Patent Number: 
Section: description

The fluid flow in a boiling water reactor will be generally described with reference to FIG. 2. Feed water is admitted into a reactor pressure vessel (RPV) 10 via a feed water inlet 12 and a feed water sparger 14, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feed water inside the RPV. A core spray inlet 11 supplies water to a core spray sparger 15 via core spray line 13. The feed water from feed water sparger 14 flows downwardly through the down corner annulus 16, which is an annular region between RPV 10 and core shroud 18. Core shroud 18 is a stainless steel cylinder, which surrounds the core 20 comprising numerous fuel assemblies 22 (only two 2xc3x972 arrays of which are depicted in FIG. 2). Each fuel assembly is supported at the top guide 19 and at the bottom by core plate 21. Water flowing through downcomer annulus 16 then flows to the reactor lower plenum 24. The water subsequently enters the fuel assemblies 22 disposed within core 20, wherein a boiling boundary layer (not shown) is established. A mixture of water and steam enters reactor upper plenum 26 under shroud head 28. Reactor upper plenum 26 provides standoff between the steam-water mixture exiting core 20 and entering vertical standpipe 30, which are disposed atop shroud head 28 and in fluid communication with reactor upper plenum 26. The steam-water mixture flows through standpipes 30 and enters steam separators 32, which are of the axial-flow centrifugal type. The separated liquid water then mixes with feed water in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus and/or through jet pump assemblies. The steam passes through steam dryers 34 and enters steam dome 36. The steam is withdrawn from the RPV via steam outlet 38. The BWR also includes a coolant recirculation system, which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 16 via recirculation pump (not shown) into jet pump assemblies 42 (only one of which is shown) via recirculation water inlets 45. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The pressurized driving water is supplied to each jet pump nozzle 44 via an inlet riser 47, an elbow 48 and an inlet mixer 46 in flow sequence. A typical BWR has 16 to 24 inlet mixers. The present disclosure is directed to a method of reducing the electrochemical corrosion potential of a component (i.e., structural material) exposed to high temperature water in a hot water system, which includes providing a reducing species to the high temperature water, providing a plurality of catalytic and dielectric nanoparticles to the high temperature water, wherein the catalytic nanoparticles provide a catalytic surface on which the reducing species reacts with the at least one oxidizing species present in the high temperature water, and wherein the dielectric nanoparticles provide insulative protection to the surfaces, regardless of the water chemistry conditions, and correspondingly reducing the electrochemical corrosion potential of the component. The introduction of the catalytic and dielectric nanoparticles into the high temperature water of reactors, such as the BWR described above, advantageously protects the reactor components and reduces the oxidizing properties of the high temperature water. The nanoparticles comprise both dielectric nanoparticles and catalytic nanoparticles and/or nanoparticles having both dielectric and catalytic properties, i.e., each nanoparticle possesses both a catalytic functionality and a dielectric functionality. Although described with respect to a BWR, the present disclosure is not intended to be limited to use in BWRs and is applicable to the primary and secondary sides of PWRs. Other suitable structures include those structural components that are exposed to high temperature water environments. Such structures include pressurized water reactors, steam driven turbines, water deaerators, and the like. As used herein, the term high temperature water refers to water having a temperature between about 50xc2x0 C. and about 320xc2x0 C., and preferably, between about 50xc2x0 C. and about 290xc2x0 C. As used herein, the term xe2x80x9cnanoparticlesxe2x80x9d is defined as discrete particles with average diameters less than about 100 nanometers (nm). More preferably, the nanoparticles have diameters of about 1 nm to about 100 nm, and with about 5 nm to about 50 nm even more preferred. Due to the large fraction of atoms located at the surface, nanoparticles possess very unique electrical, magnetic, mechanical, and optical properties, such as, but not limited to, increased surface area and the ability to form colloidal suspensions. Particles having a diameter of about 9 nm, for example, may have a surface area of about 97 m2/g when fully dispersed. In the present disclosure, the nanoparticles preferably have a surface area of about 1 m2/g to about 300 m2/g, and more preferably, a surface area of about 10 m2/g to about 100 m2/g. The nanoparticles generally comprise both dielectric non-noble metal nanoparticles and a catalytic noble metal nanoparticles, and/or nanoparticles comprising a mixture of both catalytic noble metal and dielectric non-noble metals, i.e., each nanoparticle is comprised of both catalytic and dielectric functional materials. While not wanting to be bound by theory, it is believed that upon introduction of the nanoparticles in the high temperature water, the nanoparticles are colloidally dispersed in the water to provide protection to the reactor components from the detrimental effects of the high temperature water. In particular, the catalytic nanoparticles catalytically increase the efficiency of the recombination kinetics for hydrogen and oxygen to lower the electrochemical corrosion potential of the water. The dielectric nanoparticles provide insulative barrier protection properties to reactor surfaces proximate to or in contact therewith. Some of the nanoparticles, e.g., catalytic noble metal component and/or dielectric component, may also deposit onto surfaces of the components in contact with the high temperature water to provide continued protection. Once deposited onto the component surfaces, the nanoparticles may redeposit onto other component surfaces during operation or become colloidally dispersed in the high temperature water. Upon introduction of the nanoparticles to the reactor, the nanoparticles are colloidally dispersed throughout the water and are responsive to electrostatic forces in the water. As a result, redistribution of the nanoparticles can occur on various component surfaces of the reactor. In addition, it has been found that the catalytic efficiency is greatly improved due to the increased surface area provided by the use of nanoparticles, relative to coated articles. Thus, the presence of the catalytic nanoparticles and/or the dielectric nanoparticles can reduce the oxidizing power of the water and at the same time, can lead to the formation of an insulated and/or catalytic deposits on surfaces of the reactor components. In addition, such nanoparticles are capable of penetrating or diffusing into the existing crevice and thus inhibit further growing. Examples of suitable metals for forming the various reactor components to be protected are nickel based alloys, cobalt based alloys, titanium based alloys, copper based alloys, and ferrous and non-ferrous alloys. Carbon steels and low alloy steels are further examples. In a preferred embodiment, a mixture of nanoparticles including both dielectric nanoparticles and catalytic nanoparticles are introduced into the high temperature water. In another embodiment, each nanoparticle is fabricated as a mixture of both the dielectric component and the catalytic component, wherein all or a portion of the catalytic component contacts the high temperature water upon immersion therein. Advantageously, the use of the nanoparticles as disclosed herein reduces the electrochemical corrosion potential without the need to continuously monitor variables such as dissolved hydrogen/oxygen levels, flow rates, temperature gradients, radiation fluxes, and the like, which are generally difficult to accurately monitor in BWRs and other like reactors. The nanoparticles may have a variety of morphologies, including single-lobed such as spherical, substantially spherical, cigar-shaped, rod-shaped and moon-shaped, and multi-lobed such as tetrahedral, raspberry, acorn, dumb-bell, and the like. The size distribution of the nanoparticles may be monodisperse, bimodal, or polydisperse. In a preferred embodiment, the nanoparticles have an average diameter less than about 100 nanometers. The nanoparticles are formed using conventional techniques leading to a wide variation in the amount of agglomeration of particles. As those skilled in the art will appreciate, the stoichiometry of the metals (non-noble metals and noble metals) will establish the ratio of the metal in the final product Typically, nanoparticles need to be dispersed to take advantage of their unique properties. Particle dispersion can be divided into three stages: wetting; separation of particles; and stabilization. Once wetted, the breakdown of agglomerates is usually achieved by collision or attrition. Methods used to disperse the nanoparticles include ultrasonic energy, vigorous mixing, vigorous spraying, and the like. Nanoparticles, once dispersed, can remain in a colloidal suspension indefinitely due to Brownian motion. Oxidizing species present in the high temperature water include, but are not limited to, oxygen (O2), hydrogen peroxide (H2O2), and various radicals, such as OH-, and the like. Reducing species include, but are not limited to, hydrogen (H2), hydrazine (N2H2), ammonia (NH3), alcohols, and the like. In a preferred embodiment, a catalytic nanoparticle provides a catalytic surface upon which hydrogen reacts with oxygen and hydrogen peroxide to form water. The reductants may already be present in the high temperature reactor water in equilibrium concentrations. Alternatively, the reductants may be introduced into the high temperature water and dissolved therein. In one such embodiment, an amount of hydrogen gas is introduced into the high temperature water such that the ratio of H2O2in the high temperature water has a value determined by weight of about 1:8. The dielectric nanoparticles preferably comprise a non-noble metal material. Suitable dielectric materials for fabricating the nanoparticles include, but are not intended to be limited to, inorganic or organometallic compounds, metals, zeolites, metal oxides, and the like. Examples of non-noble metals include zirconium, hafnium, niobium, tantalum, yttrium, ytterbium, tungsten, vanadium, titanium, molybdenum, chromium, cerium, germanium, scandium, lanthanum, and nickel. It is also possible to use non-noble metals that possess conducting or semiconducting properties such as carbon, or silicon. The non-noble metal identified above can be used alone or in admixture with other non-noble metals or non-metals. The catalytic nanoparticles preferably comprise at least one of platinum, palladium, osmium, rhodium, ruthenium, iridium, oxides, nitrides, borides, phosphides and mixtures of these metals. Preferably, the plurality of catalytic nanoparticles comprises at least one of palladium, platinum, rhodium, and combinations thereof. Additionally, the plurality of catalytic nanoparticles may comprise other chemical compounds containing at least one of platinum, palladium, osmium, ruthenium, iridium, and rhodium. Such compounds include intermetallic compounds formed with other elements. The ratio of catalytic nanoparticles to dielectric nanoparticles will depend on the desired application and can vary widely as any ratio can be employed. Upon introduction into the reactor, the concentration of the catalytic nanoparticles is preferably less than about 100 parts per billion (ppb), preferably about 1 parts per trillion (ppt) to about 10 ppb, and even more preferably, about 10 ppt to about 1 ppb. The concentration of the dielectric nanoparticles is preferably less than about 100 ppb, preferably about 1 ppt to about 10 ppb, and even more preferably, about 10 ppt to about 1 ppb. In one embodiment of the present invention, the nanoparticles are deposited onto the component surfaces to provide a heterogeneous catalysis site and form a protective insulative layer. In another embodiment however, the plurality of nanoparticles are sufficiently buoyant to remain in a colloidal suspension in the high temperature water and act as homogenous catalysts for the reaction between oxidizing and reducing species within the high temperature water, and also provide insulative properties due to the proximity of the dielectric nanoparticles to the reactor surfaces. The presence of a colloidal suspension of nanoparticles having a high surface area in the BWR waterxe2x80x94when coupled with the presence of a stoichiometric excess of reductantxe2x80x94may cause an increase in radioactivity resulting from increased volatility of N-16 compounds that are produced by transmutation of O-16 to N-16 in the reactor core, otherwise known as xe2x80x9cturbine shine.xe2x80x9d This method of providing the nanoparticles to the high temperature water may require that injection of the reductant (e.g., H2) be temporarily suspended when the nanoparticles are initially introduced into the reactor to minimize the production of N-16 containing species. The electrochemical corrosion potential of the reaction components can be lowered in situ by providing the nanoparticles directly to the reactor feedwater, thus eliminating the need to remove the components for treatment with noble metal powders. The nanoparticles may be provided to the BWR feedwater during reactor operation, thus avoiding expensive and complicated BWR shutdowns. Alternatively, the nanoparticles may be added to the reactor feedwater during a scheduled reactor shutdown. Depending on the needs of the respective nuclear reactor, a predetermined amount of the nanoparticles can be introduced into the high temperature water in the reactor either continuously or incrementally at predetermined time intervals. Predetermined quantities of the catalytic nanoparticles can be introduced into the BWR to obtain a predetermined concentration of the catalytic nanoparticles in the high temperature reactor water. Several options are available for introducing the catalytic nanoparticles in situ into the thigh temperature water to reduce the electrochemical corrosion potential. The nanoparticles can be introduced homogeneously so as to create colloidal floaters within the BWR, wherein the nanoparticles remain in colloidal suspension indefinitely due to Brownian motion. Alternatively, the nanoparticles can be introduced heterogeneously such that the nanoparticles deposit on the BWR component surfaces. The nanoparticles may be provided to the high temperature water by first preparing a concentrated solution or suspension of the nanoparticles, using fluid media well known to those skilled in the art, and subsequently delivering the concentrated suspension to the reactor feedwater. Suitable media for forming such concentrated solutions or suspensions include, but are not limited to: water; alcohols such as methanol, ethanol, propanol, and n-butanol; and acids such as lower carboxylic acids, e.g. acetic acid, propionic acid, and butyric acid; or ketones such as acetone and acetylacetone; and combinations thereof. The nanoparticles may also be entrained in gaseous fluid media, such as air. Alternatively, the nanoparticles may be introduced in nondispersed metallic form into the reactor feedwater. The nanoparticles may be introduced into the high temperature water during various stages of operation of the reactor. The nanoparticles may be provided to the high temperature water in any of the embodiments described above during full power operation, cool down, outage, heat-up, hot standby, or low power operation of the reactor. Moreover, the nanoparticles may be introduced into the high temperature water at any location within the reactor structure where thorough mixing of the nanoparticles in the high temperature water can occur. The locations at which the nanoparticles may be introduced into the high temperature water include, but are not necessarily limited to, residual heat removal (RHR) piping, recirculation piping, feedwater lines, core delta P lines, jet pump instrumentation lines, control rod drive cooling water lines, water level control points, reactor water clean-up (RWCU) systems, and the like. The various lines may be either open or closed to the remainder of the coolant system during introduction of the catalytic nanoparticles. The temperature of the high temperature reactor water when the catalytic nanoparticles are introduced into to the reactor water is typically in the range between about 50xc2x0 C. and about 290xc2x0 C. for BWR reactors, and between about 50xc2x0 C. and about 320xc2x0 C. for PWR reactors. The temperature is generally in the range of 100-177xc2x0 C. and, most frequently, between about 170xc2x0 C. and about 185xc2x0 C. If the nanoparticle addition is performed at full power operation, the reactor water temperature is between about 270xc2x0 C. and about 290xc2x0 C. The following examples are provided to illustrate some embodiments of the present disclosure. They are not intended to limit the disclosure in any aspect. In this example, the catalytic effect of nanoparticle addition in a simulated BWR environment was studied by introducing the platinum nanoparticles into water held at 288xc2x0 C. and containing excess hydrogen. FIG. 3 graphically illustrates the corrosion potential behavior for Type 304 stainless steel electrodes as a function of immersion time in the 288xc2x0 C. high temperature water containing excess the hydrogen (molar ratio H/O=3.0). Prior to injection of 5 ppb of platinum nanoparticles, the corrosion potential was relatively constant at about xe2x88x9250 mV over a 5-day period. Upon addition of the catalytic nanoparticles, the corrosion potential steadily dropped over the next 20 days of monitoring indicating that the catalytic nanoparticles are highly effective in lowering electrochemical corrosion potential in a simulated BWR environment, i.e., a high efficiency of recombination kinetics for oxygen and hydrogen. Thus, the results indicate that the presence of the catalytic nanoparticles catalytically enhances the kinetics in the formation of water by oxygen and hydrogen present in the high temperature water, thereby reducing the electrochemical corrosion potential of the stainless steel electrodes. FIG. 4 is a scanning electron micrograph showing the size and distribution of the platinum nanoparticles on the Type 304 stainless steel oxide surface. The platinum nanoparticles were injected for 12 days to high temperature water at 288xc2x0 C. In this example, in situ deposition of a dielectric nanoparticle was conducted to study the effectiveness of longer in situ deposition times. Coupons of Type 304 stainless steel and alloy 600 were immersed in 288xc2x0 C. water containing 300 ppb of dissolved oxygen for a period of about 31 days. Test electrodes were first immersed in the 300 ppb dissolved oxygen water for a period of about 5 days. After about 5 days, 10 ppm ZrO(NO3)2 was continuously injected into the water for a period of about 14 days, wherein a dramatic initial reduction in ECP is observed for both specimens, as shown in FIG. 5. After about 22 days, the flow of 10 ppm ZrO(NO3)2 into the water was discontinued, wherein a marginal increase in electrochemical corrosion potential was observed. It is evident that the addition of ZrO(NO3)2 to 300 ppb dissolved oxygen water decreased ECP by 50 to 100 mV. Also, the presence of the high oxygen conditions may enhance formation of ZrO2.H2O on the surface of the specimens. Advantageously, the catalytic nanoparticles provide a high efficiency of recombination kinetics, while the dielectric nanoparticles restricts mass transport of oxidants through the oxide layer to the substrate. Moreover, the use of nanoparticles provides a colloidal behavior that permits redistribution to occur. Thus, the presence of the catalytic component on the surface of the dielectric component can reduce the oxidizing power of water, and simultaneously, provide an insulative layer onto surfaces of the reactor. While the disclosure has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the disclosure. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the disclosure without departing from the essential scope thereof. Therefore, it is intended that the disclosure not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this disclosure, but that the disclosure will include all embodiments falling within the scope of the appended claims.