IE Circular No. 80-20, Changes in Safe-Slab Tank Dimensions
Accession No .:
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, D.C. 20555
August 21, 1980
IE Circular No. 80-20: CHANGES IN SAFE-SLAB TANK DIMENSIONS
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.
August 21, 1980
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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.
Page Last Reviewed/Updated Thursday, March 25, 2021