Patent Number: 047599022
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Analysis of the radiation build-up data base led to the surprising discovery that the long-term (equilibrium) dose rate is dictated by the early build-up rate, which is determined by current plant conditions and is related to the growth and morphology of the oxide film. Therefore, a physical parameter that is sensitive to the oxide film growth and morphology will provide early indication of what the long-term radiation build-up and dose levels are likely to be. I have discovered that the electrochemical potential of an unprefilmed corroding metal surface is a strong function of the thickness and character of the corrosion film. Therefore, I conclude that the rate of increase in the electrochemical potential, measured with an unprefilmed measuring electrode over a short-term period, should provide a measurement of the morphology of the oxide films and, hence, of radiation build-up rates and the eventual long-term radiation levels from the cooling system of a nuclear power plant. If the curves in FIGS. 8 and 9 are compared, it is apparent that the Hatch-2 data show the most rapid electrochemical potential increase and the earliest leveling off of the electrochemical potential; and the Hatch-2 plant has the lowest radiation build-up rate and the lowest long-term (equilibrium) radiation levels. Conversely, the Vermont Yankee data show the slowest electrochemical potential increase and latest leveling off of the electrochemical potential, and Vermont Yankee plant has had the most rapid radiation build-up rates and the highest long-term radiation levels. The normal chemistry data (no hydrogen addition) from Dresden-2 lie between the other two plants in regard to both electrochemical potential change and the long-term radiation levels. However, the radiation levels at Dresden-2 have begun to increase since hydrogen has been routinely added (initiated at about 7.6 effective full-power years), and the shape of the hydrogen water chemistry electrochemical potential curve from Dresden-2 seems to resemble the Vermont Yankee electrochemical potential curve. The discovery of these qualitative relationships prompted me to develop a more quantitative method for predicting long-term radiation dose rates from short-term electrochemical measurements. I discovered that if the electrochemical potential data are normalized by dividing by the measured or interpolated value after a short exposure time and if these normalized electrochemical potential factions are plotted versus the logarithm of time (in hours), an approximately linear relationship is produced for each of the three data sets (see FIG. 10). Any short-time value can be used, as long as the same time is used for all data sets. I further discovered that when I plotted the concentration of Co-60 in the cooling water divided by the negative slopes of these lines versus the long-term dose rates for these plants, I had prepared a standard curve (FIG. 11) that can be used to predict long-term dose rates from any properly determined set of electrochemical measurements that have been normalized to the same short-time value (a two-hour value was used in FIGS. 10 and 11). This standard curve is shown in FIG. 11 with the three points for the three plants where the necessary electrochemical potential data, long-term dose rate data, and Co-60 concentrations are available in the published literature. The process of data treatment that is a part of this invention can be illustrated by using the short-term electrochemical potential data for hydrogen injection at Dresden-2. These data have been normalized to the two-hour value and plotted versus the logarithm of time in FIG. 12. The slope of the line is -0.325. The average Co-60 concentration at Dresden-2 near the time the electrochemical potential measurements were made was about 0.15 uCi/liter (8). Using the standard curve of FIG. 11, the quotient of these two numbers (0.46 uCi/1) predicts a long-term dose rate for Dresden-2 with hydrogen injection of about 320 mR/h as shown in FIG. 13. Unfortunately, after only 1.7 effective full-power years of operation with hydrogen injection, the Dresden-2 recirculation lines were decontaminated chemically so the long-term result will never be known. However, at that time of the decontamination, the Dresden-2 dose rate had risen to about 300 mR/h from about 230 mR/h just before hydrogen injection was started. Since the build-up rate is logarithmic with time, it is likely the long-term (equilibrium) dose rate would have been about 320 mR/h. REFRENCES TO PRIOR ART 1. C. J. Wood, "Recent Developments in LWR Radiation Field Control," Progress in Nuclear Energy, Electric Power Research Institute, June 1985. PA1 2. R. A. Shaw and M. D. Naughton, "Radiation Control in Light Water Reactors," Proceedings of an International Conference on Water Chemistry of Nuclear Reactor System 2, Oct. 1980 (page 32). PA1 3. L. D. Anstine and M. Naughton, "Radiation Level Assessment and Control for Boiling Water Reactors," ibid (paper 50). PA1 4. W. E. Berry and R. B. Diegle, "Survey of Corrosion Product Generation, Transport, and Deposition in Light Water Reactors," Electric Power Research Institute, Mar. 1979 (EPRI NP-522). PA1 5. L. D. Anstine, "BWR Radiation Assessment and Control Program: Assessment and Control of BWR Radiation Fields," Electric Power Research Institute, May 1983 (EPRI NP-3114, vol. 2). PA1 6. L. D. Anstine, J. J. Zimmer and T. L. Wong, "BWR Corrosion-Product Transport Survey," Electric Power Research Institute, Sept. 1984 (EPRI NP-3681). PA1 7. W. Marble, "Control of Radiation-Field Buildup in BWRs," Electric Power Research Institute, June 1985 (NP-4072). PA1 8. "BWR Radiation Control Handouts from EPRI Contractors Meeting," Electric Power Research Institute, Nov. 1985. PA1 9. R. S. Greeley, M. H. Lietzke, W. T. Smith, and R. W. Stoughton, "Electromotive Force Studies in Aqueous Solutions at Elevated Temperatures. I. The Standard Potential of the Silver-Silver Chloride Electrode," Journal of Physical Chemistry, vol. 64, p. 652, 1980. PA1 10. M. E. Indig and J. E. Weber, "Electrochemical Potential Measurements in a Boiling Water Reactor," Electric Power Research Institute, Nov. 1983 (EPRI NP-3362). PA1 11. J. Leibovitz, W. R. Kassen, W. L. Pearl and S. G. Sawochka, Draft-BWR "In-Plant Measurements of Electrochemical Potentials," Electric Power Research Institute, May 1983 (EPRI NP-3521. PA1 12. E. L. Burley, "Oxygen Suppression in Boiling Water Reactors--Phase 2," General Electric Company, Oct. 1982 (NEDC-23856-7). As can be seen from the preceding descriptions and discussion, the invention provides an effective method for predicting the long-term radiation level changes that will be associated with the corrosion and films on the interior surfaces of the coolant system piping of a nuclear power plant. Having thus described the invention, what is believed to be new and novel and sought to be protected by letters patent of the United States is as follows: