Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
June 30, 1994
NRC INFORMATION NOTICE 94-48: SNUBBER LUBRICANT DEGRADATION IN
HIGH-TEMPERATURE ENVIRONMENTS
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 the possible degradation of the lubricant used
in mechanical snubbers manufactured by Pacific Scientific (PSA) when the
snubber is used in a high-temperature environment. 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
During the ninth refueling outage at the Joseph M. Farley Nuclear Plant,
Unit 2 at the end of 1993, the Southern Nuclear Operating Company (the
licensee) functionally tested a number of safety-related snubbers as
prescribed by its technical specification surveillance requirements. From an
original test sample of 88 mechanical snubbers, the licensee identified
8 failures. Two of these failures, a PSA-1/4 on a steam generator blowdown
line and a PSA-10 on a pressurizer safety valve, led to the discovery of four
additional snubbers that failed their functional tests for common causes
(three more PSA-1/4s on steam generator blowdown lines and another PSA-10 on
the pressurizer).
The licensee concluded that high environmental temperatures (ranging from 38
to 93 C [100 to 220 F]) for extended periods had degraded the internal
lubricants used in these PSA snubbers. The licensee determined that the
failure mechanism of the PSA-1/4s was a loss of lubricant, and for the PSA-10s
it was a breakdown of lubricant viscosity. Elevated temperatures caused the
PSA-1/4 snubber grease to bake and dry up, resulting in complete lockup when
these snubbers were tested. For the PSA-10 snubbers, the high temperatures
caused the grease to lose almost all viscosity, resulting in insufficient drag
resistance during testing.
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June 30, 1994
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The licensee subsequently replaced all failed snubbers, and conducted physical
inspections and engineering evaluations to ascertain if any structural damage
had occurred. No support damage or system operability concerns were
identified by the licensee.
Discussion
The licensee does not have the capability to overhaul mechanical snubbers.
Consequently, unlike their hydraulic counterparts, mechanical snubbers are not
part of any routine preventive maintenance program. PSA mechanical snubbers
are purchased and installed with the expectation that they are maintenance
free for the duration of their life in the plant. The physical conditions of
all snubbers are examined by visual inspections which, depending on the number
of unacceptable snubbers identified in the previous inspection interval, could
be performed once in every 36 months. Although the operability of some
snubbers is verified by functional testing each refueling outage, only a small
sample of the snubber population is exercised and tested. Past industry
experience with snubber failures has demonstrated that the failure modes of
mechanical snubbers are not readily discernible by visual inspection or even
physical stroking. Only functional testing under actual load conditions
appears to confirm their operability. For example, of all the functionally
tested mechanical PSA snubber failures identified by the licensee for both
units since commercial operation, only one was discovered by visual
inspection.
Depending on the size and distribution of the snubber population, sample
selection techniques and failure rates, it may take decades before all plant
snubbers are actually tested. In the case of Farley Unit 2, none of the
failed PSA snubbers had been tested since the unit began commercial operation
in 1981. The subject PSA-1/4 and PSA-10 snubbers were installed during
original construction and were about 15 years old.
The licensee concluded that the failure of PSA snubbers as the result of
lubricant degradation after extended exposure to high temperatures is a new
issue. The Snubber Utility Group (SNUG), of which the licensee is a member,
and PSA are aware of the issue. In December of 1993, PSA sent letters to all
users of its snubbers, including Farley Unit 2, informing them of these
grease-related problems. Although PSA snubbers are lubricated with a
radiation-resistant grease (i.e., NRRG-159), recent test results from PSA and
the industry also indicate there could be an adverse impact on the service
life of snubbers in environments with elevated temperatures. PSA and the SNUG
Lubrication Working Group have requested users to inspect and evaluate all PSA
snubbers removed from service because of preventive maintenance or functional
failure for possible lubrication degradation. SNUG also requested utilities
to forward any relevant information using a standard data sheet.
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June 30, 1994
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PSA and SNUG are establishing a database to analyze and develop criteria based
on time and temperature for a service life program to be used by all
utilities. The NRC staff is following the above industry activities and will
evaluate whether additional NRC actions are necessary.
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.
/s/'d by BKGrimes
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: Floyd S. Cantrell, RII
(404) 331-5534
Arnold J. H. Lee, NRR
(301) 504-2758
Attachment:
List of Recently Issued NRC Information Notices
