Japanese adopt ultimate solution to core shroud cracking: replacement

28 February 1998

Work is now well advanced on core shroud replacement at Tokyo Electric Power’s Fukushima Daiichi 3 BWR – the first such replacement in Japan. On this issue TEPCO is adopting essentially the ultimate in proactive stances (see previous article). Although Japan has experienced BWR core shroud cracking indications, no such cracking had been experienced in Fukushima Daiichi 3. However the shroud was made of S304 stainless steel, which is known to have a susceptibility to cracking. Therefore the decision was taken to replace the shroud and all other internals components made of S304 with new components of 316L stainless steel. Over the next few years similar replacements will be done at Fukushima Daiichi 1, 2 and 5, which also have core shrouds of S304.

The present outage at Fukushima Daiichi 3 started on 26 May 1997. Using underwater EDM, the old shroud was cut out and cut into smaller pieces, which are now stored at site. The new shroud was delivered to site in December. Installation was scheduled to start early 1998, with completion in March.

At Nuclear Engineering International’s PLIM + PLEX conference (Prague, December 1997), Kanji Kinoshita gave a description of the replacement sequence (see diagrams). In the new shroud design the number of welds is reduced significantly and vertical welds are eliminated. He described the replacement as “a complete countermeasure to improve SCC resistance for all the welds” and noted that the key technology is the welding method for joining the new core shroud and its shroud support.

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