Model Calculations of Porosity Reduction Resulting From Cement-Tuff Diffusive Interaction

1997 ◽  
Vol 506 ◽  
Author(s):  
Peter C. Lichtner ◽  
Roberto T. Pabalan ◽  
Carl I. Steefel
Keyword(s):  
Reactive Transport ◽  
Near Field ◽  
Computer Code ◽  
Diffusive Transport ◽  
Geologic Repository ◽  
Model Calculations ◽  
Porosity Reduction ◽  
Field Environment ◽  
The Matrix ◽  
High Level

ABSTRACTTo determine the potential effects of alkaline plume migration on the near-field environment of the proposed high-level radioactive waste geologic repository at Yucca Mountain, Nevada, calculations are conducted simulating interactions between cement and tuff with pure diffusive transport of solute species. The calculations used the reactive transport submodule GEM of the computer code MULTIFLO [6]. The results suggest that strong alteration of the tuff host rock and of cement in contact with the tuff could result from these interactions. Porosity reduction within the tuff could isolate the matrix from fracture pore water. The model calculations predict calcification of the cement as would be expected. In simulations involving counter-diffusive transport across the cement-tuff contact, calcification is more pronounced in a partially-saturated environment compared to a fully-saturated one.

Modeled Near-Field Environment Porosity Modification due to Coupled Thermohydrologic and Geochemical Processes

1999 ◽  
Vol 556 ◽  
Author(s):  
John J. Nitao ◽  
William E. Glassley
Keyword(s):  
Reactive Transport ◽  
Near Field ◽  
Mineral Dissolution ◽  
Rock Properties ◽  
Actual Distribution ◽  
Porosity Reduction ◽  
Field Environment ◽  
Flow Pathways ◽  
Waste Repositories ◽  
Dissolution And Precipitation

AbstractHeat generated by waste packages in nuclear waste repositories can modify rock properties by instigating mineral dissolution and precipitation along hydrothermal flow pathways. Modeling this reactive transport requires coupling fluid flow to permeability changes resulting from dissolution and precipitation. Modification of the NUFT thermohydrologic (TH) code package to account for this coupling in a simplified geochemical system has been used to model the timedependent change in porosity, permeability, matrix and fracture saturation, and temperature in the vicinity of waste-emplacement drifts, using conditions anticipated for the potential Yucca Mountain repository. The results show dramatic porosity reduction approximately 10 m above emplacement drifts within a few hundred years. Most of this reduction is attributed to deposition of solute load at the boiling front, although some of it also results from decreasing temperature along the flow path. The actual distribution of the nearly sealed region is sensitive to the time-dependent characteristics of the thermal load imposed on the environment and suggests that the geometry of the sealed region can be engineered by managing the waste-emplacement strategy and schedule.

scholarly journals Reactive transport calculations to evaluate sulphide fluxes in the near-field of a SF/HLW repository

2019 ◽  
Vol 98 ◽  
pp. 10005
Author(s):  
Marek Pękala ◽  
Paul Wersin ◽  
Veerle Cloet ◽  
Nikitas Diomidis
Keyword(s):  
Reactive Transport ◽  
Spent Nuclear Fuel ◽  
Near Field ◽  
Opalinus Clay ◽  
High Level Waste ◽  
Waste Containment ◽  
Geological Repository ◽  
High Level ◽  
Reducing Bacteria ◽  
Hlw Repository

Radioactive waste is planned to be disposed in a deep geological repository in the Opalinus Clay (OPA) rock formation in Switzerland. Cu coating of the steel disposal canister is considered as potential a measure to ensure complete waste containment of spent nuclear fuel (SF) and vitrified high-level waste (HLW) or a period of 100,000 years. Sulphide is a potential corroding agent to Cu under reducing redox conditions. Background dissolved sulphide concentrations in pristine OPA are low, likely controlled by equilibrium with pyrite. At such concentrations, sulphide-assisted corrosion of Cu would be negligible. However, the possibility exists that sulphate reducing bacteria (SRB) might thrive at discrete locations of the repository’s near-field. The activity of SRB might then lead to significantly higher dissolved sulphide concentrations. The objective of this work is to employ reactive transport calculations to evaluate sulphide fluxes in the near-field of the SF/HLW repository in the OPA. Cu canister corrosion due to sulphide fluxes is also simplistically evaluated.

Laboratory and Modelling Studies of Sodium Benionite Grounmater Interaction

1986 ◽  
Vol 84 ◽  
Author(s):  
M. Sneujman ◽  
H. Uotiia ◽  
J. Rantanen
Keyword(s):  
Near Field ◽  
Chemical Species ◽  
Fluid Phase ◽  
High Level Waste ◽  
Sodium Bentonite ◽  
Model Calculations ◽  
Buffer Material ◽  
Modelling Studies ◽  
Calcite Saturation ◽  
High Level

AbstractAccording to the present Finnish concept sodium bentonite will be used as a buffer material in the repository for high-level waste. Experimental and theoretical studies treating the effect of bentonite upon the chemical conditions in a repository have been initiated with the object of specifying the chemistry of the near field.Sodium bentonite was let react with water under anaerobic conditions at 25°C for 540 days, during which time six fluid samples were extracted for the chemical analysis of 15 chemical species. The generated fluid phase was alkaline (PH = 9…10) and contained a high amount of bicarbonate. Also a low redox-potential was measured. The fluid phase chemistry was investigated using the geochemical code PHREEM. Calcite saturation was observed in all fluid samples.A modelling of sodium bentonite interaction with water based on the main mineral components of bentonite was also performed with PHREEQE. A fairly good agreement between experimental results and model calculations was observed.

Near field evolution of a spent fuel repository in an argillaceous rock formation and impact on radionuclide migration

2020 ◽  
Author(s):  
Vanessa Montoya ◽  
Orlando Silva ◽  
Emilie Coene ◽  
Jorge Molinero ◽  
Renchao Lu ◽  
...  
Keyword(s):  
Ion Exchange ◽  
Radioactive Waste ◽  
Nuclear Fuel ◽  
Reactive Transport ◽  
Spent Nuclear Fuel ◽  
Near Field ◽  
Spent Fuel ◽  
Exchange Reactions ◽  
Radionuclide Migration ◽  
High Level

<p>In August 2015, the German government approved the national programme for the responsible and safe management of spent nuclear fuel (SNF) and radioactive waste proposed by the Federal Ministry for the Environment, Nature Conservation, Building and Reactor Safety (BMU). The assumption is that about ~ 1 100 storage casks (10 500 tons of heavy metal) in the form of spent fuel assemblies will be generated in nuclear power plants and will have to be disposed. However, a decision on the disposal concept for high-level waste is pending and an appropriate solution has to be developed with a balance in multiple aspects. All potential types of host rocks, clay and salt stones as well as crystalline formations are under consideration. In the decision process, evaluation of the risk of different waste management options and scenarios play an enormous role in the discussion. Coupled physical and chemical processes taking place within the engineered barrier system of a repository for high-level radioactive waste will define the radionuclide mobility/retention and the possible radiological impact. The objective of this work is to assess coupled processes occurring in the near-field of a generic repository for spent nuclear fuel in a high saline clay host rock, integrating complex geochemical processes at centimetre-scale. The scenario considers that radionuclides can be released during a period of thousands of years after full saturation of the bentonite barrier and the thermal phase.</p><p>Transport parameters and the discretization of the system, are implemented in a 2D axisymmetric geometry. The multi-barrier system is emplaced in clay and a solubility limited source term for the selected radionuclides is assumed. Kinetics and chemical equilibria reactions are simulated using parameters obtained from experiments. Additionally, porosity changes due to mineral precipitation/dissolution and feedback on the effective diffusion coefficient are taken into account. Protonation/deprotonation, ion exchange reactions and radionuclide inner-sphere sorption is considered.</p><p>Numerical simulations show, that, when the canister corrosion starts, the redox potential decreases, magnetite precipitates and H<sub>2</sub> is formed. Furthermore, the aqueous concentration of Fe(II) increases due to the presence of magnetite. By considering binding to montmorillonite via ion exchange reactions, the bentonite acts as a sink for Fe(II). Additionally, magnetite forms a chemical barrier offering significant sorption capacity for many radionuclides. Finally, a decrease of porosity in the bentonite/canister interface leads to a further deceleration of radionuclide migration. Due to the complexity of reactive transport processes in saline environments, benchmarking of reactive transport models (RTM) is important also to build confidence in those modelling approaches. Development of RTM benchmark procedures is part of the iCROSS project (Integrity of nuclear waste repository systems - Cross-scale system understanding and analysis) funded by both the Helmholtz Association and the Federal Ministry of Education and Research (BMBF).</p><p> </p>

Modeling of the oxic stage in a HLW disposal cell in an argillaceous host rock

2012 ◽  
Vol 1475 ◽  
Author(s):  
François Marsal ◽  
Laurent De Windt ◽  
Delphine Pellegrini
Keyword(s):  
Reactive Transport ◽  
Near Field ◽  
Pyrite Oxidation ◽  
Gas Diffusion ◽  
Radioactive Waste Disposal ◽  
Localized Corrosion ◽  
High Level Waste ◽  
Perfect Contact ◽  
High Level ◽  
Water Saturated

ABSTRACTDetermining the redox conditions in the near field of deep underground radioactive waste disposal cells is a key question regarding the performance of metallic components (e.g. waste overpack), which may undergo drastic corrosion processes in oxic conditions. This oxic transient is supposed to be short due notably to the oxygen consumption by corrosion and pyrite oxidation. However, the observed precipitation of Fe(III)-minerals as well as localized corrosion patterns on steel coupons placed during 6 years in a borehole drilled in the Toarcian argillite of Tournemire (France) may suggest that in-situ oxic conditions lasted several years, which is not consistent with reactive transport simulations performed with usual hypotheses (perfect contact between materials, high pyrite accessibility, water saturated conditions). Multicomponent reactive transport simulations considering gas diffusion were performed with the code HYTEC and reproduce correctly the observations made in Tournemire while considering imperfect interfaces and resaturation processes. The model was then applied to a disposal cell for high-level waste (HLW) representative of the design developed in France, putting into evidence the possibility of a redox contrast between the front and back of a disposal cell in an argillaceous medium, as well as a duration of the oxic stage within the cell as long as the ventilation of handling drifts is maintained.

scholarly journals A Large Block Heater Test for High Level Nuclear Waste Management,

1994 ◽  
Vol 353 ◽  
Author(s):  
Wunan Lin ◽  
D. G. Wilder ◽  
J. A. Blink ◽  
S. C. Blair ◽  
T. A. Buscheck ◽  
...  
Keyword(s):  
Nuclear Waste ◽  
Near Field ◽  
Radioactive Decay ◽  
Test Site ◽  
Constant Load ◽  
Large Block ◽  
Field Environment ◽  
Before And After ◽  
Pressure Chemical ◽  
High Level

AbstractThe radioactive decay heat from nuclear waste packages may, depending on the thermal load, create coupled thermal-mechanical-hydrological-chemical (TMHC) processes in the near-field environment of a repository. A group of tests on a large block (LBT) are planned to provide a timely opportunity to test and calibrate some of the TMHC model concepts. The LBT is advantageous for testing and verifying model concepts because the boundary conditions are controlled, and the block can be characterized before and after the experiment. A block of Topopah Spring tuff of about 3 × 3 × 4.5 m was sawed and isolated at Fran Ridge, Nevada Test Site. Small blocks of the rock adjacent to the large block were collected for laboratory testing of some individual thermal-mechanical, hydrological, and chemical processes. A constant load of about 4 MPa will be applied to the top and sides of the large block. The sides will be sealed with moisture and thermal barriers. The large block will be heated by heaters within and guard heaters on the sides so that a dry-out zone and a condensate zone will exist simultaneously. Temperature, moisture content, pore pressure, chemical composition, stress, and displacement will be measured throughout the block during the heating and cool-down phases. The results from the experiments on small blocks and the tests on the large block will provide a better understanding of some concepts of the coupled TMHC processes. The progress of the project is presented in this paper.

scholarly journals Laser Photoacoustic Spectroscopy for Trace Level Detection of Actinides in Groundwater

1986 ◽  
Vol 84 ◽  
Author(s):  
Mark M. Doxtader ◽  
Victor A. Maroni ◽  
James V. Beitz ◽  
Michael Heaven
Keyword(s):  
Photoacoustic Spectroscopy ◽  
Near Field ◽  
Trace Level ◽  
Field Environment ◽  
Waste Container ◽  
Laser Photoacoustic Spectroscopy ◽  
High Level Nuclear Waste ◽  
High Level ◽  
Radionuclide Release

The Basalt Waste Isolation Project (Rockwell Hanford Operations- BWIP) is investigating the feasibility of building a repository in the Columbia River Basalts for the permanent disposal of high-level nuclear waste. One aspect of this effort is to develop an understanding of the chemical behavior of radionuclides in the near-field environment of the waste container. Such information is needed to determine radionuclide release rates from the waste package and to make long-term projections of repository performance. To accomplish this task, ultrasensitive laser- based techniques, such as laser photoacoustic spectroscopy (LPAS) and laser induced fluorescence (LIF), are being developed as analytical methods for the trace-level detection and speciation of actinides in solutions typical of those encountered in groundwaters near the BWIP repository.

The Influence of Thermal Gradients on the Long-Term Evolution of the Near-Field Environment of High-Level Nuclear Wastes Disposal

1997 ◽  
Vol 506 ◽  
Author(s):  
C. Poinssot ◽  
P. Toulhoat ◽  
B. Goffé
Keyword(s):  
Thermal Gradient ◽  
Near Field ◽  
Coupled Processes ◽  
Elemental Distribution ◽  
Thermal Gradients ◽  
Secondary Phases ◽  
Rock Interaction ◽  
Geochemical Model ◽  
Field Environment ◽  
High Level

ABSTRACTThe initial stage of some HLW disposal systems will be characterised by a large thermal pulse in the near-field environment, due to the heat of the radioactivity decay. This will lead to the development of a transient spatial thermal gradient between the hot canister and the cold geological medium, which could significantly affect the composition and the elemental distribution within the near-field environment. A coupled experimental and modelling work is presented in order to determine the influence of a thermal gradient on water-rock interaction processes. First experiments with a simulated nuclear glass evidenced mass transfer processes leading to chemical differentiation in the solid phases between the hot and the cold end of the system. The relevance of these experimental results to the case of a HLW disposal is strongly supported by in-situ experiments at Stripa, in which a realistic EBS under thermal gradient developed exactly the same mass transfers.In order to understand the driving force of these processes, we tried to model simplified experiments by using a mixing cell geochemical model built upon the geochemical code EQ3/EQ6. The discrepancies between modelling and experiments indicate the existence of coupled processes involving irreversible precipitation.Finally, thermal gradients were applied in nuclear glass-clay interaction experiments to enhance elemental migrations. The main results are: (i) a re-crystallisation of the initial clay toward a more silicic one through incorporation of elements released by the glass, (ii) a strong influence of clay chemistry on the nuclear glass secondary phases.

scholarly journals In-Situ Observation of the Alpha/beta Cristobalite Transition Using High Voltage Electron Microscopy

1989 ◽  
Vol 176 ◽  
Author(s):  
A. Meike ◽  
W. E. Glassley
Keyword(s):  
Water Vapor ◽  
High Voltage ◽  
Amorphous Silica ◽  
Near Field ◽  
In Situ Observation ◽  
High Voltage Electron Microscopy ◽  
Voltage Electron ◽  
Field Environment ◽  
High Level ◽  
Temperature Water

ABSTRACTA high temperature water vapor phase is expected to persist in the vicinity of high level radioactive waste packages for several hundreds of years. We have begun an investigation of the structural and chemical effects of water on cristobalite because of its abundance in the near field environment. A high voltage transmission electron microscope (HVEM) investigation of bulk synthesized α-cristobalite to be used in single phase dissolution and precipitation kinetics experiments revealed the presence β cristobalite, quartz and amorphous silica, in addition to α-cristobalite. Consequently, this apparent metastable persistence of β-cristobalite and amorphous silica during the synthesis of α-cristobalite was investigated using a heating stage and an environmental cell installed in the HVEM that allowed the introduction of either dry CO2 or a CO2 + H2O vapor. Preliminary electron diffraction evidence suggests that the presence of water vapor affected the α-β transition temperature. Water vapor may also be responsible for the development of an amorphous silica phase at the transition that may persist over an interval of several tens of degrees. The amorphous phase was not documented during the dry heating experiments.

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