United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 82-29: Control Rod Drive (CRD) Guide Tube Support Pin Failures at Westinghouse PWRS

                                                            SSINS No.: 6835 
                                                            IN 82-29 

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

                               July 23, 1982  

Information Notice No. 82-29:   CONTROL ROD DRIVE (CRD) GUIDE TUBE 
                                   SUPPORT PIN FAILURES AT WESTINGHOUSE PWRS 

Addressees: 

All nuclear power reactor facilities holding an operating license (OL) or 
construction permit (CP) using a Westinghouse-designed NSSS. 

Purpose: 

This information notice is provided as notification of an event that may 
have safety significance. It is expected that recipients will review the 
information for applicability to their facilities. No specific action or 
response is required. 

Description of Circumstances: 

Since 1978, several failures of the control rod drive (CRD) guide tube 
support pins have occurred. Westinghouse has notified NRC of these 
occurrences by the following correspondence: 

     1.   June 11, 1979, NS-TMA-2099, Letter to D. Eisenhut from T. M. 
          Anderson concerning support pin and flexure failures in Japan. 

     2.   March 14, 1980, NS-TMA-2214, Letter to Victor Stello from T. M. 
          Anderson; Title 10 CFR Part 21 notification concerning CRD Guide 
          Tube Support Pin Failures at Foreign Plants. 

     3.   April 23, 1980, NS-TMA-2235, Letter to Stephen S. Pawlicki from T.
          M. Anderson summarizing Westinghouse/TVA/NRC meeting on May 20, 
          1980 on Sequoyah guide tube support pins. 

     4.   June 10, 1980, NS-TMA-2254, Letter to Stephen Pawlicki from T. M. 
          Anderson concerning inspection of support pins. 

     5.   May 20, 1982, NS.-EPR-2251, Letter to Victor Stello from E. P. 
          Rahe, Jr., concerning a pin failure at Graveline 1. 

Prior to May of this year, at which time a guide tube pin failed at North 
Anna 1, these failures had occurred only at foreign reactors (Japan and 
France). The pins are used to align the bottom of the CRD guide tube 
assembly into the top of the upper core plate. Two support pins are bolted 
into the bottom plate of each lower gUide tube, and are inserted into the 
top of the upper core plate in a manner that provides lateral support while 
accommodating thermal expansion of the guide tube relative to the core plate
(see attached pin assembly diagram). The pins are about 3 1/2 inches long 
and have a diameter of 0.507 or 0.537 inch (depending on reactor design). 
The pin assembly includes (1) a bolt section 

8204210402 
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                                                           IN 82-29  
                                                           July 23, 1982  
                                                           Page 2 of 3 

to which a nut (sleeve) is threaded to anchor the pin to the guide tube, (2)
at collet that rests against the guide tube, and (3) a leaf spring section 
with the leaf shaped somewhat like a clothespin. The material is Inconel 
X-750, which, depending on the manufacturer and the fabrication date, has 
been solution heat-treated and age hardened at various temperatures and for 
various times. For example, the solution heat treatment temperatures and 
times ranged from 1625F to 2100F and from 1/2 hour to 24 hours; 
age hardening temperatures and times ranged from 1148F to 1544F 
and from 8 hours to 20 hours, respectively. 

The first failures were detected in early 1978 at Mihama Unit 3 in Japan, at
which time the top portion of a support pin with the shank and lock nut 
engaged was found in a steam generator. Subsequent ultrasonic testing (UT) 
showed a possibility of cracks in 103 out of 105 pins at the bolt to collet 
transition region of the pin. Seven of the Mitsubishi-supplied pins were 
then removed and inspected, confirming the UT results. All pins were 
subsequently replaced and UT inspection was conducted at other Japanese 
plants. In all, there have been at least eight support pin failures where a 
pin has actually broken. These occurred with both Westinghouse and 
Mitsubishi-supplied pins. 

In a recent failure at Fessenheim Unit 1 in France, part of a broken pin 
caused considerable damage to a steam generator within 72 hours of its 
failure. It is estimated that the plant will be shutdown for about a year to
repair the steam generator. Although the broken part consists of the bolt 
section including the nut, only the lock nut of the pin has been found and 
the bolt portion is still missing. Previous to the Fessenheim failure, a 
leaf from a support pin was found in ah accumulator check valve at Graveline 
1 in France. It is not known how the leaf traveled to the check valve. 

The only domestic pin failure occurred in May 1982 at North Anna 1. The lock
nut of a support pin was found in steam generator "A" and a smaller piece of
material, also identified as part of a support pin, was found in steam 
generator "C." Damage to the steam generators is considerable, with about 
75% of the tube ends sustaining damage. It is our understanding that the 
plant was shutdown in less than 24 hours after detecting the loose parts in 
the steam generators. It is also our understanding that the reactor 
internals will be video inspected to determine the status of the remaining 
support pins. 

Westinghouse's analysis indicated that the failures are caused by stress 
corrosion cracking (SCC) of pins that are solution heat treated at less than
1800F after which they are age hardened, and then highly stressed 
(60,000 psi nominal on the shank and 130,000 psi on the, leaf spring section
of the pin). The solution heat treatment of the North Anna 1 support pin was
1625F for 1 hour followed by an age hardening treatment.  The torque on
the nut was 210 ft-lb. Westinghouse now recommends that the pins be solution
heat treated at 2000F for 1 hour and age hardened at 1300F for 20 
hours to minimize the SCC problem. Westinghouse also recommends that the 
torque on the lock nut be reduced to 130 to 140 ft-lb. 

The consequences of pin failure for plants with the upper head injection 
(UHI) design was originally considered to be more acute than those for 
non-UHI plants. This concern resulted from the potential for CRD 
misalignment in UHI plants on 
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                                                           IN 82-29 
                                                           May 23, 1982 
                                                           Page 3 of 3 

pin failure. However, domestic operating UHI plants now have support pins 
meeting the recommended material process standards and the pin body design 
has been revised to prevent control rod misalignment on pin failure. 

Westinghouse does not consider CRD misalignment as credible in non-UHI 
plants. The safety consequence of a support pin as a loose part, however, is
still under consideration by NRC. It is important to note that, although a 
single-pin failure is of limited safety significance, the common-mode 
failure mechanism of stress corrosion cracking could cause several pins to 
fail. We are concerned that, if not properly detected, multiple pin failures 
may occur that could affect redundant safety systems. 

If you have any questions regarding this matter, please call the appropriate
regional administrator or this office. 


                              Edward L. Jordan, Director 
                              Division of Engineering and 
                                Quality Assurance 

Technical Contact:  I. Villalva, IE 
                    301-492-9635 

Attachments: 
1. Pin Assembly Diagram 
2. List of Recently Issued IE Information Notices 

Page Last Reviewed/Updated Tuesday, November 12, 2013