Structural analysis of ORNL underground gunite waste storage tanks

The North Tank Farm (NTF) and the South Tank Farm (STF) located at ORNL contains 8 underground waste storage tanks which were built around 1943. The tanks were used to collect and store the liquid portion of the radioactive and/or hazardous chemical wastes produced as part of normal facility operations at ORNL, but are no longer part of the active Low Level Liquid Waste system of the Laboratory. The tanks were constructed of gunite. The six STF tanks are 50 ft in diameter, and have a 12 ft sidewall, and an arched dome rising another 6.25 ft. The sidewall are 6 in. thick and have an additional 1.5 in. gunite liner on the inside. There is a thickened ring at the wall-dome juncture. The dome consists of two 5 in. layers of gunite. The two tanks in the NTF are similar, but smaller, having a 25 ft diameter, no inner liner, and a dome thickness of 3.5 in. Both sets of tanks have welded wire mesh and vertical rebars in the walls, welded wire mesh in the domes, and horizontal reinforcing hoop bars pre-tensioned to 35 to 40 ksi stress in the walls and thickened ring. The eight tanks are entirely buried under a 6 ft layer of soil cover. The present condition of the tanks is not accurately known, since access to them is extremely limited. In order to evaluate the structural capability of the tanks, a finite element analysis of each size tank was performed. Both static and seismic loads were considered. Three sludge levels, empty, half-full, and full were evaluated. In the STF analysis, the effects of wall deterioration and group spacing were evaluated. These analyses found that the weakest element in the tanks is the steel resisting the circumferential (or hoop) forces in the dome ring, a fact verified separately by an independent reviewer. However, the hoop steel has an adequate demand/capacity ratio. Buckling of the dome and the tank walls is not a concern.