METHOD FOR NUCLEAR POWER PLANT DECONTAMINATION

Abstract: Method effecting decontamination of metal components carrying an oxide layer in a nuclear powered water cooled reactor wherein a low concentration of zinc ions is introduced into the water to cause loosening and removal of the oxide layer from the metal surface.

Description: METHOD FOR NUCLEAR POWER PLANT DECONTAMINATION The present application relates to a method for decontamination of nuclear power plant systems. More particularly, the invention relates to the use of small amounts of zinc to change the properties of the oxide film present on internal walls and tubing surfaces of nuclear reactors to facilitate removal of radioactive substances such as 60 Co therefrom. BACKGROUND OF THE INVENTION A major problem in water-cooled nuclear reactors is the accumulation of radioactive substances in the structural portions of the reactor system. For example, during reactor shut-down, workers are exposed to radiation emanating from internal walls and tubing surfaces, and radioactive materials retained in oxide films which have accumulated on these surfaces are a major source of radiation exposure. The build-up of radioactive cobalt ( 60 Co) in recirculation piping of nuclear power reactors, including boiling water reactors, is a major source of radiation exposure, especially during reactor shutdown. Efforts have been made during recent years to identify parameters which affect the rate and magnitude of 60 Co buildup, with a view to developing methods for limiting the buildup. It has been shown in prior work that the majority of 60 Co buildup in recirculation piping occurs by incorporation into the oxide film during formation of the oxide film on stainless steel surfaces. U.S. Patent No. 4,950,449 describes the use of zinc ions to remove or lessen deposition of radioactive substances and reduce intergranular stress corrosion cracking in water-cooled nuclear reactors. The zinc may be added in the form of zinc oxide paste, slurry or aqueous solution. U.S. Patent No. 4,756,874 describes the use of zinc having a lower content of the M Zn isotope in order to reduce accumulation of radioactive cobalt without increasing the presence of the 65 Zn activation product of ^Zn. The zinc in this form may be added to the reactor water in the form of a zinc salt or zinc oxide. U.S. Patent No. 4,759,900 relates to the inhibition of deposition of radioactive cobalt by continuous injection of zinc oxide into the reactor water. The zinc oxide may be prepared in the form of a paste, slurry or aqueous solution. A need exists for reducing the buildup of radioactive materials in nuclear power systems. The present invention seeks to satisfy that need. SUMMARY OF THE INVENTION It has been discovered, according to the present invention, that it is possible to introduce changes in the structure of an oxide film present on a metal surface by exposure of the oxide bearing surface to zinc ions at a low concentration, typically 1-3000 part per billion (ppb), more usually 10-100 ppb, over a period of at least 100 hours, typically in the region of 300 hours or more, and at a temperature of at least 450, for example 455 to 5.50°F. Such exposure results in the oxide film changing from a tightly adherent structure to a loosely adherent structure that can be removed by the circulating water of the system, thereby facilitating removal of at least a portion of the film, including radioactive materials contained therein. In this way, through changing the structure of the oxide film, the invention provides a way of effecting decontamination of the reactor to reduce the buildup of radioactive materials, including 60 Co buildup, on the inner wall surfaces and recirculation piping of nuclear power boiling water reactors. It is also possible according to the present invention to incorporate zinc into a passivating oxide film wherein stainless steel, initially free of oxide film, is subjected to high temperature, typically greater than 300°F, high purity water (i.e. water having a quality consistent with that typically found in operating nuclear reactors) used under conditions where the zinc concentration is no higher than 500 ppb for an exposure time greater than or equal to 32 about 100 hours. A passivating film covers the surface of the base metal (e.g. stainless steel) and inhibits subsequent corrosion/oxide formation Under these conditions, an oxide film is produced which is substantially thinner, i.e. usually less than 0.5 microns, more usually less than 0.2 microns, than non-oxide-bearing oxide films formed under similar conditions in the absence of zinc. Typically, film thicknesses in absence of zinc range from about 1 to 3 microns. With the thinner films present on a stainless steel surface, it is observed the at Co 60 buildup is less than when the oxide film is formed in the absence of zinc. When zinc is present, 60 Co levels are typically less than 10μCi/cm 2 , while without zinc, the 60 Co levels are typically between about 1 OμCi/cm 2 and 500μCi/cm 2 . Moreover, removal of the loosened oxide layer can be effected by the circulating water in the pipe. In accordance with one aspect of the present invention, there is provided a method for effecting decontamination of metal components carrying an oxide layer comprising exposing the component to an aqueous solution of zinc ions to cause loosening of the oxide layer from the metal surface. In accordance with another aspect there is provided a method for effecting decontamination of metal components carrying an oxide layer in a nuclear powered water cooled reactor comprising introducing a low concentration of zinc ions into the reactor water to cause loosening of the oxide layer from the metal surface. In accordance with yet another aspect, there is provided a method of passivation of a metal surface comprising immersing the metal surface initially free of oxide surface layer in water at elevated temperature, typically in the region of 230 to 300°C in the presence of zinc at a low concentration for example 1-300 ppb for a time of at east 300 hours. It has been found according to the invention that incorporation of low amounts of zinc in the passivated film limits the buildup of radioactive materials therein, including 60 Co. The zinc may be added in the form of a salt, for example zinc chromate or zinc oxide. It is also possible to use zinc which has been treated to eliminate or reduce its M Zn content, such as described in U.S. patent 4,756,874 (herein incorporated by reference). The zinc oxide may be added in any form which permit it to be dissolved in the reactor water, such as by way of a slurry, paste or preformed solution. A paste will typically have a concentration of zinc oxide of about 25% to about 95% by weight, whereas a slurry will have a zinc oxide content in the region of about 0.1% to 20% by weight. Examples of ways in which the zinc oxide is introduced are set forth in U.S. patent 4,756,874, referred to above. An advantage associated with the method of the present invention is that it may be applied directly to existing boiling water reactor recirculation piping and other components without prior oxide film removal. It may also be applied to oxide-free metal components before their introduction into the reactor. The inventors have found that the properties of the oxide film are changed when subjected to the conditions of the present method. At least part of the film is changed from a tightly adherent structure to a loosely adherent structure, with simultaneous release of constituents. Thus, according to the invention, the application of the process not only inhibits further Co 60 buildup but also effects a decontamination, including a decrease in Co 60 levels. EXAMPLE The following example illustrates the present invention. Small stainless steel autoclaves were used to contain test specimens for the formation of the oxide films in high temperature water. The autoclaves used in these tests were previously employed for corrosion specimen testing under similar conditions without zinc present in the water. Under these conditions, after long exposure (many thousands of hours) the inner surfaces had attained a heavy black oxide coating which was tightly adherent and could not be removed by w i p i n g . During a shut-down period for specimen removal, it was noted that the inner autoclave surfaces which had been treated according to the method of the present invention appeared lighter in color. Wiping of the surfaces resulted in removal of some of the dark outer surface oxide, leaving a dark bronze-colored oxide inner film. The autoclaves have been operated with high temperature water containing zinc for approximately 2,000 hours. During the initial exposure, with the zinc containing water, a zinc depletion was also noted. The zinc concentration in the water from the autoclaves was significantly lower than the inlet concentration, indicating absorption on the stainless steel surfaces. Zinc can thus be incorporated into an oxide film even in the presence of an already existing oxide film. This results in release of material from the previously existing film and also gives rise to reduced Co 60 buildup on the metal surfaces. While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.

Claims: WE CLAIM; 1. A method for effecting decontamination of metal components carrying an oxide layer in a nuclear powered water cooled reactor comprising introducing a low concentration of zinc ions into the water to cause loosening of the oxide layer from the metal surface. 2. A method according to claim 1, wherein said zinc ions are introduced in a concentration of at least 30 ppb. 3. A method according to claim 2, wherein said zinc ion concentration is 1-500 ppb. 4. A method according to claim 1, wherein said water is at a temperature of at least about 450°F. 5. A method according to claim 4, wherein said temperature is about 455 to 550°F. 6. A method according to claim 1, wherein said metal surface carrying said oxide layer is exposed to said zinc ions for at least 100 hours. 7. A method according to claim 6, wherein said exposure time is about 2000 hours. 8. A method according to claim 1, where the zinc ion is substantially devoid of ^Zn. 9. A method according to claim 1 wherein the zinc ion is added by dissolving zinc oxide in water. 10. A method according to claim 1 wherein the zinc ion is added as a water-based paste, a water-based slurry or a water- based solution. 11. A method of passivation of a metal surface comprising immersing the metal surface initially free of oxide surface layer in water at elevated temperature in the presence of zinc at a low concentration for a time of at least 100 hours. 12. A method according to claim 11, wherein said water is at a temperature of at least about 450°F. 13. A method according to claim 1 1, wherein said zinc is at a concentration of 1-300 ppb. 14. A method for effecting decontamination of metal components carrying an oxide laver comprising exposing the component to an aqueous solution of zinc ions to cause loosening of the oxide layer from the metal surface.

Inventors: MARBLE WILLIAM JOEL, PETERSON GEORGE ERVIN, ROBINSON RANDALL NORMAN, RUIZ CARL PHILLIP, MARBLE, WILLIAM, JOEL, PETERSON, GEORGE, ERVIN, ROBINSON, RANDALL, NORMAN, RUIZ, CARL, PHILLIP
Applicants: GEN ELECTRIC, GENERAL ELECTRIC COMPANY
Application year: 1999
Main classification: G21C17
IPC codes: G21C17, G21C19, G21F9