Patent Number: 056028881
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is an in situ technique to coat or dope the oxide film or crud formed on metal surfaces of reactor components with noble metal while the reactor is shutdown or during heatup with recirculation pump heat only, i.e., without nuclear heat generation. The noble metal is brought into contact with the oxide layer by injecting a noble metal-containing compound into the coolant water during an outage or during pump heatup. Preferably the noble metal compound is injected at a point upstream of the feedwater inlet. While the rate of thermal decomposition of the noble metal compound during shutdown is diminished relative to the decomposition rate at the operating temperature of the reactor, the gamma and neutron radiation in the reactor core act to decompose the compound even during shutdown. This decomposition frees noble metal ions/atoms for deposition on or incorporation in the oxide film or crud on reactor components which have been in service for an extended period of time. The preferred embodiment of the present invention involves radiation-induced palladium doping of reactor components performed at ambient temperature or heatup temperatures lower than the operating temperature of 550.degree. F. The radiation-induced doping causes deposition/doping of the reactor component surfaces with palladium in an amount that provides catalytic activity for H.sub.2 and O.sub.2 recombination sufficient to reduce the ECP of stainless steel and other alloy surfaces to required levels for protection against intergranular stress corrosion cracking. Radiation-induced palladium doping was tested with a Type 304 stainless steel surface using ultraviolet (UV) radiation. A Type 304 stainless steel constant extension rate tensile (CERT) specimen (preoxidized) was immersed in a well-stirred solution of palladium acetylacetonate (Pd(CH.sub.3 COCHCOCH.sub.3).sub.2). The palladium acetylacetonate solution was prepared by dissolving/suspending 43 mg of palladium acetylacetonate in 20 ml of ethanol and diluting the resulting mixture to one liter with deionized water. The solution was vigorously mixed to obtain a uniform distribution of the compound. The mixture so prepared had a palladium content of 15 mg/liter (15 ppm) as palladium. This stock solution was diluted to obtain a 100 ppb Pd solution. After immersing the CERT specimen in the palladium acetylacetonate solution, a UV lamp was also immersed in the same solution so that the distance of separation between the UV lamp and the CERT specimen was approximately 1 cm. The temperature of the solution was 78.degree. F. After a 10-min exposure of one side of the specimen to UV radiation, the lamp was moved to the other side, exposing that side to another 10 min of UV radiation to obtain a uniform doping of the CERT specimen with palladium. After radiation treatment, the specimen was washed well with deionized water and then tested at 550.degree. F. for its ECP response as a function of the molar ratio of H.sub.2 /O.sub.2 in the high-purity water environment being tested. The results of this study are shown in FIG. 1. Clearly, the ECP of the specimen responded to hydrogen better than the Type 304 stainless steel autoclave without palladium, showing the presence of palladium on the UV-treated specimen. The response was not as good as thermal doping, e.g., at a temperature of 550.degree. F., presumably due to a lower palladium content on the surface. The present invention is not limited to UV radiation. Any form of electromagnetic radiation, including visible light or higher-energy radiation such as gamma radiation, is expected to provide doping. The extent of doping, however, is expected to depend on the energy of the ionizing radiation and the exposure time. Higher-energy radiation assists in doping the surface with palladium faster and more effectively. For example, experiments have proven that gamma radiation is effective in causing palladium doping of surfaces. The results of experiments involving gamma-assisted palladium doping are shown in FIG. 2. In FIG. 2, one curve shows the ECP response of pure platinum and the other curve shows the ECP response of a Type 304 stainless steel specimen (Type 304 stainless steel/crud/palladium) that had been in a reactor for 10 years. The Type 304 stainless steel specimen was believed to have a truly representative oxide layer (or crud) of thickness 1-2 .mu.m on its surface because of its lengthy immersion in reactor water. After being removed from the reactor, the crudded Type 304 stainless steel specimen was palladium doped (100 ppb Pd at 200.degree. F. for 24 hr) in a hot cell facility. The purpose of this gamma-assisted palladium doping test was to determine the effects of gamma radiation (inherent on the specimen due to activation) and moderately high temperature (200.degree. F.) on the palladium doping process, which may be used to coat or dope reactor internals with palladium or other noble metal. Another objective of the test was to determine the effectiveness of palladium doping on a crud-deposited surface. The test involved the following steps. In Step I, a specimen was cut from surveillance basket material (Type 304 stainless steel) which had been in the mid-core region of a boiling water reactor for 10 years in the expectation that the material would have a representative crud layer on its surface. The presence of crud was confirmed by analysis as well as by measuring the thickness (1-2 .mu.m) using scanning electron microscopy. Therefore, machining was done so as to minimize any crud spalling. The specimen was cut so that a minimum number of cutting edges were generated. The specimen dimensions were approximately 1 cm.times.2 cm. In Step II, a stainless steel wire was spot-welded to the specimen. The amount of cleaning required for the spot welding process was minimized. This specimen was then immersed in a 100 ml aqueous solution of palladium acetylacetonate (100 ppb as palladium) containing 0.01% ethyl alcohol. The solution was stirred and heated to 200.degree. F. in a flask fitted with a reflux condenser over a period of about one day. After the test, the specimen was removed from the flask, washed and set aside for the ECP test in Step III. In Step III, the specimen from Step II was installed inside an autoclave attached to a recirculating flow system containing high-purity water. An ECP test of the specimen was performed at 550.degree. F. at H.sub.2 /O.sub.2 molar ratios ranging from 0.5 to about 5. The total test time was one week. The results of these experiments are plotted as open diamonds in FIG. 2. These results are compared with experimental data showing the ECP response of pure platinum. The following conclusions can be drawn from FIG. 2. First, the ECP response, at 550.degree. F., of the palladium-doped crud-coated Type 304 stainless steel specimen was very similar to that of pure platinum. Second, palladium doping is feasible on heavily oxidized (crudded) surfaces, which is desirable for in-reactor application. Third, it is possible to perform palladium doping at temperatures, e.g., 200.degree. F., below the temperature of an operating boiling water reactor, which is typically at least 550.degree. F., due to the high radiation levels present in the reactor, even during an outage. FIG. 3 shows the influence of palladium doping temperature on the ECP response in the absence of radiation. Type 304 stainless steel specimens were doped with palladium at temperatures of 200.degree. F., 400.degree. F. and 550.degree. F. at different H.sub.2 /O.sub.2 molar ratios ranging from 0 to about 7.5. The doping done at 200.degree. F. without any radiation influence showed that the lowest ECP reached at a H.sub.2 /O.sub.2 molar ratio of 7 was approximately -0.330 V(SHE). The data in FIG. 3 show that, in the absence of electromagnetic radiation, palladium doping is more effective when performed at higher temperatures. In contrast, as seen in FIG. 2, with gamma radiation assisting the doping process, the ECP reached at a H.sub.2 /O.sub.2 molar ratio of 7 decreased to almost -0.460 V(SHE), i,e, a benefit of 160 mV at a much lower doping temperature of 200.degree. F. In particular, it is believed that the radioactivity of this crudded specimen contributed to an increase in the rate at which the palladium acetylacetonate in the coolant water decomposed during palladium doping. Thus, the good ECP response of the crudded specimen taken from a reactor was attributable to radiation-assisted palladium doping of the specimen. In the foregoing experiment, the radiation in aid of noble metal doping was inherent, i.e., was emitted by radioisotopes (due to activation as a result of reactor exposure) contained in the material of the specimen. Specifically, the crudded Type 304 stainless steel (reactor exposed) specimen had an inherent radiation dose of 50 to 60 mRad/hr, because of its exposure to reactor water over a period of 10 years. It follows that the rate at which palladium acetylacetonate decomposes inside a boiling water reactor will be increased dramatically due to the effect of gamma radiation emitted by the nuclear fuel core. The contribution to the decomposition rate made by electromagnetic radiation will allow palladium doping of equal effect to be achieved at relatively lower doping temperatures. The main conclusions to be drawn from the foregoing experimental data are as follows: (1) gamma radiation assists the palladium doping process; (2) the presence of gamma radiation facilitates palladium doping at a lower temperature than the temperature at which thermal doping is performed (i.e., approximately 550.degree. F.); and (3) palladium doping of in-reactor surfaces (crudded) is possible. A significant advantage of gamma-induced doping is that when thermal doping is practiced in nuclear reactor applications, gamma radiation would inherently be present even if the reactor is in an outage, thereby providing an additional benefit of radiation-induced palladium doping of surfaces. Thus, when thermal doping of palladium is practiced in nuclear reactors, the doping will be due to a combined effect of both thermally induced as well as radiation-induced effects. The foregoing method has been disclosed for the purpose of illustration. Variations and modifications of the disclosed method will be readily apparent to practitioners skilled in the art of reactor kinetics. For example, the noble metals which can be applied using this technique include palladium, platinum, ruthenium, rhodium, osmium, iridium, and mixtures thereof. The noble metal can be injected in the form of an organic or organometallic compound to reduce the potential of reactor components made of stainless steel or other alloys even in the absence of hydrogen injection. Alternatively, the noble metal can be injected in the form of an inorganic compound in conjunction with hydrogen injection to reduce the ECP at the surface of reactor components. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.