Patent Number: 
Section: description

The fluid flow in a boiling water reactor will be generally described with reference to FIG. 1. Feedwater is admitted into a reactor pressure vessel (RPV) 10 via a feedwater inlet 12 and a feedwater sparger 14, which is a ringshaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. A core spray inlet 11 supplies water to a core spray sparger 15 via core spray line 13. The feedwater from feedwater sparger 14 flows downwardly through the downcomer 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. 1). Each fuel assembly is supported at the top by top guide 19 and at the bottom by core plate 21. Water flowing through downcomer annulus 16 then flows to the core 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 core upper plenum 26 under shroud head 28. Core upper plenum 26 provides standoff between the steam-water mixture exiting core 20 and entering vertical standpipes 30, which are disposed atop shroud head 28 and in fluid communication with core 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 feedwater in the mixing plenum 33, which mixture then returns to the core via the downcomer annulus. 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 The steam is withdrawn from the RPV via steam outlet 38.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 water outlet 43 and forced by a centrifugal 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 invention is based on the discovery that it is possible to control the amount of metals deposited on an oxided metal surface in high temperature water, as well as the ratio of metal deposit from a mixture of metals, by careful choice of the temperature of the water, concentration of the metal and time. In the following discussion, for convenience of description, reference will be made to the use of platinum as a typical noble metal. When mixtures are being considered, platinum and rhodium will be described for ease of reference. It is understood, however, that the invention is not limited to the use of platinum and rhodium, and other platinum group and/or non-platinum group metals may be used alone or as mixtures. Compounds of the platinum group metals are preferred. The term xe2x80x9cplatinum group metalxe2x80x9d, as used herein, means platinum, palladium, osmium, ruthenium, iridium, rhodium and mixtures thereof. It is also possible to use compounds of non-platinum group metals, such as for example zinc, titanium, zirconium, niobium, tantalum, tungsten and vanadium. Mixtures of platinum group compounds may also be used. Mixtures of platinum group compounds and non-platinum group compounds may also be used in combination, for example platinum and zinc. The compounds may be organometallic, organic or inorganic and may be soluble or insoluble in water (i.e. may form solutions or suspensions in water and/or other media such alcohols and/or acids). Generally, when mixtures of platinum and non-platinum group metals are used, the platinum group metal is in excess of the other metal. Examples of preferred platinum group metal compounds which may be used are palladium acetyl acetonate, palladium nitrate, palladium acetate, platinum acetyl acetonate, hexahydroxyplatinic acid, Na2Pt(OH)6, Pt(NH3)4(NO3)2, Pt(NH3)2(NO3)2, K3Ir(NO2)6 and K3Rh(NO2)6. Other examples are platinum(IV) oxide (Pt(IV)O2), platinum(IV) oxide-hydrate (Pt(IV)O2.xH2O, where x is 1-10), rhodium(II) acetate (Rh(II)ac2), Rh(III) nitrate (Rh(III)(NO3)3), rhodium(III) oxide (Rh(III)2O3), rhodium(III) oxide-hydrate (Rh(III)2O3.xH2O, where x is 1-10), rhodium(II) phosphate (Rh(III)PO4) and rhodium(III) sulphate (Rh(III)2(SO4)3). Examples of mixtures of the compounds which may be used are mixtures containing platinum and iridium, and platinum and rhodium. Use of such mixtures results in incorporation of noble metals on the oxided stainless steel surfaces of both noble metals. The presence of iridium or rhodium with the platinum gives good long-term durability. It has been found that a combination of about 40-80 ppb Pt and 10-35 ppb Rh, for example concentrations of about 60 ppb Pt and about 20 ppb Rh in water, provides good adherent properties over extended periods of time. The metal compound may be injected in situ in the form of an aqueous solution or suspension, or may be dissolved in the water before the metal surface to be treated is introduced. As used in the claims hereafter, the term xe2x80x9csolutionxe2x80x9d means solution or suspension. Solutions and suspensions may be formed using media well known to those skilled in the art. Examples of suitable media in which solutions and/or suspensions are formed, are water, alkanols such as ethanol, propanol, n-butanol, and acids such as lower carboxylic acids, e.g. acetic acid, propionic acid and butyric acid. FIGS. 2A and 2B show the effect of variation of temperature on metal deposit loading rate as well as the effect of distance from the point of introduction of the compound to the region of deposit on the metal surface. As demonstrated in FIGS. 2A and 2B, surprisingly enhanced loading is observed over the temperature range of 200xc2x0 to 500xc2x0 F., more especially in the range of 300xc2x0 to 450xc2x0 F., and particularly at about 340xc2x0 to 360xc2x0 F. As seen from FIGS. 2A and 2B, the loading observed in the temperature range of 300 to 450xc2x0 F. extends from about 10 xcexcg/cm2 at about 300xc2x0 F. to a maximum of about 62 xcexcg/cm2 at about 340xc2x0 F., and then drops off to about 10 xcexcg/cm2 and lower as the temperature rises towards 500xc2x0 F. This peaking effect is surprising and affords the advantage that loading of the metal species on the metal surface can be controlled by careful selection of the water temperature and point of introduction of the metal to be deposited. When the metal compound solution or suspension enters the high-temperature water, the compound decomposes very rapidly to produce atoms, which are incorporated into the metal (typically stainless steel) oxide film. In accordance with the process, only the solution or suspension of the compound is introduced into the high-temperature water initially. No further agents, such as hydrogen, other reducing agents, acids or bases are introduced into the high-temperature water when the compound solution or suspension is injected into and decomposes in the high-temperature water. FIG. 3 shows the effect of temperature on the ratio of platinum and rhodium deposited on the metal surface. The presence of rhodium renders the deposit more durable. As the temperature reaches 300xc2x0 to 500xc2x0 F., the ratio of deposited platinum to rhodium drops to within the range of about 5:1 to 10:1. Thus, knowing this relationship, it is possible to control the ratio of platinum to rhodium in the deposited layer based on the prevailing temperature conditions of the water. FIG. 4 shows that the deposition rate for a 60 ppb platinum and 20 ppb rhodium solution is a negative exponential with temperature in the 180 to 350xc2x0 F. range. From this it is possible to predict the effect of temperature on the ratio of deposit of the metals and the time required to deposit a given quantity of noble metal in the oxide. Higher xcex94E for rhodium indicates slower rhodium deposition rate. This figure can be used to select the conditions required to select the conditions required for depositing the desired platinum/rhodium ratio and quantity FIGS. 5 and 6 show that the deposition rate is approximately linear within the concentration range investigated (0-60 ppb). FIGS. 7A and 7B show the deposition of platinum and rhodium is approximately linear with time. The bulk concentration of platinum and rhodium, time and temperature are the variables that can be used to produce a desired platinum to rhodium deposit ratio and total noble metal loading. The process of the present invention is distinguished from the processes of U.S. Pat. Nos. 5,130,080 and 5,130,181 to Niedrach. The Niedrach patents teach that it is possible to electrolessly plate oxide films using conventional electroless plating techniques. Conventional electroless plating is carried out at relatively low temperatures, typically in the region of 50 to 80xc2x0 C., possibly lower, and requires the presence of an added reducing agent, typically sodium hypophosphite, to supply electrons for reduction of the noble metal ions to the metal. The reaction takes place only on a catalytic surface which has been sensitized/activated beforehand, for example with stannous chloride, and the process results in a build-up of metal coating on the surface which eventually coats the entire surface with deposited metal. The electroless plating bath typically contains high ionic concentrations, of the order of thousands of ppm, of chemicals, including, for example, palladium (II) chloride, ammonium hydroxide, ammonium chloride, disodium EDTA and hydrazine, as well as a reducing agent (e.g. sodium hypophosphite). The pH of the electroless bath is usually in the region of 9.0 to 10.5 in view of the presence of base (ammonium hydroxide and ammonium chloride). The process of the present invention does not rely on the use of electroless plating techniques or other techniques which result in the metal being plated on the oxide surface. In the present process, the metal compound or mixture of metal compounds is introduced into the high-temperature water in an amount such that the concentration of the metal(s) in the water is very low, i.e. in the ppb range, but is sufficient such that when present on the metal component, the ECP is lowered below the critical potential required for protection from stress corrosion cracking. Typically, the metal compound is added in such an amount to produce a metal concentration of no higher than 2000 ppb, for example 0.1 to 1000 ppb, typically 1 to 500 ppb, more usually 5 to 100 ppb. The compound solution or suspension may be injected into the high-temperature water while the reactor is operating and generating nuclear heat (full power operation), or during cool down, during outage, during heat-up, during hot standby, or during low power operation. The noble metal may be introduced into residual heat removal (RHR) piping, recirculation piping, feedwater line, core delta P line, jet pump instrumentation line, control rod drive cooling water lines, water level control points, or any other location which provides introduction of the noble metal into the reactor water and good mixing with the water. As used herein, the term xe2x80x9chigh-temperature waterxe2x80x9d in the present invention means water having a temperature of about 200xc2x0 F. or greater, steam, or the condensate thereof. High temperature water can be found in a variety of known apparatus, such as water deaerators, nuclear reactors, and steam-driven power plants. The temperature of the water when noble metal is added to the reactor water is typically in the range of 200-500xc2x0 F., for example 200-450xc2x0 F., more usually about 340xc2x0-360xc2x0 F. When the compound is in the high-temperature water, it decomposes very rapidly and the metal atoms are incorporated in the oxide surface. At the very low levels of metal(s) introduced into the reactor, the stainless steel oxide surface is not covered completely with metal. Typically, the oxide surface has metal present in an amount of about 0.1-15 atomic %, for example 0.5-10 atomic %, more usually 2-5 atomic %. The depth of metal in the oxide surface is generally in the range of 100 to 1000 Angstroms, more usually 200 to 500 Angstroms. The external appearance of the oxided alloy treated according to the present process does not differ from the appearance of untreated stainless steel oxide. The noble metal containing surface does not have a bright metallic luster as is generally obtained with electroplating or electroless coating processes. In the present process, only very dilute compound solution or suspension is injected into the high-temperature water. No reducing agents (including hydrogen), acids and bases, are added. As a result, the typical pH of the water at ambient temperature is in the region of 6.5 to 7.5, and at higher operating temperatures is lower, generally in the region of about 5.5-5.8, for example 5.65. This is due to increased dissociation of the water at the higher temperatures. An operating BWR has very stringent coolant water conductivity levels which must be observed. Typically, the conductivity of the coolant water must not exceed 0.3 xcexcS/cm, and more usually must be less than 0.1 xcexcS/cm. Such conductivity levels are adversely impacted by high concentrations of ionic species, and every effort is made in the present process to ensure that reactor ionic concentrations are maintained as low as possible after clean-up, preferably less than 5 ppb. The process in particular excludes the use of chloride ion in view of its corrosive nature. The present process does not involve any catalytic activation/sensitization of the stainless steel oxide surface. The use of stannous chloride to achieve such activation would be incompatible with operation of the BWR and the stringent conductivity limits on the coolant water referred to above. While not being bound by theory, it is understood that the metal, for example platinum and/rhodium, is incorporated into the stainless steel oxide film via a thermal decomposition process of the compound wherein metal ions/atoms apparently replace iron, nickel and/or chromium atoms in the oxide film, resulting in a metal-doped oxide film. The metal, such as platinum/rhodium, may for example be incorporated within or on the surface of the oxide film and may be in the form of a finely divided metal. The oxide film is believed to include mixed nickel, iron and chromium oxides. The ECPs of the stainless steel components all drop by approximately 0.30 V after injection of the noble metal and subsequent addition of low levels of hydrogen. It is possible to reduce the ECP of Type 304 stainless steel to IGSCC protection values without injecting hydrogen when an organic metal compound has been injected into the water. The catalytic oxidation of organics on noble metal-doped surfaces consumes oxygen, thereby lowering the dissolved oxygen content in the high temperature water. Good results are also obtained when an inorganic metal compound(s) is used. Moreover, clean-up of the water is easier when inorganic(s) such as nitrates are used as compared to organics such as formates and acetates. For this reason, inorganic compounds, particularly inorganic platinum group metal compounds (e.g. noble metal nitrates and nitrites), are typically used. Following injection and incorporation of the metal(s) in the oxided stainless steel surfaces, the water is subjected to a conventional clean-up process to remove ionic materials such as nitrate ions present in the water. This clean-up process is usually carried out by passing a fraction of the water removed from the bottom head of the reactor and recirculation piping through an ion exchange resin bed, and the treated water is then returned to the reactor via the feedwater system. Hydrogen may subsequently be introduced into the water some time after the doping reaction, for example 1 to 72 hours after injection and incorporation of the metal atoms in the oxided surface, to catalyze recombination of hydrogen and oxygen on the metal doped surfaces. As hydrogen is added, the potential of the metal-doped oxide film on the stainless steel components is reduced to values which are much more negative than when hydrogen is injected into a BWR having stainless steel components which are not doped with the noble metal. The noble metal-containing compound is injected in situ into the high-temperature water of a BWR in an amount such as to produce, upon decomposition of the compound, a metal concentration of up to 2000 ppb, for example about 1 to 850 ppb, more usually 5 to 100 ppb. Preferably, the palladium compound is injected at a point downstream of the recirculation water outlet 43 (see FIG. 1). The high temperatures as well as the gamma and neutron radiation in the reactor core act to decompose the compound, thereby freeing noble metal ions/atoms for deposition on the surface of the oxide film. As used herein, the term xe2x80x9catomsxe2x80x9d means atoms or ions. It has been shown in other commonly assigned cases, for example U.S. Ser. No. 08/635,539, filed Apr. 22, 1996 (herein incorporated by reference) that palladium treatment in accordance with the invention, the ECP value of the stainless steel surfaces remains quite negative and below the required IGSCC protection potential of xe2x88x920.230 V(SHE) even without the addition of any hydrogen when organics are present in the water. The noble metal injection solution may be prepared for example by dissolving the noble metal compound in ethanol. The ethanol solution is then diluted with water. Alternatively, a water-based suspension can be formed, without using ethanol, by mixing the noble metal compound in water. The noble metal either deposits or is incorporated into the stainless steel oxide film via a thermal decomposition process of the noble metal compound. As a result of that decomposition, noble metal ions/atoms become available to replace atoms, e.g., iron atoms, in the oxide film, thereby producing a noble metal-doped oxide film on stainless steel. The present invention offers the advantage that steel surfaces can be doped with noble metal using an in situ technique (while the reactor is operating) which is simple in application and also inexpensive. However, the technique is not limited to in situ application. The application technology can be implemented even for doping ex situ components. The technique can be applied to operating BWRs and PWRs and their associated components, such as steam generators. In practice, the noble metal concentration in the reactor water is preferably in the range of 1 to 1000 ppb, for example 2 to 900 ppb, more usually 5 to 100 ppb. 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 hydrogen water chemistry. For example, metals other than platinum/rhodium can be applied using this technique, e.g., other platinum group metals. A platinum group metal can be injected in the form of an organic, organometallic or inorganic compound to reduce the potential of stainless steel reactor components even in the absence of hydrogen injection. Alternatively, the platinum group metal can be injected in the form of an inorganic compound to reduce the potential of stainless steel reactor components. It may also be possible to dope oxide films on stainless steel components with non-platinum group metals, e.g., zirconium and titanium, using the technique of the invention. All such variations and modifications are intended to be encompassed by the claims set forth hereinafter.