Please use this identifier to cite or link to this item: http://dx.doi.org/10.18419/opus-11291
Authors: Nigbur, Corbinian
Title: Advanced modelling of the high-temperature oxidation of zirconium in steam exploiting self-consistent thermodynamic data
Issue Date: 2020
Publisher: Stuttgart : Institute of Nuclear Technology and Energy Systems
metadata.ubs.publikation.typ: Dissertation
metadata.ubs.publikation.seiten: xv, 161
Series/Report no.: IKE (Institut für Kernenergetik . Bericht);2-163
URI: http://elib.uni-stuttgart.de/handle/11682/11308
http://nbn-resolving.de/urn:nbn:de:bsz:93-opus-ds-113080
http://dx.doi.org/10.18419/opus-11291
ISSN: 0173-6892
Abstract: In a loss-of-coolant accident, deteriorated cooling conditions facilitate the high-temperature oxidation of zirconium in steam. This exothermic reaction entails the acceleration of core degradation and causes the generation of hydrogen. Due to the significance of oxidation reaction modeling for accident progression, it is an essential part of severe accident simulation codes. However, due to high computation cost, integral codes commonly rely on simple parabolic rate laws as their oxidation models. These models are subject to assumptions such as isothermal conditions, semi-infinite media, and unlimited steam supply, which are infrequently met under accident conditions. Integral models, as considered here, relax these restrictive assumptions and have the potential to provide significant improvement, and they share the advantage of being relatively computationally inexpensive. Nonetheless, a satisfactory solution is yet to be found for their extension to cover zirconium oxidation with its various phase transitions and accompanying heat effects. The obstacles inhibiting further advances in oxidation reaction modeling using the integral approach are twofold. The first challenge arises from the high complexity of a thermodynamic system that can undergo phase transitions due to changes in both composition and temperature and that is complicated by chemical reactions, which cause a variety of heat effects. Therefore, a suitable numerical model must be both physically precise regarding the heat and mass transport mechanisms and must incorporate detailed thermodynamic data. The second challenge consists in the lack of experimental data providing insight into the oxidation reaction, especially under transient conditions, which, however, is much needed for the validation of new modeling approaches. In this light, this thesis presents the development of a fast-running advanced integral model and its novel coupling to a lookup table that comprises physically self-consistent thermodynamic data. It addresses the challenge of missing experimental data, which is suitable for its validation, using model tests in comparison to a spatially discretized model, which has also been developed in this thesis for this very purpose. Furthermore, this work draws comparisons with models proposed in the literature and models that are implemented in severe accident analysis codes today. It demonstrates the capabilities of the advanced integral model to describe complex heat effects and phase changes, which surpass the abilities of state-of-the-art modeling approaches. In terms of structure, this thesis can be divided into three parts. The first part is dedicated to a detailed physical description of the oxidation reaction and an investigation of the state of the art regarding its mathematical modeling in today's severe accident analysis codes. On that basis, limitations of existing modeling approaches are identified and requirements are defined that an advanced model must satisfy. In the second part, a description is given of the development of the thermodynamic lookup table, a material and transport property library, the advanced integral model, and the spatially discretized model. Following this, the third part presents a series of model tests, starting with verification measures, continuing with comparisons to models from the literature and the model used in ATHLET-CD, and finally addressing a test case, whose complexity regarding the occurring phase changes exceeds the range of applicability of today's modeling approaches. The result of this systematic development is an advanced integral model that captures both chemically- and thermally-induced phase transitions, and which describes heat sinks and sources with precise thermodynamic data. Hence, it overcomes the limitations of the commonly used parabolic rate approaches and surpasses the capabilities of existing integral models. Thus, in a single model, it achieves the urgently required capability of following the oxidation reaction through different phases of core degradation, starting from the classic oxidation of cladding tube surfaces, through the phases of steam starvation, over the melting and chemical dissolution processes, and ending with the solidification and oxidation of crusts. Consequently, the advanced integral model's improved prediction of heat and hydrogen generation has the potential to reduce the uncertainty associated with today's severe accident analysis codes.
Appears in Collections:04 Fakultät Energie-, Verfahrens- und Biotechnik

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