As sent to me by a
minion helpful reader.
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The reactors at Fukushima are “boiling water reactors” that have space at the top of the reactor vessel for the water to flash to steam. Alternative techniques include Pressurized Water reactors where the water in the primary coolant loop is kept under pressure and never turns to steam, and liquid sodium reactors, which use pressurized liquid sodium as the coolant instead of water. Our navy has experimented with LSRs, but now uses PWRs in submarines and aircraft carriers. In all cases, the primary coolant loop is connected via a heat exchanger to a secondary coolant look, which flashes to steam, and is used to turn a turbine, either to generate electricity, or in the case of ships, to turn the propeller shafts.
As seen from the presentation, the real danger in Fukushima isn’t so much a failure of the reactor itself (even in the event of a primary containment failure, there’s a relatively small footprint of contamination), but rather the risk that the spent fuel pool will lose coolant, and the resulting dispersal of spent fuel will send airborne contaminants over a wide area.
The other think I liked about this presentation was the way it explained how the hydrogen explosions came about, and just what risk they posed.