8 research outputs found
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EVALUATION OF ORGANIC VAPOR RELEASE FROM CEMENT-BASED WASTE FORMS
A cement based waste form was evaluated to determine the rates at which various organics were released during heating caused by the cementitious heat-of-hydration reaction. Saltstone is a cement-based waste form for the disposal of low-level salt solution. Samples were prepared with either Isopar{reg_sign} L, a long straight chained hydrocarbon, or (Cs,K) tetraphenylborate, a solid that, upon heating, decomposes to benzene and other aromatic compounds. The saltstone samples were heated over a range of temperatures. Periodically, sample headspaces were purged and the organic constituents were captured on carbon beds and analyzed. Isopar{reg_sign} L was released from the saltstone in a direct relationship to temperature. An equation was developed to correlate the release rate of Isopar{reg_sign} L from the saltstone to the temperature at which the samples were cured. The release of benzene was more complex and relied on both the decomposition of the tetraphenylborate as well as the transport of the manufactured benzene through the curing saltstone. Additional testing with saltstone prepared with different surface area/volume also was performed
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SALTSTONE VARIABILITY STUDY - MEASUREMENT OF POROSITY
One of the goals of the Saltstone Variability Study is to identify the operational and compositional variables that control or influence the important processing and performance properties of Saltstone mixes. One of the key performance properties is porosity which is a measure of the volume percent of a cured grout that is occupied by salt solution (for the saturated case). This report presents (1) the results of efforts to develop a method for the measurement of porosity of grout samples and (2) initial results of porosity values for samples that have been previously produced as part of the Saltstone Variability Study. A cost effective measurement method for porosity was developed that provides reproducible results, is relatively fast (30 to 60 minutes per sample) and uses a Mettler Toledo HR83 Moisture Analyzer that is already operational and routinely calibrated at Aiken County Technology Laboratory. The method involves the heating of the sample at 105 C until no further mass loss is observed. This mass loss value, which is due to water evaporation, is then used to calculate the volume percent porosity of the mix. The results of mass loss for mixes at 105 C were equivalent to the results obtained using thermal gravimetric analysis. The method was validated by comparing measurements of mass loss at 105 C for cured portland cement in water mixes to values presented in the literature for this system. A stereopycnometer from Quantachrome Instruments was selected to measure the cured grout bulk densities. Density is a property that is required to calculate the porosities. A stereopycnometer was already operational at Aiken County Technology Laboratory, has been calibrated using a solid stainless steel sphere of known volume, is cost effective and fast ({approx}15 minutes per sample). Cured grout densities are important in their own right because they can be used to project the volume of waste form produced from a given amount of salt feed of known composition. For mixes made from either Modular Caustic Side Solvent Extraction Unit (MCU) or Salt Waste Processing Facility (SWPF) simulants in premix, the porosities averaged near 62 % with an uptake of water through hydration reactions equivalent to a water to cementitious materials ratio (w/cm) of 0.04. For a mix made from a Deliquification, Dissolution and Adjustment (DDA) simulant and premix, the porosity is slightly lower at 57 % with an uptake of water through hydration reactions equivalent to a w/cm ratio of 0.07. Data are presented which demonstrate that porosity is inversely related to the heat of hydration, a measure of the extent of the hydration reactions. Modeling of porosities from three of the statistically designed phases of the Saltstone Variability Study demonstrated that the data could be fit to a linear model with an R2 of 0.74 and no statistical evidence for a lack of fit. The model revealed that w/cm ratio plays a significant role in the total porosity with porosity increasing as the w/cm ratio increases. Other elements of the model include positive correlations with the free hydroxide ion concentration and the total nitrate plus nitrite ion concentration. For a series of mixes in which the composition of the salt solution remained constant (MCU baseline) the porosity increased from {approx}60 to 65 % as the w/cm ratio increased from 0.55 to 0.65
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ISOPAR L RELEASE RATES FROM SALTSTONE USING SIMULATED SALT SOLUTIONS
The Modular Caustic-Side Solvent Extraction (CSSX) Unit (MCU) and the Salt Waste Processing Facility (SWPF) will produce a Decontaminated Salt Solution (DSS) that will go to the Saltstone Production Facility (SPF). Recent information indicates that solvent entrainment in the DSS is larger than expected. The main concern is with Isopar{reg_sign} L, the diluent in the solvent mixture, and its flammability in the saltstone vault. If it is assumed that all the Isopar{reg_sign} L is released instantaneously into the vault from the curing grout before each subsequent pour, the Isopar{reg_sign} L in the vault headspace is well mixed, and each pour displaces an equivalent volume of headspace, the maximum concentration of Isopar{reg_sign} L in the DSS to assure 25% of the lower flammable limit is not exceeded has been determined to be about 4 ppm. The amount allowed would be higher if the release from grout were significantly less. The Savannah River National Laboratory was tasked with determining the release of Isopar{reg_sign} L from saltstone prepared with a simulated DSS with Isopar{reg_sign} L concentrations ranging from 50 to 200 mg/L in the salt fraction and with test temperatures ranging from ambient to 95 C. The results from the curing of the saltstone showed that the amount of Isopar{reg_sign} L released versus time can be treated as a percentage of initial amount present; there was no statistically significant dependence of the release rate on the initial concentration. The majority of the Isopar{reg_sign} L that was released over the test duration was released in the first few days. The release of Isopar{reg_sign} L begins immediately and the rate of release decreases over time. At higher temperatures the immediate release rate is larger than at lower temperatures. Initial curing temperature was found to be very important as slight variations during the first few hours or days had a significant effect on the amount of Isopar{reg_sign} L released. Short scoping tests at 95 C with solvent containing all components (Isopar{reg_sign} L, suppressor trioctylamine (TOA), and modifier Cs-7SB) except the BOBCalixC6 extractant released less Isopar{reg_sign} L than the tests run with Isopar{reg_sign} L/TOA. Based on these scoping tests, the Isopar{reg_sign} L releases reported herein are conservative. Isopar{reg_sign} L release was studied for a two-month period and average cumulative release rates were determined from three sets of tests each at 95 and 75 C and at ambient conditions. The overall average releases at were estimated for each temperature. For the 95 and 75 C data, at a 5% significance level, the hypothesis that the three test sets at each temperature had the same average percent release can be rejected, suggesting that there was a statistically significant difference among the three averages seen in the three experimental tests conducted. An upper confidence limit on the mean percent release required incorporation of variation from two sources: test-to-test variation as well as the variation within a test. An analysis of variance that relies on a random effects model was used to estimate the two variance components. The test-to-test variance and the within test (or residual) variance were both calculated. There is no indication of a statistically significant linear correlation between the percent Isopar{reg_sign} L release and the Isopar{reg_sign} L initial concentration. From the analysis of variance, upper confidence limits at confidences of 80-95% were calculated for the data at 95 and 75 C. The mean Isopar{reg_sign} L percent releases were 67.33% and 13.17% at 95 and 75 C, respectively
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ISOPAR L RELEASE FROM SALTSTONE CURED AT 55 C
The decontaminated salt solution waste stream from the Modular Caustic Side Solvent Extraction Unit and the Salt Waste Processing Facility is anticipated to contain entrained extraction solvent. The decontaminated salt solution is scheduled to be processed through Tank 50 into the Saltstone Production Facility. This study, among others, has been undertaken because the solvent concentration in the decontaminated salt solution may cause flammability issues within the Saltstone Disposal Facility that may need to be addressed prior to operation. Previous work at the Savannah River National Laboratory determined the release of Isopar{reg_sign} L from saltstone prepared with a simulated DSS with Isopar{reg_sign} L concentrations ranging from 50 to 200 {micro}g/g in the salt fraction and with test temperatures ranging from ambient to 95 C. The results from the curing of the saltstone showed that the Isopar{reg_sign} L release data can be treated as a percentage of initial concentration in the concentration range studied. The majority of the Isopar{reg_sign} L that was released over the test duration was released in the first few days. The release of Isopar{reg_sign} L begins immediately and the rate of release decreases over time. At higher temperatures the immediate release is larger than at lower temperatures. In this study, saltstone was prepared using a simulated decontaminated salt solution containing Isopar{reg_sign} L concentrations of 50 {micro}L/L (30 {micro}g/g) and 100 {micro}L/L (61 {micro}g/g) and cured at 55 C. The headspace of each sample was purged and the Isopar{reg_sign} L was trapped on a coconut shell carbon tube. The amount of Isopar{reg_sign} L captured was determined using NIOSH Method 1501. The percentage of Isopar{reg_sign} L released after 20 days was 1.4 - 3.7% for saltstone containing 50 {micro}L/L concentration and 2.1 - 4.3% for saltstone containing 100 {micro}L/L concentration. Given the measurement uncertainties in this work there is no clearly discernible relationship between percentage release and initial Isopar{reg_sign} L concentration
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SIMULANT DEVELOPMENT FOR SAVANNAH RIVER SITE HIGH LEVEL WASTE
The Defense Waste Processing Facility (DWPF) at the Savannah River Site vitrifies High Level Waste (HLW) for repository internment. The process consists of three major steps: waste pretreatment, vitrification, and canister decontamination/sealing. The HLW consists of insoluble metal hydroxides (primarily iron, aluminum, magnesium, manganese, and uranium) and soluble sodium salts (carbonate, hydroxide, nitrite, nitrate, and sulfate). The HLW is processed in large batches through DWPF; DWPF has recently completed processing Sludge Batch 3 (SB3) and is currently processing Sludge Batch 4 (SB4). The composition of metal species in SB4 is shown in Table 1 as a function of the ratio of a metal to iron. Simulants remove radioactive species and renormalize the remaining species. Supernate composition is shown in Table 2
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DIRECT DISPOSAL OF A RADIOACTIVE ORGANIC WASTE IN A CEMENTITIOUS WASTE FORM
The disposition of {sup 137}Cs-containing tetraphenylborate (TPB) waste at the Savannah River Site (SRS) by immobilization in the cementitious waste form, or grout called ''saltstone'' was proposed as a straightforward, cost-effective method for disposal. Tests were performed to determine benzene release due to TPB decomposition in saltstone at several initial TPB concentrations and temperatures. The benzene release rates for simulants and radioactive samples were generally comparable at the same conditions. Saltstone monoliths with only the top surface exposed to air at 25 and 55 C at any tetraphenylborate concentration or at any temperature with 30 mg/L TPB gave insignificant releases of benzene. At higher TPB concentrations and 75 and 95 C, the benzene release could result in exceeding the Lower Flammable Limit in the saltstone vaults
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ANNULUS CLOSURE TECHNOLOGY DEVELOPMENT INSPECTION/SALT DEPOSIT CLEANING MAGNETIC WALL CRAWLER
The Liquid Waste Technology Development organization is investigating technologies to support closure of radioactive waste tanks at the Savannah River Site (SRS). Tank closure includes removal of the wastes that have propagated to the tank annulus. Although amounts and types of residual waste materials in the annuli of SRS tanks vary, simple salt deposits are predominant on tanks with known leak sites. This task focused on developing and demonstrating a technology to inspect and spot clean salt deposits from the outer primary tank wall located in the annulus of an SRS Type I tank. The Robotics, Remote and Specialty Equipment (RRSE) and Materials Science and Technology (MS&T) Sections of the Savannah River National Laboratory (SRNL) collaborated to modify and equip a Force Institute magnetic wall crawler with the tools necessary to demonstrate the inspection and spot cleaning in a mock-up of a Type I tank annulus. A remote control camera arm and cleaning head were developed, fabricated and mounted on the crawler. The crawler was then tested and demonstrated on a salt simulant also developed in this task. The demonstration showed that the camera is capable of being deployed in all specified locations and provided the views needed for the planned inspection. It also showed that the salt simulant readily dissolves with water. The crawler features two different techniques for delivering water to dissolve the salt deposits. Both water spay nozzles were able to dissolve the simulated salt, one is more controllable and the other delivers a larger water volume. The cleaning head also includes a rotary brush to mechanically remove the simulated salt nodules in the event insoluble material is encountered. The rotary brush proved to be effective in removing the salt nodules, although some fine tuning may be required to achieve the best results. This report describes the design process for developing technology to add features to a commercial wall crawler and the results of the demonstration testing performed on the integrated system. The crawler was modified to address the two primary objectives of the task (inspection and spot cleaning). SRNL recommends this technology as a viable option for annulus inspection and salt removal in tanks with minimal salt deposits (such as Tanks 5 and 6.) This report further recommends that the technology be prepared for field deployment by: (1) developing an improved mounting system for the magnetic idler wheel, (2) improving the robustness of the cleaning tool mounting, (3) resolving the nozzle selection valve connections, (4) determining alternatives for the brush and bristle assembly, and (5) adding a protective housing around the motors to shield them from water splash. In addition, SRNL suggests further technology development to address annulus cleaning issues that are apparent on other tanks that will also require salt removal in the future such as: (1) Developing a duct drilling device to facilitate dissolving salt inside ventilation ducts and draining the solution out the bottom of the ducts. (2) Investigating technologies to inspect inside the vertical annulus ventilation duct
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DWPF FLOWSHEET STUDIES WITH SIMULANTS TO DETERMINE MCU SOLVENT BUILD-UP IN CONTINOUS RUNS
The Actinide Removal Process (ARP) facility and the Modular Caustic Side Solvent Extraction Unit (MCU) are scheduled to begin processing salt waste in fiscal year 2007. A portion of the streams generated in these salt processing facilities will be transferred to the Defense Waste Processing Facility (DWPF) to be incorporated in the glass matrix. Before the streams are introduced, a combination of impact analyses and research and development studies must be performed to quantify the impacts on DWPF processing. The Process Science & Engineering (PS&E) section of the Savannah River National Laboratory (SRNL) was requested via Technical Task Request (TTR) HLW/DWPF/TTR-2004-0031 to evaluate the impacts on DWPF processing. Simulant Chemical Process Cell (CPC) flowsheet studies have been performed using previous composition and projected volume estimates for the ARP sludge/monosodium titanate (MST) stream. Initial MCU incorporation testing for the DWPF flowsheet indicated unacceptable levels of Isopar{reg_sign}L were collecting in the Sludge Receipt and Adjustment Tank (SRAT) condenser system and unanticipated quantities of modifier were carrying over into the SRAT condenser system. This work was performed as part of Sludge Batch 4 (SB4) flowsheet testing and was reported by Baich et al. Due to changes in the flammability control strategy for DWPF for salt processing, the incorporation strategy for ARP changed and additional ARP flowsheet tests were necessary to validate the new processing strategy. The last round of ARP testing included the incorporation of the MCU stream and identified potential processing issues with the MCU solvent. The identified issues included the potential carry-over and accumulation of the MCU solvent components in the CPC condensers and in the recycle stream to the Tank Farm. Solvent retention in the DWPF condensers contradicts the DWPF solvent control strategy. Therefore, DWPF requested SRNL to perform additional MCU flowsheet studies to better quantify the organic distribution in the CPC vessels. The earlier rounds of testing used a Sludge Batch 4 (SB4) simulant since it was anticipated that both of these facilities would begin salt processing during SB4 processing. The same sludge simulant recipe was used in this round of MCU testing to minimize the number of changes between the three phases of testing so a better comparison could be made. The MCU stream simulant was fabricated to perform the testing. The MCU stream represented the ''Maximum Volume'' case from the material balances provided by Campbell. ARP addition was not performed during this set of runs since the ARP evaluation had been completed in earlier runs. The MCU stream was added at boiling during the normal reflux phase of the SRAT cycle. SRAT cycle completion corresponded to the end of MCU stream addition. A total of ten 4-liter SRAT runs were performed to meet the objectives of the testing. The first series of five tests evaluated the organic portioning and mass balance for the addition of 50 mg/kg solvent. The second series of five tests evaluated the organic portioning and mass balance for the addition of 125 mg/kg solvent. A solvent concentration of 50 mg/kg is close to the nominal concentration anticipated in the effluent from the Salt Waste Processing Facility (SWPF). The organic solvent used in the testing was fabricated by the Chemical Science & Technology section. BOBCalixC6 was not added to this solvent due to the high cost and limited availability. All runs targeted 150% acid stoichiometry and 1% Hg in the sludge slurry dried solids