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

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.