Information Notice 1997-79, Failures of Main Steam Isolation Check Valves Resulting in Disc Separation

Failures of Main Steam Isolation Check Valves Resulting in Disc Separation

ML031130139
Person / Time
Site:	Beaver Valley, Millstone, Hatch, Monticello, Calvert Cliffs, Dresden, Davis Besse, Peach Bottom, Browns Ferry, Salem, Oconee, Mcguire, Nine Mile Point, Palisades, Palo Verde, Perry, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Oyster Creek, Watts Bar, Hope Creek, Grand Gulf, Cooper, Sequoyah, Byron, Pilgrim, Arkansas Nuclear, Three Mile Island, Braidwood, Susquehanna, Summer, Prairie Island, Columbia, Seabrook, Brunswick, Surry, Limerick, North Anna, Turkey Point, River Bend, Vermont Yankee, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Vogtle, Waterford, Duane Arnold, Farley, Robinson, Clinton, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Quad Cities, Humboldt Bay, La Crosse, Big Rock Point, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, FitzPatrick, McGuire, LaSalle, Fort Saint Vrain, Shoreham, Satsop, Trojan, Atlantic Nuclear Power Plant
Issue date:	12/20/1990
From:	Rossi C Office of Nuclear Reactor Regulation
To:
References
IN-90-079, NUDOCS 9012140086
Download: ML031130139 (10)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR REACTOR REGULATION

WASHINGTON, D.C. 20555 December 20, 1990

NRC INFORMATION NOTICE NO. IN 90-79: FAILURES OF MAIN STEAM ISOLATION CHECK

VALVES RESULTING IN DISC SEPARATION

Addressees

All holders of operating licenses or construction permits for nuclear power

reactors.

Purpose

This information notice is intended to alert addressees to potential problems

involving the design and location of main steam isolation check valves (MSCVs)

that could result in disc separation as a consequence of fatigue failure of the

post (or stud). 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 do

not constitute NRC requirements; therefore, no specific action or written

response is required.

Description of Circumstances

On October 8, 1990, the Tennessee Valley Authority (the licensee) noted that

three of four MSCVs had failed at the Sequoyah Nuclear Power Plant, Unit 1. Each

failure involved the disc separating from the swing arm. In 1982, the disc on

one of the same Sequoyah MSCVs had become disconnected, and in 1983, the disc on

a main steam isolation valve (MSIV) at the Joseph M. Farley Nuclear Plant, Unit 2 (effectively the same design as the Sequoyah MSCVs) became disconnected, too.

(See 'Discussion' below.)

In March 1990, before the recent MSCV failures, the licensee inspected an MSCV at

Sequoyah Unit 1 in response to Electric Power Research Institute (EPRI) guidance

(see below). The licensee found that this MSCV was severely worn where the

surface of the swing arm collar met the surface of the disc. The valve post that

connects the disc to the swing arm was worn at the unthreaded portion that meets

the swing arm collar; and both washers, originally located where the threaded and

unthreaded portions of the post meet, were missing. As a result of these

findings, the licensee inspected the other three MSCVs and found that two of them

had similar levels of degradation. To correct the problem, the licensee, in

consultation with the valve manufacturer, modified the posts of all three

affected MSCVs by machining down the unthreaded portion of the posts, adding weld

buildup using high-hardness filler material, and remachining the posts to their

original diameter. They also redistributed the stresses at the post-to-disc

connections by reducing the thread torque preload on the post from 2800 to 2400

f t

~8

K-' V IN 90-79 December 20, 1990 In September and October 1990, personnel at Sequoyah Unit 1 heard loud noises

coming from the steam piping. A radiograph revealed that an MSCV disc had

separated from its swing arm. Further investigation revealed that the discs in

two other MSCVs had also separated from their respective arms. The licensee

found that the detached disc in one MSCV had lodged against its valve stop, and

the detached discs from the other MSCVs had traveled several hundred feet through

the piping and had lodged Just upstream of the turbine throttle valves. All '

valve posts had broken at a machined fillet where the portion threaded into the

disc joins the unthreaded portion that passes through the swing arm collar (see

Figure 1). In addition, an inspection of the MSCVs in Unit 2 revealed cracks

similar to those that led to the 1982 failure at Unit 1. The Unit 2 valves had

not received the weld buildup modification noted above.

Discussion:

The MSCVs at Sequoyah Unit 1 were manufactured by Atwood & Morrill Co., Inc.

These valves are 32-inch, articulated, swing check valves with a single-post

design and are located just downstream of the main steam isolation valves

(MSIVs). All of the MSCVs that experienced disc post failures are actually

welded to their corresponding MSIVs; the MSCV with the disc still intact was

located 2.5 pipe diameters downstream of its corresponding MSIV and was not

severely worn. The NRC staff believes that although the valve location may have

made the MSCVs susceptible to high-cycle, low-stress fatigue failure, the weld

buildup modification in 1990 appears to have so accelerated the rate of fatigue

wear that the posts failed within seven months.

EPRI studied the mechanisms that cause check valves to degrade and the methods to

prevent this degradation. The results of this study are documented in EPRI

NP-5479, Application Guidelines for Check Valves in Nuclear Power Plants,"

January 1988. The EPRI study indicates that placing check valves just downstream

of a source of turbulence could cause failure and is 'deserving of priority

attention." Flow disturbances Just upstream of a check valve can cause discs to

flutter and subject valve internals to cyclic loads and premature failure.

The backstop design may be another possible contributor to the MSCY failures at

Sequoyah Unit 1. The configuration'is such that a single backstop makes contact

with the disc edge. The swing arm is beveled where it contacts the disc, per- mitting oscillation of the disc that is further exaggerated by a 40-mil clear- ance between the post and swing arm collar. This arrangement allows a maximum

resultant disc oscillation of up to approximately 0.25 inch at the rim of the

disc. In the case of Sequoyah Unit 1, the effects of this design were magnified

by having the post built up with filler material'; this modification concentrated

the stresses on the post where it meets the disc and accelerated the failure of

the post. Atwood & Morrill recommended, as corrective action, modifying

Sequoyah's single-backstop MSCV design to a three-point backstop design that may

help limit lateral disc oscillation. The NRC staff has identified that other

Atwood & Morrill and Schutte A Koerting swing check valves with single posts may

be of this design and may, therefore, be susceptible to this mode of failure.

Other valves with a similar design may also be affected.

IN 90-79 December 20, 1990 In previous instances, MSCV discs have separated from their respective swing arms

when the post(s) failed before approaching its design fatigue life. For

instance, the previous Sequoyah Unit 1 failure occurred in 1982 when the post

came unscrewed from its disc when a tack'weld connecting the post to the disc

failed. At Farley Unit 2, in 1983, an Atwood & Morrill, 32-inch MSIV disc

separated when the posts failed. Although neither of these failures involved the

1990 weld buildup modification performed at Sequoyah Unit 1, both cases involved

the articulated check valve design that helps ensure proper disc-to-seat

alignment, but also allows flexure of the disc during flow conditions while the

valve is on the open'backstop. This flexure, caused by flow disturbances, exposed the posts of both the Sequoyah Unit 1 and Farley Unit 2 valves to cyclic

loads and, in the case of Farley Unit 2, resulted in excessive wear and eventual

fatigue failure of the posts.

It appears that any check valve with disc flexure is susceptible to cyclic

fatigue failure -f-the posts. Such features as single-post design, backstop

contact with the disc edge instead of the disc nut, and valve location near a

source of turbulence, increase the susceptibility of a check valve to cyclic

fatigue failure. The result may be that the post unscrews from the disc, unscrews from the nut, or simply fails; and the disc separates. An example of

such a failure is a main feedwater regulator check valve (manufactured by

Pacific Valve Company) disc separation at the San Onofre Nuclear Generating

Station, Unit 1, in 1985, when the post failed because of high flow velocity and

valve installation near a source of turbulence. (Other recent examples of

failures of check valve internals are discussed in Information Notice 90-03,

"Malfunction of Borg-Warner Bolted Bonnet Check Valves Caused by Failure of the

Swing Arm", dated January 23, 1990, Information Notice 89-62, Malfunction of

Borg-Warner Pressure Seal Bonnet Check Valves Caused by Vertical Misalignment of

Disk", dated August 31, 1989, NRC Bulletin No. 89-02, "Stress Corrosion Cracking

of High-Hardness Type 410 Stainless Steel Internal Preloaded Bolting in Anchor

Darling Model S350W Swing Check Valves or Valves of Similar Design", dated

July 19, 1989, and Information Notice 83-54, *Common Mode Failure of Main Steam

Isolation Nonreturn Check Valves", dated August 11, 1983.)

Among the corrective actions taken by the industry in response to these check

valve failures are: tack welding the post to the disc, better disc nut locking

devices, interference threads for the post, multiple-backstop designs, and

multiple-post designs. Despite these actions, check valves continue to fail

prematurely. Most recently, a potential problem was found at Sequoyah Unit 2 during their 1990 refueling outage, where an MSCV had cracks in its post and disc

near the post-to-disc weld similar to those that led to the 1982 post-to-disc

weld failure at Unit 1. These cracks occurred despite the corrective actions--

i.e., reweld of the disc and increased torque preload on the post--taken by the

licensee in response to the 1982 post failure.

Check valves are used widely in the nuclear industry and can play a key role in

the safe operation of the plant. -EPRI has taken steps to improve the longevity

of these valves by providing guidance for improving check valve reliability. The

NRC staff also has developed an action plan (NUREG-1352) aimed at improving check

valve reliability. An underlying concern with check valve reliability appears to

N-' - IN 90-79 December 20, 1990 be that certain features, such as valve installation location and backstop

design, may increase the susceptibility of check valve internals to premature

cyclic fatigue failure.

This information notice requires no specific action or written response. If

you have any questions about this matter, please contact one of the technical

contacts listed below or the appropriate NRR project manager.

Charles E. Ross , D rector

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Angie P. Young, NRR

(301) 492-1167 Yun-Seng Huang, NRR

(301) 492-1417 Edward Girard, RII

(404) 841-4186 Attachments:

1. Figure 1. Articulated, Swing Check Valve with Single Backstop

2. List of Recently Issued NRC Information Notices

Attachment 1 IN 90-79 December 20, 1990 Figure 1. Articulated, Swing Check Valve

with Single Backstop

'

Attachment 2 IN 90-79 December 20, 1990 ,

LIST OF RECENTLY ISSUED

NRC INFORMATION NOTICES

Information Date of

Notice No. Subject Issuance Issued to

90-78 Previously Unidentified 12/18/90 All holders of OLs or

Release Path from Boiling CPs for boiling water

Water Reactor Control Rod reactors (BWRs).

Hydraulic Units

90-77 Inadvertent Removal of Fuel 12/12/90 All holders of OLs or

Assemblies from the Reactor CPs for pressurized- Core water reactors (PWRS).

88-23, Potential for Gas Binding of 12/10/90 All holders of OLs or

Supp. 3 High-Pressure Safety Injection CPs for pressurized- Pumps During A Loss-Of-Coolant water reactors (PWRs).

Accident

90-76 Failure Of Turbine Overspeed 12/7/90 All holders of OLs or

Trip Mechanism Because Of CPs for nuclear power

Inadequate Spring Tension reactors.

90-75 Denial Of Access To Current 12/5/90 All Michigan holders

Low-Level Radioactive Waste of NRC licenses.

Disposal Facilities

90-74 Information on Precursors To 12/4/90 All holders of OLs or

Severe Accidents CPs for nuclear power

reactors.

90-73 Corrosion Of Valve-To- 11/29/90 All holders of OLs or

Torque Tube Keys In CPs for nuclear power

Spray Pond Cross Connect reactors.

Valves

90-72 Testing of Parallel Disc 11/28/90 All holders of OLs or

Gate Valves In Europe CPs for nuclear power

reactors.

90-71 Effective Use of Radi- 11/6/90 All NRC licensees

ation Safety Committees authorized to use by- to Exercise Control Over product material for

Medical Use Programs medical purposes.

90-70 Pump Explosions Involving 11/6/90 All uranium fuel

Ammonium Nitrate fabrication and

conversion facilities.

OL = Operating License

CP = Construction Permit

IN 90-79 December 20, 1990 be that certain features, such as valve installation location and backstop

design, may increase the susceptibility of check valve internals to premature

cyclic fatigue failure.

This information notice requires no specific action or written response. If

you have any questions about this matter, please contact one of the technical

contacts listed below or the appropriate NRR project manager.

KgInaM Signed by

Charles -. rNo, 5 °,stbirector

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Angie P. Young, NRR

(301) 492-1167 Yun-Seng Huang, NRR

(301) 492-1417 Edward Girard, RII

(404) 841-4186 Attachments:

1. Figure 1. Articulated, Swing Check Valve with Single Backstop

2. List of Recently Issued NRC Information Notices

OFC :OEAB:DOEA :EMEB:DET :OEAB:DOEA :RII :RPB:ADM :C:OEAB:DOEA :

NAME :APYOUNG* :YSHUANG* :DFISCHER* :EGIRARD* :TECH ED* :ACHAFFEE*  :

DATE :11/02/90 :11/08/90 :11/14/90 :11/14/90 :12/07/90  : 11/16/90  :

OFC :D:DET :C:OGCB:DOEA :D:DOEA  :

NAME :JERICHARDSON*:CHBERLINGER

---

 :------------- :------------ ------------------ ------------ ------------:---

DATE :11/21/90 :12/11/90  :/J / /90  :

OFFICIAL RECORD COPY

Document Name: CHECKM *See previous concurrence

IN 90-

November XX, 1990 This information notice requires no specific action or written response. If you

have any questions about this matter, please contact one of the the technical

contacts listed below or the appropriate NRR project manager.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Angie P. Young, NRR

(301) 492-1167 Yun-Seng Huang, NRR

(301) 492-1417 Edward Girard, RII

(404) 841-4186 Attachments: (1) Diagram of Articulated, Swing Check Valve with Single Backstop

(2) List of Recently Issued KRC Information Notices

• See previous concurrence

OFC :OEAB:DOEA :EMEB:DET :RII

MOEAB;DEA :RPB:ADM :C:OEAB;DOEA :

---

---- z----- t ar---------r-^-------------}6tt--- ---

NAME :APYOUNG*  : G :DF ER EGIRARD :CHAFFEE:

_ _  :------H D------------------------:

DATE :11/02/90  :(T /90 :/g/o/*S/9 :1I  : (' /1'/90 :

.D.C:OGCB:DOEA

E :D:DOEA

' NAM4E :JERICHA ON:CHBERLINGER :CEROSSI  :  :  :  :

DATE :11/i I/90  : / /90: / /90:

OFFICIAL RECORD COPY

Document Name: CHECK1

The previous Sequoyah Unit 1 failure occurred in 1982 and involved separation

of the disc from the post due to the failure of the post-to-disc weld. At

Farley in 1983, the disc separated when the bolts that attach the disc to the

swing anm failed in an MSCV of similar size and design as those at Sequoyah.

Neither of these failures involved the 1990 weld buildup modification performed

at Sequoyah 1.

During the current refueling outage, a Sequoyah Unit 2 MSCV was found to have

cracks in the its post and disc near the post-to-disc weld. This appears to be

the same potential failure mechanism which caused the MSCV failure at Sequoyah

Unit 1 in 1982.

This information notice requires no specific action or written response. If

you have any questions about this matter, please contact one of the the

technical contacts listed below or the appropriate NRR project manager.

Charles E. Rossi, Director

Division of Operational Events Assessment

Office of Nuclear Reactor Regulation

Technical Contacts: Angie P. Young, NRR

(301) 492-1167 Yun-Seng Huang, NRR

(301) 492-1417 Edward Girard, RII

(404) 841-4186 Attachments: (1) Diagram of Atwood and Morrill Co.,Inc. Check Valve

(2) List of Recently Issued NRC Information Notices

OFC :OEAB:DOEA :EMEB:DET :OEAB:DOEA :RX :RPB:ADM :C:OEAB:DOEA :

-- - - - - -.- K

NAWE :APYOUNG :YSHUANG :DFISCHER :EGIRARD :TECH EDJ 42; :ACHAFFEE  :

DATE : II/ cg /90  : / /90 / /90 / /90 : I IS/g  :/ / /90

OFC :D:DET :C:OGCB:DOEA :D:DOEA  :  :  :

NAME :JERICHARDSON:CHBERLINGER :CEROSSI  :  :  :

DATE : / /90: / /90: / /90:  :  :

OFFICIAL RECORD COPY

Document Name: (CHECK)