United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 88-31: Steam Generator Tube Rupture Analysis Deficiency

                                  UNITED STATES
                          NUCLEAR REGULATORY COMMISSION
                             WASHINGTON, D. C. 20555

                                  May 25, 1988



All holders of operating licenses or construction permits for Westinghouse 
(W)- and Combustion Engineering (CE)-designed nuclear power reactors.  


This information notice is being provided to alert addressees to potential 
problems resulting from a non-conservatism in the safety analysis for rupture 
of a steam generator tube which may increase offsite dose consequences.  It is 
expected that recipients will review the information for applicability to 
their facilities and consider actions, as appropriate, to avoid similar 
problems.  However, suggestions contained in this information notice do not 
constitute NRC requirements; therefore, no specific action or written response 
is required.  

Description of Circumstances:  

Following the steam generator tube rupture at North Anna Unit 1 on July 15, 
1987, the Virginia Electric and Power Company (VEPCO) modified the flow 
resistance of the steam generator downcomers at North Anna by the addition of 
flow baffle plates.  This modification necessitated the reanalysis of certain 
design basis events including rupture of a steam generator tube.  The new 
analysis utilized a revised Westinghouse method for calculating steam 
generator water mass and indicated that during the event, the water level on 
the secondary side could fall below the top of the steam generator tubes for a 
10-minute period at the beginning of the event.  

Steam generator tube uncovery is significant because, if the break location 
becomes uncovered, a direct path might exist for fission products contained in 
the primary coolant to be released to the atmosphere without partition with 
the secondary coolant.  VEPCO and Westinghouse reanalyzed the design basis 
steam generator tube rupture accident for Surry using the revised methods and 
determined that the steam generator tubes at Surry could also become uncovered 
even though the Surry plants were not modified by the addition of flow baffle 

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                                                            May 25, 1988 
                                                            Page 2 of 3 

The licensee further concluded that the offsite dose consequences exceeded 
those calculated in the Surry Updated Final Safety Analysis Report (UFSAR) 
because tube uncovery could produce a direct path for fission product release.
Based on the Surry results, the analysis of steam generator inventory during a 
steam generator tube rupture at other plants may show that the steam generator 
tubes may uncover. Thus, for those plants where the steam generator tubes were 
thought to remain covered following tube rupture, the previously calculated 
safety analysis offsite doses might be exceeded and since the primary coolant 
activity limit in Technical Specifications is based upon the occurrence of 
this accident, the allowable technical specification limit may be too high.  


A postulated steam generator tube rupture is one of the design basis accidents 
analyzed in plant Safety Analysis Reports (SARs).  Using conservative assump-
tions of single failure and loss of offsite power, it must be shown that the 
offsite dose consequences will be limited to the guideline doses of 10 CFR 100 
or a fraction of the guideline doses depending on the assumptions made for 
iodine spiking.  The iodine in the reactor coolant may be previously dissolved 
from allowable operational fuel failure or may result from an iodine spike 
which is the sudden increase in coolant iodine concentration produced by the 
transient conditions during the accident.  Mechanisms for transport of the 
iodine that exits the reactor system to the atmosphere are discussed in 
Standard Review Plan (NUREG-0800) Section 15.6.3.  In determining the amount 
of iodine that is trans-ported to the atmosphere, credit may be given for 
"scrubbing" of iodine contained in the steam phase and in the atomized primary 
coolant droplets sus-pended in the steam phase for release points which are 
below the steam generator water level.  

The Surry UFSAR assumed that the break is always covered with water so that 
99% of the iodine would remain within the steam generator coolant and only 1% 
would be released through the atmospheric relief valves.  The break location 
is assumed to be always covered in the UFSAR calculations because an initial 
steam generator water mass that may be non-conservatively large was assumed in 
order to conservatively account for the possibility of overfill and because 
steam generator tube failures were thought only to occur close to the tube 
sheet.  The North Anna tube rupture demonstrated that steam generator tube 
failures can occur near the top of the tube bundle.  The revised steam 
generator water mass calculations by Westinghouse with the assumption that the 
break occurs at the top of the tube bundle led to the conclusion that the 
break could be uncovered for a significant period of time.  Tube uncovery 
occurs because of the level shrink that accompanies reactor trip/turbine trip 
during the tube rupture event.  The tubes would be recovered by the flow of 
auxiliary feedwater into the ruptured steam generator and by the reactor 
coolant which would be added due to the ruptured tube; however, the amount of 
iodine released from the ruptured steam generator could be larger than that 
previously calculated. 

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                                                            Page 3 of 3 

The NRC staff is evaluating the impact that these non-conservative assumptions 
may have on calculated offsite doses and the need for further generic action. 

No specific action or written response is required by this information notice.
If you have any questions about this matter, please contact the technical con-
tact listed below or the Regional Administrator of the appropriate regional 

                              Charles E. Rossi, Director 
                              Division of Operational Events Assessment 
                              Office of Nuclear Reactor Regulation 

Technical Contact:  Walton L. Jensen
                    (301) 492-21190

Attachment:  List of Recently Issued NRC Information Notices  
.                                                            Attachment
                                                            IN 88-31 
                                                            May 25, 1988 
                                                            Page 1 of 1

                             LIST OF RECENTLY ISSUED
                            NRC INFORMATION NOTICES 
Information                                  Date of 
Notice No._____Subject_______________________Issuance_______Issued to________

88-30          Target Rock Two-Stage         5/25/88        All holders of OLs
               SRV Setpoint Drift                           or CPs for nuclear
               Update                                       power reactors. 

88-29          Deficiencies in Primary       5/24/88        All holders of OLs
               Containment Low-Voltage                      or CPs for nuclear
               Electrical Penetration                       power reactors. 

88-28          Potential for Loss of         5/19/88        All holders of OLs
               Post-LOCA Recirculation                      or CPs for nuclear
               Capability Due to                            power reactors. 
               Insulation Debris Blockage 

88-27          Deficient Electrical          5/18/88        All holders of OLs
               Terminations Identified                      or CPs for nuclear
               in Safety-Related                            power reactors. 

85-35,         Failure of Air Check          5/17/88        All holders of OLs
Supplement 1   Valves to Seat                               or CPs for nuclear
                                                            power reactors. 

88-26          Falsified Pre-Employment      5/16/88        All holders of OLs
               Screening Records                            or CPs for nuclear
                                                            power reactors and
                                                            all major fuel 

88-25          Minimum Edge Distance for     5/16/88        All holders of OLs
               Expansion Anchor Bolts                       or CPs for nuclear
                                                            power reactors. 

88-24          Failures of Air-Operated      5/13/88        All holders of OLs
               Valves Affecting Safety-                     or CPs for nuclear
               Related Systems                              power reactors. 
OL = Operating License
CP = Construction Permit 
Page Last Reviewed/Updated Friday, May 22, 2015