Patent Number: 053751523
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The method of the invention comprises a chemical process for preventing the formation of cobalt-contaminated oxide films on the surfaces of metal structures providing the cooling water circuits of water-cooled nuclear reactors, such as inner portions of pipes, conduits, vessels, tanks, chambers, etc. Cobalt derived from metal alloy materials utilized in nuclear reactor plants is known as a major source of radiation and, in turn, is a health hazard to operating and maintenance personnel working about the nuclear reactor structures. Cobalt, particularly the Co-60 isotope, is carried in the cooling water throughout the nuclear reactor coolant circuit or system and becomes entrained and/or embedded in the mass of oxides commonly forming and accreting over the exposed metal surfaces of vessels, conduits, etc. of the cooling water circuit system. Reducing the presence of cobalt by replacing cobalt-containing alloys with alloys free of cobalt to minimize its source is expensive and most often impractical. Chemical decontamination procedures for removing cobalt-contaminated oxide films from inside surfaces of the cooling water circuit have been proposed whereby the hazardous radiation fields are substantially reduced through oxide film removal by chemical means. However, due to extremely high corrosion rates, the decontaminated surfaces rapidly pick up Co-60 isotope from the circulating cooling water and retain it in the accreting body of oxides forming on exposed metal surfaces. Thus, radiation levels measured one cycle after decontamination are frequently as great as before decontamination. In accordance with the method of the present invention, a chemical technique is provided which controls and/or minimizes contamination in water-cooled nuclear fission reactor systems prior to any significant initial contamination or following decontamination. By minimizing recontamination, the method of this invention can be a more effective means of reducing radiation exposure of personnel in a boiling water nuclear reactor. The chemical measures of this invention entail a combination of conditions that reduce the soluble (ionic) Co-60 concentration in reactor cooling water and preoxidize the surfaces of the cooling water circuit with an oxide film which is substantially free of Co-60. The steps of the invention comprise adjusting the pH of the cooling water circulating within the cooling water circuit to a slightly basic condition of about 7.5 to about 8 when measured at a water temperature of about 25.degree. C., and adding a solution of an iron compound, including, but not limited to, freshly prepared insoluble species such as Fe(OH).sub.3, Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, or water-soluble compounds such as ferrous oxalate and ferric citrate, in amounts sufficient to maintain a soluble iron concentration in the cooling water in the range of 50 to 200 ppb. Under these conditions, preferably augmented by elevated water temperatures, the soluble (ionic) Co-60 in the reactor cooling water is effectively scavenged. Moreover, while the soluble Co-60 concentration in the cooling water is reduced, the surfaces of the cooling water circuit can be oxidized to form a substantially cobalt-free protective film prior to initial operation or following cobalt purging. Preferred conditions for the practice of this invention comprise adjusting the cooling water pH to about 7.5 to about 8 with adequate Fe(OH).sub.3 addition to maintain the iron concentration at approximately 200 ppb with the cooling water at a temperature of at least about 230.degree. C. Generally optimum effects are obtained when these conditions of pH, iron concentration and temperature are maintained in the cooling water of the reactor coolant system over a period of at least about 500 hr. The elevated temperatures of the cooling water can be provided without nuclear fission-produced heat in accordance with a pre-startup treatment by any suitable means or source, such as heat generated by recirculation pumps which drive the cooling water through the reactor coolant system. In a typical reactor pre-startup treatment in accordance with the invention, suitable amounts of ferric hydroxide in a slightly basic water solution are injected into the reactor coolant for attaining the desired pH of about 8 and iron concentration of about 200 ppb, with the pH being determined at a water temperature of about 25.degree. C. The temperature of the cooling water is maintained at about 230.degree. C. or higher. Given these conditions, the soluble Co-60 in the cooling water can be reduced to less than about 1% of the total Co-60 concentration in the reactor water. To foster oxidation of the surfaces of the cooling water circuit upon purging of Co-60 from the cooling water, the dissolved oxygen content in the reactor cooling water is maintained at about 200 to about 400 ppb. The oxygen can be provided by introducing oxygenated water, such as control rod drive water, or by injecting oxygen. Preferably the operations of pH adjustment combined with iron solution addition for Co-60 purging of the cooling water system, and oxygen level control are carried out as long as is practical before startup of the nuclear reactor, for example, at least about 500 hr. Following starting up of the water-cooled nuclear fission reactor, the iron content of the cooling water may be depleted rapidly, whereby a high iron solution injection rate can be appropriate or required to maintain the iron content in the range of about 50 to about 100 ppb. The pH of the water should be maintained in the range of about 7.5 to about 8. Then the nuclear reactor is operated under the given conditions for approximately 500 hr before the iron solution injection is terminated. At this stage the iron content of the cooling water should be maintained at about 5 ppb. This can be achieved by means of feedwater quality control.