Patent Number: 056087667
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiment of the present invention is a technique to dope stainless steel surfaces with palladium in situ by injecting a palladium-containing compound into the high-temperature water of a BWR while oxide film is forming on the stainless steel surface. Preferably the palladium compound is injected in the form of a solution or suspension at a point upstream of the feedwater inlet. The high temperatures as well as the gamma and neutron radiation in the reactor core act to decompose the compound, thereby freeing palladium species for incorporation in the oxide film as its grows. As used herein, the term "species" means ions or atoms. One Pd-containing compound successfully used for this purpose is an organometallic compound, palladium acetylacetonate. However, other noble metal compounds of organic, organometallic and inorganic nature can also be used for this purpose. The palladium acetylacetonate compound is dissolved in an ethanol/water mixture or in water alone to form a solution or suspension which is injected into the reactor coolant. The palladium gets incorporated into the stainless steel oxide film via a thermal decomposition process of the organometallic compound. As a result of that decomposition, Pd species become available to replace atoms, e.g., Fe atoms, in the oxide film, thereby producing a Pd-doped oxide film on stainless steel. The method of the present invention involves in situ removal of some or all of the oxide film from the surfaces of wetted reactor component and co-deposition of noble metal during subsequent growth of oxide film on the same wetted surfaces. The result is a noble metal-doped oxide film having a relatively longer catalytic life in the reactor operating environment. Incorporation of palladium into the film provides greatly increased catalytic life as compared to palladium coatings which lie on the oxide surface. In accordance with the broad concept of the present invention, several approaches are possible. In the simplest approach, mechanical cleaning (e.g., by flapper wheel or ultra-high-pressure water jet) is used to remove most or all of the oxide film from the reactor component to be treated. Because the oxide film formed on a reactor component reaches a limiting thickness, some portion of the oxide film must be removed before more oxide film, which forms the matrix for the metal dopant, can be grown. After removal of some oxide film, the appropriate aqueous noble metal compound is added to the reactor water prior to initial heat up. This can be accomplished without the nuclear fuel being present by using the recirculation pumps. As the oxide film reforms, palladium will be incorporated into the film. While it is desirable to use the highest possible palladium concentrations consistent with plant and cost considerations, levels in the preferred range of 5 to 100 ppb Pd should be sufficient. In accordance with the preferred method, after the oxide film has been thinned, noble metal doping of newly formed oxide film can be performed at regular intervals to produce a noble metal concentration which varies cyclically in the thickness direction or can be performed continuously to produce a noble metal concentration which is generally constant in the thickness direction. Since mechanical cleaning is expensive, complex and limited to reactor components that are readily accessible, more attractive approaches for preparing the oxidized alloy surfaces include chemical decontamination (which is periodically performed in many plants to reduce the radioactivity, e.g., of piping from Co.sup.60 and other elements which incorporate into the oxide) and exposure to hydrogen water chemistry, which will thin the existing oxide film. Additions of zinc will also reduce the oxide film thickness. However, it may be desirable to halt the zinc additions during the palladium doping process since zinc appears to densify the film. The formation of ZnO on alloy surfaces has been shown to yield many benefits in BWRs, including reduced incorporation of Co.sup.60 in films (thereby lowering the radiation level, e.g., in piping) and reduced susceptibility to SCC. A further aspect of the present invention is that cycling the temperature during the palladium doping process (e.g., by repeatedly raising the water temperature to 550.degree. F. and then cooling the water to 100.degree. F.) should be beneficial, since the solubility of the metal oxides, film thickness and semiconducting properties of the oxide film change with change in temperature. This may be especially valuable following zinc exposure, since zinc desorbs from the oxide films at lower temperatures, providing more sites for the deposition of palladium and more opportunities for film growth. The advantage of the method of the invention, in which the oxide film on alloy surfaces is removed or thinned before palladium deposition, is that palladium is distributed throughout the oxide film in the thickness direction. In contrast, when pre-oxidized alloy surfaces are treated with, e.g., palladium acetylacetonate, the palladium is deposited only on the surface of the oxide. If this deposited palladium is removed from the surface, e.g., by very high flow rates of the reactor coolant, the catalytic response of the surface coating with palladium is decreased, whereas in the case of co-deposition of palladium during oxide film growth, the catalytic response may be sustained due to the presence of palladium species throughout the thickness of the oxide film. Cylindrical coupons of as-machined Type 304 stainless steel were exposed in 288.degree. C. water containing about 300 ppb O.sub.2 for 16 hr. Thereafter, the coupons were exposed in 288.degree. C. water containing about 300 ppb O.sub.2 and 100 ppb Pd as palladium acetylacetonate for 6-8 hours. This cycle was repeated six times. During palladium doping cycles, palladium acetylacetonate was injected. During oxidizing cycles, palladium acetylacetonate was not injected and the palladium acetylacetonate injected during the doping cycle had been removed by the water cleanup system. During the doping cycle, palladium deposits on the high-temperature oxide film and as this oxide films thickens over time, palladium is incorporated throughout the layer of oxide in the thickness direction. However, the palladium concentration in the thickness direction of the oxide film varies as a function of the amount of palladium in the solution in which the coupon is exposed. During this experiment, the incorporation of palladium was observed by depth profiling the Auger electron spectroscopy of the as-exposed surface. The cyclical variation of the palladium doping in the thickness direction can be seen in FIG. 7. The excellent corrosion potential response of this palladium co-deposited specimen is shown in FIG. 8 by the sharp decrease in corrosion potential at H.sub.2 /O.sub.2 molar ratios in the range of about 1.5-2. The method of the present invention can also be used to dope oxide films on reactor components with corrosion-inhibiting non-noble metal. In accordance with this method, the component or structural material is immersed in a solution or suspension of a compound containing the non-noble metal. The non-noble metal must have the property of increasing the corrosion resistance of the stainless steel or other metal surface when incorporated therein or deposited thereon. The selected compound must have the property that it decomposes under reactor thermal conditions to release species of the selected non-noble metal which incorporate in or deposit on the oxide film formed on the stainless steel or other metal surfaces. The non-noble metals which can be used are selected from the group consisting of zirconium, niobium, yttrium, tungsten, vanadium, titanium, molybdenum, chromium and nickel. The preferred compounds in accordance with the invention are those containing zirconium, e.g., the organometallic compounds zirconium acetylacetonate and inorganic compounds zirconium nitrate and zirconyl nitrate. The present invention offers the advantage that alloy surfaces can be doped with palladium or other metal using an in-situ technique (while the reactor is operating) which is simple in application and also inexpensive. However, this technique can also be implemented for coating ex-situ components. In addition, the technique can be applied to operating BWRs and PWRs and their associated components, such as steam generators. The foregoing method have 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 mitigating stress corrosion cracking. For example, noble metals other than palladium can be applied using this technique. The noble metal can be injected in the form of an organic or inorganic compound in conjunction with injection of small amounts of hydrogen to reduce the potential of stainless steel reactor components. One option is to inject the palladium acetylacetonate solution or suspension via the same port by which dissolved hydrogen is injected. The corrosion-inhibiting non-noble metals can be used even in the absence of hydrogen injection. In addition, the doping technique of the invention is not restricted to use with stainless steel surfaces, but also has application in reducing the ECP of other metals which are susceptible to IGSCC, e.g., nickel-based alloys. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.