Information Notice No. 93-68: Failure of Pump Shaft Coupling Caused by Temper Embrittlement During Manufacture

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

                               September 1, 1993



All holders of operating licenses or construction permits for nuclear power


The U.S. Nuclear Regulatory Commission (NRC) is issuing this information
notice to alert addressees to problems caused by temper embrittlement of
American Iron and Steel Institute Type 410 stainless steel couplings supplied
by Byron Jackson.  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

On June 20, 1991, a river water pump shaft coupling at Beaver Valley Nuclear
Power Plant, Unit 1, failed during operation when a large section of one end
of the coupling broke away from the rest of the coupling.  This coupling,
which was threaded internally, was used to join two shafts of a Byron Jackson
vertical circulator river water pump.  During its investigation of the
failure, the licensee found that two more couplings from the same pump (river
water pump 1A) had cracks.  All three of the Unit 1 pump shafts had at least
one of the defective couplings.  The licensee at Beaver Valley noted that
increased vibration levels on pump 1A caused by a worn bearing, pump shaft
misalignment or both contributed to the failure.

All of these couplings were part of a group of 13 couplings that had been
acquired in 1977 from Byron Jackson under the same purchase order.  However,
the licensee was only able to locate six of the couplings for examination and
surmised that the rest were used and had been disposed of during routine
maintenance.  All of these couplings had been fabricated from the same heat
treatment batch of Type 410 stainless steel (steel heat number HT821336).

Metallurgists at Lehigh University conducted a root cause analysis of the
damaged Unit 1 couplings and concluded that failure was caused by the low
impact strength of the couplings due to temper embrittlement resulting from 


                                                            IN 93-68
                                                            September 1, 1993
                                                            Page 2 of 3    

improper heat treatment.  They also concluded that all of the couplings from
this heat of steel were temper embrittled.  Under NRC contract, Brookhaven
National Laboratory conducted a detailed metallurgical failure analysis of the
broken coupling taken from pump 1A.  Brookhaven similarly concluded that the
failure was caused by temper embrittlement of the Type 410 stainless steel
brought about by improper heat treatment of the coupling during manufacture. 

In 1992, three couplings in Beaver Valley Unit 2 river water pumps were found
to have cracks:  two in pump 2A and one in pump 2C.  These pumps together with
the couplings were also supplied by Byron Jackson.  The Unit 2 couplings are
keyed sleeves instead of threaded couplings.  The cracks were located in the
corners of the keyway slots.  Lehigh University metallurgists examined these
couplings and found them to be temper embrittled also.  However, the Unit 2
couplings were produced from material with a heat number (heat number 72337)
different from that of the damaged couplings previously found in the Unit 1


The Brookhaven analysis of the first failed coupling included impact tests on
subsized Charpy V notch samples that were machined from the coupling material. 
These samples were about 7.5 mm [0.295 in.] thick and 10 mm [0.394 in.] wide. 
Two of these samples were treated at 704�C [1300�F] for 1 hour and oil
quenched.  In its report, Brookhaven concluded that the impact energy of the
as-received material was extremely low (11 to 20 joules [8 to 15 ft-lbf]) and
was markedly increased (+136 to +163 joules [+100 to +120 ft-lbf]) after
corrective heat treatment.

The Beaver Valley licensee also reported that Charpy impact tests on material
from the damaged Unit 1 couplings indicated low measured energy absorption
values of 11 joules [8 ft-lbf] or less using a 7 mm by 10 mm specimen.  The
licensee replaced all of the failed and questionable couplings used on the
river water pumps of both units with couplings that had been screened for
toughness and proper heat treatment on the basis of impact properties.  The
licensee also modified the purchase specifications for the couplings to
include minimum impact energy requirements for the coupling material.  The
licensee issued a 10 CFR Part 21 notification to the NRC on November 1, 1991,
regarding the Unit 1 Byron Jackson river water pump couplings and a 
10 CFR Part 21 notification on October 29, 1992, regarding the Unit 2

The NRC staff understands that Byron Jackson vertical circulator pumps with
coupling configurations similar to those at Beaver Valley are commonly used. 
Because couplings manufactured from two different heats of Type 410 stainless
steel have been found with temper embrittlement at Beaver Valley, Byron
Jackson stainless steel couplings at other locations also may have low impact
strength due to temper embrittlement.  Pump shafts containing temper.

                                                            IN 93-68
                                                            September 1, 1993
                                                            Page 3 of 3    

embrittled couplings could fail during operation if the pump has worn
bearings, the pump shaft is misaligned, or shaft motion is impeded by silt or
debris ingestion.

This information notice requires no specific action or written response.  If
you have any questions about the information in this notice, please contact
the technical contact listed below or the appropriate Office of Nuclear
Reactor Regulation (NRR) project manager.

                                    /s/'d by BKGrimes

                                    Brian K. Grimes, Director
                                    Division of Operating Reactor Support
                                    Office of Nuclear Reactor Regulation

Technical contact:  James A. Davis, NRR
                    (301) 504-2713

List of Recently Issued NRC Information Notices.

Page Last Reviewed/Updated Wednesday, March 24, 2021