High-Temperature Precipitation Kinetics of Metal Oxide Corrosion Products

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The use of metals and alloys in nuclear power plants often involves exposure to corrosive, aqueous, high-temperature, and high-pressure environments. In CANDU reactors, carbon steel is used for the feeders (piping that transports the ~300 degree Celsius and 10 MPa heavy-water coolant) as it exhibits good corrosion resistance due to the formation of protective, double-oxide magnetite layers. The temperature gradient in the heat exchangers or steam generators in the primary circuit results in an oversaturation in the coolant iron concentration relative to the temperature-dependent equilibrium solubility of magnetite and, thus, in precipitation that exceeds the thin layers required for corrosion resistance. This results in unfavourable outcomes such as reduced heat transfer, and the accumulation of radioactive species, which incorporate into available oxide deposits throughout the primary side. An accurate understanding of material degradation mechanisms and the behaviour of the key corrosion products of the materials of construction is essential in minimizing radiation fields to ensure nuclear worker safety. These mechanisms are incorporated into comprehensive computational simulation packages, allowing for predictive modelling of corrosion mitigation strategies and fouling removal events. Knowledge on the kinetics of magnetite is limited. Precipitation is generally assumed to follow first-order kinetics, and no high-temperature data on the precipitation constant are available in the literature. Estimates are derived from station data, but the presence of mixed metal oxides makes it challenging to extract fundamental kinetic information solely for magnetite. The lack of agreement between mechanistic predictions and station observations has often led model developers to de-emphasize water chemistry. Instead, a mass transfer-based approach to material transport is often used despite evidence that a more complex combination of mass transfer and kinetics is necessary. This research program proposes to determine novel kinetic data for magnetite and other mixed metal oxides relevant to CANDU environments and to integrate this into an existing UNB CANDU code, which is used for prediction of corrosion rates, heat transfer, and surface radioactivity on pipe surfaces. The bulk of the short-term work will be performed in a high-temperature (260-310 degree C) flow-through loop, with measurements over a sufficient temperature range to obtain Arrhenius parameters for kinetic precipitation constants, while also remaining applicable to station conditions. This research supports both fundamental (Arrhenius parameters and overall precipitation kinetics) and applied industry work (continuing to support the operation of the existing nuclear reactor fleet through predictive modelling). The technology development derived from the proposed activities has the potential to establish the UNB CANDU Code as a substantial tool in corrosion and radioactivity modelling.