Interpretation of Corrosion Potential Data from Boiling-Water Reactors Under Hydrogen Water Chemistry Conditions

A method was devised to estimate electrochemical conditions at the entrance to the recirculation piping of a boiling water reactor (BWR) under hydrogen (H2) water chemistry (HWC) conditions from electrochemical corrosion potential (ECP) measurements made in remote autoclaves. The technique makes use of the mixed potential model (MPM) to estimate ECP in the autoclaves and compares estimates to measured values in an optimization on the concentrations of hydrogen peroxide (H2O2) and oxygen (O2) in the recirculation system. The algorithm recognizes that H2O2 decomposes in sampling lines and that transit times between the recirculation system and monitoring points depend upon flow rates and sampling line diameters. An analysis was made of ECP data from three monitoring locations in the Barseback BWR in Sweden, as a function of H2 concentration in the feedwater for two flow rates (5,500 kg/s and 6,300 kg/s [12,100 lb/s and 13,860 lb/s] for the four recirculation loops). HWC did not displace ECP below a critical value of –0.23 VSHE at the lower flow rate until the reactor water [H2] exceeded 0.15 ppm, corresponding to a feedwater H2 level of > 0.93 ppm. At the higher flow rate of 6,300 kg/s (divided equally between four recirculation loops), protection was not predicted until the feedwater [H2] exceeded 1.2 ppm, corresponding to a reactor water [H2] of ~ 0.195 ppm. The difference was attributed to the greater persistence of H2O2 at high feedwater [H2] at the higher flow rate, possibly because of the lower transit time from the core to the recirculation system.