GE said that the USNRC issued guidance in 1980 to reinforce the torus structure. It said that it understands that the Fukushima Daiichi units with Mark I containments ‘addressed these issues and implemented modifications in accordance with Japanese regulatory requirements’.
These 1980 modifications included three main changes. Quenchers were installed to distribute the bubbles of steam pumped into the torus to produce rapid condensation underwater and reduce loads on the unit. Deflectors were installed in the torus to break up the pressure caused by incoming steam. The leg-like structures that support the torus were strengthened.
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The Nuclear Energy Institute has published a timeline of other BWR changes in the USA, partly in response to incidents there (available on www.neimagazine.com/mark1bwrmods).
Also, in the late 1990s, suction strainers in the torus were enlarged to avoid the potential of debris filters plugging, which could prevent the functioning of the emergency core cooling system, according to a NEI Mark I containment report published on 19 March.
