RHEOLOGICAL AND ELEMENTAL ANALYSES OF SIMULANT SB5 SLURRY MIX EVAPORATOR-MELTER FEED TANK SLURRIES

Fernandez, A.

Savannah River Site (S.C.)

关键词: Leaching;    Sludges;    12 Management Of Radioactive Wastes, And Non-Radioactive Wastes From Nuclear Facilities;    Flow Blockage;    Radioactive Waste Processing;   

DOI  :  10.2172/973192
RP-ID  :  SRNL-STI-2009-00751
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  973192

美国|英语

来源: UNT Digital Library

【 摘 要 】

The Defense Waste Processing Facility (DWPF) will complete Sludge Batch 5 (SB5) processing in fiscal year 2010. DWPF has experienced multiple feed stoppages for the SB5 Melter Feed Tank (MFT) due to clogs. Melter throughput is decreased not only due to the feed stoppage, but also because dilution of the feed by addition of prime water (about 60 gallons), which is required to restart the MFT pump. SB5 conditions are different from previous batches in one respect: pH of the Slurry Mix Evaporator (SME) product (9 for SB5 vs. 7 for SB4). Since a higher pH could cause gel formation, due in part to greater leaching from the glass frit into the supernate, SRNL studies were undertaken to check this hypothesis. The clogging issue is addressed by this simulant work, requested via a technical task request from DWPF. The experiments were conducted at Aiken County Technology Laboratory (ACTL) wherein a non-radioactive simulant consisting of SB5 Sludge Receipt and Adjustment Tank (SRAT) product simulant and frit was subjected to a 30 hour SME cycle at two different pH levels, 7.5 and 10; the boiling was completed over a period of six days. Rheology and supernate elemental composition measurements were conducted. The caustic run exhibited foaming once, after 30 minutes of boiling. It was expected that caustic boiling would exhibit a greater leaching rate, which could cause formation of sodium aluminosilicate and would allow gel formation to increase the thickness of the simulant. Xray Diffraction (XRD) measurements of the simulant did not detect crystalline sodium aluminosilicate, a possible gel formation species. Instead, it was observed that caustic conditions, but not necessarily boiling time, induced greater thickness, but lowered the leach rate. Leaching consists of the formation of metal hydroxides from the oxides, formation of boric acid from the boron oxide, and dissolution of SiO{sub 2}, the major frit component. It is likely that the observed precipitation of Mg(OH){sub 2} and Mn(OH){sub 2} caused the increase in yield stress. The low pH run exhibited as much as an order of magnitude greater B and Li (frit components) leachate concentrations in the supernate. At high pH a decrease of B leaching was found and this was attributed to adsorption onto Mg(OH){sub 2}. A second leaching experiment was performed without any sludge to deconvolute the leach rate behavior of Frit 418 from those of the SB5 sludge-Frit 418 system. At high pH, the sludgeless system demonstrated very fast leaching of all the frit components, primarily due to fast dissolution of the main component, silica, by hydroxide anion; various semiconductor studies have established this reactivity. Overall, the frit-water system exhibited greater leaching from a factor two to almost three orders of magnitude (varying by conditions and species). The slower leaching of the sludge system is possibly due to a greater ionic strength or smaller driving force. Another possible reason is a physical effect, coating of the frit particles with sludge; this would increase the mass transfer resistance to leaching. Based on this study, the cause of clogs in the melter feed loop is still unknown. A possible explanation is that the SME product, like the simulant, is too thin and could contribute to a loss of two-phase flow which could cause plugging of a restricted and poorly mixed zone like the melter feed loop. This is feasible since a previous study of a slurry showed an increase in resistance to flow at lower flow rates. Testing with a radioactive SME sample is underway and should help understand this potential mechanism.