Michael S. Rearick

Bentonite evolution at elevated pressures and temperatures: An experimental study for generic nuclear repository designs

Abstract Geologic disposal of spent nuclear fuel in high-capacity metal canisters may reduce the repository footprint, but it may yield high-thermal loads (up to 300 °C). The focus of this experimental work is to expand our understanding of the hydrothermal stability of bentonite clay barriers interacting with metallic phases under different geochemical, mineralogical, and engineering conditions. The hydrothermal experiments were performed using flexible Au/Ti Dickson reaction cells mounted in an externally heated pressure vessel at 150-160 bars and temperatures up to 300 °C for five to six weeks. Unprocessed Wyoming bentonite, containing primarily montmorillonite with minor amount of clinoptilolite, was saturated with a K-Ca-Na-Cl-bearing water (~1900 mg/L total dissolved solids) at a 9:1 water:rock mass ratio. The bentonite and solution combination contained either steel plates or Cu-foils and were buffered to low Eh using magnetite and metallic iron. During reactions, pH, K+, and Ca2+ concentrations decreased, whereas SiO2(aq), Na+, and SO42- concentrations increased throughout the experiments. Pyrite decomposition was first observed at ~210 °C, generating H2S(aq,g) that interacted with metal plates or evolves as a gas. The aqueous concentrations of alkali and alkaline earth cations appear to be buffered via montmorillonite and clinoptilolite exchange reactions. Illite or illite/smectite mixed-layer formation was significantly retarded in the closed system due to a limited K+ supply along with high Na+ and SiO2(aq) concentrations. Precursor clinoptilolite underwent extensive recrystallization during the six weeks, 300 °C experiments producing a Si-rich analcime in addition to authigenic silica phases (i.e., opal, cristobalite). Analcime and feldspar formation partially sequester aqueous Al3+, thereby potentially inhibiting illitization. Associated with the zeolite alteration is a ~17% volume decrease (quartz formation) that translates into ~2% volume loss in the bulk bentonite. These results provide chemical information that can be utilized in extending the bentonite barriers’ lifetime and thermal stability. Zeolite alteration mineralogy and illitization retardation under these experimental conditions is important for the evaluation of clay barrier long-term stability in a spent nuclear fuel repository.

Rapid dissolution of surrogate nuclear debris using ammonium bifluoride fusion and indirect sonication dissolution methods

Prompt analysis of elemental and isotopic information from post-detonation nuclear debris is critical for rapid attribution analysis. In this work, the capabilities of dissolution methods that use ammonium bifluoride (ABF, NH4HF2) and pressurized microwave digestion with HF acid are reported for NIST Surrogate Post-detonation Urban Debris (SPUD). Elemental concentration and isotope ratios were measured using inductively coupled plasma mass spectrometry. The SPUD material was also analyzed for U concentration using instrumental neutron activation analysis. The ABF dissolution method with a 10:1 ABF to sample ratio and the pressurized microwave digestion methods resulted in comparable results.

What’s that yellow powder? A nuclear forensic case study

In this paper the utilization of three analytical chemistry techniques including gamma spectrometry, XRF, and ICP-MS/OES is described for performing nuclear forensic analyses on an unknown powder. We have demonstrated that each method was unique in providing specific material characteristics, yet they were also complementary for extracting useful nuclear forensic signatures. It is the integral effort of all three analytical chemistry tools in the nuclear forensic tool box that ultimately allowed us to reveal the identity of the unknown nuclear material as Nb2O5 mixed with ~ 9% HEU.

Combining accuracy and precision of traceable standards to estimate uncertainties in trace element content measurements

AbstractThe accuracy and precision systematic of traceable certified reference material (CRM) standards can be combined to estimate the uncertainties that are achieved in analytical measurements. Standards (CRMs or other well characterized materials) in matrices similar to the unknown sample can be used for evaluation of the accuracy. The standards should be prepared following same preparation steps as the unknown samples and analyzed in the same analytical sequence as the samples using similar instrumental methods. The accuracy and precision of these quality control (QC) standards are combined to calculate uncertainty estimates for trace element contents. The uncertainties calculated from the QC standards are compared with those estimated following the guide to expression of uncertainty in measurements (GUM). For trace element content measurements using ICP-AES and ICP-MS instruments, the uncertainty estimates obtained by combining the accuracy and precision of the QC standards are found to be more conservative than those calculated by a model following GUM methodology. LA-UR-17-31180.

Water quality and hydrogeochemistry of a basin and range watershed in a semi-arid region of northern New Mexico

Hundreds of domestic wells in northern New Mexico, have concentrations of U, As, and NO3− that exceed the Environmental Protection Agency’s (EPA) maximum contaminant level (MCL) for drinking water consumption. As part of a case study in groundwater quality, we collected groundwater samples from 749 domestic wells throughout the eastern half of the Española Basin. All water samples were analyzed for major ions, trace metals, and alkalinity. Selected samples were also analyzed for stable isotopes of O, H, and N. Of the wells we measured, 15, 173, and 99 had respective NO3−, U, and As concentrations that exceeded the EPA’s MCL. Total dissolved solids (TDS), U, and HCO3− were elevated in the Sangre de Cristo mountain block and around the town of Nambé. Our findings suggest that roll-front U deposits and devitrification of volcanic ash result in elevated U near Nambé, while weathering of granitic rocks accounts for high U in the mountain block. Arsenic concentrations were high in much of the study area with the exception of the Santa Fe metro region and the mountain block. Elevated As concentrations can be explained by devitrification of volcanic ash, anion exchange with clays, and mixing with hydrothermal fluids. In wells with high NO3− concentrations, analysis of N isotopes are consistent with contamination from domestic wastewater effluent. Our findings suggest that the geochemistry of the region is largely influenced by local geology while groundwater contamination from domestic water treatment and wastewater effluent is an emerging issue.