Coolant-cladding interaction models in FINIX fuel behaviour module

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Title: Coolant-cladding interaction models in FINIX fuel behaviour module
Author: Peltonen, Jussi
Contributor: University of Helsinki, Faculty of Science
Publisher: Helsingin yliopisto
Date: 2019
Language: eng
Thesis level: master's thesis
Discipline: Fysiikka
Abstract: FINIX is a nuclear fission reactor fuel behaviour module developed at VTT Technical Research Centre of Finland since 2012. It has been simplified in comparison to the full-fledged fuel performance codes to improve its usability in coupled applications, by reducing the amount of required input information. While it has been designed to be coupled on a source-code level with other reactor core physics solvers, it can provide accurate results as a stand-alone solver as well. The corrosion that occurs on the interface between nuclear fuel rod cladding and reactor coolant is a limiting factor for the lifespan of a fuel rod. Of several corrosion phenomena, oxidation of the cladding has been studied widely. It is modelled in other fuel performance codes using semiempirical models based on several decades of experimental data. This work aims to implement cladding oxidation models in FINIX and validate them against reference data from experiments and the state-of-the-art fuel performance code FRAPCON-4.0. In addition to this, the models of cladding-coolant heat transfer and coolant conditions are updated alongside to improve the accuracy of the oxidation predictions in stand-alone simulations. The theory of the cladding oxidation, water coolant models and general structure of FINIX and reactor analysis will be studied and discussed. The results of the initially implemented cladding oxidation models contain large errors, which indicates that FINIX does not account for the axial temperature difference between the bottom and the top of the rod in the coolant. This was corrected with the updates to the coolant models, which calculate various properties of a water coolant based on International Association for the Properties of Water and Steam (IAWPS) industrial water correlations to solve the axial temperature increase in a bulk coolant. After these updates the predictions of cladding oxidation improved and the validity of the different oxidation models were further analyzed in the context of FINIX.

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