Patent Number: 051715155
Section: summary

BACKGROUND OF THE INVENTION The invention relates to a process for inhibiting corrosion in a pressurized water nuclear reactor, and more particularly for inhibiting corrosion within the primary circuit of a pressurized water nuclear reactor. Corrosion is a particular concern for nuclear reactors in which water is present as a coolant. Several methods have been proposed to deal with the problem, including dissolving oxide scale from the structure of the nuclear reactor, as disclosed in U.S. Pat. No. 3,664,870 to Oberhofer et al. and U.S. Pat. No. 4,042,455 to Brown. Another approach, as disclosed in U.S. Pat. No. 4,364,900 to Burrill, is to inhibit corrosion formation within the reactor system. Burrill adds from about 120 to about 200 milligrams of ammonia per kilogram of coolant water to reduce crevice corrosion in the core of pressurized water nuclear reactors. Zinc ions are thought to inhibit corrosion within boiling water nuclear reactors, and such ions from zinc oxide and zinc chloride have been described for use in boiling water nuclear reactors. As discussed by W. J. Marble in "Control of Radiation-Field Buildup in BWRs", Electric Power Research Institute NP-4072, Project 189-2, Interim Report, June 1985, zinc in the form of ZnO was used in a boiling water reactor plant. It was discussed therein that soluble zinc, by acting as a corrosion inhibitor for stainless steel, significantly reduces the amount of oxide formed on the pipes and thus the amount of Co-60 incorporated in the system. The hypothesis proposed on page 4-2 of that reference was that zinc cations normally found in zinc oxide crystals will tend to modify the normal magnetite crystal defect structure so that a more protective film is formed and corrosion significantly inhibited. Further, the presence of zinc ions in the reactor coolant water of boiling water reactors having brass tubing, as discussed at page 6-1, has been correlated with a reduced amount of corrosion and radioactive cobalt transport throughout the reactor. Zinc introduced in the form of zinc chloride (ZnCL.sub.2) was also laboratory tested as a corrosion inhibitor under power plant operating conditions, as discussed by L. W. Niedrach and W. H. Stoddard in "Effect of Zinc on Corrosion Films that Form on Stainless Steel", Corrosion 42, 546 (1986). The use of zinc oxide as a source of zinc ions has the drawback that zinc oxide is not particularly soluble in water, and, therefore, the zinc oxide must be added to the coolant as a slurry or suspension, rather than as a solution. Pressurized water nuclear reactors are thermal reactors in which water is used as the coolant and as the moderator. The water is circulated by pumps throughout a primary circuit, that includes a pressure vessel, which houses the heat generating reactor core, and a plurality of flow loops. The heat absorbed by the water as it passes through the reactor core is transferred by means of a heat exchanger to a readily vaporizable liquid (water) in a secondary circuit in which the thermal energy is used to produce electricity. The water is then returned to the pressure vessel. The water in the primary circuit, which normally contains boric acid as a moderator, passes through numerous metal, generally stainless steel and Alloy 600, conduits, all of which are subject to corrosion. Further, some radioactive cobalt from the reactor core is dissolved in the water as the metal ion and transported throughout the primary circuit. The transport of radioactive cobalt throughout the primary circuit results in a level of residual radioactivity throughout the primary circuit that is not desired. Thus, it is desired to develop a process for inhibiting corrosion in a pressurized water nuclear reactor. An object of the invention is to provide a process for inhibiting corrosion in a nuclear reactor by the addition of zinc to the coolant water flowing therethrough in a soluble form. SUMMARY OF THE INVENTION The present invention provides a process for inhibiting corrosion caused by the presence of coolant water passing through a pressurized water nuclear reactor. An effective amount of an aqueous solution of zinc borate is added to the reactor coolant water. As a result of the process, the transport of corrosion products and radioactive cobalt ions through the reactor system, as well as levels of radioactivity within the reactor system, are reduced. The aqueous solution of zinc borate is preferably an aqueous solution of zinc borate in boric acid, with the zinc borate preferably added to the reactor coolant water so that zinc ions are present in the reactor coolant water in an amount of from about 10 to about 200 parts per billion.