Information Notice 2006-24, Recent Operating Experience Associated with Pressurizer and Main Steam Safety/Relief Valve Lift Setpoints
| ML062910111 | |
| Person / Time | |
|---|---|
| Issue date: | 11/14/2006 |
| From: | Michael Case NRC/NRR/ADRA/DPR |
| To: | |
| Carla P. Roquecruz, 301-415-1455 | |
| References | |
| IN-06-024 | |
| Download: ML062910111 (5) | |
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR REACTOR REGULATION
WASHINGTON, D.C. 20555
November 14, 2006 NRC INFORMATION NOTICE 2006-24:
RECENT OPERATING EXPERIENCE
ASSOCIATED WITH PRESSURIZER AND MAIN
STEAM SAFETY/RELIEF VALVE LIFT
SETPOINTS
ADDRESSEES
All holders of operating licenses, except those who have permanently ceased operations and
have certified that fuel has been permanently removed from the reactor vessel.
PURPOSE
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
addressees of operating experience insights associated with the pressurizer safety valves
(PSVs), main steam safety valves (MSSVs) and boiling-water reactor (BWR) safety/relief valves
(SRVs) involving valves that exceeded the lift setpoint tolerance required by technical
specifications (TS). 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 IN are not NRC requirements; therefore, no specific action or
written response is required.
DESCRIPTION OF CIRCUMSTANCES
The NRC staff reviewed licensee event reports (LERs) that were submitted in accordance with
Title 10 of the Code of Federal Regulations (10 CFR), Section 50.73, and found that from 2001 through August 2006, 72 LERs involved instances where PSVs, MSSVs or SRVs failed to meet
the TS-allowed lift setpoint tolerance. Specifically, the NRC staff noted there were 11 LERs in
2001, 17 LERs in 2002, 9 LERs in 2003, 6 LERs in 2004, 14 LERs in 2005, and 8 LERs in 2006 (as of August).
The PSVs, MSSVs and SRVs are typically tested during outages, or removed during outages
and tested afterwards. When found outside their acceptance criteria, the valves have most
likely been in use during plant power operation with the lift setpoint outside the required
tolerance limit specified by TS. Therefore, it is important that appropriate preventive and
corrective measures be taken to ensure the valves reliable operation prior to installation.
Many LERs describe comprehensive root cause analyses and corrective actions. The LERs of
2005 and 2006 indicated that some of the main and contributing root causes included: random
setpoint drift, corrosion bonding/binding of the valve disk and seat assembly, micro galling of the
valve seat, foreign material, and manufacturing or installation defects. Setpoint drift is the result
of many random variables and is usually considered a function of time since calibration and
setting. Corrosion bonding and micro galling have been common industry problems related to
design problems and operational conditions. Some of the corrective actions that were planned
or taken include: replacing the valve disks with the pre-oxidized X-750 Inconel material (which
is believed to be less susceptible to corrosion), revision of test periodicity, and enhanced use of
operating experience to identify emerging failure mechanisms and prevent them.
Operating experience shows that new failure mechanisms or root causes can emerge. For
example, a licensee reported in its LER in 2005 that one SRV pilot setpoint remained out of
tolerance high, and it was caused by internal binding between the SRV pilot rod and spherical
collar, which resulted from a lack of a chamfer on the spherical collar. This was considered a
unique case, as industry operating experience and the vendor refurbishment experience
showed that there were no previous similar failures. (The additional details for this particular
event are available in Agencywide Documents Access and Management System (ADAMS)
Accession No. ML051660231.)
The NRC staffs evaluation of the LERs for 2005 and 2006 revealed that the safety significance
of individual events was generally low or minimal since the as-found valve setpoints were within
the American Society of Mechanical Engineers (ASME) tolerance limit or within the accident
analyses. However, a licensee reported in its LER in 2005 that one of the SRVs failed to
reclose when tested. If the valve had actuated during plant operation, it could have stayed
open, causing a plant transient or complicating a transient that caused the valve actuation. The
licensee determined that likely cause was improper orientation of the main spring during valve
re-assembly at an offsite facility. (The additional details for this particular event are available in
ADAMS Accession No. ML053270220.)
Since 2005, NRC inspectors identified several findings involving a non-cited violations of
10 CFR Part 50, Appendix B, Criterion XVI, Corrective Actions, for the licensees lack of timely
actions to promptly identify and correct out-of-tolerance lift setpoints for the valves specified.
Although the findings were all Green, i.e., issues of very low significance, the number of findings
along with the number of reported events highlight the extent of the valve setpoint issue at
various plants.
BACKGROUND
The issue of pressurizer and main steam safety or relief valve lift setpoints found to be out of
tolerance has been a longstanding industry issue that has been the subject of a number of
previous NRC generic communications including:
IE Bulletin 80-25, Operating Problems with Target Rock Safety-Relief Valves at BWRs
IN 82-41, Failure of Safety/Relief Valves to Open at a BWR
IN 83-82, Failure of Safety/Relief Valves to Open at a BWR - Final Report
IN 84-33, Main Steam Safety Valve Failures Caused by Failed Cotter Pins
IN 85-05 and its Supplement, Main Steam Safety Valve Test Failures and Ring Setting
Adjustments
IN 86-56, Reliability of Main Steam Safety Valves
IN 86-12, Target Rock Two-Stage SRV Setpoint Drift
IN 88-30, Target Rock Two-Stage SRV Setpoint Drift Update
IN 94-56, Inaccuracy of Safety Valve Set Pressure Determinations Using Assist
Devices
IN 96-03, Main Steam Safety Valve Setpoint Variation as a Result of Thermal Effects
DISCUSSION
The PSVs, MSSVs, and SRVs are used to satisfy ASME Code requirements for overpressure
protection and are designed to prevent the system pressure from exceeding a limit of 110% of
the design pressure. The valves are also credited in mitigating the effects of postulated
accidents (e.g., loss of external electrical load, loss of normal feedwater). The consequences of
exceeding the ASME pressure limit could include damage to system components, increased
leakage, or a requirement to perform additional stress analyses prior to resumption of reactor
operation.
Despite previous industry efforts to address this longstanding problem, 72 LERs were submitted
during the period 2001 through August 2006 involving the as-found lift setpoint being outside
the specified TS limit. Random setpoint drift is a recognized phenomenon that is valid for
licensees to include their LER root cause discussion. However, when several out-of-tolerance
valves are routinely identified at particular plant, this suggests that other causes may also be a
factor. There is a great deal of operating experience available to licensees to assist them in
identifying and correcting these other causes. While the safety significance of recent instances
of out-of-tolerance valves has been minimal, some plants have found it necessary to perform
the surveillance and maintenance more frequently in order to achieve acceptable valve
performance.
CONTACT
This information notice does not require any specific action or written response. Please direct
any questions about this matter to the technical contacts listed below.
/RA by Ted Quay for/
Michael Case, Director
Division of Policy and Rulemaking
Office of Nuclear Reactor Regulation
Technical Contacts: Ian Jung, NRR/DE
Gary Hammer, NRR/DCI
301-415-5562
301-415-7363 E-mail: ixj@nrc.gov
E-mail: cgh@nrc.gov
Note: NRC generic communications may be found on the NRC public Web site, http://www.nrc.gov, under Electronic Reading Room/Document Collections.
ML062910111 OFFICE
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NAME
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CHammer
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DATE
10/23/2006
10/23/2006
10/23/2006
10/24/2006
10/25/2006 OFFICE
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CHawes by e-mail
DBeaulieu
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MCase
DATE
10/18/2006
11/09/2006
11/08 /2006
11/13/2006
11/14/2006