CORROSION INHIBITION OF CLOSED COOLING WATER AUXILIARY SYSTEM FOR NUCLEAR POWER PLANTS

Abstract: A potassium inhibitive blend and corrosion protection method of closed cooling water systems. The closed cooling system circulates about 35 to 40 gallons of corrosion inhibitor per 10,000 gallons of circulating water. The corrosion inhibitor contains potassium molybdate, potassium nitrite, and potassium borate in an aqueous solution so that if leakage of the circulated water occurs, the leaked corrosion inhibitive liquid does not cause dangerous radiation in any amounts dangerous to the environment. The potassium molybdate and potassium nitrate each constitute about 12% of the aqueous solution. Potassium based borate at 2 to 2.5%, ethylene glycol preferably at about 10%, and benzotriazole or tolyltriazole of about 0.4% with up to 1% polymeric scale suppressant are also included in the corrosion inhibitive blend.

Description: This invention relates to inhibiting corrosion in closed cooling water systems under radiation, especially in nuclear power plants. Background of the Invention Closed cooling water systems are important parts of power plants and other industrial systems. These systems have traditionally employed chromates for their corrosion protection. Concern about toxicity of chromates and their ecological effects had led to a consideration of the use of other anodic inhibitors such as sodium molybdates. Sodium molybdates are compatible with all the widely used biocides/polymers, scale suppressants, and sludge control agents. Therefore they are used as a part of multicompositional products (blends) along with HEDP (1-hydroxy-ethylidene-1, 1-disphonic acid), MPT (2-mercapto-benzothiazole), tolyltriazole, benzotriazole, nitrites, sodium hydroxide, pH chelants etc. to prevent corrosion of materials. See Burda, P.A., Proceedings of an International Symposium, "Surfaces, Inhibition and Passivation," The Electrochemical Society, Volume 86-7, Pages 91-103 (1986). These molybdate-based blends are much lower in toxicity than the chromates. Some of these blends have been found to be as effective as chromates for corrosion protection of mild and cast steels, cast iron and copper alloys in deionized and nonpotable water. However, it has been discovered in closed cooling water auxiliary systems of nuclear plants that such sodium molybdate based blends tend to evolution of sodium radioactive isotopes in the presence of radiation and flux of neutrons, causing contamination of cooling systems. Thus, sodium molybdate and all chemicals incorporating substantial amounts of sodium have been unsatisfactory for nuclear application. An object of the present invention is to provide an improved closed cooling water system which can operate safely under radiation conditions. Summary of the Invention The present invention rests on the discovery that, although sodium and potassium are often considered to be equivalent metals and ions, that is not true in the case of nuclear systems. The surprising result has been obtained that potassium molybdate can replace sodium molybdate without resulting in any significant production of dangerous radioactivity. The new potassium-based corrosion inhibitive blend may also include, potassium nitrate and other materials used in other blends before, such as ethylene glycol, benzotriazole, tolyltriazole, and polymers, for scale suppression. Brief Description of the Drawings The drawing is a schematic view or flow sheet of a component cooling water system embodying the principles of the invention. Detailed Description of the Invention Laboratory and field tests have revealed that sodium-molybdate-based blends could successfully replace chromate corrosion inhibitors in closed cooling water systems, including those in power plants. It was, however, found that in nuclear applications, radioactive sodium isotopes, especially Na²⁴, were produced, causing closed cooling water auxiliary systems to be dangerously radioactive and hazardous to health in case of leakages or repairs. Therefore, in a preferred closed cooling water system for each 37,850 L (10,000 gal.) water, 132.5 to 151.4 L (35 to 40 gallons) of the following mixture may be used for corrosion protection of materials: Potassium molybdate (K₂MoO₄) 12% Potassium nitrite (KNO₂) up to 12% Potassium borate (K₂B₄O₇·5H₂O) 2-2.5% Ethylene glycol 10% Benzotriazole or tolyltriazole 0.4% Polymeric scale suppressant up to 1% DI water Remainder . Since absolute purity is nearly impossible to attain and is impractical, there may be some sodium present, but this can be kept under 160 ppm. There may be also up to 10 ppm of chloride (as Cl⁻) and up to 10 ppm of fluoride (as F⁻), allowing less than 0.05 ppm of Cl⁻ and F⁻ and less than 0.6 ppm of Na in the closed cooling water system. The effectiveness of the potassium molybdate system on toxicity is about the same as that of sodium molybdate and is far better than that of chromates, as shown in the following table: Tests made with potassium based molybdate as a corrosion inhibitor in auxiliary closed cooling water systems revealed that there was no secondary radiation detected in the presence of potassium. Based on calculations of radiation levels, potassium in potassium molybdate causes thirteen times lower radiation than sodium; so the radiation level caused by the presence of less than 0.6 ppm of Na has an insignificant effect on the radioactivity of the cooling system. Also, the concentration of chlorides and fluorides is below the 0.15 ppm level permitted by Nuclear Operating Procedures for Chemistry Control limits in nuclear plants (Pacific Gas and Electric Gas Co., Department of Nuclear Plant Operations, Diablo Canyon Power Plant, Chemistry Control Limits and Action Guidelines for the Plant Support Systems N° OP F-5: 111, 1/28/84). The buildup of significant amounts of radioactive isotopes in this corrosion inhibitor has been found to be, for practical purposes, substantially eliminated. It has been found that this inhibitive blend, kept practically without sodium, will not produce secondary radiation in any significant amount, and at the same time results in prevention of corrosion in nuclear power plant auxiliary and/or other closed cooling water systems. To those skilled in the art to which this invention relates, many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosures and the descriptions herein are purely illustrative and are not intended to be in any sense limiting.

Claims: A method of cooling nuclear power plants while providing corrosion protection, comprising: circulating coolant in a closed water system, said coolant containing about 132.5-151.4 L (35 to 40 gallons) of a liquid corrosion inhibitor per 37,850 L (10,000 gallons) of water, said liquid corrosion inhibitor being an aqueous solution of potassium molybdate, potassium nitrite, and potassium borate with no more than 160 ppm of sodium, so that if leakage of said circulating water occurs, the leaked corrosion inhibitor does not produce dangerous radiation isotopes in any amounts and it is not dangerous to the environment. The method of claim 1 wherein said liquid corrosion inhibitor contains about 12% of said potassium molybdate, about 12% of said potassium nitrite, and 2 to 2.5% of potassium borate. The method of claim 2 wherein said liquid corrosion inhibitor also contains about 10% ethylene glycol, about 0.4% benzotriazole or tolyltriazole and up to 1% polymeric scaling retardant (potassium based). A liquid corrosion inhibitor for closed water-based cooling systems suitable for the application in the method according to any of claims 1-3 and comprising water containing potassium molybdate, potassium nitrite, and potassium borate and containing less than 160 ppm sodium ions. The liquid corrosion inhibitor of claim 4 also containing practically sodium-free ethylene glycol and benzotriazole or tolyltriazole and/or scaling retardant. A liquid corrosion inhibitor according to claim 4 or 5 containing about 12% potassium molybdate and about 12% potassium nitrite and 2 to 2.5% potassium borate in aqueous solution and containing less than 160 ppm sodium ions. The liquid corrosion inhibitor of claim 6 also containing about 10% ethylene glycol and about 0.4% of benzotriazole or tolyltriazole respectively and up to 1% polymeric potassium based scaling retardant.

Inventors: BURDA, PAUL A.
Applicants: PACIFIC GAS AND ELECTRIC COMPANY
Application year: 1989
Main classification: C23F11
IPC codes: C23F11, G21C15