United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 90-68: Stress Corrosion Cracking of Reactor Coolant Pump Bolts

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

                               October 30, 1990

Information Notice No. 90-68:  STRESS CORROSION CRACKING OF REACTOR 
                                   COOLANT PUMP BOLTS

Addressees:

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

Purpose:

This information notice is intended to alert addressees of a significant
event  which occurred at a foreign reactor.  The event involves the use of
a material  sensitive to intergranular stress corrosion cracking (IGSCC)
in the  fabrication of the bolts fastening the turning vanes of the
reactor coolant  pumps.  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:

The U.S. Nuclear Regulatory Commission (NRC) has received information 
concerning cracking of the bolts fastening the turning vanes of the
reactor  coolant pumps at a foreign reactor plant.  There are 23 bolts
fastening the  turning vanes in each of three reactor coolant pumps.  Five
of these bolts had  experienced stress corrosion cracking.  The bolts were
made of a stainless  steel alloy which is designated by the American
Society for Testing and  Materials as A453 grade 660. The material is
commonly described commercially  as alloy A-286.  The reactor coolant
pumps are similar in design to those  manufactured by the Westinghouse
Electric Company.

Considerable information has previously been available regarding the 
susceptibility of alloy A-286 to IGSCC.  For example, licensees of U.S. 
reactors designed by Babcock and Wilcox (B&W) have noted stress corrosion 
cracking of bolts fabricated of alloy A-286.  Specifically, bolts
fastening  the B&W reactor internals, including the core barrel and lower
thermal shield,  were fabricated of alloy A-286.  Between 1981 and 1984,
bolt cracking and  indication of cracking were documented at Oconee,
Rancho Seco, Crystal River  and Arkansas Unit 1.  In response, the B&W
owners group formed a special task  force to study the internal bolts of
the reactor vessel. This task force  documented its conclusions in
BAW-1842, "The B&W Owners Group Evaluation of  Internals Bolting Concerns
in 177 FA Plants," August 1984.  The task force  concluded that bolts
fabricated

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                                                 October 30, 1990 
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of alloy A-286 are subject to IGSCC at peak stresses above 100 ksi.  The 
occurrence of IGSCC also appeared to be a function of chromium content, 
fabrication practice and environment.  As a result of the task force
findings,  the design of the reactor internal bolts was modified at B&W
plants to use  fastener materials that are less sensitive to IGSCC than
alloy A-286 or to  reduce the maximum stress loadings of bolts fabricated
of alloy A-286 to less  than 100 ksi.

In addition, the Brookhaven National Laboratory (BNL) performed an
extensive  study of bolting which is documented in NUREG/CR-3604, "Bolting
Applications,"  May 1984.  The report discusses the direct relationship
between loading and  IGSCC of bolts fabricated of alloy A-286.  In this
report, BNL recommended  that alloy A-286 not be used as a reactor
structural material because of its  susceptibility to IGSCC.

This information notice requires no specific action or written response. 
If  you have any questions about this matter, 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:  Clifford Sellers, NRR 
                     (301) 492-0703 

                     Walton Jensen, NRR 
                     (301) 492-1157 

Attachment:  List of Recently Issued NRC Information Notices

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