Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
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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