The torus tube style Suppression Chamber (SC) used in the Mark 1 BWR containment system is the design type used for units 1 – 5 at Fukushima Daiichi. This system, intended to absorb and condense steam back into water that can be used to cool the reactor was involved in 3 of the 3 meltdowns at Fukushima Daiichi.
“Mark I containment design is a drywell in the shape of an inverted common incandescent light bulb containing the reactor vessel and primary piping attached with
several large vent pipes to a torus shaped suppression chamber located below the drywell. The steam escaping from the break in the reactor coolant piping would vent, along with the drywell atmosphere, down into the suppression chamber. It would be distributed through a header to many downcomer pipes whose open ends were submerged in the suppression pool, which fills about half the suppression chamber. ” Source
During the failure progressions of units 1-3 at Fukushima Daiichi, the suppression chamber of each unit rapidly lost the ability to quench and turn steam back into water. The heat generated eventually turned all of the suppression chamber water to steam, defeating the scrubbing ability of torus routed emergency venting systems. The Mark 1 containment and specifically the torus-shaped suppression chamber had a number of known design flaws.
