Advanced treatment of zircaloy cladding high-temperature oxidation in severe accident code calculations PART Ⅱ. Best-fitted parabolic correlations

摘要

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. The kinetic data base, to be defined for recommended application in severe accident codes, developed by critical assessment of the essential experimental results in Part Ⅰ, is further verified and refined. The experiments of Prater and Courtright, the main high temperature data source, require re-evaluation to account for the temperature gradients across the test specimens. This task is performed by Pawel' model extension for treatment of temperature gradients within scales and development of the solution algorithm. Statistical evaluation of the original data within reasonable parameter limits allows to deduce corrected rate coefficients for the corresponding isothermal case. Moreover, the co-existence of two oxide phase sub-layers, giving rise to a transitional kinetic response in an intermediate temperature range, is treated by model extension and described in terms of an "effective" diffusivity; a smooth transition to the high temperature range is obtained. In total, the deduced kinetics with certain reduction in reaction rate at high temperatures, compared to original evaluation of Prater and Courtright, is able to verify the combined original experimental results. At the same time contradictory trends are explained and reconciled on basis of the specific experimental procedures and constraints. The examination results support the conviction that the presented correlations are adequate transfer functions for the direct application in codes. Their use can be recommended for every code, apt for the correlations approach. In Part Ⅲ the verification of obtained correlations in frames of ICARE/CATHARE code against separate-effect and bundle experiments will be presented.