An electron microscopy study on the Zr oxides formed during the transition to breakaway oxidation
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An electron microscopy study on the Zr oxides formed during the transition to breakaway oxidation Dong Jun Park, Byoung Kwon Choi, and Jeong Yong Park LWR Fuel Technology Division, Korea Atomic Energy Research Institute, 1045 Daedeok-daero, Yuseong-gu, Daejeon 305-353, Korea ABSTRACT In order to characterize the microstructure of oxide layers formed on Zircaloy-4 tubes during the breakaway transition, oxidation tests in a flowing steam environment were performed at 1000ഒ with a different oxidation time. It was found that breakaway oxidation occurred after the oxidation time of 3000s, and zirconium dioxide layers existed in two mixed crystallographic forms of the tetragonal and monoclinic phase in all samples. The zirconium oxide layers showed enhanced crystallinity, increase in grain size, and fine pores at the grain boundary after breakaway oxidation. We found that the initiation of breakaway-oxidation instability originated from these microstructural changes. INTRODUCTION Zirconium (Zr) based alloys have been extensively studied due to their fairly good resistance to corrosion from water at elevated temperatures and a very low absorption crosssection of thermal neutrons, and also their potential use in the nuclear industry. The main application of Zr alloys is thin-walled tubes used to clad radioactive fuel in light water reactors (LWRs). When a small break loss of coolant accident (LOCA) occurs, the temperature of the fuel system rises rapidly and the cladding undergoes an oxidation for relatively longer periods of time at environment of the mixture of water and steam until it is quenched by emergency core cooling water. This can cause breakaway oxidation phenomenon which is described by linear oxidation kinetics with a sudden increase in the oxidation rate which follows initial parabolic regimes. This phenomenon could severely promote embrittlement of the cladding, therefore it is very important to explain precisely with microstructural characterization why this breakaway oxidation takes place during the LOCA for safety analysis. However, no mechanism has been generally accepted for the explanation of pre- to post-breakaway kinetics and its actual process is still a matter of controversy, although extensive investigations have been carried out. Several mechanisms suggested are the change of the stoichiometry of oxide layer [1, 2], change in crystalline structure of oxide from cubic or tetragonal to monoclinic [3], mechanical failure of the oxide [4], the spontaneous formation of small pores in oxide layer [5], and recrystallization of oxide [6]. EXPERIMENTAL DETAILS A Zircaloy-4 tube which has a 200 mm length was used in this study and specimen temperature was measured by a pyrometer. Direct heating by an ohmic resistance with feedback between the pyrometer and the power supply enabled the specimen to heat up to temperatures of
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1000ഒ. Then the specimen was oxidized in a flowing steam with different exposure times in the range of 300 s to 4000 s. Finally, the specimen was cooled at an intermediate temperatu
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