ML14206A841

Lessons Learned

ML14206A841
Person / Time
Site:	San Onofre
Issue date:	08/13/2014
From:	Hiland P Division of Engineering
To:	Emily Larson FirstEnergy Nuclear Operating Co
AJohnson, NRR/DE/ESGB, 415-1475
References
TAC MF3704
Download: ML14206A841 (3)

Text

August 13, 2014 Eric Larson, Executive Chairman Steam Generator Management Program First Energy Nuclear Operating Company PO Box 4 Shippingport, PA 15077-0004

Subject:

San Onofre Nuclear Generating Station Lessons Learned Mr. Larson:

The steam generator tubes at San Onofre Nuclear Generating Station (SONGS), Unit 3, became susceptible to in-plane fluid-elastic instability during the first cycle of normal operation.

This phenomenon resulted in the steam generator tubes impacting each other in the U-bend region and causing tube wear. As a result of this wear, eight tubes failed to meet the steam generator performance criteria specified in the plants technical specifications. The susceptibility of the tubes to fluid-elastic instability was attributed to aggressive thermal hydraulic conditions coupled with the ineffectiveness of the anti-vibration bars in mitigating in-plane motion of the U-bends. The anti-vibration bars are intended to provide out-of-plane support to the U-bend region of the tubes, although friction between the tube and the anti-vibration bar may provide some resistance to in-plane motion.

In a memorandum to the U.S. Nuclear Regulatory Commission (NRC) staff dated March 20, 2014 (ADAMS Accession No. ML14028A028), the Executive Director of Operations (EDO) tasked the NRC staff with performing an evaluation of the lessons learned from the SONGS tube degradation event. One of the eight topical areas of review outlined in the EDO memorandum was a steam generator (SG) technical review, which was to be performed by the Office of Nuclear Reactor Regulation. This letter addresses Item of Consideration (4) outlined under the SG Technical Review topical area of the EDO memorandum, which involves reviewing standards and criteria for SGs.

The events at San Onofre not only raise questions about industry standards for designing and fabricating steam generators, but also raise questions about the operation of existing steam generators. The issue could affect operating steam generators since steam generator secondary side thermal hydraulic conditions could change as a result of SG tube plugging, changes to reactor Thot or Tave, power uprates, and buildup of deposits on the secondary side of the steam generator. In addition, the support provided by anti-vibration bars may change (become ineffective) as a result of wear between the tubes and the anti-vibration bars, changing the tubes susceptibility to fluid-elastic instability.

As a result of the steam generator event at San Onofre in 2012, the NRC staff is assessing whether any changes are needed to industry standards (e.g., Appendix N of Section III of the

E. Larson American Society of Mechanical Engineers Boiler and Pressure Vessel Code) and whether NRC review guidance (regulatory guides, standard review plan, and inspection procedures) should be modified.

Although there does not appear to be an immediate safety concern for the operating plants based on current operating experience, the events at San Onofre raise questions about whether the tubes may become susceptible to fluid-elastic instability as the steam generator operating conditions change. As a result, the NRC staff requests the industry discuss with the NRC staff specific actions planned or taken by the industry addressing the San Onofre operating experience, including any actions to add, develop, or modify industry design and fabrication standards/guidance and any actions to develop guidance on critical thermal hydraulic parameters to eliminate the potential for fluid-elastic instability.

We look forward to interacting with the industry on this issue.

Sincerely, Patrick L. Hiland, Director /RA/

Division of Engineering Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission cc: Kevin Ennis, ASME Ralph Hill, ASME

ML14206A841 OFFICE NRR/DE NRR/DE NRR/DE/ESGB NRR/DE NAME KKarwoski KManoly GKulesa PHiland DATE 07/ 30 /2014 07/ 31 /2014 08/ 13 /2014 08 / 13 /2014