Samuel D. Fink

Strontium and Actinide Separations from High Level Nuclear Waste Solutions Using Monosodium Titanate 1. Simulant Testing

Pretreatment processes at the Savannah River Site will separate {sup 90}Sr, alpha-emitting and radionuclides (i.e., actinides) and {sup 137}Cs prior to disposal of the high-level nuclear waste. Separation of {sup 90}Sr and alpha-emitting radionuclides occurs by ion exchange/adsorption using an inorganic material, monosodium titanate (MST). Previously reported testing with simulants indicates that the MST exhibits high selectivity for strontium and actinides in high ionic strength and strongly alkaline salt solutions. This paper provides a summary of data acquired to measure the performance of MST to remove strontium and actinides from actual waste solutions. These tests evaluated the effects of ionic strength, mixing, elevated alpha activities, and multiple contacts of the waste with MST. Tests also provided confirmation that MST performs well at much larger laboratory scales (300-700 times larger) and exhibits little affinity for desorption of strontium and plutonium during washing.

Development of an Improved Titanate-Based Sorbent for Strontium and Actinide Separations under Strongly Alkaline Conditions

High-level nuclear waste produced from fuel reprocessing operations at the Savannah River Site (SRS) requires pretreatment to remove 134,137Cs, 90Sr, and alpha-emitting radionuclides (i.e., actinides) prior to disposal onsite as low level waste. The separation processes at SRS include the sorption of 90Sr and alpha-emitting radionuclides onto monosodium titanate (MST) and caustic side solvent extraction of 137Cs. The MST and separated 137Cs is encapsulated along with the sludge fraction of high-level waste (HLW) into a borosilicate glass waste form for eventual entombment at a federal repository. The predominant alpha-emitting radionuclides in the highly alkaline waste solutions include plutonium isotopes 238Pu, 239Pu, and 240Pu; 237Np; and uranium isotopes, 235U and 238U. This article describes recent results evaluating the performance of an improved sodium titanate material that exhibits increased removal kinetics and capacity for 90Sr and alpha-emitting radionuclides compared to the current baseline material, MST.

Determination of Actinide Loadings onto Monosodium Titanate (MST) under Conditions Relevant to the Actinide Removal Process Facility

Researchers at the Savannah River Site (SRS) have studied adsorption of uranium, plutonium, and neptunium onto monosodium titanate (MST) at conditions relevant to operation of the Actinide Removal Process (ARP) facility. This study measured actinide loadings at a large phase ratio of simulated wastes solution to mass of MST. The large phase ratio was designed to mimic the maximum phase ratio that would occur during a single process cycle of the ARP facility. Uranium and plutonium loadings measured in this study proved much higher than previous measurements at lower phase ratios.

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

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.

Processing Macrobatch 2 at the Savannah River Site Integrated Salt Disposition Process (ISDP)

The Savannah River Site (SRS) is currently removing liquid radioactive waste from the tanks in its Tank Farm. To treat waste streams that are high in 137Cs, 90Sr, and/or actinides, SRS developed the Actinide Removal Process (ARP) and the Modular Caustic Side Solvent Extraction (CSSX) Unit. Collectively, these two processes make up the Integrated Salt Disposition Process (ISDP). The ARP part is responsible for the removal of strontium and actinides, while the MCU part is responsible for removing cesium. This paper discusses the qualification testing of the second batch of caustic waste that is being processed through ISDP currently. This paper also describes the tests conducted and compares results with current facility performance. The ARP contacts the salt solution with monosodium titanate (MST) to sorb strontium and select actinides. After MST contact, the resulting slurry is filtered to remove the MST (and sorbed strontium and actinides) and entrained sludge. The filtrate is transferred to the MCU for further treatment to remove cesium. The solid particulates removed by the filter are concentrated to ∼5 wt%, washed to reduce the sodium concentration, and transferred to the Defense Waste Processing Facility (DWPF) for vitrification. The CSSX process extracts the cesium from the radioactive waste using a customized solvent to produce a Decontaminated Salt Solution (DSS), then strips and concentrates the cesium from the solvent with dilute nitric acid. The DSS is incorporated in grout while the strip acid solution is transferred to DWPF for vitrification. In order to predict waste behavior, the Savannah River National Laboratory (SRNL) personnel performed tests using actual radioactive samples of the second waste batch – Macrobatch 2 – for processing prior to the start of the operation. Testing included MST sorption to remove strontium and actinides followed by CSSX batch contact tests to verify expected cesium mass removal and concentration. This paper describes the tests conducted and compares results from MCU facility operations. The results include strontium, plutonium, and cesium removal, cesium concentration, and organic entrainment and recovery data. Our work indicates that the bench scale tests are a conservative predictor of actual waste performance.

The Effect of Magnetic Fields on Uranium and Strontium Sorption on Monosodium Titanate

The presence of a magnetic field gradient enhanced the rate of strontium and uranium sorption onto monosodium titanate. The enhancement was evident only in the early contact times and did not impact the equilibrium capacity of the sorbent. No enhancement was seen when the sorption test was conducted under a homogeneous magnetic field. Further studies are needed to determine if the enhancement is also seen with other cations and/or anions.

Analysis of solids remaining following chemical cleaning in tank 6F

Following chemical cleaning, a solid sample was collected and submitted to Savannah River National Laboratory (SRNL) for analysis. SRNL analyzed this sample by X-ray Diffraction (XRD) and scanning electron microscopy (SEM) to determine the composition of the solids remaining in Tank 6F and to assess the effectiveness of the chemical cleaning process.

Copper‐Catalyzed Peroxide Oxidation Testing for Tetraphenylborate Decomposition

Abstract A new processing option, copper‐catalyzed hydrogen peroxide oxidation of tetraphenylborate under alkaline conditions, was demonstrated in laboratory testing. Laboratory‐scale tests were conducted to evaluate the use of copper‐catalyzed hydrogen peroxide oxidation to treat simulants of the Savannah River Site tank waste. The oxidation process involves the reaction of hydrogen peroxide with a copper catalyst to form hydroxyl free radicals. With an oxidation potential of 2.8 volts, the hydroxyl free radical is a very powerful oxidant, second only to fluorine, and will react with a wide range of organic molecules. The goal is to oxidize the tetraphenylborate completely to carbon dioxide, with minimal benzene generation. Testing was completed in a lab‐scale demonstration apparatus at the Savannah River National Laboratory. Greater than 99.8% tetraphenylborate destruction was achieved in less than three weeks. Offgas benzene analysis by a gas chromatograph demonstrated low benzene generation. Analysis of the resulting slurry demonstrated >82.3% organic carbon destruction. The only carbon compounds detected were formate, oxalate, benzene (vapor), carbonate, p‐terphenyl, quaterphenyl, phenol, and phenol 3‐dimethylamino.