## 1 Introduction  
Nuclear power plants (NPPs) prevent the release of radioactive materials to the public through the concept of physical defense in depth. Since the cladding oxidation phenomenon that occurs in severe accidents threatens the integrity of the cladding, the second barrier of physical defense in depth, many studies have been conducted to date. For the safety of NPPs and accident management, it is very important to determine the timing of cladding failure in severe accidents, and the oxidation model established through the TGA (Thermogravimetric Analysis) experiment is used in safety codes, such as the MELCOR. However, these models are a function of time and temperature, and do not reflect the effect of the flow rate of the oxidant, such as steam, air, and oxygen. The oxidant flow rate can have a significant influence on the oxidation behavior, because the flow rate affects the oxidation heat and the initial temperature rise in the cladding. Although researchers have already shown that the reaction heat affects oxidation behaviors [1; 2; 3], most studies have not investigated this in detail. The relationship between the oxidant flow rate and peak temperature induced by reaction heat was identified from data trends by [PERSON] et al. [1]. From the results, the peak temperature increased with increasing the air flow rate and oxidation temperature, accompanied by an abrupt increased oxidation rate. [PERSON] et al. [2] reported on the effect by comparing IRSN [1] and FZK [4] data and the results of the theoretical models. The predictions of time to breakaway by models were in good agreement with relatively low air flow experiments at 1000 \({}^{\circ}\)C; however, the time and reaction kinetics were underestimated. It was assumed that the differences between experimental and theoretical results are caused by sharp increases in temperature generated by the reaction heat and quick growth in the oxide. Recently, [PERSON] et al. [5] intensively studied the effect of the flow rate on the reaction of zirconium under specific atmosphere conditions, i.e., vapor, vapor-air mixture, etc., at 850 \({}^{\circ}\)C and 1100 \({}^{\circ}\)C. At 850 \({}^{\circ}\)C, the time to the breakaway is shorter with the decreased gas flow in both air and a steam-nitrogen mixture, and the reaction rate also decreases with the increased gas flow rate. These trends differed completely at 1100 \({}^{\circ}\)C. This previous research mainly focused on the effect on oxidation behaviors in terms of global and/or local oxygen starvation, which can strongly affect the activity of nitrogen in reactions and their diffusivity; however, the effects of reaction heat on oxidation behaviors have not yet been studied deeply. If the reaction heat can have an impact on the breakaway time, this phenomenon should be considered an important parameter for estimating NPPs' safety. Therefore, in this study, the effect of reaction heat on the oxidation kinetics and breakaway time was observed by conducting oxidation experiments at 1000 \({}^{\circ}\)C, where the breakaway behaviors change dramatically. Additionally, the reason for the change in the breakaway behavior around this temperature was explained in terms of reaction heat and oxidation kinetics.