United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 80-36: Failure of Steam Generator Support Bolting

                                                       SSINS NO.: 6835 
                                                       ACCESSION NO.: 
                                                       8006190085 
                                                       IN 80-36 

                               UNITED STATES 
                       NUCLEAR REGULATORY COMMISSION 
                    OFFICE OF INSPECTION AND ENFORCEMENT 
                           WASHINGTON, D.C. 20555 
                                     
                              October 10, 1980 

Information Notice No. 80-36:   FAILURE OF STEAM GENERATOR SUPPORT 
                                   BOLTING 
                                   
Description of Circumstances: 

On September 4, 1980, the NRC staff was informed by the Northern States 
Power company that extensive cracking of the bolting for the supports 
attached to bottom of the steam generators on Prairie Island Unit 1 had 
occurred. The licensee initially discovered the bolting problem when two 
bolts failed during detensioning. The licensee was attempting to remove the 
pre-load on the bolting (specified at 1400 ft-lbs) to minimize the potential 
for stress corrosion cracking of the high strength 1-1/2 in. dia. 250 ksi 
maraging steel bolting (Vascomax 250). 

Ultrasonic examination of the support bolting revealed cracks or breaks in 
47 of 48 support bolts for the two steam generators at the Prairie Island 
Unit 1 facility. The licensee subsequently shut down the Prairie Island Unit
2 facility and performed ultrasonic examinations of the same steam generator
support bolting, that failed on Unit 1. The results of the UT examinations 
revealed cracks in three of the 48 steam generator support bolts. The 
licensee attempted to remove the cracked bolts from the steam generator 
supports. Two cracked bolts were successfully removed while the third failed
during removal. Two bolts that were judged as defect free by the ultrasonic 
examination were also removed. Magnetic particle examinations confirmed the 
results of the ultrasonic examinations on the bolts which were removed. As 
part of the short term corrective actions taken, the licensee has unloaded 
and retensioned the bolting at approximately 50 to 100 ft.-lbs. 

Inservice ultrasonic examination of the affected bolting had been performed 
previously. In the case of Unit 1 a significant number of the bolts were  
examined at the last refueling outage 14 months ago. No cracks were found at
that time. 

The susceptibility for stress corrosion cracking was identified in 
NUREG-0577 (for comment). The NUREG identifies those facilities which have 
utilized maraging steels for reactor coolant pump and steam generator 
support bolting. Further the NUREG recommends augmented inspections for the 
facilities included in Group I. 

Licensees are encouraged to review the bolting materials used at their 
respective facilities and consider inspection of maraging steel bolting that
are either pre-loaded or significantly loaded during operation. 

This Information Notice is provided as an early notification of a possibly 
significant matter that is still under review by the NRC staff. It is 
expected that recipients will review the information for possible 
applicability to their facility. No specific action or response is requested
at this time. If you have any questions regarding this matter, please 
contact the Director of the appropriate NRC Regional Office. 

Page Last Reviewed/Updated Tuesday, November 12, 2013