A New Model to Predict the Oxidation Kinetics of Zirconium Alloys in a Pressurized Water Reactor

Abstract

The previous CEA corrosion code COCHISE provided satisfactory simulations of in-reactor corrosion of the fuel cladding when used in its validity range. In contrast, it could lead to hazardous predictions if applied out of this range due to the strongly linked parameters mainly based on the analysis of French pressurized water reactor (PWR) data. To predict the oxidation kinetics for new operating conditions or new materials, the CEA and EDF decided to develop a new model, named CORCY, which is based on a more phenomenological approach and uses separate parameters deduced from analytical experiments. The aim of this paper is to present the new model for Zircaloy-4 in PWR. The phenomenological approach is described. It is based on out- and in-pile data. Typically, since (1) the oxidation kinetics of zirconium alloys in autoclave are periodic, and (2) the oxide films formed in autoclave, in out-of-pile loop, and in-reactor all exhibit periodic lateral cracks with a period similar to the oxide thickness to transition, the oxidation kinetics in CORCY are characterized by a cyclic repetition of semi-parabolic law. Each model parameter is detailed. They are deduced separately from (a) oxidation tests performed in autoclave on fresh alloys to determine their kinetics; (b) oxidation tests performed in the out-of-pile corrosion loops Corail and Reggae to quantify the effects of thermo-hydraulic conditions; (c) data provided by Testing Material Reactors (OSIRIS and Halden reactor) during isothermal oxidation to determine the effects of irradiation; and (d) oxidation tests performed on pre-hydrided alloys to take into account the accelerated corrosion phase occurring, in reactor, for Zircaloy-4 at high burn-up. After describing both the phenomenological approach and the different model parameters, a comparison of measured and calculated corrosion data from French PWRs at a burn-up up to 65 GWd/tU is provided.

Influence of light ion irradiation of the oxide layer on the oxidation rate of Zircaloy-4

“…However, it was recently shown that the ion irradiation of the metal/oxide interface results in a significant increase of the oxidation rate of the Zircaloy-4 alloy by increasing the vacancy concentration gradient. The accelerating factor (∼2) is close to that measured between cladding corroded in autoclave and in PWR at low burnup [3,17].…”

“…Four major assumptions are proposed in the literature in order to explain this acceleration [2,3]. The first one is that the hydrides precipitated beneath the metal/oxide interface in the metallic part of the cladding, resulting from the hydrogen uptake by the matrix, may be corroding at a faster rate than the metal itself [4][5][6][7].…”

Corrosion Model for Zirconium-Niobium Alloys in Pressurized Water Reactors

“…For this reason, the exponent m can lie between the limiting cases 0.5 < m < 1. A similar expression is used in the model in [3]. Fig.…”

“…The engineering model was constructed using an approach similar to foreign models [2,3]. The stages of oxidation before and after the turning point are examined.…”

“…Microstructural investigations [3,4] show that because of internal mechanical stresses the continuous protective layer of the oxide film near the metal periodically cracks when the critical thickness is reached. The synchronicity of the cracking in different sections of the sample is disrupted after several periods.…”