On Friday, the Nuclear Regulatory Commission made public a redacted version of Mitsubishi Heavy Industries' root cause analysis on prematurely thinning tubes within steam generators at the San Onofre nuclear plant.The report can be downloaded here.It offers a detailed technical breakdown of the operational conditions and design elements that caused tubes in the steam generators' U sections to wear against support bars and each other. Among its conclusions, MHI found that sections of tubes were not held in place firmly enough to control the vibration that caused the wear. The report listed the root causes of the premature wear as: "1. Insufficient programmatic requirement to assure effective AVB (anti-vibration bar) contact force to prevent in-plane fluid elastic instability and random vibration and subsequent wear under high localized thermal-hydraulic conditions (steam quality (void fraction), flow velocity and hydro-dynamic pressure).2. The design control process did not provide sufficient direction to assure that an evaluation of the need for an analysis of flow induced vibration of the retainer bar was performed and verified."Both units at the Southern California Edison plant have been offline since the tube wear was discovered early last year. The report's release follows political pressure from Sen. Barbara Boxer (D-Calif.) and Rep. Edward Markey (D-Mass.) to make the RCA public. Friends of the Earth, an anti-nuclear environmental group, used sections of the report Friday to allege that SCE knew about the steam generator design issues years ago but did not address them to avoid a license amendment – a charge that SCE executives have strongly denied.
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Studies have also shown that if the plant was run at around 80% power, they would reduce the vibrations to a negligible amount, and could potentially run that way for the foreseeable future with no increased risk of additional tube wear. This problem has become far more political than technical and that is a shame.
