Information Notice 1997-24, Failure of Packing Nuts on One-Inch Uranium Hexafluoride Cylinder Valves

Failure of Packing Nuts on One-Inch Uranium Hexafluoride Cylinder Valves

ML031050319
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:	05/08/1997
From:	Teneyck E NRC/NMSS/FCSS
To:
References
IN-97-024, NUDOCS 9705020259
Download: ML031050319 (5)

UNITED STATES

NUCLEAR REGULATORY COMMISSION

OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS

WASHINGTON, D.C. 20555 May 8, 1997 NRC INFORMATION NOTICE NO. 97-24: FAILURE OF PACKING NUTS ON ONE-INCH

URANIUM HEXAFLUORIDE CYLINDER VALVES

Addressees

All U.S. Nuclear Regulatory Commission (NRC) licensees and certificatees authorized to

handle uranium hexafluoride in 30- and 48-inch diameter cylinders.

Purpose

The U.S. Nuclear Regulatory Commission is issuing this information notice to alert

addressees to incidents in which packing nuts on one-inch uranium hexafluoride cylinder

valves were found to be cracked. 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 require- ments; therefore, no specific action or written response is required.

Descrimtion of Circumstances

On March 14, 1997, the United States Enrichment Corporation (USEC) reported that

cracks were observed in packing nuts on one-inch valves designed for use in 30- and 48- inch diameter uranium hexafluoride (UF,) cylinders. The cracks have been observed in the

Copper Development Association (CDA) 636 alloy nuts on valves supplied by Hunt Valve

Co., Inc.

In two instances, a UF, release was observed during pigtail operations on a full cylinder.

The release was visible as a small wisp of smoke after the pigtail was pressurized above

atmospheric pressure. In both instances, a visible crack in the valve packing nut was

observed. The two failed nuts were examined metallurgically and were found to exhibit

intergrannular cracking. As part of the investigation, USEC conducted valve packing nut

inspections. In addition to visual inspections, eddy current inspection techniques were

used to help identify potentially degraded valve packing nuts. Of the 136 Hunt CDA 636 packing nuts, owned by USEC, that were inspected, 13 exhibited cracks. At least one of

the valves with a cracked nut was a valve that was in storage and had not been exposed

to plant process chemicals. Metallurgical analysis was performed on this nut and

confirmed the presence of intergrannular cracks. This indicates that the concern is a

material or manufacturing problem with the nuts. In addition, 20 Hunt CDA 636 nuts from

customer cylinders were examined. Three nuts showed indications of cracking; one had

visible indications. Three nuts that showed no indications of cracking were chosen for

902 PDR X gNOTICEq7-0Zq 9765.668 CI)020 90 A

-I IN 97-24 May 8, 1997 metallurgical examination; one showed slight indications of cracking that had not been

picked up by the eddy current testing. Hunt B1 50 613 nuts and Superior valve nuts of

CDA 636 were also eddy-current-tested; no problems with cracking were identified.

Discussion

Packing nuts with intergrannular cracks were found in valves that were in storage and had

not been exposed to the plant process chemicals or handling practices that affect the

packing nuts. This indicates that the problem is likely a material or manufacturing

problem with the nuts. Three valve packing nut materials are currently used: monel, ASTM B1 50 613 alloy, and CDA 636 alloy. The cracking problem has been observed only

in the CDA 636 alloy nuts on valves supplied by Hunt Valve Co., Inc.

Failure of the packing nut during plant operations to fill or empty a UF6 cylinder could allow

the packing rings to be forcefully ejected from the valve, resulting in a UF, release, which

is a hazard to plant personnel and a challenge to plant safety systems. Failure of a

cylinder valve packing nut and ejection of the valve packing with an open valve on a full

cylinder would constitute a failure of a UF,, confinement system and, therefore, could

create a substantial safety hazard. Cracking of the packing nut on closed cylinder valves

does not pose a hazard.

Previous problems with packing nuts have resulted from heat-treating problems that

resulted in soft nuts that did not meet the required hardness requirements. One previous

investigation of cracking problems concluded that the cracking is the combined result of

the following:

1. Excessive stresses placed on the nut by cold flow and thermal expansion of the

teflon packing rings, resulting from retightening of the nut and repeated heating of

the valves;

2. Mechanical and structural characteristics of the packing nut material are

incompatible with these stresses; and

3. The presence of uranium hexafluoride, hydrofluoric acid, and nascent hydrogen

facilitates cracking.

The American National Standards Institute (ANSI) provides criteria for packaging of

uranium hexafluoride for transport. The criteria are found in ANSI N14.1-1995, "Uranium

Hexafluoride-Packaging for Transport."' This standard provides, in part, specific

information on design, fabrication, and assembly requirements for the one-inch valve

installed in uranium hexafluoride cylinders.

'Copies of industry codes and standards used in a substantive manner in the NRC

regulatory process are maintained at the NRC Library and are available there for reference

by the public. Codes and standards are usually copyrighted and may be purchased from

the originating organization or, if they are American National Standards, from the American

National Standards Institute, 11 West 42nd Street, 13th floor, New York, NY 10036.

IN 97-24 May 8, 1997 The standard states that the valve packing nut is initially to be torqued in the range of 120

to 150 foot-pounds, to compact the teflon packing rings. If leakage at the valve stem

occurs, the packing nut may be retightened. However, excessive force is not to be used

in an attempt to eliminate the leak. The maximum torque permitted by the standard for

retightening the packing nut is 150 foot-pounds.

Related Generic Communications

NRC Information Notice 89-78, dated November 22, 1989, "Failure of Packing Nuts on

One-inch Uranium Hexafluoride Cylinder Valves," concerned cracking of packing nuts from

Descote and Superior Valve Company.

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 NRC regional office.

Or1nd So md By

Elizabeth Ten Eyck, Director

Division of Fuel Cycle Safety

and Safeguards

Office of Nuclear Material Safety

and Safeguards

Technical contacts: Merri Horn, NMSS

(301) 415-8126 E-mail: mlhl@ nrc.gov

Christopher Tripp, NMSS

(301) 415-7733 E-mail: cst@nrc.gov

Attachments:

1. List of Recently Issued NMSS Information Notices

2. List of Recently Issued NRC Information Notices

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97-24.IN

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DATE 4/22/97 j 4/23/97 4/22/97 4/22/97 [ 4/24/97 [ 4/25/97 J_5/01/97_J

C = CUVEK t = WUVLK & tECLUbUI1L rd = NOU 'Ouri

OFFICIAL RECORD COPY

IN 97-xx

May , 1997 -he standard states that the valve packing nut is initially to torqued in the range of 120

t150 foot-pounds, to compact the teflon packing rin If leakage at the valve stem

0CC , the packing nut may be retightened. How , excessive force is not to be used in

an atte to eliminate the leak. The maximu orque permitted by the standard for

retightenin e packing nut is 150 foot-p ds.

NRC Information Notice 89- , d November 22, 1988, "Failure of Packing Nuts on One- Inch Uranium Hexafluori Cylinder ves," concerned cracking of packing nuts from

Descote and Super alve Company.

This informa notice requires no specific act or written response. If you have any

question out the information in this notice, pie e contact one of the technical contacts

listed low or the appropriate NRC regional office.

Sincerely, Elizabeth Ten Eyck, Vir -t

Division of Fuel Cycle Safi

and Safeguards

Office of Nuclear Material

and Safeguards

Technical Contacts: Merri Horn, NMSS

(301) 415-8126 E-mail: mlhl@nrc.gov

Christopher Tripp, NMSS

(301) 415-7733 E-mail: cst@nrc.gov

Attachments:

1. List of Recently Issued NMSS Information Notices

2. List of Recently Issued NRC Information Notices 06C R3 Pe.lew D /717 G:\INVALVE fI", l411 Se nmrvintis concurrence

OC *SPB I - NMSS I - FCOB I *SPB I *SPB I *SPB I FC

NAME MHorn:ij EKraus CTripp DHoadley DMartin RPierson ETen yck

DATE 4/22/97 4/23/97 4/22/97 4/22/97 4/24/97 4/25/97 ijl /97 C = COVER COVER & ENCLOSURE N = NO COPY

OFFICIAL RECORD COPY

IN 97-xx

April , 1997 Nicensees and certificatees should review their retightening procedures to ensure that

op tors are not misapplying the ANSI torquing requirement by retightening packing nuts

to the ximum torque with each valve use. Furthermore, since packing nuts are more

likely to c k while in service, regulatees should have operators check for cracked packing

nuts before a dfer each valve use.

RltdGneric Co nications:

NRC Information Notice 89- dated November 22, 1988, 'Failure of Packing Nuts on

One-inch Uranium Hexafluoride ylinder Valves," concerned cracking of packing nuts from

Descote and Superior Valve Compn y

This information notice requires no spec action or written response. If you have any

questions about the information in this noti , please contact one of the technical contacts

listed below or the appropriate NRC regional oace.

Sincere yK

Elizabeth Ten Eyck, Di ctor

Division of Fuel Cycle Sa ty

and Safeguards

Office of Nuclear Material Sa ty

and Safeguards

Technical Contacts: Merri Horn, NMSS

(301) 415-8126 Christopher Tripp, NMSS

(301) 415-7733 Attachments:

1. Referenced Codes and Standards

2. List of Recently Issued NMSS Information Notices

3. List of Recently Issued NRC Information Notices

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