United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 89-78: 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

                              November 22, 1989


Information Notice No. 89-78:  FAILURE OF PACKING NUTS ON ONE-INCH 
                                   URANIUM HEXAFLUORIDE CYLINDER VALVES

Addressees:

All U.S. Nuclear Regulatory  Commission (NRC) licensees authorized to 
possess and use source material and/or special nuclear material for the 
heating, emptying, filling, or shipping of uranium hexafluoride in 30- and 
48-inch diameter cylinders.

Purpose:

This notice is provided to advise licensees about 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 activities, distribute it to responsible operations 
staff, and consider actions, if appropriate, to identify and correct similar 
occurrences at their facilities.  However, suggestions contained in this 
Notice do not constitute any new NRC requirements, and no written response 
is required.

Description of Circumstances:

Allied-Signal, Inc., has reported that cracks were observed in packing nuts 
on three one-inch valves designed for use in 30- and 48-inch diameter 
uranium hexafluoride cylinders.  The cracks were discovered when the valves 
were removed from storage and visually inspected by the licensee.  The 
affected valves were unused and had been in storage since receipt in early 
1987 from the French firm, Descote.  Both the U.S. Department of Energy 
(DOE) and the Canadian Atomic Energy Control Board have reported similar 
cracking in packing nuts on one-inch valves supplied by Superior Valve 
Company.  However, unlike the Descote valves, the Superior valves were in 
service at the time the cracks were observed.

Cracked nuts on uranium hexafluoride cylinder valves can result in leakage 
of uranium hexafluoride through the packing material and valve stem.

DOE's contractor, Martin Marietta Energy Systems, Inc. (MMES), examined one 
of the affected Descote valves to determine possible causes of the cracking. 
Its investigation found that the failure mode of the packing nut was inter-
granular cracking.  The metallographic structure of the examined nut was 
different from Descote and Superior nuts produced from other bar stock.  






8911160028
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                                                       IN 89-78
                                                       November 22, 1989
                                                       Page 2 of 3


The cracked packing nuts were produced from material designated heat lot 
R91.  The MMES investigation also indicated that the nut had significant 
residual stresses, possibly resulting from surface finishing operations.  
These findings suggest that the packing nuts produced from heat lot R91 
and/or subjected to surface grinding are susceptible to stress corrosion 
cracking.

Descote has also investigated the cracking problem and 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 valve;

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.

Based on its findings and those of MMES, Descote has recalled all one-inch 
valve packing nuts produced from heat lot R91.

Discussion:

The American National Standards Institute (ANSI) provides criteria for 
packaging of uranium hexafluoride for transport.  The criteria are found in 
ANSI N14.1-1987, "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.

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.  

Although NRC does not consider the cracked packing nut to be a major safety 
issue, licensees should consider actions to identify and reduce the 
occurrence of cracking.  Licensees should review their retightening 
procedures to ensure that operators are not misapplying the ANSI torquing 
requirement by retightening packing nuts to the maximum torque with each 
valve use.  Furthermore, since packing nuts are more likely to crack while 
in service, licensees should have operators check for cracked packing nuts 
before and after each valve use.

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                                                       IN 89-78
                                                       November 22, 1989
                                                       Page 3 of 3


No written response is required by this information notice.  If you have any 
questions about this matter, please direct them to the technical contacts 
listed below or to the appropriate NRC regional office.




                              Richard E. Cunningham, Director 
                              Division of Industrial and
                                Medical Nuclear Safety


Technical Contacts: W. Scott Pennington, NMSS
                    (301) 492-0693

                    George H. Bidinger, NMSS
                    (301) 492-0683

Attachments:  
1.  List of Recently Issued NMSS Information Notices
2.  List of Recently Issued NRC Information Notices
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