Thermotransport of Hydrogen in the Modified Zircaloy-4 as a Function of Hydrogen and Oxygen Concentrations
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Thermotransport of Hydrogen in the Modified Zircaloy-4 as a Function of Hydrogen and Oxygen Concentrations Hyun Sook Kim*, Kyung Sub Lee and Seon Jin Kim Department of Materials Science and Engineering, Hanyang University, 17 Haengdang-dong, Seongdong-ku, Seoul 133-791, Korea, [email protected] ABSTRACT The hydrogen redistribution induced by the thermotransport in the modified Zircaloy-4 at temperatures likely to be encountered in nuclear power reactors (300-340°C) was investigated by means of steady state techniques. The modified Zircaloy-4 was prepared by changing the chemical compositions of Zircaloy-4, which is used widely as a nuclear fuel cladding material in pressurized water reactors. The change of Q* for hydrogen, which describes the direction and magnitude of the thermotransport, with increasing hydrogen and oxygen concentrations was investigated in the modified Zircaloy-4. The value of Q* for hydrogen in the modified Zircloy-4 alloys was found to be about 7 kcal/mol and it was not affected by hydrogen concentration in the hydrogen concentration range from 63.3 ppm to 91.7 ppm. While the value of Q* for hydrogen decreased from 6.8 kcal/mol to 4.5 kcal/mol with increasing oxygen concentration from 0.2 wt% to 1.0 wt% and it was considered to be due to the trapping of hydrogen by oxygen. In addition, the hydrogen redistribution and Q* in Zircaloy-4 was also investigated in order to compare the characteristics of thermotransport of hydrogen between Zircaloy-4 and modified Zircaloy-4. The hydrogen redistribution and Q* in Zircaloy-4 showed the same results to those of the modified Zircaloy-4. INTRODUCTION The recent trend towards extended burned up and high pH operation of the nuclear power plants has led to an increased demand on the higher corrosion resistance of nuclear fuel cladding materials as well as improvement of mechanical properties. Recently the modified Zircaloy-4 was developed by changing the chemical composition of Zircaloy-4. The tin content decreased from 1.5 wt% to 0.5 wt% to improve the corrosion resistance while oxygen increased from 0.1 wt% to 0.8 wt% to compensate the decrease in strength resulted from the reduction of tin content. Besides, 0.1 wt% Nb was added and the amounts of Fe and Cr were kept 0.1 wt% and 0.2 wt% respectively. The oxygen addition caused a marked improvement in the mechanical strength, and the modified Zircaloy-4 containing 0.2 wt% oxygen showed a better oxidation resistance than that of Zircaloy-4 [1]. JJ11.62.1
Zircaloy-4 picks up hydrogen resulted mainly from the oxidation reaction of Zircaloy-4 [2,3]. Some of the hydrogen atoms are accumulated at the ZrO2-Zirconium interface and the remainders are diffused into zirconium matrix [4]. It has been known for a long time that hydrogen dissolved in Zircaloy-4 can be redistributed in the presence of a temperature gradient [5,6]. Under the influence of a temperature gradient, hydrogen in Zircaloy-4 moves towards the colder regions until the steady state reached [5,7]. Zirconium hydride is precipitated at the cold end
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