Modeling the behavior of colloidal corrosion products in the primary circuit of pressurized water reactors

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University of New Brunswick
In Pressurized Water Reactors (PWRs), uniform corrosion of surfaces of the primary circuit leads to formation of oxide layers and to the release of ionic species. Due to the erosion of oxides or to local solubility differences, particles are present in the coolant. The activation of particles in regions under neutron flux induces volume or surface contamination inside the primary system, depending upon whether they remain in the coolant or deposit. The modeling and prediction of activity transport within the primary circuit of PWRs is of the utmost importance for the radiological protection of workers and for the availability of reactors. The OSCAR calculation code developed by the CEA, with EDF and Framatome, aims at accurately predicting contamination levels by activated corrosion products, fission products and actinides. It relies on the development of comprehensive models to fully describe the numerous and complex interactions underlying activity transport. At the heart of this work, the current deposition model, based on the Beal model developed in the 1970s, shows results in good agreement with the field experience. However, preferential deposition in certain regions has yet to be reproduced in OSCAR (tOol for Simulating ContAmination in Reactors). A non-negligible proportion of particles in the Reactor Cooling System (RCS) exists under colloidal size, exhibiting specific behavior as surface interactions are decisive for their deposition. Such interfacial interactions are outside the scope of the Beal model and should be included in an updated comprehensive deposition model. The characterization of surface interactions between colloids and a wall requires the knowledge of their respective zeta potential in the corresponding conditions. As no data for zeta potential values of typical corrosion products in the RCS are available, a test-section for the measurement was designed and manufactured. Installed in a high-pressure and -temperature recirculating loop, it enabled the measurements of the zeta potentials of magnetite and nickel ferrite between 20 and 240 ◦C for representative chemical conditions (operation cycle and cold shutdown process). A comprehensive deposition model was developed and using these zeta potential values enabled one to simulate the deposition of both colloidal and inertial particles. Calibration using CIRENE loop experiments at the CEA was performed. Finally, the OSCAR code and its new comprehensive deposition model allowed one to reproduce a preferential deposition of particles in certain regions of the primary system highlighted by gamma spectrometry measurements of surface activities using the EMECC device.