United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 84-73: Downrating of Self-Aligning Ball Bushings Used in Snubbers

                                                 SSINS No.: 6835           
                                                 IN 84-73

                                 UNITED STATES
                        NUCLEAR REGULATORY COMMISSION 
                     OFFICE OF INSPECTION AND ENFORCEMENT
                             WASHINGTON, DC 20555

                              September 14, 1984 

Information Notice No. 84-73: DOWNRATING OF SELF-ALIGNING BALL BUSHINGS 
                                 USED IN SNUBBERS

Addressees:                                      

All nuclear power facilities holding an operating license permit (CP).

Purpose:

This information notice is provided as a notification of a potentially 
significant problem pertaining to the downrating of self-aligning ball 
bushings used in both mechanical and hydraulic snubbers. It is expected that 
recipients will review the information for applicability to their facilities 
and consider actions, if appropriate, to preclude a similar problem occurring 
at their facilities. However, suggestions contained in this information
notice  do not constitute NRC requirements and, therefore, no specific action
or  written response is required.

Description of Circumstances:

On May 25, 1984, the Carolina Power and Light Company notified the NRC 
resident inspector of the failure of a steam generator hydraulic snubber at
H.  B. Robinson Plant Unit 2 to pass its operability testing at a rated load
of  470,000 pounds (470 kips). One of the contributors to this failure was
the  failure of the self-aligning ball bushing at a static load of 412 kips. 
Bushing failure could prevent the snubber from properly aligning itself with 
the load and thus lead to failure of the snubber itself. At the time of 
manufacture in 1969, one of the 12 snubbers used at H. B. Robinson Plant Unit 
2 had been successfully tested at its rated load of 470 kips. However, the 
snubbers had not been in-service tested since installation because they were 
exempt from testing requirements as a result of their large size and their 
being difficult to remove. The snubbers (mounted on the steam generators)
were  removed as a part of the steam generator replacement program and were
being  rebuilt for preventive maintenance reasons. The snubber was being
functionally  tested in the as-found condition before rebuilding when the
failure occurred.

Baxter Fluidpower Group (Anker-Holth), the snubber manufacturer, indicated
in  their June 5, 1984, 10 CFR 21 report that the rating for the self-
aligning  ball bushing was 899 kips at the time the snubbers were
manufactured in 1969.  The report continued by noting that subsequent to
manufacture the bushing  manufacturer,


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the Torrington Company, had revised their limit-load rating downward to 300 
kips for this model bushing. The report also indicated that the only other 
snubber with this bushing in it was a 500 kip model used at Point Beach 
Nuclear Plant Units 1 and 2 and that the affected utility had been notified.

In their supplemental 10 CFR 21 report, Baxter Fluidpower Group (Anker-
Holth)  identified two other snubber models which may be affected by this
downrating.  They are an 800 kip model at Point Beach Nuclear Plant Units 1
and 2, and a 90  kip model at Prairie Island Nuclear Station Units 1 and 2.
In both cases, the  affected utilities have been notified.

Conversations with the Torrington Company indicated that this downrating of 
their self-aligning ball bushings took place in 1972 and that they had issued 
a notice of this to their primary customers. They indicated that this 
downrating had been across their entire product line and that it was the 
result of taking a more conservative approach in considering the methods of 
use actually encountered in the industries they were servicing. Before this, 
the ratings had been predicated on their use in the aviation industry where 
the bushing's use more closely approached the ideal conditions used in 
determining the theoretical maximum load the bushing could carry. They also 
indicated that they had supplied self-aligning ball bushings to other snubber 
manufacturers, but that their records are not specific enough to permit them 
to determine which other nuclear facilities, if any, might be affected.

The conversations with the Torrington Company also indicated that at least
one  other self-aligning ball bushing manufacturer in the United States, 
Roller  Bearing Company of America, had also down rated their bushings at 
approximately the same time. This was confirmed by conversations with the 
Roller Bearing Company of America who indicated that, as with the Torrington 
Company, their records were not specific enough to permit them to determine 
which nuclear facilities, if any, might have their bushings.

SKF, a major foreign manufacturer of self-aligning ball bushings, currently 
uses ratings similar to those used by the United States manufacturers before 
1972.

While the bushings at H. B. Robinson Unit 2 had a limit load of only 300
kips,  their ultimate or static fracture rating was 450 kips. Thus, they
should have  most likely survived a test at the snubber rated load of 470
kips. A recent  finite element analysis of the end housing of the bushing
that failed has  indicated that local stresses exceeded the 44,000 psi (44
ksi) certified yield  strength of the housing material. This local yielding
of the housing caused an  unusual load distribution on the bushing, which
contributed to its premature  failure. For a usage such as this, the
Torrington Company's catalogue  recommends that the yield strength of the
housing material be a minimum of 75  ksi. The material specification for the
housing at the end that did not fail  had a minimum yield of 80 ksi.

Paul Munroe Hydraulics, Inc. has assumed the role of responsible organization 
for the snubber product line formerly designed and manufactured by Anker-
Holth. Their 10 CFR 21 reports of July 12 and August 10, 1984 confirmed the 
cause of the failure to be that described above and also indicated that the 
result of

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the failure is total inoperability of the snubber as a safety-related 
component support for the steam generators. A design review by Paul Munroe 
Hydraulics, Inc.  considered both the bushing downrating and the yield 
strength of the end housings.  This review indicated that similar problems 
exist at four additional sites (Arkansas Nuclear One Unit 1, Crystal River 
Nuclear Plant Unit 3, Kewaunee Nuclear Power Plant, and Trojan Nuclear Plant 
Unit 1) and with snubbers with ratings from 90 kips to 2000 kips.

While the actions taken by Paul Munroe Hydraulics, Inc. appear to resolve
this  issue for their product line, there is still the possibility that
snubbers  from other manufacturers may have self-aligning ball bushings with
improper  ratings or end housings.

No written response to this information notice is required. If you have any 
questions about this matter, please contact the Regional Administrator of the 
appropriate NRC Regional Office, or this office.


Edward L. Jordan, Director
Division of Emergency Preparedness 
and Engineering Response
Office of Inspection and Enforcement

Technical Contact: R. J. Kiessel, IE
                   (301) 492-8119

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