Information Notice No. 92-35: Higher Than Predicted Erosion/Corrosion in Unisolable Reactor Coolant Pressure Boundary Piping Inside Containment at a Boiling Water Reactor

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

                                 May 6, 1992


NRC INFORMATION NOTICE 92-35:  HIGHER THAN PREDICTED EROSION/CORROSION IN 
                               UNISOLABLE REACTOR COOLANT PRESSURE BOUNDARY 
                               PIPING INSIDE CONTAINMENT AT A BOILING 
                               WATER REACTOR 


Addressees

All holders of operating licenses or construction permits for nuclear power 
reactors.

Purpose

The U.S. Nuclear Regulatory Commission (NRC) is issuing this information 
notice to alert addressees to erosion/corrosion rates that could be higher 
than predicted in certain unisolable reactor coolant pressure boundary 
piping inside the containment drywell at boiling water reactors.  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 are not 
NRC requirements; therefore, no specific action or written response is 
required.

Description of Circumstances

The Pennsylvania Power and Light Company (the licensee) recently performed 
erosion/corrosion inspections at the Susquehanna Steam Electric Station, 
Unit 1, and may have identified an unexpectedly high rate of erosion/ 
corrosion in certain main feedwater (FW) piping inside containment 
(Attachment 1).  Erosion of this portion of FW piping is of particular 
concern since this portion cannot be isolated from the reactor vessel, and 
erosion/corrosion inspection strategies may not direct attention to that 
part of the FW system. 

When the licensee began operating the unit commercially in June 1982, the 
nominal wall thickness for the pipe was about 0.688 inch.  During the 
current refueling outage, wall thinning was found in one of the 20 inch by 
12 inch reducing tee risers approximately 10 inches downstream from the tee 
in the 12 inch pipe section, immediately above a circumferential pipe weld.  
During the previous refueling outage (18 months ago) the licensee had 
measured the pipe wall as 0.619 inch thick at that location.  During the 
current refueling outage, the licensee measured a thickness of 0.521 inch at 
the same location.  The licensee measured a thickness of 0.482 inch within 
about 2 inches of that location.  The licensee calculated a minimum 
allowable wall thickness of 0.440 inch for that portion of FW pipe.  
Previous experience and models had indicated an erosion wear rate of no more 
than 0.085 inch each cycle.  However, the most 


9205010009
.

                                                            IN 92-35 
                                                            May 6, 1992 
                                                            Page 2 of 3 


recent measurement indicates a higher wear rate that may be greater than 
0.100 inch each cycle.  

The licensee evaluated the data for the FW system and determined that 
continued operation could not be justified for another fuel cycle.  
Therefore, the licensee repaired, rather than replaced, the FW pipe in 
accordance with Section XI of the American Society of Mechanical Engineers 
(ASME) Boiler and Pressure Vessel Code.  

Discussion

The licensee determined that although it had expected to find erosion/ 
corrosion at this location, the magnitude of wall thinning exceeded 
expectations.  The licensee is continuing its investigation to determine the 
root cause of the unexpected erosion/corrosion rate.  

Related Generic Communications

Following a pipe rupture at the Surry Power Station in 1986, the NRC issued 
Bulletin 87-01, "Thinning of Pipe Walls in Nuclear Power Plants," July 9, 
1987.  In this bulletin, the staff requested licensees and applicants to 
inform the NRC about their programs for monitoring the wall thickness of 
carbon steel piping in both safety-related and nonsafety-related high energy 
fluid systems.

In 1989, following an audit of the erosion/corrosion programs at 10 plants, 
the NRC issued Generic Letter (GL) 89-08, "Erosion/Corrosion-Induced Pipe 
Wall Thinning," May 2, 1989.  In this generic letter, the staff requested 
licensees and applicants to implement long term erosion/corrosion monitoring 
programs.  The staff made this request to obtain assurances that the 
addressees had implemented procedures or administrative controls to maintain 
the structural integrity of all carbon steel systems carrying high energy 
fluids.

The NRC also issued several information notices on the subject of erosion/ 
corrosion.
.

                                                            IN 92-35 
                                                            May 6, 1992 
                                                            Page 3 of 3 


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 Office of 
Nuclear Reactor Regulation (NRR) project manager.




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

Technical contacts:  K. I. Parczewski, NRR      S. Barber, Region I
                     (301) 504-2705             (717) 542-2134

                     David Gamberoni, NRR       John White, Region I
                     (301) 504-1171             (215) 337-5114

Attachments:
1.  Feedwater System Erosion/Corrosion Location
2.  List of Recently Issued NRC Information Notices

.