Determination of Oxidation and Metallic Cations Release Kinetics on Nickel Base Alloys in PWR: Description of the On-Line Measurement Techniques

A common issue in Nuclear Power Plants (NPP) is the release of corrosion products from the steam generator tubes in primary circuit which gives rise, after their neutronic activation, to a residual activity very problematic during chemistry transients. The objectives of this work are not to solve directly this problem but rather to improve the understanding of this phenomenon by determining experimentally the kinetics of oxidation and metallic cations release of nickel base alloys. These results will contribute to progress in the building of a physical mechanism. The specificity of the test facility is (i) the on-line measurements of cation release rate by a highly sensitive analytical device and (ii) the in-situ measurement of oxidation rate by Diffuse Reflectance Spectroscopy (DRS). The primary water chemistry is controlled by means of a circulating micro-loop in titanium to avoid any interference with the test samples. A specific test vena is connected between the titanium micro-loop and the analytical device for on-line measurements. Metallic cation release from SG tube-samples is deduced from the measurement of nickel and iron concentrations in upstream the test vena by use of differential polarography and ICP-AES techniques. The precision on the determination of Ni and Fe concentrations is assumed to be a few tens of ppt. Oxidation or corrosion measurements are performed off-line by optical diffuse reflectance measurements specifically developed in this framework. The corrosion sample is mounted in a High Temperature and Pressure (HT-HP) spectroscopic flow-through cell connected to the test vena and fed with the primary chemistry from the titanium micro-loop. Analysis of reflection spectra provides the thickness of oxide layer. A set of parameters such as (i) temperature, (ii) hydrogen content and (iii) metallurgical surface state in nominal primary chemistry for test durations of 1500–2000 hours is planned to be investigated in the frame of metallic cation release and oxidation kinetics measurements of Ni base alloys. The aim of this paper is focused on the presentation of the online technique specifically developed in the French Technical Centre of AREVA NP to determine the metallic cation release and oxidation kinetic of Ni base alloys in primary water chemistry. First results of cation release and oxidation rate are given for shorter duration test.