Abstract: This invention relates to a method of predicting the long-term dose rates from radioactive material on the interior, wetted surfaces of the primary coolant piping of nuclear power reactors. The electrochemical potential of the cooling water of a nuclear power plant is measured over a short-term period with an electrochemical potential measuring device that has an unprefilmed measuring electrode. The results of these electrochemical potential measurements are divided by the result at a prescribed short period of exposure, and these normalized electrochemical potential fractions are plotted versus the logarithm of time. The negative of the slope of the straight line through the plotted data is divided into the measured average Co-60 concentration in the cooling water, and a standard curve of long-term dose rate versus this parameter is used to predict the eventual long-term radiation build-up performance of the nuclear power plant in which the electrochemical potential measurements are made.