Patent Application
20070003001
The present invention relates to a method for mitigating, minimizing or eliminating deposition of metal or metal oxide particles on various components in high temperature/flow water, by applying one or more noble metals to the surface, such that the suspended particles and the component surface interact to preclude or minimize fouling of the component surface.
Typically, during the operation of boiling water nuclear reactors (BWRS) under normal water chemistry conditions containing high oxidizing species, such as oxygen and hydrogen peroxide, intergranular stress corrosion cracking (IGSCC) of reactor components e.g. sensitized 304 stainless steel, is known to be a major environment-related material performance concern. It has also been observed that a thick, dense layer of crud, (mostly metal oxides) is deposited on components exposed to high temperature/high flow water, such as a jet pump inlet mixer assembly for the BWR. This crud buildup substantially reduces water velocities, results in reduction in core flow capability, and thus, at refueling outages, the crud deposited surfaces typically need to be cleaned.
Particularly, and as a representative example, a recirculation pump in a BWR causes a downward flow of coolant in the annular space between the core shroud and the reactor pressure vessel wall. The coolant is pumped to a high pressure, and distributed through a manifold to the jet pumps, where the coolant flows in a upward direction through the jet pump risers. The coolant then splits in a transition piece, changes direction, and is accelerated downwardly through the nozzles and into a mixer section of the jet pump. The nozzles cause a high velocity coolant flow that is approximately one third of the core flow and discharges into the throat section of the inlet-mixers.
Over time, an oxide layer builds-up on the inside of the inlet-mixers including the jet pump nozzles, forming a layer of crud. There is also the potential of stress corrosion cracking along these surfaces. The build-up of crud is believed to be caused by electrically charged metallic/metal oxide particles suspended in the coolant which interact with the metallic inner surface of the inlet-mixer, including a triboelectrostatic charge on the surface. This charge creates an electrostatic potential that attracts the suspended particles in the coolant to the metallic surface where they form a layer. The highest deposition of crud is observed in areas that experience the highest flow rates.
As the crud layer becomes excessive, the performance of the recirculation system will be degraded. This degradation will also adversely affect the efficiency of the plant because the recirculation pumps must be run at a higher speed to maintain core flow. Degradation of jet pump performance can further result in extreme vibration and damage to jet pump components. Eventually, the inlet-mixer must be mechanically or chemically cleaned or replaced during regular maintenance and refueling outages, which is costly and time consuming.
Consequently, it is important that the crud layer be eliminated, substantially minimized or its rate of build-up curtailed. One such method for accomplishing that objective is disclosed in U.S. Pat. No. 6,633,623 issued Oct. 14, 2003 of common assignee herewith. Under normal water chemistry conditions, and according to that patent, the fouling of metal oxides on component surfaces can be reduced by applying a ceramic coating to the surfaces. For example, a coating formed of TiO2 or Ta2O5, SiO2 or yttria stabilized zirconia may be applied to the surfaces. That dielectric coating reduces the electrical potential between the metal of the component surfaces e.g., an inlet mixer and the charged metallic particles in the water minimizing or eliminating the build-up of crud on the surfaces of the inlet mixer. While that dielectric coating has been demonstrated to satisfactorily reduce the deposition of crud on the component surfaces, it has been found less effective for use on component surfaces in hydrogenated water. Accordingly, there is a need to provide apparatus and methods to mitigate, reduce or eliminate the deposition of metal or metal oxide particles in high temperature/flow water on component surfaces in hydrogen water chemistry (HWC). The present invention seeks to provide a solution to this problem.
In a preferred embodiment of the present invention, there is provided a method of reducing, minimizing or eliminating deposition of charged particulates on metal surfaces of a component subject to high temperature/high flow water, the method comprising the step of applying at least one noble metal or an alloy thereof to the surface.
In a further preferred embodiment of the present invention, there is provided a method of reducing, minimizing or eliminating deposition of charge particulates on interior metal wall surfaces defining a coolant flow passage in a jet pump for a nuclear reactor comprising the step of: depositing one of a noble metal or noble metal alloy on the interior metal wall surfaces of one of a nozzle and a mixing section forming part of an inlet mixer of the jet pump for disposition in the radioactive environment of the nuclear reactor to reduce, minimize or eliminate an electric potential between the metal wall surfaces and the charged particulates.
In another embodiment of the invention, there is provided a method for protecting interior metal wall surfaces defining a coolant water flow passage in a jet pump for a nuclear reactor comprising steps of reducing, minimizing or eliminating deposition of charged particulates in the coolant water flowing through the jet pump on the metal wall surfaces by depositing a noble metal or a noble metal alloy on the interior metal wall surfaces of one of a nozzle and a mixing section forming part of an inlet mixer of the jet pump, thereby to reduce, minimize or eliminate any electric potential between the metal wall surfaces and the charged particulates.
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In accordance with a preferred embodiment of the present invention, the inlet-mixer 23 is provided with a coating that inhibits or eliminates “crud” build-up. To accomplish this, one of the noble metals is applied to the component surface e.g., the surfaces of the inlet/mixer 23. Platinum is a preferred noble metal for use in this application, although other noble metals such as rhodium, iridium, ruthenium, palladium, silver and gold or noble metal alloys, or chemicals containing one or more noble metals thereof may be applied to the component surface. Various processes may be utilized to apply the noble metal or noble metal alloys to the surfaces such as plasma spray, chemical vapor deposition, physical vapor deposition, HVOF, electroplating or electroless plating. It will be appreciated that in hydrogenated water chemistry, the noble metal surface has the same charge of electric potential as metal oxides of structural materials.
The dielectric coating noted in the above-identified patent works reasonably well in water containing a certain magnitude of oxygen and hydrogen. However, the ceramic coating does not completely mitigate or eliminate fouling i.e., deposition of crud on the component surfaces, mainly due to unknown water chemistry conditions. The reduced oxygen content in those BWRs using hydrogenated water is beneficial in many respects including improving the longevity of the piping and reactor internals by reducing the IGSCC susceptibility. However, deposition of crud and fouling in BWRs using hydrogenated water has remained a problem. By applying a noble metal,a noble metal alloy, or a noble metal chemical coating to the component surface, for example platinum to a thickness about of between 0.1 μm-10 μm, the resulting coating mitigates, reduces or eliminates crud deposition i.e. fouling, particularly in BWRs using reduced oxygen or hydrogenated water chemistry. It is believed that by applying a noble metal surface coating, the sign of the charge on the noble metal coated-surface changes to the same one as the suspended metal or metal oxide particles in the hydrogenated water and are either less attracted to or repelled by the component surface, having the noble metal or noble metal alloy coating.
As a representative example, typical fouling materials in solution may include Fe2O3 and Fe3O4, Fe3O4-type spinels such as Fe-chromate, Ni-ferrite, these of which develop a surface charge. The potential difference between the shear plane and the bulk solution is identified as the zeta potential. The zeta potential is a characteristic of the solid substrates/electrolytic solution system. The zeta potentials of the charged fouling material in the component surface must therefore be opposite in sign for fouling or deposition of crud to occur. The zeta potential-of a given particle of metal oxide or hydroxide depends on the metal element, the oxidation state, the degree of oxide hydration and H+/OH− concentrations. The pH at which the surface charge disappears is called point of zero charge (PZC). This corresponds to the isoelectric point of surface where the surface has an equal tendency to release positive and negative ions. Metal oxides in normal water chemistry have an IEPS in excess of 7 providing a positive surface charge, whereas metal oxides in the same water have an IEPS less than 7 form a negative surface charge. The surfaces of oxide particles dispersed in water tend to coordinate water molecules to form hydroxylated surfaces. The surfaces may become positively or negatively charged depending on pH. The-stability of oxide particles is related to the zeta potential of the oxide particles. The zeta potential is positive at low pH and negative at high pH. Thus, by controlling the zeta potential by applying a surface coating of noble metal or noble metal alloys thereof to the component surfaces, a neutral or negative surface charge is provided. As a consequence, it will be appreciated that the coating material i.e. noble metal or noble metal alloys has the same sign or close to the same sign on the component surface as the sign of the metallic oxide particulates in the solution. Stated differently, the coating material avoids the opposite sign of the particulate charge.
Consequently, with the application of noble metal, noble metal alloys, or noble metal chemical coating to the component surfaces, the fouling or crud deposition onto the component surfaces is mitigated or eliminated which eliminates or minimizes the need to clean the component surfaces. Moreover, the coating is durable in the high flow environment, does not delaminate and does not erode or corrode the piping. The coating is also benign to the system and cost effective.
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.
