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Oxidative Dissolution of Spent Fuel and Release of Nuclides from a Copper/Iron Canister: Model Developments and Applications
KTH, Superseded Departments, Chemical Engineering and Technology.ORCID iD: 0000-0001-6801-9208
2001 (English)Doctoral thesis, comprehensive summary (Other scientific)
Abstract [en]

Three models have been developed and applied in the performance assessment of a final repository. They are based on accepted theories and experimental results for known and possible mechanisms that may dominate in the oxidative dissolution of spent fuel and the release of nuclides from a canister. Assuming that the canister is breached at an early stage after disposal, the three models describe three sub-systems in the near field of the repository, in which the governing processes and mechanisms are quite different.

In the model for the oxidative dissolution of the fuel matrix, a set of kinetic descriptions is provided that describes the oxidative dissolution of the fuel matrix and the release of the embedded nuclides. In particular, the effect of autocatalytic reduction of hexavalent uranium by dissolved H2, using UO2 (s) on the fuel pellets as a catalyst, is taken into account. The simulation results suggest that most of the radiolytic oxidants will be consumed by the oxidation of the fuel matrix, and that much less will be depleted by dissolved ferrous iron. Most of the radiolytically produced hexavalent uranium will be reduced by the autocatalytic reaction with H2 on the fuel surface. It will reprecipitate as UO2 (s) on the fuel surface, and thus very little net oxidation of the fuel will take place.

In the reactive transport model, the interactions of multiple processes within a defective canister are described, in which numerous redox reactions take place as multiple species diffuse. The effect of corrosion of the cast iron insert of the canister and the reduction of dissolved hexavalent uranium by ferrous iron sorbed onto iron corrosion products and by dissolved H2 are particularly included. Scoping calculations suggest that corrosion of the iron insert will occur primarily under anaerobic conditions. The escaping oxidants from the fuel rods will migrate toward the iron insert. Much of these oxidants will, however, be consumed by ferrous iron that comes from the corrosion of iron. The nonscavenged hexavalent uranium will be reduced by ferrous iron sorbed onto the iron corrosion products and by dissolved hydrogen.

In the transport resistance network model, the transport of reactive actinides in the near field is simulated. The model describes the transport resistance in terms of coupled resistors by a coarse compartmentalisation of the repository, based on the concept that various ligands first come into the canister and then diffuse out to the surroundings in the form of nuclide complexes. The simulation results suggest that carbonate accelerates the oxidative dissolution of the fuel matrix by stabilizing uranyl ions, and that phosphate and silicate tend to limit the dissolution by the formation of insoluble secondary phases.

The three models provide powerful tools to evaluate "what if" situations and alternative scenarios involving various interpretations of the repository system. They can be used to predict the rate of release of actinides from the fuel, to test alternative hypotheses and to study the response of the system to various parameters and conditions imposed upon it.

Place, publisher, year, edition, pages
Stockholm: KTH , 2001. , viii, 68 p.
Series
Trita-KET, ISSN 1104-3466 ; 154
Keyword [en]
model, oxidative dissolution, spent fuel, radiolysis, release, mass transport, radionuclide, hydrogen, corrosion, canister, near field, repository
National Category
Chemical Engineering
Identifiers
URN: urn:nbn:se:kth:diva-3292ISBN: ISBN 91-7283-220-7 (print)OAI: oai:DiVA.org:kth-3292DiVA: diva2:9077
Public defence
2002-02-01, 00:00 (English)
Note
QC 20100521Available from: 2002-01-22 Created: 2002-01-22 Last updated: 2010-05-21Bibliographically approved
List of papers
1. A coupled model for oxidative dissolution of spent fuel and transport of radionuclides from an initially defective canister
Open this publication in new window or tab >>A coupled model for oxidative dissolution of spent fuel and transport of radionuclides from an initially defective canister
2001 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 135, 273-285 p.Article in journal (Refereed) Published
Abstract [en]

An earlier model for oxidative dissolution of spent fuel was developed by including the release behavior of actinides from the fuel surface and the barrier effect of Zircaloy claddings. The aim here is to explore the possibility and consequences of precipitation in the water film around the fuel pellets due to solubility, and transport limitations of nuclides. The model has been applied in the performance assessment of a damaged canister under natural repository conditions, by coupling to a redox-front-based model for transport of nuclides. The simulation results identify? that the time of penetration of the canister, the size of the damage, and the initial free volume of the fuel rods are important factors that dominate the dissolution behavior of the fuel matrix and thus the transport behavior of actinides in the nearfield of a repository.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12972 (URN)
Note
QC 20100521Available from: 2010-05-21 Created: 2010-05-20 Last updated: 2017-12-12Bibliographically approved
2. The effect of hydrogen on oxidative dissolution of spent fuel
Open this publication in new window or tab >>The effect of hydrogen on oxidative dissolution of spent fuel
2002 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 138, 69-78 p.Article in journal (Refereed) Published
Abstract [en]

An earlier model for the oxidative dissolution of spent fuel is further developed by including the reductive effect of H-2, which is formed by both the radiolysis of ground-water and the anoxic corrosion of the cast iron insert of the canister. The kinetics of reduction of dissolved uranium species by dissolved hydrogen is derived from a series of previously published experimental studies. The simulation results suggest that the effect of autocatalytic reduction of hexavalent uranium by hydrogen may play an important role in controlling the dissolution of the fuel matrix within a canister. Further experimental studies are required to firmly verify these findings.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12974 (URN)
Note
QC 20100521Available from: 2010-05-21 Created: 2010-05-20 Last updated: 2017-12-12Bibliographically approved
3. A reactive transport model for oxidative dissolution of spent fuel and release of nuclides within a defective canister
Open this publication in new window or tab >>A reactive transport model for oxidative dissolution of spent fuel and release of nuclides within a defective canister
2002 (English)In: Nuclear Technology, ISSN 0029-5450, E-ISSN 1943-7471, Vol. 137, 228-240 p.Article in journal (Refereed) Published
Abstract [en]

In this study, we develop a mechanism-based model to take into account most of the important processes that may influence the dissolution behavior of spent fuel and subsequently the release behavior of nuclides within a defective canister in a final repository for high-level nuclear waste. The model is, in essence, a redox-controlled reactive transport model that provides a description of the mass transport of multiple species involved in both local equilibrium and kinetically controlled reactions in the system. The complexity of the kinetics of the various redox reactions involved and the requirement of the long-term prediction, however, make numerical implementation of the fully coupled model computationally inefficient. A series of scoping calculations was performed to highlight the local characteristics and behaviors of the system, and to provide a basis for refinement of the reactive transport model. The results indicate that the rapid buildup of hydrogen within the system is mainly attributed to corrosion of the cast-iron insert that primarily occurs under anaerobic conditions, rather than to radiolysis of water. The system that is rapidly in equilibrium with 50 bar hydrogen would then keep pH constant throughout the system. In addition, simulations suggest that reduction of dissolved hexavalent uranium by ferrous iron adsorbed onto the corrosion products and by dissolved H-2 are the most important mechanisms to retard the release of uranium out of the canister. More importantly, it is found that the pseudo stationary state approximation may well be applied to the system. This greatly simplifies the numerical implementation of the reactive transport model.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12975 (URN)000173859000005 ()
Note

QC 20100521

Available from: 2010-05-21 Created: 2010-05-20 Last updated: 2017-12-12Bibliographically approved
4. A Model for Radionuclide Release from a Defective Canister
Open this publication in new window or tab >>A Model for Radionuclide Release from a Defective Canister
(English)Article in journal (Other academic) Submitted
National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12991 (URN)
Note
QS 20120328Available from: 2010-05-21 Created: 2010-05-21 Last updated: 2012-03-28Bibliographically approved
5. Sensitivity Analysis of Uranium Solubility under Strongly Oxidizing Conditions
Open this publication in new window or tab >>Sensitivity Analysis of Uranium Solubility under Strongly Oxidizing Conditions
1998 (English)In: Proc. Scientific Basis for Nuclear WasteManagement XXII,, 1998, 1001- p.Conference paper, Published paper (Other academic)
Abstract [en]

To evaluate the effect of geochemical conditions in the repository on the solubility of uranium under strongly oxidizing conditions, a mathematical model has been developed to determine the solubility, by utilizing a set of non linear algebraic equations to describe the chemical equilibria in the groundwater environment. The model takes into account the predominant precipitation-dissolution reactions, hydrolysis reactions and complexation reactions that may occur under strongly oxidizing conditions. The model also includes the solubilitylimiting solids induced by the presence of carbonate, phosphate, silicate, calcium, and sodium in the groundwater. The thermodynamic equilibrium constants used in the solubility calculations are essentially taken from the NEA Thermochemical Data Base of Uranium, with some modification and some uranium minerals added, such as soddyite, rutherfordite, uranophane, uranyl orthophosphate, and becquerelite. By applying this model, the sensitivities of uranium solubility to variations in the concentrations of various groundwater component species are systematically investigated. The results show that the total analytical concentrations of carbonate, phosphate, silicate, and calcium in deep groundwater play the most important role in determining the solubility of uranium under strongly oxidizing conditions.

National Category
Chemical Engineering
Identifiers
urn:nbn:se:kth:diva-12992 (URN)
Conference
Materials Research Society, Boston, Massachusetts, November 30-December 4, 1998
Note
QC 20100521Available from: 2010-05-21 Created: 2010-05-21 Last updated: 2010-05-21Bibliographically approved

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