Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
February 16, 1993
NRC INFORMATION NOTICE 93-13: UNDETECTED MODIFICATION OF FLOW CHARACTERISTICS
IN THE HIGH PRESSURE SAFETY INJECTION SYSTEM
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 potential for changes of system flow
characteristics to go undetected as occurred at Arkansas Nuclear One (ANO)
Unit 2. 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
On September 23, 1992, with Unit 2 shut down, Entergy Operations, Inc. (the
licensee) evaluated flow imbalances that were found during testing of the high
pressure safety injection (HPSI) system. The licensee determined that the
flow rates through five of the system valves were less than required. Because
of the low flow rates, the licensee concluded that the sum of the flow rates
of the three injection paths with the lowest flow rates was less than that
assumed in the plant design basis calculations which support the plant safety
analysis.
The licensee investigated the event and determined that replacement stem disc
assemblies, supplied as "like for like" and installed as early as 1982, were
not identical to the original assemblies. Subsequent to the event, the
licensee had the vendor rework spare valve discs to meet the design
requirements and installed these in the five affected valves. The licensee
then conducted flow balance testing to ensure that all system flow
requirements were met.
9302100227.
IN 93-13
February 16, 1993
Page 2 of 3
Discussion
The licensee reviewed documentation for the affected discs and determined
that, as a result of a vendor drawing error in 1978, certain design changes of
the stem disc assemblies had not been incorporated into drawings for
ANO Unit 2. The most significant of these was a 2 to 3 mm [1/10-inch] change
in diameter at one location on the replacement discs. This change was not
easily detected by visual examination. However, the as-found flow through the
affected injection paths was degraded an average of 23 percent.
The technical specifications for ANO Unit 2, require flow balance testing
after system modifications which could affect flow characteristics, but do not
require periodic flow balance testing. The licensee did not recognize that
the replacement stem disc assemblies were different from the original stem
disc assemblies and, therefore, did not test the system after changing the
components.
The licensee discovered the flow imbalances and degraded flow rates during a
full flow test performed in response to Generic Letter 89-04, "Guidance on
Developing Acceptable Inservice Testing Programs." During this test, the
total indicated flow was found to be lower than the actual flow. After
investigation, the difference was found to have been caused by a flow orifice
that had been installed backwards. While investigating the problem, the
licensee determined the actual system flow by isolating the hot leg injection
paths and summing the indicated flows of the four cold leg injection paths.
From these measurements, the licensee found that the flows varied greatly.
The large variation in indicated flows of the cold leg injection paths led the
licensee to find the improper discs.
Although the individual loop flows varied greatly, the total flow of the
system met the acceptance criteria. Performing full flow testing alone would
not have caused the licensee to find the degraded flow in parts of the system.
Therefore, the flow imbalances might have remained undetected if the flow
orifice had been installed correctly and investigation of the flow orifice
problem not been required.
The licensee reviewed the technical specifications for similar surveillance
test requirements. The licensee then evaluated the similar tests to determine
whether they were adequate to fully test the system and were required to be
performed at appropriate times. Upon completing this review and another
review to find systems with similar valves, the licensee performed flow
balance testing on the low pressure safety injection system with satisfactory
results. The licensee revised the HPSI flow testing procedure to provide for
confirmation of satisfactory flow balance and capacity during each refueling
outage.
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IN 93-13
February 16, 1993
Page 3 of 3
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.
ORIGINAL SIGNED BY
Brian K. Grimes, Director
Division of Operating Reactor Support
Office of Nuclear Reactor Regulation
Technical contacts: William Johnson, RIV
(817) 860-8148
Linda Smith, RIV
(501) 968-3290
Dennis Kelley, RIV
(817) 860-8289
Attachment: List of Recently Issued NRC Information Notices
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