United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 97-24: Failure of Packing Nuts on One-Inch Uranium Hexafluoride Cylinder Valves

                                 UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
               OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
                            WASHINGTON, D.C. 20555

                                  May 8, 1997

                                       
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 requirements;
therefore, no specific action or written response is required.

Description 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 (UF6) 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 UF6 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 


9705020259.                                                       IN 97-24
                                                                  May 8, 1997
                                                                  Page 2 of 3


metallurgical examination; one showed slight indications of cracking that had
not been picked up by the eddy current testing.  Hunt B150 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 B150 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 UF6 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 UF6 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. 
.                                                                 IN 97-24
                                                                  May 8, 1997
                                                                  Page 3 of 3


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.

                                    


                                          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:  mlh1@nrc.gov      

                     Christopher Tripp, NMSS
                     (301) 415-7733
                     E-mail:  cst@nrc.gov
Page Last Reviewed/Updated Tuesday, December 03, 2013