Calibrating a Large Slab Vessel: A Battle of the Bulge

The accurate measurement of volume in slab vessels can be difficult because slab vessels expand--in spite of internal or external supports--as they are filled. One form of bulging is elastic deflection, a gradual expansion of the vessel wall resulting from an increased weight of contained solution. As part of an upgrade to the Idaho Chemical Processing Plant, slab tanks were proposed as accountability measurement vessels. A 1960 liter slab tank prototype was set up for preliminary calibrations. Two series of calibrations were conducted: the first using water, and the second using aluminum nitrate. It was conjectured that the increased weight of aluminum nitrate would cause the vessel walls to deflect more than they did for an equal level of water, resulting in a greater volume. As expected, a significant expansion was observed with the aluminum nitrate, but some of the deflection proved to be permanent rather than elastic. The consequence is that considerably more effort will be required to calibrate slab vessels for uranium accountability. Not only must a calibration curve (or family of curves) be developed giving volume as a function of both liquid level and density, but, if possible, a determination must be made as to when the deflection is no longer temporary.