Influence of the transient conditions on release of corrosion products and oxidation of alloy 690 tubes during pressurized water reactor restart after steam generators replacement

The radioactivity of the Reactor Coolant System (RCS) of Pressurized Water Reactors (PWRs) mainly comes from the release of corrosion products of Steam Generator (SG) tubes made of Ni-base alloys. In order to reduce this activity and thus the radiation exposure of PWR operators during maintenance operations, it is necessary to minimize the release. That requires prior understanding of the various mechanisms involved. EDF R&D constructed a loop, BOREAL, to specifically measure rates of release of SG tubes in various conditions of primary environment. Tubes were usually tested at high temperature, under constant conditions of primary chemistry. So, it is necessary to carefully investigate the impact of transient conditions during a PWR restart after SG replacement. Tests were performed on the industrial material with curvature, roughness, defects and heterogeneities, regularly observed on this type of component. Characterisations of the inner surface were done on as-received and corroded specimens of SG tubes and were correlated with the obtained release kinetics. The native oxide layer is formed of a very thin layer (1-2 nm) of oxidised matrix, without specific enrichment. During the restart, the most critical step for the release phenomenon is revealed from 170 degrees C to 297 degrees C. The majority of the metal is indeed released into the fluid during this step. The characterisations after release tests have made it possible to propose oxidation and release mechanism during a PWR restart after SG replacement. Up to 170 degrees C, a thin layer of amorphous chromium oxide is formed by selective dissolution of iron and nickel. When the temperature rises, this chromium oxide layer is not stable enough to be protective and the diffusion phenomena are activated. At 325 degrees C, the oxide does not exhibit any particular enrichment and corresponds to an oxidised metal layer; an equilibrium is established and the rate of release reaches a pseudo-stationary regime. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

In order to reduce the radioactivity of PWRs, it is necessary to understand the release mechanisms, with research showing the critical temperature range during restart as the key phase for the release phenomenon. Tests and characterizations on SG tube materials provided insights into oxidation and release mechanisms during a PWR restart after SG replacement.