United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-19: Steam Generator Feedwater Distribution Piping Damage

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
                    OFFICE OF NUCLEAR REACTOR REGULATION
                           WASHINGTON, D.C.  20555

                               March 12, 1991


Information Notice No. 91-19:  STEAM GENERATOR FEEDWATER DISTRIBUTION 
                                   PIPING DAMAGE


Addressees:

All holders of operating licenses or construction permits for pressurized 
water reactors (PWRs).

Purpose:

This information notice is intended to alert addressees to potential 
problems resulting from degradation of feedwater distribution piping in 
steam generators due to thermal stress, cracking, erosion and corrosion.  
Depending on the design of the steam generator feedwater system, these 
problems may affect operation of the auxiliary feedwater system.  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.

Background:

The degradation noted below of the feedwater distribution system piping in 
the steam generators at San Onofre Units 2 and 3 may be applicable to 
Combustion Engineering steam generator designs predating the System 80 
design and to similar designs in other steam generators at other nuclear 
power plants.  This matter is considered safety-significant because the 
feedwater distribution system piping degradation may affect the delivery of 
auxiliary feedwater flow in some of these steam generators and because of 
the potential for consequential damage to the steam generator tubes from 
resulting debris.  The NRC has issued several generic communications dealing 
with one or more aspects of such degradation (Attachment 3).

At San Onofre Units 2 and 3, both main feedwater and auxiliary feedwater 
enter the steam generators through a feedwater nozzle.  The feedwater enters 
a distribution box and 12-inch diameter piping (feedring) that distributes 
the flow through top-mounted discharge elbows (J-tubes) around the periphery 
of the steam generator shell (Figure 1).  The feedring is attached by two 
U-bolts at each of four supports that are welded to the shell wall.  A 
3-inch elbow and tee vent assembly is attached to the upper portion of the 
innermost (toward the interior of the steam generator) end of the 
distribution box (Figure 2).  


9103060156 
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                                                            IN 91-19
                                                            March 12, 1991
                                                            Page 2 of 3


Combustion Engineering originally designed the distribution box without the 
vent assembly.  However, in 1980 during preoperational testing at San Onofre 
Unit 2, a test of the auxiliary feedwater system caused a partial vacuum 
within both halves of the feedring in one of the steam generators, and the 
feedring collapsed.  The licensee, Southern California Edison Co., 
determined that the inadequate flow area of the discharge elbows and the 
relatively thin-walled Schedule 40 piping constituting the feedring had 
contributed to the feedring collapse.  Corrective actions included replacing 
most of the feedring with Schedule 120 piping (except for 9-inch segments on 
each side of the distribution box), enlarging the diameter of the discharge 
elbows from 1.5 inches to 3.5 inches, and installing the vent assembly on 
the distribution box.  

Description of Circumstances:

San Onofre Unit 3:

On May 10, 1990, the licensee found several pieces of carbon steel debris 
during a routine inspection of the secondary side of the tubesheet of one 
steam generator (LER 50-362/90-05-01).  During further inspection of the 
internal components of this and the other steam generator, the licensee 
found material missing from the lower portion of the feedring at its 
intersection with the distribution box, surface cracks in the heat-affected 
zone at the toe of the weld at that intersection, erosion and corrosion 
indications on the interior surfaces of the distribution boxes, erosion of 
the vent assemblies, "T" section tops missing from the vent assemblies, and 
deformation of several U-bolt supports. 

San Onofre Unit 2:

On July 23, 1990, the licensee shut down Unit 2 to perform a similar inspec-
tion.  The damage found was significantly less than on Unit 3.  No material 
was found missing from the distribution box-feedring junction.  One U-bolt 
was fractured.  

Discussion:

The licensee determined the root cause contributing to the degradation of 
the feedwater distribution system piping to be inadequate design of the 
feedring and feedring supports.  The design did not adequately consider the 
thermal stresses resulting from normal operating conditions, in particular 
the batch process of auxiliary feedwater addition during startup operations.  
In addition, the design of the vent assembly had not properly considered the 
potential for erosion and corrosion resulting from localized high velocity 
flow.  The corrective actions taken by the licensee included replacing the 
remaining Schedule 40 piping material with Schedule 120 piping material, 
replacing the distribution box-feedring weld configuration with weld-o-let 
forgings, removing the distribution box vents from the design, repairing 
local thinning of the distribution box by weld buildup and removal of local 
interior surface discontinuities, modifying the feedring supports to provide 
flexibility for thermal expansion, and using stronger U-bolts.  The licensee 
had previously modified the auxiliary feedwater system to provide continuous 
feeding of the steam generator rather than the batch feeding that was used 
during startup operations.
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                                                            IN 91-19
                                                            March 12, 1991
                                                            Page 3 of 3


On September 20, 1990, the steam generator vendor, Combustion Engineering, 
issued an information bulletin (Combustion Engineering Infobulletin 90-04, 
"Feedwater Distribution System Degradation"), recommending that its client 
utilities perform a baseline inspection during their next refueling outage 
to detect wall thinning in the feedwater distribution system.  

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please contact 
one of the technical contacts listed below or the appropriate NRR project 
manager.




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


Technical Contacts:  Vern Hodge, NRR
                     (301) 492-1861

                     Lawrence E. Kokajko, NRR
                     (301) 492-1380


Attachments:
1.  Figure 1.  Top View of the Feedwater Distribution Piping
2.  Figure 2.  Side View of the Feedwater Distribution Piping
3.  List of References
4.  List of Recently Issued NRC Information Notices 
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                                                            Attachment 3 
                                                            IN 91-19
                                                            March 12, 1991
                                                            Page 1 of 1


                             LIST OF REFERENCES

1.   Bulletin No. 87-01:  "Thinning of Pipe Walls in Nuclear Power Plants," 
     July 9, 1987 

2.   Bulletin No. 79-13:  "Cracking in Feedwater System Piping," June 25, 
     1979

3.   Ibid., Revision No. 1:  "Cracking in Feedwater System Piping," 
     August 30, 1979

4.   Ibid., Revision No. 2:  "Cracking in Feedwater System Piping," 
     October 16, 1979

5.   Generic Letter No. 89-08:  "Erosion/Corrosion-Induced Pipe Wall 
     Thinning,"  May 2, 1989

6.   Generic Letter No. 79-20:  Untitled, on Cracking In Feedwater Lines, 
     May 25, 1979

7.   Information Notice No. 88-17: "Summary of Responses to NRC Bulletin 
     87-01, 'Thinning of Pipe Walls in Nuclear Power Plants,'" April 22, 
     1988

8.   Information Notice No. 87-36: "Significant Unexpected Erosion of 
     Feedwater Lines," August 4, 1987

9.   Information Notice No. 86-106: "Feedwater Line Break," December 16, 
     1986

10.  Ibid., Supplement 1: "Feedwater Line Break," February 13, 1987

11.  Ibid., Supplement 3: "Feedwater Line Break," November 10, 1988 

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