Many penetrations and connections within a reactor vessel can cause weak points where structural failure or corium (melted fuel) leakage may happen. The outer containment structures can also be plagued by this problem. One such problem includes the bellows expansion joint in the torus vent pipes for the GE Mark 1 design used at Fukushima Daiichi.
Diagrams of the torus vent pipe and the bellows expansion joint below.
The bellows joint features a pipe within a pipe along with an internal expansion joint that allows the pipe to expand along the length. This does not provide for lateral or axial movement of the pipe in the design. The gap between the inner diameter of the expansion joint and outer diameter of the vent pipe creates enough room for lateral movement or torque that could stress the welds and structure of the joint.
Expansion joints are prone to stress and aging over time and use. The nature of this particular expansion joint makes it difficult to inspect inside for fatigue or corrosion. The downward angle of the pipe and the welded connection being on the downward side lends itself to water or condensation collection. This could accelerate failure of this joint through corrosion.
The lack of lateral flexibility in the design of the joint could cause damage from lateral forces in a strong quake or during large explosions in the reactor building. The potential for failures in this expansion joint could lead to leaking of hydrogen gas or leaking of water out of the drywell into the suppression chamber room during conditions such as we have seen at Fukushima Daiichi where containment flooding has been attempted.
The torus structures have also been the subject of repeated design fixes as flaws were discovered in the design. Quenchers and deflecters were later added to the torus design. Additional supports were added to torus structures after a water hammer effect was realized during operation. Torus structures in some US Mark 1 reactors have been found to have corrosion problems that required repairs.
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