United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 93-19: Slab Hopper Bulging

                                   UNITED STATES
                         NUCLEAR REGULATORY COMMISSION
                            WASHINGTON, D.C. 20555

                                March 17, 1993



All nuclear fuel cycle licensees


This information notice is provided to alert addressees to possible bulging
that may occur in slab hoppers.  It is expected that licensees 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 new Nuclear Regulatory Commission 
requirements; therefore, no specific action or written response is required.

Description of Circumstances

In October 1992, a licensee notified an NRC Regional Office that a visual
inspection had identified the bulging of an empty slab hopper used to store
calcined UO2 powder.  This bulging caused the slab thickness of the hopper to
exceed the safe value specified in the nuclear criticality safety analysis. 
As such, this event constituted a nuclear criticality hazard.  At that time,
the acting shift supervisor, in conjunction with the safety supervisor, tagged
the slab hopper "out of service," pending further investigation.  The bulging
occurred about one-half of an inch from the lower end of the hopper and
comprised an area of about 1 ft2.  Subsequently, the event was reported to the
NRC Operations Officer, in accordance with NRC Bulletin 91-01.
The licensee convened an investigation to determine the cause(s) of the slab
hopper bulging and to identify needed corrective actions, to preclude
recurrence.  This investigation revealed that, in addition to the originally
cited slab hopper, all other slab hoppers had bulged and exceeded their design
thicknesses, in the same general area as the initial slab hopper. 
Accordingly, all were removed from service.  Additional analyses of the
initially bulged slab hopper, performed using an ultrasonic examination,
revealed that the specified wall thickness was actually thinner than that
specified in the drawings of the slab hopper.  At this point, the licensee is
unclear as to whether the hopper was fabricated with thinner material or
material thinning had taken place because of extended use.  As a result of
these findings, it was decided that all slab hoppers will remain "locked out"
until corrective actions identified by the investigation have been

Although the licensee has not completed its investigation, the preliminary
conclusion is that bulging occurred because of metal fatigue, caused by 


                                                             IN 93-19
                                                             March 17, 1993
                                                             Page 2 of 3

extended service, and heat and powder expansion that occurs when UO2 powder
oxidizes while in the slab hoppers.  The licensee's final investigation will
be reviewed in a subsequent NRC inspection and is considered as an inspector
followup item.


The bulging of the slab hoppers used to store UO2 constitutes a nuclear
criticality threat.  This is so because the inside thickness of the slab
hopper is used as the primary criticality safety control.  Therefore, it is
imperative that all possible events that might perturb the thickness of the
slab hopper be examined.  In this instance, however, the licensee did not
examine the possibility of slab bulging as a consequence of the chemical
reaction of UO2 with oxygen to form U3O8 (which releases energy), or the
hydrostatic forces that result from the associated volume expansion.  In fact,
an NRC inspection revealed that the licensee did not have documentation of the
design loading and design criteria for the existing slab hoppers. 
Furthermore, no quality assurance program or material specifications for the
slab hoppers existed.

To ensure that adequate safety for the slab hoppers is provided, licensees
with slab hoppers may wish to review their nuclear safety programs and
consider the need to incorporate the following elements:

1.  Specifications that identify design loadings, criteria, and methods and    
    acceptance criteria for the slab hoppers.

2.  A QA program for slab hoppers to address all activities, including         
     design, purchase, fabrication, inspection, operation, and maintenance.

3.  A preventive maintenance program that provides for the performance of      
    routine surveillance and periodic dimensional checks of the slab           

In addition, when geometric control is used for nuclear criticality safety,
the choice between geometric shapes should be based on the inherent ability of
the equipment to retain its integrity.  This is of particular concern for
equipment subject to bending stresses in the side walls during normal and
accident conditions.  The following geometric shapes are listed in descending
order of stability:  spherical, cylindrical, annular, and slab geometry.  It
should be noted that for reasonable sizes and comparable wall thicknesses,
each descent of the scale decreases the inherent pressure failure limit of the
vessel by about an order of magnitude.


                                                             IN 93-19
                                                             March 17, 1993
                                                             Page 3 of 3

This information notice requires no specific action or written response.  If
you have questions about the information in this notice, please contact the
technical contact listed below or the appropriate regional office.

                                    ORIGINAL SIGNED BY

                                 Robert F. Burnett, Director
                                 Division of Fuel Cycle Safety
                                   and Safeguards
                                 Office of Nuclear Material Safety
                                   and Safeguards

Technical contact:  Marc Klasky, NMSS
                    (301) 504-2504

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

Page Last Reviewed/Updated Tuesday, November 12, 2013