Removal of Sludge Heels in Savannah River Site Waste Tanks with Oxalic Acid

Abstract

The Savannah River Site (SRS) is preparing two tanks for closure. The first step in preparing the tank for closure is mechanical sludge removal. In mechanical sludge removal, a liquid such as inhibited water or salt solution is added to the tank, the liquid is mixed with the sludge to form a slurry, and the slurry is transported from the tank. Mechanical cleaning removes a large fraction of the sludge in the tank, but it leaves a sludge heel of several thousand gallons. SRS employs chemical cleaning to remove this sludge heel. In chemical cleaning, oxalic acid is added to the tank to dissolve the sludge, and the liquid, containing the dissolved sludge, is transported from the tank.

The authors conducted demonstrations of the chemical cleaning process with simulated SRS waste and actual SRS waste to assess the effectiveness of oxalic acid in dissolving SRS sludge. Following these demonstrations, SRS conducted chemical cleaning in two waste tanks (referred to as Tank A and Tank B). During chemical cleaning, the authors analyzed samples to assess the effectiveness of the chemical cleaning in removing the sludge heel.

The conclusions from this work follow. With the exception of iron, the dissolution of sludge components from Tank A agreed with results from the actual waste demonstration performed in 2007. The fraction of iron removed from Tank A by chemical cleaning was significantly less than the fraction removed in the SRNL demonstrations. The likely cause of this difference is the high pH following the first oxalic acid strike. The dissolution of sludge components from Tank B agreed with results from the actual waste demonstration performed in 2007. The fraction of plutonium removed from Tank B by chemical cleaning was slightly higher than the fraction removed in the SRNL demonstrations. Most of the sludge mass remaining in the tank is iron and nickel. The remaining sludge contains significant amounts of barium, chromium, and mercury. Most of the radioactivity remaining in the residual material is beta emitters and ⁹⁰Sr. The chemical cleaning removed a large fraction of the uranium, aluminum, calcium, sodium, strontium, and cesium. The chemical cleaning was not effective at removing nickel, mercury, plutonium, americium, and curium.

Keywords:

• chemical cleaning
• oxalic acid
• sludge

Notes

¹In this paper, we refer to the tanks as Tank A and Tank B. Tank A and Tank B are not the actual names of the Tank. A third tank is referred to as Tank C.

n.d. = not detected.

^a Peroxide fusion better method for this species.

^b Aqua regia better method for this species.

^c Uncertainties are ±10%, except for Hg, which is ±20%.

^a Uncertainties are ±20%.

n.m. = not measured.

*Species not detected.

^a Peroxide fusion better method for this species.

^b Aqua regia better method for this species.

^c Uncertainty is ±10%, except for Hg which is ±20%.

^a uncertainty is ±20%.

n.m. = not measured.

n.m. = not measured.

n.m. = not measured.

n.m. = not measured.

^a all samples less than detection limit – fraction removed should be the same as other isotopes.

^b all samples less than detection limit.

n.m. = not measured.