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关于氧化动力的计算
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Oxidation fo Zircaloy-4 Zircaloy-4 is used principally as fuel cladding in pressurized water nuclear reactors (PWRs). The alloy’s mean composition is as follows: Zr – 1.5 % Sn – 0.2 % Fe – 0.1 %Cr. In the primary circuit, liquid metal exists under 100 bar pressure at 315°C and alloy oxidation leads to the almost exclusive formation of zirconia, ZrO2. In order to simulate this behaviour in a lab out of a reactor, oxidation in autoclaves is carried out under pressurized water vapour or even in oxygen under atmospheric pressure. The kinetic data has been obtained in the latter case, at 415 and 470°C, respectively, is given in Table 1. It will be assumed in the following exercise that only ZrO2 is formed during zircaloy oxidation. Q： 1) Plot the kinetic curves for both temperatures. Describe the curves as a series of three stages. 2) Identify the zirconia thickness at which the kinetic transition takes place and verify that this thickness is the same for both temperatures. 3) Pre-transition oxidation: a. Can the pre-transition oxidation of Zircaloy-4 be considered to be parabolic? b. If this is not the case, then the oxidation kinetics may be described by the general rate equation: (Δm)n = k t. Determine the value of n. Is this value the same at both temperatures? c. Indicate the rate limiting step. What type of further phenomenon may lead to the same exponent ‘n’? (hint: calculate the Pilling and Bedworth ratio and recall that zirconia is a n-type semiconductor with oxygen vacancies as predominant ionic defects). d. Is it reasonable to determine the apparent activation energy of the constant k in the rate equation: (Δm)n = k t? 4) Time and nature of the transition: (in order to determine the transition using a mathematical method, plot the transforms y = f[t/y] ) a. Under what conditions are these transforms straight lines? What does one observe if the rate equation is linear? b. What are the transition times determined using this method? c. Explain the origin of this transition. 5) Post-transition oxidation a. Determine, at 415 and 470°C, the linear rate constants kl expressed in terms of the thickness of corroded metal (assumed to be pure zirconium) per unit of time. b. The lifetime of PWR fuel rods is defined as the time necessary to corrode 10% of the zircaloy clad thickness, originally 570 μm thick at start of life. What is the lifetime deduced from the plots of the two kinetic curves, assuming, as a simplification, that the rate equation is linear from time zero? c. What is the apparent activation energy of kl ? d. What is the lifetime at 315°C, which is the operating temperature of the fuel rods? (it should be noted that in industrial reactors, three additional phenomenon that effect cladding lifetime are observed but are not considered here: 1) hydridation of the alloy; 2) thermal flux through the cladding and the oxide scale; and 3) massive neutron irradiation)  Screen Shot 2015-05-20 at 11.56.10.jpg |
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