Boric Acid Chemistry under Pressurized Water Reactor Coolant Conditions by Flow AC Conductivity
A state-of-the-art high precision flow-through conductance instrument has been used to measure very accurate transport properties and equilibrium constants for aqueous borate and polyborate species from 25 to 350 °C. These results provide a significant improvement to the database that has been used to adjust mass transport thermodynamic models used for chemistry control in Pressurized Water Nuclear Reactors (PWRs). Frequency dependent molar conductivities were measured for over 500 aqueous solutions of boric acid, sodium borate, potassium borate, and lithium borate to yield (i) limiting conductivities of borate, diborate, and triborate (ii) ionization constants of boric acid, (iii) diborate and triborate formation constants, and (iv) sodium borate, potassium borate, and lithium borate ion-pair formation constants using the Turq-Blum-Bernard-Kunz (“TBBK”) theoretical model. These results are the first reported accurate (poly)borate limiting conductivities above 50 °C and only the second high temperature study to report polyborate formation constants, the first lithium borate ion-pair formation constants reported above 45 °C, and the first accurate boric acid ionization constants reported above 290 °C. The significance of this work lies in providing the new thermodynamic and transport properties measured under extreme conditions which are necessary (i) to develop our basic theoretical understanding of aqueous systems and (ii) to model the crevice chemistry of fuel deposits in PWRs under the “hideout” conditions that can lead to crud-induced power shifts. The results are expected to form the basis for a new database for borate species in the EPRI software MULTEQ, used to model PWR primary coolant chemistry.