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).

ML062910111
IN 2006-24 

Page 2 of 4

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 X750 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 staff’s 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. 
IN 2006-24 

Page 3 of 4

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 95-56, “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-oftolerance 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. 
IN 2006-24 

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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.