An Improved Evaluation Model for Zircaloy Oxidation

Recent measurements of the Zircaloy-steam oxidation reaction kinetics are reviewed for the purpose of proposing a replacement for the Baker-Just evaluation model currently used in licensing calculations. These data are shown to fall into two groups according to the method used to heat the specimens. It is concluded that the parabolic reaction rate constants derived from the experiments that used internal heating of the specimens would be a conservative Zircaloy oxidation rate for use in licensing calculations. A best-estimate fit to the recent data yielded A = 3.33 × 10⁵ (mg Zr/cm²)²/s and B = 140.6 kJ/mol for the constants in the equation K_p = A exp[-B/RT]. Results from earlier investigations are in agreement with this equation if comparisons are made over appropriate temperatures. A comparison of measured and calculated oxidation for specimens subjected to anisothermal exposure histories suggests that the actual oxidation rate under these conditions is about one-fourth that which would be calculated by using the above equation.