IE Circular No. 80-20, Changes in Safe-Slab Tank Dimensions

                                                           SSINS: 6830    
                                                           Accession No .: 
                                                           IEC 80-20      

                                UNITED STATES
                           WASHINGTON, D.C. 20555

                               August 21, 1980


Description of Circumstances: 

During a routine inspection at a fuel facility, an NRC inspector received a 
report of significant dimensional changes in safe geometry tanks. The tanks 
were used to store low-enriched uranium solutions from a scrap dissolver. 
The problem was first identified when a sight-glass mounted on the large 
face of one of three safe-slab tanks cracked from the strain of tank 
dimensional changes. Further investigation and measurements revealed that 
the large faces of two of the tanks had bulged and increased the thickness 
dimensions of the tanks. 

The maximum bulge occurred at the center of one of the tanks, increasing the
tank thickness 2 1/2 in. beyond the design thickness of 5 1/2 in. The bulge 
tapered from the tank centers to the designed 5 1/2 in. at the tank edges. 
The tanks were made of 1/8-in. type 304 stainless steel with the tank edges 
and large faces supported and stiffened by 3/16-in. angle iron. 

The cause of the bulging was believed to be overpressurization due to 
partial plugging of the tank vent lines. Contributing factors may have been 
excessive transfer rates, metal fatigue aggravated by corrosion, and the 
high specific gravity of the stored solution. 

Replacement tanks were provided with tie-bars and heavier 1/4-in. angle iron
stiffeners to preserve the thickness dimensions. Conservative calculations, 
taking into account the weight of solution, tank dimensions, and tie-bar and
stiffener strength, indicated that required dimensions would be maintained. 
A precondition for the calculations required that the tank vent system be 
designed to prevent accidental pressurization. 

To prevent accidental pressurization, a 1-1/2-in. overflow line was 
connected to the 1-1/2-in. vent line to the process offgas (POG) system at a 
point immediately above each tank. This provided a positive overflow as well 
as a second vent in case a plug occurred in the main POG line. Additionally, 
each tank was provided with a 2" x 4" inspection port at the top with a 
cover free to "float" should pressurization occur. 

The tank dimensions were approximately 62" x 62" x 5 1/2". The tank 
locations were parallel to and about 1 ft removed from the scrap recovery 
area walls. The visual detection of the 2-1/2-in. deflection of the large 
tank face from a point in front of the tank is usually difficult so that 
measurements should be made with calipers or similar equipment to assure 
detection of significant distortion. 

                                                           IEC 80-20      
                                                           August 21, 1980 
                                                           Page 2 of 2    

Notice to Licensees: 

All licensees using safe-slab tanks should be aware of the possible changes 
in tank dimensions resulting from hydraulic or pneumatic forces. Certain 
steps should be taken to prevent or detect changes in safe-slab tank 
dimensions. These actions include the following: 

(1)  Structural analyses should be reviewed for all vessels designed to be 
     geometrically safe to assure that the possibility of pressurization has
     been adequately considered. The use of tie-bars and rigid steel 
     supports should be considered in designs for new tanks. 

(2)  The possibility of vent lines becoming plugged should be studied and 
     special overflow mechanisms should be provided if pressurization by 
     vent line plugging is possible. 

(3)  Provision should be made for routine dimensional checks of 
     geometrically safe vessels. These checks should be made whether or not 
     the vessels are subject to pressurization. 

No written response to this circular is required. If additional information 
regarding this subject is required, contact the Director of this office. 


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