Increased Hydrogen Uptake of Zirconium Based Claddings at High Burnup

INCREASED HYDROGEN UPTAKE OF ZIRCONIUM BASED CLADDINGS AT HIGH BURNUP by ADRIENN BARIS A thesis submitted to the University of Birmingham for the degree of DOCTOR OF PHILOSOPHY School of Metallurgy and Materials University of Birmingham March 2019 0 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. ABSTRACT In light water reactors the fuel is encapsulated in Zr-based claddings that withstand the harsh environment (neutron bombardment, high temperature and water under pressure); without absorbing too many neutrons to sustain the chain reaction in the reactor core. Relatively high corrosion resistance of Zr is achieved when alloyed (e.g. with Sn, Fe, Cr, Ni, or Nb). Some elements form second phase particles (SPPs) and provide protection against rapid corrosion. The cladding undergoes compositional and microstructural changes, such as irradiation- induced SPP dissolution. Zr oxidizes at the metal-oxide interface by diffusion of the oxidizing species through the oxide layer. Therefore, a protective inner barrier oxide is essential to prevent the metal from fast reaction with different species. Hydrogen is released as a by-product of the oxidation, and by the radiolysis of the coolant. If H enters the metal it precipitates as brittle Zr- hydrides degrading the cladding’s mechanical properties. The H-uptake is a critical safety issue. Although, extensive literature is available on this topic, there are some aspects that need better understanding. Increasing H-uptake of certain cladding types at high burnups was reported. The causes are not yet fully understood. To better understand the causes of increased H-uptake at high burnups, an extremely high burnup cladding (9 cycle LK3/L Zircaloy-2) from boiling water reactor provided the basis of the study. The same type of cladding after different service times was examined revealing the compositional and microstructural evolution. Two types of cladding from pressurized water reactor with medium burnup were studied to separate the reactor- and alloy-specific parameters from the generic ones. FIB tomography was used for the 3D reconstructions of the microstructure; EPMA and ChemiSTEM for the micro- and nanometric chemical analysis. It is revealed that regardless of alloy- and reactor-type, crack-free oxide and the absence of large hydrides in the vicinity of the metal-oxide interface; undulated interface; and presence of SPPs are among the essential factors for the cladding’s high performance. It is demonstrated that the oxidation of the hydrides at the metal-oxide interface induces crack formation in the oxide, reducing its protectiveness. High level of SPP dissolution, large hydride phases in the metal and high level of porosity in the oxide at the interface, straight metal-oxide interface, stoichiometric oxide, increased Ni concentration in the inner oxide, segregation of Fe, Ni, Sn and slightly Cr in the metal grain boundaries, Sn segregation at the interface oxide are identified as the causes of increased H- uptake of the LK3/L cladding at high burnups. Although all of these factors are present after 9 cycles, the cladding does not show extremely fast oxidation and H-uptake even beyond the designed service time. ACKNOWLEDGEMENTS First of all, I would like to express my sincere gratitude to my supervisor at PSI, Dr. Sousan Abolhassani, who guided me patiently for four years to find good directions in my research, and who always supported me. This work would not have been possible without her immense knowledge in the field and her valuable advices for the thesis writing. I greatly appreciate her positive attitude both at a professional and personal level. She kept smiling and encouraging me even during hard times; this helped me to overcome every difficulties I faced in these years. I feel very lucky to have a supervisor like her. I would like to thank my supervisors at Birmingham, Dr. Yu-Lung Chiu and Prof. Hugh Evans, for their insightful comments and suggestions throughout the PhD work and for the support and help in the writing of this thesis. I could not have imagined a better group of supervisors and it was a pleasure to work with all three of them. I would like to thank the Nuclear Fuels group for the great working environment and all the opportunities to participate in various scientific events and conferences that helped to widen my knowledge and perspective. I would like to thank swissnuclear for the financial support of this project. Westinghouse Electric Co., KKL, KKG are thanked for providing the investigated materials. This project is a partner of the MUZIC-3 collaboration which I thank for the very stimulating discussions and for broadening my perspective and deepening my understanding of the topic. Many thanks to Matthias Martin, Robin Grabherr, Stephane Portier, and Andrej Bullemer for making possible all my measurements in the Hotlab through providing access to the instruments and sample preparation tools. I am grateful to Dr. Elisabeth Müller for her patience and kind help whenever I had some trouble with the FIB or TEM. She was always available to help or answer any questions with a high professional level. I wish to thank Dr. Robin Schäublin for his support in the ChemiSTEM measurements at ETH Zürich, and for giving very useful advices that considerably improved the quality of the results. I thank Jinsen Tian for assistance during the ChemiSTEM measurements at the University of Birmingham even during his PhD thesis write-up. I thank my friends and groupmates at PSI - Dipa, Barbi, Jonny, Aaron, Harry, and Weijia - for making the working days even more fun and that they were always available for some coffee break and chatting. I cannot express enough my appreciation to my beloved Shantanu for making life easier with his understanding and easy-going attitude, for the emotional support, for the good advices and motivation all the time. I would like to thank my family for the emotional support and patience in these years, and that I could always count on them. TABLE OF CONTENTS 1. INTRODUCTION .......................................................................................... 1 1.1. Background of light water nuclear reactors ................................................................. 1 1.2. Brief description of the differences between BWR and PWR ..................................... 2 1.3. Background of Zr based claddings .............................................................................. 5 1.4. Motivation and aim of the thesis .................................................................................. 7 1.5. Structure of the thesis ................................................................................................ 12 2. LITERATURE BACKGROUND ................................................................ 14 2.1. Brief introduction to the development of Zr-based alloys ......................................... 14 2.2. Oxidation of the Zr-based alloys ............................................................................... 21 2.2.1. General description of the oxidation .................................................................. 21 2.2.2. Oxidation kinetics ............................................................................................... 24 2.2.3. Microstructure of the formed oxide and the metal-oxide interface .................... 27 2.2.4. Oxidation of the Secondary Phase Particles ....................................................... 29 2.3. Hydrogen uptake of the Zr-based alloys .................................................................... 31 2.4. Current knowledge regarding the mechanisms of oxidation and hydrogen uptake ... 35 2.4.1. The transition in the oxidation kinetics .............................................................. 35 2.4.2. Possible factors influencing the hydrogen uptake .............................................. 37 2.5. The impact of irradiation on the oxidation and hydrogen uptake –Studies on irradiated and high burnup materials .................................................................................................... 49 2.5.1. Differences between out-of-pile and in-reactor corrosion behavior ................... 49 2.5.2. Influencing parameters on the in-reactor behaviour ........................................... 52 2.5.3. Current understanding of the changed cladding behaviour in-reactor ............... 57 2.5.4. Short summary on the impact of irradiation and the objective of this study ...... 64 3. MATERIALS AND CHARACTERIZATION METHODS ....................... 66 3.1. Materials .................................................................................................................... 66 3.1.1. Zircaloy-2 - Grade of LK3/L .............................................................................. 67 3.1.2. Low-tin Zircaloy-4 ............................................................................................. 70 3.1.3. Zr-2.5Nb ............................................................................................................

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