Magnetite (Fe3O4) and nickel ferrite (NiFe2O4) zeta potential measurements at high temperature: Part I—Design, materials and preliminary characterization of an apparatus implementing the streaming potential method

Abstract

An apparatus for measuring the streaming potential of particles at high temperature has been designed and tested in a recirculating water loop operating at temperatures up to 300 °C. The ultimate objective is to derive values of the zeta potential of corrosion-product particles under the conditions of the primary coolant of pressurized water reactors (PWRs), quantities that until now have been unavailable. They are required for modeling the transport and radioactivation of particulate oxides around the coolant system and the consequential growth of radiation fields around components. The experimental technique employed, measuring the potential difference across a packed bed of particles through which the coolant flows, involved fabricating a tube of temperature-resistant polymer that insulated the particles from the containing stainless steel pressure boundary of the loop. The particle bed was contained between porous ceramic membranes and the whole sealed with shrunk-on PTFE to avoid by-passing flows. Platinum wires, inserted into the loop through Conax fittings and insulated from contact with it by coils of oxidized zirconium wire, served as electrodes upstream and downstream of the bed to measure the potential difference The detailed design and testing of the apparatus containing magnetite powder to verify its correct functioning under operating conditions are described. Since measuring streaming potential involved obtaining the voltage difference across the particle bed alone at the same time as the pressure drop through it from the flowing coolant was measured, the preparatory tests first established baseline values, from measurements on an empty test section, which were subtracted from the measurements on the full test section. Also, immediately before each streaming potential determination, a rapid measurement provided the overall electrical resistance of the test section. The initial measurements of the streaming potential of magnetite, between 20 and 240 °C in a solution containing 1800 ppm of boron and 3.5 ppm of lithium, led to corresponding values of zeta potential. The values show that the apparatus provided reproducible results. The results of the complete series of tests under conditions of the primary system of PWRs, involving nickel ferrite as well as magnetite particles, are the subject of a separate paper.