Patent Number: 052456420
Section: description

DETAILED DESCRIPTION OF THE INVENTION This invention consists of a chemical process for removing cobalt contaminated oxide films that form on the surfaces of metal structures providing the coolant water circuits of water cooled nuclear fission 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 cobalt-60 isotope, is carried in the coolant 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 coolant 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 coolant water containing structures 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 cobalt-60 from the circulating coolant water and retain it in the accreting body of oxides forming over exposed surfaces. Thus, radiation levels measured one cycle after decontamination are frequently as great as before decontamination. In accordance with this invention a chemical technique is provided which controls and/or minimizes contamination in water cooled nuclear fission reactor system following decontamination. By minimizing recontamination, the chemical method of this invention can be a more effective means of reducing radiation exposure of personnel in a nuclear reactor plant. The chemical measures of this invention entail a combination of conditions that reduce the soluble (ionic) Co-60 concentration in reactor coolant water and pre-oxidize the surface of the coolant water retaining system with a oxide film substantially free of Co-60. The means of the invention comprise adding a solution of an iron compound, including, but not limited to, freshly prepared insoluble species Fe(OH).sub.3, Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, or water soluble compounds ferrous oxalate and ferric citrate in amounts sufficient to maintain a soluble iron concentration in the coolant water of the coolant system at about 200 parts per billion (ppb). With these conditions, preferably augmented by elevated water temperatures, the soluble (ionic) Co-60 in the reactor coolant water is effectively scavenged. Moreover, while the soluble Co-60 concentration in the coolant water is reduced, the surfaces of the coolant water retaining system can be oxidized to form a substantially cobalt free, protective film following a cobalt purging. Preferred conditions for the practice of this invention comprise adequate Fe(OH).sub.3 addition to maintain the iron concentration of approximately 200 ppb with the coolant water at a temperature of at least about 230.degree. C. Generally optimum effects are obtained when these conditions of iron concentration and temperature are retained in the coolant water of the reactor coolant system over a period of at least about 500 hours. The elevated temperatures of the coolant water can be provided without nuclear fission produced heat in a pre-startup treatment by any suitable means or source, such as heat generated by recirculation pumps which drive the coolant water through the reactor coolant system. In a typical reactor pre-startup treatment procedure of this invention, apt amounts of ferric hydroxide in a slightly basic water solution are injected into the reactor coolant for attaining the iron concentration conditions of about 200 ppb. Coolant water temperature is maintained at least about 230.degree. C. Given these conditions, the soluble Co-60 in the coolant water can be reduced down to less than about one percent of the total Co-60 concentration in the reactor water. To foster oxidation of the surfaces of cooling water circuit system upon purging of Co-60 from the coolant water, the dissolved oxygen content in the reactor coolant water is maintained at about 200 to about 400 parts per billion (ppb). This can be provided by introducing oxigated water, such as control rod drive water or other sources, or injecting oxygen. Preferably the operation of pH adjustment with iron solution addition for Co-60 purging of the coolant water system, and oxygen level control is carried out as long as is practical before startup of the nuclear reactor, for example at least about 500 hours. Following starting up of the water cooled nuclear fission reactor, the iron content of the coolant water may be depleted rapidly, whereby a high iron solution injection rate can be appropriate or required to maintain the iron content at least about 50 to about 100 parts per billion. Then the nuclear reactor is operated under the given conditions for approximately 500 hours before the iron solution injection is terminated. At this state the iron content of the coolant water should be maintained at about 5 ppb. This be can achieved by means of feedwater quality control.