmosphere difference, it's not a submersible), a magnet quench (which does store a lot of energy that can be released at once, just ask CERN, though the result isn't exactly catastrophic because it's part of the design, but it might not be good for the magnet) or some mundane but dramatic electrical, hydraulic, pneumatic or steam system failure that could happen in a coal power station. Future reactors may have a cooling blanket that could leak liquid lithium.