Using reactive transport modeling to evaluate the source term at Yucca Mountain

Chen, Yueting

doi:10.1016/s0098-3004(03)00013-x

Cited by 10 publications

(3 citation statements)

References 25 publications

(22 reference statements)

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“…Several research groups are examining the potential for Np incorporation or association with U 6+ phases that would lead to improvements in performance assessment models (Chen 2003). A fundamental problem addressing the issue of Np behaviour in weathered waste forms is the low level of this actinide in the starting material.…”

Section: Neptunium Behaviourmentioning

confidence: 99%

The geochemical behaviour of Tc, Np and Pu in spent nuclear fuel in an oxidizing environment

Buck

Hanson

McNamara

2004

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Spent fuel from commercial nuclear reactors consists mainly of uranium oxide. However, the changes that occur during reactor operations have a profound effect on chemical and physical properties of this material. Heat build-up in the fuel pellet during reactor operations can cause redistribution of fission products. The fission products may aggregate in one of three types of precipitates; gaseous, metallic, or oxide, depending on the burn-up and in-core treatment. Radiation damage and variations in fission and neutron capture yields across the fuel pellets lead to Pu enrichment and increased porosity with increasing burn-up. A more porous surface may make the fuel more susceptible to oxidative dissolution. As the level of actinides and fission products increases, the fuel may become more resistant to oxidation. These changes may limit the usefulness of natural uraninite (U02) analogues for predicting the geological behaviour of spent fuel disposed in a high-level waste (HLW) repository. In this Chapter, an overview of spent fuel microstructure, radiolytic effects, and alteration processes is presented. Evidence for Np incorporation into U 6+ phases, the nature of Pu surface precipitates on spent fuel, and evidence for the preferential removal of 4d-metals from e-particles in corroded spent fuel is discussed. Understanding the potential mechanisms of radionuclide attenuation through sorption and/or incorporation requires techniques with both high spatial resolution and excellent elemental sensitivity.

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Section: Neptunium Behaviourmentioning

confidence: 99%

The geochemical behaviour of Tc, Np and Pu in spent nuclear fuel in an oxidizing environment

Buck

Hanson

McNamara

2004

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“…Accurate prediction of the release rate of metals and radionuclides from the rock forming minerals, and the assessment of the environmental impacts into superficial and groundwater are critical factors to be addressed and will determine the remediation strategies that will be put in place. Chen (2003) pointed out that reactive models, instead of K d based ones, reflect more accurately the several concomitant reactions taking place in the waste-rock environment, while the K d based models lack of the appropriate representation of these phenomena, because they assume an equilibrium condition that is hardly found in these systems in which the pollutant releases are dominated by kinetic and thermodynamic equilibrium reactions. In addition to this, K d values vary spatially and temporarily.…”

Section: Introductionmentioning

confidence: 99%

Preliminary Results of the Water Flow Modeling in an Acid Drainage Generating Waste Rock Pile Located at the Uranium Mining Site of Poços De Caldas – Brazil

Franklin

Fernandes

Yeh

et al. 2006

JASMR

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Abstract:The first uranium production center in Brazil began operation in 1982. After 13 years of a non-continuous operation, the mining activities were suspended definitively. Uranium was extracted by open pit mining. Operations gave rise to approximately 12.4×10 6 m 3 of waste rock, while the mill process generated a volume of approximately 2.39×10 6 m 3 of tailings. Regardless the fact that some studies developed in this area exist, a well defined plan of action, aimed at the remediation and rehabilitation of the site, has not been implemented yet. The main sources of pollutants to the environment are the tailings dam, the waste rock piles and the open pit. Pyrite oxidation was found to be the driving force in the leaching of metal and radionuclides into environment. It was estimated that acid drainage generation will last for 600 and 200 years from the waste rock and tailings respectively. Accurate prediction of the release rate of metal and radionuclides from these sources and their transport in the subsurface is a critical factor to the assessment of environmental impact and to the development of effective remediation strategies. In prevailing practice, the source term is evaluated using the dissolution rate of waste form and the solubility of radionuclides. The fate of pollutants is addressed by the use of Kd-based ''reactive'' transport models. This standard practice has obvious shortcomings, mainly because it can not produce a realistic representation of the system under study. The alternative to overcome these shortages is using more sophisticate models that could represent real complex problems. Reactive transport codes are powerful tools in the evaluation of coupled thermal-hydrological-chemical processes and in the prediction of the long-term performance of remediation strategies. The difference between the predictions from these two approaches can be as high as several orders of magnitude. Generally, conventional approaches produce predicted values higher than the measured ones. On the other hand, the use of reactive transport model requires a good knowledge of the simulated hydrogeochemical system, along with the choice of appropriated algorithms that can represent the most important processes.

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“…The possibility of other Np phases such as NpO 2 (cr) in contact with Np(V) containing solutions has also been investigated [15,16]. It has been suggested that Np partitioning to secondary U phases to form a solid solution may lower the dissolved concentration of Np below that predicted for discrete Np(V)containing solids [17]. Such a prediction is consistent with SNF dissolution experiments, which have shown Np concentrations below that expected if Np 2 O 5 was the solubility limiting solid.…”

Section: Introductionmentioning

confidence: 99%

Microscale characterization of uranium(VI) silicate solids and associated neptunium(V)

Douglas¹,

Clark²,

Friese³

et al. 2005

Radiochimica Acta

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The uranium(VI) silicate phases uranophane, Ca[(UO 2 )(SiO 3 OH)] 2 ·5H 2 O, and sodium boltwoodite, Na[(UO 2 )(SiO 3 OH)]·1.5H 2 O, were synthesized in the presence of small, variable quantities (0.5-2.0 mol % relative to U) of pentavalent neptunium (Np(V), as NpO 2 + ), to investigate the nature of its association with these U(VI) solid phases. Solids were characterized by X-ray powder diffraction (XRD), gamma spectrometry (GS), scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS), and transmission electron microscopy (TEM) with electron energy-loss spectroscopy (EELS). Neptunium concentration was determined in the bulk solid phases by GS and was found to range from 780-15 800 µg/g. In some cases, Np distributions between the aqueous and solid phases were monitored, and 78%-97% of the initial Np was associated with the isolated solid. Characterization of individual crystallites by TEM/EELS suggests the Np is associated with the U(VI) phase. No discrete Np phases, such as Np oxides, were observed. Because the U(VI) silicates are believed to be important solubility-controlling solids on a geologic timescale, these results suggest that the partitioning of the minor actinides to these solids must be considered when assessing the performance of a waste repository for spent nuclear fuel.

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Using reactive transport modeling to evaluate the source term at Yucca Mountain

Cited by 10 publications

References 25 publications

The geochemical behaviour of Tc, Np and Pu in spent nuclear fuel in an oxidizing environment

The geochemical behaviour of Tc, Np and Pu in spent nuclear fuel in an oxidizing environment

Preliminary Results of the Water Flow Modeling in an Acid Drainage Generating Waste Rock Pile Located at the Uranium Mining Site of Poços De Caldas – Brazil

Microscale characterization of uranium(VI) silicate solids and associated neptunium(V)

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