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Modification de la réactivité de surface d'un alliage base nickel afin de limiter le relâchement du nickel en milieu primaire des réacteurs à eau pressurisée

Magali Moeglen 1, 2
2 LECA - Laboratoire d'Etude de la Corrosion Aqueuse
SCCME - Service de la Corrosion et du Comportement des Matériaux dans leur Environnement : DEN/DPC/SCCME/LECA
Abstract : Steam generator tubes in pressurized water reactors (PWR) are made of nickel based alloy and represent nearly 75 % of the primary coolant loop surface. Due to generalized corrosion phenomena, nickel cations are released in the primary loop water. After neutron flux exposure in the reactor core, nickel cations can turn into radioactive products, such as 58Co. If carried through the circuit, these radioactive products precipitate and contaminate the loop, making maintenance more difficult.The present study goal is to form a very protective scale on the Alloy 690 surface by a pre-oxidation treatment performed at 700 °C. The impact of different parameters such as oxygen partial pressures of the gaz used during high temperature oxidation (low P(O2)=10 -6 bar or high P(O2)=0.2 bar) and the initial surface state of the sample (cold-worked or surface roughness) is studied. To minimize nickel cation release, the scale obtained must be continuous, homogeneous, rich in chromium and nickel free. The oxidation kinetics, studied using thermogravimetric analysis (TGA), show the existence of parabolic law independent of P(O2). The sample's surface aspect, observed using scanning electron microscopy (SEM), is similar for all P(O2). Top views show a preferential chromium oxidation along the grain boundaries and the formation of a chromium rich oxide scale, confirmed by X-ray photoelectron spectroscopy (XPS) and glow discharge optical emission spectrometry (GDOES); cross sectional views of the material exhibit holes beneath the oxide corresponding with the alloy's grain boundaries. The latter observation and an inert marking test with gold plots suggest a cationic growth of the oxide scale. X-ray diffraction (XRD) indicates the presence of chromia (Cr2O3) for low and high P(O2). For low P(O2), a Mn and Cr spinel is observed near the outer surface. For higher P(O2), a similar spinel layer is made up of Ni, Fe, Mn and Cr.Pretreatment conditions set to be applied on alloy 690 can be proposed thanks to the data collected. Samples, pretreated that way, are tested in simulated primary medium water and their nickel release is clearly cropping compared to non-thermaly-treated samples.
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Submitted on : Thursday, February 25, 2016 - 10:42:07 AM
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  • HAL Id : tel-01278920, version 1



Magali Moeglen. Modification de la réactivité de surface d'un alliage base nickel afin de limiter le relâchement du nickel en milieu primaire des réacteurs à eau pressurisée. Matériaux. Université Grenoble Alpes, 2015. Français. ⟨NNT : 2015GREAI099⟩. ⟨tel-01278920⟩



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