New insight studies of the secondary phase formation under repository conditions

ABSTRACTIn a deep geological repository (DGR) scenario, uranium oxidized in aqueous systems will be stabilized as UO22+ (hexavalent uranium), as a consequence of tetravalent uranium oxidation by radiolytic byproducts. Uranyl cationic species (UO22+) in different speciation forms are expected to be found at the whole pH range conditions. The importance of UO22+ lies in its potential incorporation of trace radioelements onto secondary uranyl phases. In view of the difficulty of U chemistry in natural groundwater, it is necessary to improve speciation assessment techniques so as to understand chemical processes. Raman spectroscopy has been shown as a powerful tool to analyze the speciation of various actinyl (UO22+,NpO2+ and PuO22+) and to determine the distribution of those elements which are more likely to be stable in a near-field groundwater environment. Therefore, the aim of this work is to follow UO22+ changes as a consequence of γ radiation in aqueous media under DGR conditions, and to understand the behavior of UO22+ as a function of aqueous media, which help to understand and predict the potential precipitation of the solid phases formed. In this work, the use of Raman spectroscopy adapted to the empirical analysis of different nuclear applications for initial uranium concentrations 0.04M at ambient atmosphere is shown, i.e. as monitoring tool for UO22+ precipitation as a function of pH, studying UO2(NO3)2·6H2O stability in aqueous solutions representative of groundwater, in particular at ionic strength I = 0.02 – 0.4 M and pH from 7 to 13.2; and to evaluate the effect of γ radiation fields. At 10−4-10-3 M of radiolytically formed H2O2 concentration, the amount of uranium in solution decreased, as a results of secondary phases precipitation. The results obtained will be useful to the performance assessment studies of the Spent Nuclear Fuel (SNF) stored in DGRs. The work performed provides a partial picture of secondary phase formations, as a result of corrosion of SNF in a DGR.

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The behavior of unirradiated UO2and uraninite under repository conditions characterized by Raman

MRS Advances ◽

10.1557/adv.2017.203 ◽

2016 ◽

Vol 1 (62) ◽

pp. 4157-4162 ◽

Cited By ~ 2

Author(s):

L. J. Bonales ◽

J.M. Elorrieta ◽

C. Menor-Salván ◽

J. Cobos

Keyword(s):

Raman Spectroscopy ◽

Spent Nuclear Fuel ◽

Secondary Phase ◽

Storage Conditions ◽

Oxidation Products ◽

Deep Geological Repository ◽

Secondary Phases ◽

Secondary Phase Formation ◽

Geological Repository ◽

Spectroscopy Studies

ABSTRACTRaman spectroscopy studies have been performed on one hand to identify different materials related to spent nuclear fuel (SNF), and on the other hand to study the behavior of SNF at different storage conditions. Specifically, the expected oxidation of the SNF matrix under dry storage conditions and the formation of secondary phases (SP), as a result of corrosion of SNF in a deep geological repository, have been studied. In order to perform these experiments, two protocols based on the Raman spectroscopy technique have been developed. The results show U4O9/U3O7and U3O8as oxidation products of UO2powder at high temperatures in air, and the secondary phase formation (rutherfordine, UO2(CO3), soddyite, (UO2)2SiO4•2H2O, uranophane alpha Ca(UO2)2(SiO3OH)2•5H2O and kasolite, PbUO2SiO4•H2O), due to uraninite corrosion under the conditions of Sierra Albarrana (Spain).

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The Influence of Ligands and Precipitates on the Release of Nuclides from the Near Field under Natural Repository Conditions

MRS Proceedings ◽

10.1557/proc-663-531 ◽

2000 ◽

Vol 663 ◽

Author(s):

L. Liu ◽

I. Neretnieks

Keyword(s):

Uranium Oxide ◽

Spent Nuclear Fuel ◽

Transport Model ◽

Radiation Power ◽

Near Field ◽

Reference Scenario ◽

Spent Fuel ◽

Secondary Phases ◽

Fuel Surface ◽

Fuel Pellets

ABSTRACTOnce groundwater intrudes into a damaged canister and wets the spent fuel pellets, radiation emitted from the spent nuclear fuel splits nearby water into oxidizing and reducing species. This may lead to an oxidizing condition near the fuel pellets. As a result, uranium oxide that makes up the fuel matrix will become more soluble, and the incorporated radionuclides will be released more rapidly. The dissolution process is, however, a dynamic one that can be influenced by many factors. Of great importance are the radiation power of the fuel matrix, the concentration of ligands near the fuel surface, and the transport resistance of the near field. Consequently, the escape of nuclides from the damaged canister is dominated mainly by the intrusion of ligands, and the precipitation/dissolution of secondary phases within the fuel rods. To investigate the possible effects of ligands and precipitates, a coupled dissolution and transport model, which includes the barrier effect of the Zircaloy claddings, is developed. The application of the model to a SKB-specified reference scenario indicates that by far the largest fraction of the oxidized uranium will reprecipitate within the canister. This may significantly decrease the fuel surface available for oxidation and the water available for radiolysis. Subsequently, much less fuel matrix will be dissolved and much less of the other nuclides will be released. Simulations further identify that carbonate and silicate have the greatest influences on the formation of secondary phases, and on the release of nuclides, under natural repository conditions.

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Coupling of Chemical Processes in the Near Field

MRS Proceedings ◽

10.1557/proc-932-119.1 ◽

2006 ◽

Vol 932 ◽

Cited By ~ 10

Author(s):

B. Grambow ◽

E. Giffaut

Keyword(s):

Spent Nuclear Fuel ◽

Hydrogen Generation ◽

Pore Space ◽

Near Field ◽

Secondary Phase ◽

Solution Chemistry ◽

High Level Waste ◽

Coupled Modelling ◽

Geochemical Transport ◽

High Level

ABSTRACTCoupled modelling has been performed using geochemical/transport codes and radiolysis models to describe the chemical evolution of the waste forms “high-level waste glass” and “spent nuclear fuel” together with its waste package and engineered barrier surroundings. Near field processes considered include container corrosion, hydrogen generation, mass transfer for radionuclides and other waste matrix components in corrosion products and buffer materials, geochemistry of near field solution chemistry, sorption of radionuclides on surface sites in the nano-sized pore space of near field materials and the radiolytic decomposition of pore water. The rate limiting steps in waste form dissolution and secondary phase formation mechanism and the associated radionuclide mobilisation chemistry (solubility, solid solution formation, speciation, redox stability) are strongly influenced by the near field constraints.

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Modelling Radionuclide Mobility of Spent Fuel in a Deep Geological Repository Located in a Clay Bedrock

ASME 2009 12th International Conference on Environmental Remediation and Radioactive Waste Management, Volume 1 ◽

10.1115/icem2009-16240 ◽

2009 ◽

Cited By ~ 1

Author(s):

Lara Duro ◽

Abel Tamayo ◽

Jordi Bruno ◽

Aurora Marti´nez-Esparza

Keyword(s):

Source Term ◽

Spent Nuclear Fuel ◽

Near Field ◽

Radioactive Decay ◽

Secondary Phase ◽

Diffusive Transport ◽

Deep Geological Repository ◽

Spent Fuel ◽

Geological Repository ◽

Source Term Model

Source term models are widely used to assess the behaviour of spent nuclear fuel after final disposal. However, most models do not take into account some phenomena which are expected to control the transport of radionuclides through the near field. Some uncertainties arise from this fact, thus making it difficult to obtain proper simulations of radionuclide behaviour in the near field. In this work, we have used a compartmental code to build up an integrated source term model in an attempt to overcome the abovementioned drawbacks. The model developed takes into account radiolytically-mediated matrix dissolution, radioactive decay chains, diffusive transport, and retardation by sorption and secondary phase precipitation, among other processes. In addition, this model has been used to estimate radionuclide mobility from spent fuel located in a conceptual clay geological repository.

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Some Important Mechanisms and Processes in the Near Field of the Swedish Repository for Spent Nuclear Fuel

MRS Proceedings ◽

10.1557/proc-294-675 ◽

1992 ◽

Vol 294 ◽

Author(s):

Ivars Neretnieks

Keyword(s):

Molecular Diffusion ◽

Spent Nuclear Fuel ◽

Dominant Role ◽

Near Field ◽

Water Flow Rate ◽

Crystalline Rock ◽

Diffusive Transport ◽

Actual Design ◽

Natural Analogues ◽

And Performance

ABSTRACTIn repositories for nuclear waste there are many processes that will be instrumental in damaging the canisters and releasing the nuclides. Based on experiences from studies of the performance of repositories and of an actual design, the major mechanisms influencing the integrity and performance of a repository are described and discussed. The paper addresses only conditions in crystalline rock repositories. The low water flow rate in fractures and channels plays a dominant role in limiting the interaction between water and waste. Molecular diffusion in the backfill and rock matrix, as well as in the mobile water, is an important transport process, but actually limits the exchange rate because diffusive transport is slow. Solubility limits of both waste matrix and of individual nuclides are also important. Complicating processes include alpha-radiolysis, which may change the water chemistry in the near-field. The sizes and locations of water flowpaths and damages in the canisters considerably influence the release rates. Uncertainties in data are large. Nevertheless the system is very robust in the sense that practically no reasonably conceivable assumptions or data will lead to large nuclide releases. Several natural analogues have been found to exhibit similarities with a waste repository and help to validate concepts and to increase our confidence that all major issues have been considered.

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