Allen J. Bakel

Measurement of kinetic rate law parameters on a NaCaAl borosilicate glass for low-activity waste

Abstract The dissolution kinetics of a NaCaAl borosilicate glass, being studied for immobilization of low-activity waste, were measured between 20 and 90°C and solution pH between 6 and 12 using the single-pass flow-through method. Dissolution kinetics measurements are needed to parameterize a mechanistic model that is being used to compute the corrosion rate of the glass waste form as a function of temperature, pH, and the concentrations of the other glass components in water percolating through a proposed shallow-land disposal facility. The key factors that were found to influence the test results include test duration and background subtraction of the raw data. Background subtraction is shown to be important to prevent a non-physical increase in the computed rate with increasing flow rate, particularly in tests run at higher flow rates. Experimental factors that were found to have no detectable influence on the test results included the glass particle size and buffer type. We also illustrate how flow rate variations can be used to obtain information about the reaction order and equilibrium constant parameters in a conventional transition-state theory rate law.

Nitrogen isotope geochemistry of organic matter and minerals during diagenesis and hydrocarbon migration

The magnitude of isotopic variations between organic and inorganic nitrogen was examined in samples from three stacked hydrocarbon reservoirs in the Fordoche Field (Louisiana Gulf Coast Basin, USA). Measurements were made of δ 15N in kerogen, bitumen, oil, formation water, and fixed-NH4 extracted from mudstones, nonproductive sandstones, and productive sandstones. Nitrogen isotope fractionation occurs because 14N is released preferentially to 15N from organic molecules during thermal maturation. Released 14N goes into solution, or may be adsorbed by minerals, leaving crude oil enriched in 15N. Diagenetic clay minerals (e.g., illite) commonly form in the temperature range of hydrocarbon generation, and NH4+ may be fixed in clay interlayers with an isotopic ratio similar to that of the migrating fluids. Results indicate that the influence of organic matter on mineral δ 15N depends on the timing of authigenic mineral formation relative to fluid migration. The average δ 15N of kerogen (3.2 ± 0.3‰) and fixed-NH4 from mudstones (3.0 ± 1.4) is similar, while bitumen increases from +3.5 to +5.1‰ with depth. In deep reservoir sandstones (>100°C), the δ 15N of crude oil averages +5.2 ± 0.4‰, similar to the δ 15N of bitumen in the proposed source rocks. Formation waters are 14N-enriched with an average δ 15N of −2.2 ± 2.6‰. Fixed-NH4 δ 15N values lie between that of the oil and water. The average δ 15N of fixed-NH4 is 3.0 ± 1.2‰ in productive sandstones, and 0.2 ± 2.4‰ innonproductive sandstones. In the shallower reservoir sandstones (<90°C) fixed-NH4 is apparently not influenced by the presently associated fluids. Productive and nonproductive sandstones have distinctly low average δ 15N values (−1.2 ± 0.8‰), yet crude oil (+11.1 ± 0.3‰) and water (+3.8 ± 0.1‰) have been 15N-enriched by ∼6‰ relative to the deeper reservoirs. This suggests that the present fluids migrated into the reservoir after authigenic illite had formed. Fluids become enriched in 15N during migration and the amount of enrichment may be a function of the amount of interaction with argillaceous sediments.

Compound specific isotopic variability in Uinta Basin native bitumens: paleoenvironmental implications

Abstract We have analyzed the solvent extracts from three different types of native bitumens from the Uinta Basin in northeastern Utah by a variety of analytical techniques, including GC-IRMS, to evaluate variations in the paleodepositional environment during two periods of Lake Uinta deposition. The gilsonite and tabbyite bitumens are associated with Parachute Creek Member sediments deposited during a major expansion of ancient Lake Uinta. Compound specific isotopic analyses of s-carotane and phytane ( δ 13 C = −32.6 to −32.1% 0 ) from these bitumens reflect input from primary photosynthetic producers such as cyanobacteria. Sterane δ 13C values (−34.5 to −29.2%0) refflect contributions from lacustrine algae, while extremely depleted δ 13C values for methylhopanes (−58.1 to −61.5%0) suggest input from methanotrophic bacteria. Variations in the δ 13C values of the αβ-hopanes (−51.4 to 37.7%0) imply additional input from other bacterial sources. The wurtzilite bitumen was generated from the Saline Facies of the Green River Formation deposited during a later regression of Lake Uinta. Compound specific isotopic analyses of phytane ( δ 13 C = −30.1% 0 ) and steranes δ 13 C = −29.6 to −26.7% 0 ) from this bitumen indicate continued input from primary producers and eukaryotes. The higher relative concentrations of gammacerane ( δ 13 C = −26.9% 0 ) indicate increasing input from aerobic protozoa. We observed a slight enrichment in δ 13C in the wurtzilite extract (and several biomarkers) and suggest that this is a result of sulfate-reducing bacteria outcompeting methanogens, thereby, eliminating the influence of methanotrophs in this later saline stage of deposition.

Extraction behaviour of actinides and lanthanides in TALSPEAK, TRUEX and NPEX processes of UREX+

Abstract Bench-scale studies to determine the extraction behavior of Pu, Np, Am and lanthanides with the organophosphorus extractants TBP, CMPO and HDEHP have been carried out. Based on the results obtained using actual spent nuclear fuel solutions, enhancements to the NPEX, TRUEX and TALSPEAK processes have been successful. In NPEX, >99.94% of both Np and Pu were separated from the fission products. In TRUEX, essentially complete recovery of the actinides (An) and the lanthanides (Ln) was achieved. In TALSPEAK, the complete separation of Pu, Np and Am from the lanthanides was demonstrated several times under various process conditions. The recovery of transuranics (TRU), including Am and Cm, is nearly 100% (below detection limit in the Ln stream), while the total recovery of Ln in the product stream exceeded 99.97%.

Crystal chemistry of uranium (V) and plutonium (IV) in a titanate ceramic for disposition of surplus fissile material

Abstract We report X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra for the plutonium LIII and uranium LIII edges in titanate pyrochlore ceramic. The titanate ceramics studied are of the type proposed to serve as a matrix for the immobilization of surplus fissile materials. The samples studied contain approximately 10 wt% fissile plutonium and 20 wt% natural uranium, and are representative of material within the planned production envelope. Based upon natural analogue models, it had been previously assumed that both uranium and plutonium would occupy the calcium site in the pyrochlore crystal structure. While the XANES and EXAFS signals from the plutonium LIII are consistent with this substitution into the calcium site within pyrochlore, the uranium XANES is characteristic of pentavalent uranium. Furthermore, the EXAFS signal from the uranium has a distinct oxygen coordination shell at 2.07 A and a total oxygen coordination of about 6, which is inconsistent with the calcium site. These combined EXAFS and XANES results provide the first evidence of substantial pentavalent uranium in an octahedral site in pyrochlore. This may also explain the copious nucleation of rutile (TiO2) precipitates commonly observed in these materials as uranium displaces titanium from the octahedral sites.

Ultrasonic vacuum extraction of gases from water for chemical and isotopic analysis

A method is described for ultrasonic vacuum extraction of gases from groundwater samples. Ultrasonic vacuum extraction provides near-quantitative gas yields within 30 min, and allows on-line preparation of carbon dioxide, nitrogen and oxygen for isotopic analyses. This method eliminates uncertainties involved in correcting headspace gas analyses for solubilities and liquid-vapor isotopic fractionation. In addition to developmental data, some field results are given to demonstrate the applicability of the method to geochemical studies.

Carbon isotopic analysis of individual n-alkanes: evaluation of accuracy and application to marine particulate organic material

The accuracy and precision of δ 13c analyses for individual n-alkanes determined by gas chromatography/isotope ratio mass spectrometry (GC/IRMS) were determined. A solution of n-alkanes with known δ 13C values (ANL-1) was prepared and analyzed by GC/IRMS, and both the precision and accuracy of these measurements were within 0.5%. Silica gel gravity liquid chromatography was carried out on ANL-1 in order to determine if isotopic fractionation occurred during compound class separation. Analyses of the eluates showed that little isotopic fractionation ( 0.5 nA in this case) which do not coelute with other components, the background can be accurately subtracted using the standard software. Therefore, differences of more than 0.5% in the δ 13C values of individual n-alkanes determined by GC/IRMS appear to be significant even in the presence of background. Normal alkanes were extracted and isolated from several samples of particulate organic matter (POM) and a Recent marine sediment. Samples collected during periods of low productivity contained n-C19-n-C24n-alkanes having higher δ 13c values (0.5% or more) than those collected when productivity was high. Normal alkanes (n-C24-n-C28) isolated from sediment samples were depleted in 13C by more than 0.5% relative to POM taken during the productivity maximum. Similarly, several sediment n-alkanes (n-C24, n-C25n-C27−n-C29, and n-C31) were depleted in 13C by 1.0% or more relative to those associated with POM collected when productivity was low. Differences in the δ 13C of n-alkanes in these samples can be related to changes in the source and to degradation processes during residence in the water column and in sediments.