Advanced treatment of zircaloy cladding high-temperature oxidation in severe accident code calculations: Part I. Experimental database and basic modeling

Abstract

Progress in the treatment of zirconium oxidation kinetics in different severe accident (SA) codes and convergence towards an agreed data base are required for the reliable verification of sophisticated LWR core degradation models. Focused on the comprehensive experimental studies the available information on the high-temperature oxidation kinetics of zircaloy (Zry) is evaluated. Important discrepancies between results for the high-temperature range are interpreted in terms of different experimental and evaluation procedures. The critical assessment identifies the following items, which require separate consideration in the simplified convention of using fixed reaction rate correlations for the high and low temperature ranges: the co-existence of two oxide phase sub-layers gives rise to a transitional kinetic response in an intermediate temperature range. Towards higher temperatures the validity of the correlations approach (assuming reaction rate control as in a semi-infinite solid-state diffusion system) is restricted further, on the one hand by kinetic control within the gas phase (initial oxidation range), on the other hand by metal matrix consumption (final oxidation range). The analytical treatment of the two last-mentioned concomitant phenomena is given, and the consideration allows to reconcile apparent discrepancies between experimental results. A reasonable base of combined data is thus identified, for a moment with certain uncertainty, which is not suitable for direct application. In Part II, the kinetic base will be further verified and refined by statistical evaluation of complementary experimental data and transferred to the form, allowing implementation in codes. In Part III, the satisfactory application in ICARE2 code calculations of separate-effect and bundle experiments will be presented.