Patent Number: 053751523
Section: summary

This invention relates to the operation and safety of water-cooled nuclear reactors, and in particular to measures for reducing the hazards of possible exposure of operating and maintenance personnel to a source of radiation dispersed throughout the cooling water circulating system of such nuclear reactors. BACKGROUND OF THE INVENTION A major hazard in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor. The buildup of radioactive nuclides occurs on the inner surfaces of components which are in contact with the reactor water. This includes both the primary recirculation circuit and the reactor water cleanup system. During reactor shutdown, workers are exposed to radiation emanating from stainless steel internal walls and inner surfaces of piping. Radioactive materials retained in oxide films which have accumulated on wall and piping surfaces are a major source of radiation exposure. The radioactivity has been found to be predominantly due to the Co-60 isotope. As a result, a substantial effort has been made to identify the key parameters which affect Co-60 buildup and to determine and implement methods for limiting that buildup. Cobalt, derived from a number of different alloys commonly employed in components of the reactor's mechanisms or structures, is subject to induced radioactivity, especially the Co-60 isotope. This radioactive Co-60 isotope, or ions or compounds thereof, can be carried in the circulating cooling water flowing through the cooling water circuit, whereby the radioactive substances are spread and deposited throughout the cooling water circuit or primary loop system of the reactor plant. Such radioactive substances are prone to be taken up and incorporated into the normally occurring oxide films which form and progressively accumulate on the inner surfaces of the cooling water circuit. A number of proposals or potential solutions to this problem of dispersion of radioactive substances throughout the cooling water circuit or system and incorporation of radioactive substances into the inherently produced oxide films forming on the surfaces of cooling water circuit or system have been considered or made. One approach to controlling the potentially hazardous cobalt source of such radiation has been the application of zinc as disclosed in U.S. Pat. No. 4,756,874 to Ruiz et al. and U.S. Pat. No. 4,759,900 to Peterson et al. SUMMARY OF THE INVENTION The present invention comprises a method for preventing or reducing the contamination of cooling water circuit surfaces by Co-60 isotope in a water-cooled nuclear reactor. The method encompasses the addition of iron compounds and the creation of attendant physical conditions which scavenge Co-60 isotope from the cooling water and also impede subsequent buildup of the Co-60 isotope on the surface of metal components or structures that form the primary cooling circuit system of a water-cooled nuclear reactor. In particular, the method for preventing or reducing the formation of material contaminated with Co-60 on the surfaces of a circuit carrying cooling water in a nuclear reactor comprises the step of adding at least one iron compound to the cooling water in an amount sufficient to maintain an iron concentration in the cooling water in a range from 50 to 200 ppb. The iron compound is added while the reactor is shut down. Prior to the addition of the iron compound, the cooling water is treated to attain a pH thereof in the range of about 7.5 to about 8.0 measured at a cooling water temperature of about 25.degree. C. Thereafter the cooling water is heated to a temperature of at least about 230.degree. C. and maintained at that temperature while the iron compound is present in the aforementioned concentration. The result of this treatment is that Co-60 is scavenged from the cooling water in the cooling water circuit. The method further comprises the step of maintaining a dissolved oxygen concentration in the cooling water in a range from 200 to 400 ppb. The oxygen is added for the purpose of accelerating the formation of oxide film on the cooling water circuit surfaces, which oxide films, because Co-60 has been scavenged from the system, will be substantially free of radioactive Co-60 isotope. Such oxide films act as a barrier to the formation of further oxide films The method of the invention further comprises the step of adding at least one iron compound to the cooling water in an amount sufficient to maintain an iron concentration in the cooling water in a range from 50 to 100 ppb during operation of the reactor after the shutdown.