Method for Coating Nuclear Power Plant Components

Abstract

A method for depositing divalent metal compounds on the surface of a nuclear power plant component, the component being a nickel-based or austenitic stainless steel alloy includes: providing within the component an aqueous treatment solution containing at least one soluble metal-containing compound such as a zinc salt and at least one source of oxygen; allowing the treatment solution to remain in the component until the compound is deposited on the wetted surface of the component; and, removing the aqueous solution after exposure. The treatment may be applied more than once, using more than one divalent metal compound, and the surface may further be exposed to a solution containing a noble metal species and a reducing agent. The treatment temperature is preferably below 100? C.

Claims

1. A method for treating a water-contacting surface of a nuclear power plant component, comprising the steps of: a) selecting a component to be treated; b) introducing an aqueous treatment solution into contact with the water contacting surface of said component, said treatment solution comprising: a source of divalent metal cations, a source of oxygen, and, a pH controlling agent; c) heating said treatment solution to a selected temperature; d) maintaining said treatment solution in contact with said water contacting surface for a selected time; and, e) draining said treatment solution from said component.

2. The method of claim 1 wherein said component comprises a material selected from the group consisting of: wrought austenitic Type 316 and 304 stainless steels including their low carbon varieties; nickel-based Alloy 600 or Alloy 690; weld metals including Type 309, Alloy 182, Alloy 82, Alloy 152, Alloy 52; and cast stainless steel materials CF3, CF3M, CF8, CF8M.

3. The method of claim 1 wherein said divalent metal cation is selected from the group consisting of: Zn.sup.2+ and Mn.sup.2+.

4. The method of claim 3 wherein said divalent metal cation comprises Zn and said Zn is depleted in Zn.sup.64.

5. The method of claim 1 wherein said source of divalent metal cations comprises a soluble salt selected from the group consisting of: metal acetates and metal nitrates.

6. The method of claim 1 wherein said source of oxygen comprises a species selected from the group consisting of: water; oxygen; ozone; and hydrogen peroxide.

7. The method of claim 1 wherein said pH controlling agent comprises a compound selected from the group consisting of: ammonia; ammonium hydroxide; ethylenediamine (EDA); triethanolamine (TEA); sodium hydroxide; and potassium hydroxide.

8. The method of claim 7 wherein said pH controlling agent is added in an amount sufficient to maintain said treatment solution in the range of pH>8.

9. The method of claim 1 wherein said selected temperature is 120? C. or less, and said selected time ranges from 10 minutes to 120 hours.

10. The method of claim 1 further comprising the step of: f) depositing a noble metal on said water contacting surface.

11. The method of claim 10 wherein said noble metal comprises Pt and said deposition uses an aqueous solution of 0.2 to 15 ppm sodium hexaplatinate and 1 to 1000 ppm hydrazine, maintained at a temperature of 90? C. or less.

12. The method of claim 1 wherein said selected component is a new component to be treated prior to installation.

13. The method of claim 1 wherein said selected component is an existing component that has been removed from high temperature service to be treated at low temperature, after which it will be returned to high temperature service.

14. The method of claim 1 further comprising the step of: g) providing a skid-mounted treatment unit comprising fluid handling systems, fluid heating and cooling systems, and valves, wherein said treatment unit is connected to a portion of the primary system of said nuclear power plant.

15. The method of claim 1 further comprising the step of: h) decontaminating said water contacting surface of said selected component prior to treating said surface.

16. The method of claim 15 wherein said decontaminating step comprises mechanical cleaning by a process selected from the group consisting of: water jet cleaning and ultrasonic cleaning.

17. The method of claim 15 wherein said decontaminating step comprises chemical cleaning by exposure at a selected temperature between ambient and 120? C. to complexing/chelating solutions selected from the group consisting of: oxalic acid; citric acid; nitric acid; ethylendiamine tetra acetic acid (EDTA); oxidizing agents; and reducing agents.

18. The method of claim 1 wherein: steps b) through e) are carried out using a source of a first divalent metal cation to deposit a first metal oxide film of a first selected thickness; and, steps b) through e) are carried out using a source of a second divalent metal cation to deposit a second metal oxide film of a second selected thickness.

19. The method of claim 18 wherein said first and second divalent metal cations are selected from the group consisting of: natural Zn.sup.2+; depleted Zn.sup.2+; and natural Mn.sup.2+.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0063] The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention, and of the components and operation of systems provided with the invention, will become more readily apparent by referring to the exemplary, and therefore non-limiting embodiments illustrated in the drawing figures, wherein like numerals (if they occur in more than one view) designate the same elements. The features in the drawings are not necessarily drawn to scale.

[0064] FIG. 1 is a Pourbaix diagram for the PtH.sub.2O system.

[0065] FIG. 2 is a Pourbaix diagram for the ZnH.sub.2O system.

[0066] FIG. 3 is a graphical representation of the relative affinities or preferences of some divalent and trivalent metals in passive oxide films.

[0067] FIG. 4 is a schematic diagram of a metal oxide deposition process in accordance with some aspects of the invention.

[0068] FIG. 5 is a schematic diagram of another metal oxide deposition process in accordance with some aspects of the invention.

[0069] FIG. 6 is a schematic diagram of a nuclear power plant cooling system and treatment method in accordance with some aspects of the invention.

[0070] FIG. 7 is a photomicrograph in cross section of a ZnO deposit on passivated Type 316L stainless steel.

[0071] FIG. 8 presents EDS analysis of as-deposited ZnO layer on Type 316L stainless steel.

[0072] FIG. 9 presents EDS analysis of ZnO layer on Type 316L stainless steel after 96 h exposure to simulated BWR water chemistry.

DETAILED DESCRIPTION OF THE INVENTION

[0073] A method is described herein for depositing adherent ZnO or ZnO.sub.2.sup.2? (herein collectively referred to as zinc oxide or ZnO) on a nuclear component after chemical decontamination with or without noble metal deposition at low temperature (40 to 100? C.), but potentially at higher temperatures (up to 120? C. if the system is pressurized to about 1 bar (14.5 psi) to prevent boiling using chemicals that are non-corrosive to the RCS components (absence of halogens, lead (Pb) or mineral acids). ZnO may also be deposited on the to-be wetted surfaces of new components at new plants or at existing plants such as replacement SGs on the wetted tube surfaces.

[0074] Once deposited and exposed to high temperature, the ZnO has been found to be incorporated in the metal surface or admixed with the existing or newly formed passive layer or CRUD.

[0075] One skilled in the art would recognize that is desirable as it would allow for incorporation of both zinc and optionally noble metal for the few months after startup after a plant outage whereafter normal NMCA or zinc addition may be resumed. Incorporation of zinc before the normal waiting period after a refueling outage with new or fresh fuel in the core would have the advantage of reducing worker exposure during future outages. The incorporation of zinc may also reduce the potential for IGSCC of stainless steel after plant startup and operation.

[0076] One or more non-limiting examples include taking the power plant out of service, providing the treatment solution containing the divalent metal compound (e.g., a zinc salt) and at least one species as a source of oxygen within a component or portion of the primary system at low temperature, providing the solution with an additive to control pH, allowing the treatment solution to remain in the component at low temperature for a period of time to form an adherent oxide, removing the treatment solution and then returning the plant to operation at high temperature.

[0077] The treatment may be performed during a refueling outage at the nuclear power plant.

[0078] The treatment may be performed on a component or system prior to installation of that component at a nuclear plant or after a new component is installed at the plant but before high temperature operations commence.

[0079] The treatment temperature may be less than 120? C. and more preferably less than 100? C.

[0080] The treatment period may be less than 5 days.

[0081] The treatment period may be less than 24 hours but more than 30 minutes.

[0082] A plant component previously exposed to high temperature operations during power generation mode of the plant may be partially or completely decontaminated with chemical solutions or by mechanical means such as ultrasonic cleaning or water jetting to remove CRUD and passive films leaving behind bare metal, residual passive film or residual CRUD, after which the compound is deposited on these surfaces/materials.

[0083] The treatment solution may be applied to a portion of the primary system that is constructed from austenitic stainless steel or nickel-based alloy upon which passive films have formed because of contact with primary coolant during operations or plant commissioning and deposition may occur on the passive film.

[0084] The wetted surface of the component or system may have accumulated radioactive CRUD on its surfaces and the treatment creates a deposited film on the CRUD.

[0085] The passive film and CRUD may have incorporated radioactive species into their structures rendering them radioactively contaminated and acting as a source of radiation leading to worker exposure. The radioactive species may be Co.sup.60.

[0086] The treatment solution may contain a divalent metal species in the form of a soluble chemical compound. The divalent metal compound may be zinc acetate or zinc nitrate. The zinc compound may be depleted in Zn.sub.64. Alternatively, analogous compounds of manganese, a constituent of stainless steels, may be used. Referring to FIG. 3, it can be seen that Mn also has a greater affinity for tetrahedral sites than does Co. Naturally occurring manganese consists entirely of the isotope Mn.sup.55 which can activate to Mn.sup.56 by neutron capture but it exhibits a half-life of only 2.578 hours.

[0087] The treatment solution may contain a source of oxygen such as water, oxygen, ozone or hydrogen peroxide.

[0088] More than one cycle of treatment may performed, producing sequential additive deposition of a compound on the surface including bare metal surfaces of surfaces if passive films or CRUD. Furthermore, two different divalent metal oxides (e.g., Zn and Mn) may be deposited together or in two different deposition cycles, in any desired order and thickness.

[0089] The treatment solution may be applied for a period sufficient to deposit at least 0.1 ?g/cm.sup.2 of divalent metal oxide on the surface being treated. The treatment solution may be applied for a period sufficient to deposit at least 10 ?g/cm.sup.2 of divalent metal oxide. The treatment solution may be applied for a period sufficient to deposit at least 100 ?g/cm.sup.2 of divalent metal oxide.

[0090] The treatment solution may be drained and stored, and then reused in another system.

[0091] The treatment may be conducted in a series of steps to build up a layer of divalent metal species having an affinity for tetrahedral sites in a metal spinel present as a passive film or CRUD to achieve a desired final thickness (?m) or loading (?g/cm.sup.2). If two different divalent oxides are deposited in two separate steps, the desired thickness of one oxide might be the same as or different than the desired thickness of the other oxide.

[0092] A separate treatment solution may be applied containing a noble metal such as platinum at a concentration of at least 0.5 ppm with no divalent metal species but including a reducing agent, and said treatment may be performed at a temperature less than 100? C.

[0093] The noble metal treatment may be applied before the application of the treatment solution containing the divalent metal species such as zinc.

[0094] The plant may be returned to service allowing the deposited species to become incorporated into newly formed passive layers after a decontamination of a primary system during operation of the plant at high temperature.

[0095] The deposited species may become incorporated into residual passive films or residual CRUD left behind after decontamination after operating of the plant at high temperature.

[0096] Illustrative examples of the inventive method for depositing a divalent metal compound are presented below.

Example

[0097] FIG. 4 summarizes one preferred example of a divalent compound (e.g., zinc oxide) deposition process. The method includes identifying, 10, a system with one or more fluidically connected components at an operational nuclear plant where the deposition of zinc at low temperature would be advantageous as described herein. The system may be isolated, 11, to contain zinc deposition reagents during the process. Prior to deposition of the zinc compound the wetted surfaces of the system may be fully or partially decontaminated by chemical or physical means, 12, to remove CRUD or passive layers exposing at least in part bare metal. Low temperature zinc oxide deposition is then performed, 13. This deposition may be performed in one step or multiple steps to build up a deposit of desired thickness and composition. After zinc oxide deposition, a noble metal such as platinum or palladium may be deposited, 14, on the surfaces now covered in whole or in part with zinc oxide. One skilled in the art will recognize the noble metal deposition may be performed before the deposition of zinc oxide. Finally, the system and plant is returned to high temperature operation 15 resulting in the formation of zinc bearing passive film or CRUD as the deposited zinc oxides comingles with or diffuses into the bare metal surfaces, passive layers or residual CRUD, which retards the incorporation of radioactive cobalt species at the surface.

Example

[0098] FIG. 5 summarizes another preferred embodiment of a divalent compound (e.g., zinc oxide) deposition process. The method includes identifying a new component, 20, to be used at a nuclear power plant prior to installation at the plant or prior to operation of the plant but after installation if the component is a replacement component. Low temperature zinc oxide deposition is then performed, 21. The component is installed at the plant, 22. The component is then placed in service at high temperature 23 which results in the incorporation of zinc into the newly formed passive layers or CRUD deposits.

Example

[0099] FIG. 6 depicts one example of a system selected for treatment. A nuclear power plant of the boiling water reactor type includes a reactor vessel, 100, a nuclear core, 101, a feedwater system 102 admitting reactor coolant from the RCS via valve 103, and steam supply to a turbine 104 controlled by valve 105. The turbine is in turn connected to an electrical generator, not shown, to produce electricity when the reactor operates at high temperature and elevated pressure. To improve the operation of a BWR plant, a separate reactor coolant recirculation system (RRS) 106a is used to pump fluid from and back to the reactor vessel 100 using one or more pumps 106b. The reactor recirculation system may be configured as one or more recirculation loops. During shutdown for maintenance or refueling, the reactor system is reduced to temperatures below 100? C. and preferably below 40? C. The reactor recirculation system is physically located in areas of the plant where workers can be exposed to radiation emanating from CRUD and passive films on the interior wetted surfaces of the system if these layers contain radioactive cobalt species, as an example. To reduce worker exposure during a maintenance or refueling outage or during futures outages, a chemical decontamination may be performed using a chemical decontamination skid 107 that is temporarily attached to the reactor recirculation system 106a. The flow to and from the skid 107 to the recirculation skid may be isolated by valves 113. One skilled in the art would understand that the decontamination system 107 may contain necessary tanks, piping, valves, pumps, heaters and coolers that allow for mixing, heating, filling, control, cooling and draining of the decontamination solutions. After chemical decontamination, a zinc treatment may be performed at low temperature (e.g., less than 100? C.) using a zinc treatment skid 108 connected directly to the recirculation system 106a using connection 110 or to the chemical decontamination skid via connection 109 which includes the piping, pumps, heaters, and cooling system that may be required to implement the zinc treatment process. Otherwise, these components may be part of the zinc treatment skid 108. Zinc treatment reagents may be prepared ahead of time or mixed in the zinc skid from concentrated chemicals supplied from containers 112.

[0100] Additional aspects and exemplary values and variants of the invention are described as follows.

[0101] A method for deposition of a compound on the wetted surface of a nuclear plant component in references to FIGS. 4 and 5 is described. The method includes optional deposition of a noble metal on the surface before or after deposition of the compound. The invention may involve the deposition of a zinc oxide compound as ZnO. The zinc is preferably depleted in Zn.sup.64. Alternatively the deposition may include the deposition of ZnO.sub.2.sup.2?, or the deposition of a combination of ZnO and ZnO.sub.2.sup.2?. The exact ratio of ZnO to ZnO.sub.2.sup.2? is typically pH dependent. The deposition on the surface is performed from a liquid solution at low temperature, typically up to 120? C., but preferably below 100? C. and more preferably about 90? C.

[0102] While the divalent metal oxide may consist of either or both of ZnO and/or ZnO.sub.2.sup.2?, they are collectively hereinafter referred to simply as zinc oxide.

[0103] The zinc oxide may be deposited on bare metal, passive films or CRUD (metallic oxides) which are wetted during plant operation at a nuclear power plant. The bare metal surface may be a surface that has been decontaminated at an operating nuclear power plant (see FIG. 4) or the surface of a new or replacement component (see FIG. 5). After deposition, the nuclear plant components selected for treatment with the deposition process are returned to, or in the case of a new or replacement component placed in service, at high temperature, typically 260? C. to 275? C. at a BWR or at about 300? C. at a PWR. Exposure to high temperature reactor coolant after low temperature deposition incorporates the zinc into tetrahedral sites in newly formed or existing passive film surface spinels or CRUD. Such incorporation mitigates the incorporation of radioactive cobalt species into the passive films or CRUD, and may help mitigate IGSCC.

[0104] As shown in FIG. 6, the zinc treatment system or equipment may be integral to or separate from a chemical decontamination system. The zinc treatment chemicals may be supplied as concentrated reagents and mixed with water during injection or after the injection into the system. The reagent chemicals are prepared at room (ambient) temperature, typically 20-40? C. At these cold temperatures the solutions or concentrates are stable; in other words, no precipitation or deposition of zinc oxide occurs on surfaces or in the bulk solutions.

[0105] For a new or replacement component to be installed at or placed in service at a plant, a process like that shown in FIG. 5 is followed.

[0106] The solutions or concentrates may be heated to the application temperature of say, 90? C., before injection, after injection, or during injection using heating systems quite commonly used for chemical cleaning or decontamination, including electrical heaters or steam heating. The treatment time once the desired temperature has been reached may be less than 10 minutes, less than 1 hour, less than 4 hours, less than 24 hours or up to 5 days (120 hours).

[0107] The zinc oxide deposition reagents in the treatment solution consist of (1) a zinc salt, (2) a pH control agent, (3) and a source of oxygen. The zinc salt can be zinc acetate or zinc nitrate. Other zinc salts that could be used are zinc chloride and zinc citrate. Other anion reagents that complex with zinc may be added in addition to the zinc salt to stabilize the solution such as tartrates. Analogous reagents may be used for the deposition of manganese oxides.

[0108] The pH control agent used to raise pH and promote deposition and stabilize the treatment solution are ammonia, ammonium hydroxide, ethylenediamine (EDA), triethanolamine (TEA), sodium hydroxide or potassium hydroxide. As seen in the previously described Pourbaix diagram from the zinc-water system, zinc oxides are stable solid phases at pH>8, and more favorably at pH>11. For the zinc oxide process described herein, the pH is greater than 8, preferably greater than 11 and most preferably >12.

[0109] According to various examples, the concentration of zinc salt (e.g., zinc acetate where the zinc is preferably depleted in Zn-64) in the treatment solution is at least (1) 1?10.sup.?7, 1?10.sup.?6, 1?10.sup.?5 moles/liter (M) as zinc, (2) less than 1?10.sup.?3, 1?10.sup.?4, 1?10.sup.?5 and 1?10.sup.?6 moles per liter (M) as zinc and or (3) any zinc concentration between any two such values (e.g., between 1?10.sup.?7 and 1?10.sup.?3 moles per liter zinc such as between 1?10.sup.?6 and 1?10.sup.?4 moles per liter (M)). The equivalent concentration of a 1?10.sup.?4 M zinc solution is about 6.5 ppm. The equivalent concentration of a 1?10.sup.?3 M zinc solution is about 65 ppm. At such concentration ranges it has been found that deposits of zinc oxides from 0.1 to 10 ?g/cm.sup.2 can be achieved in about 4 hours at 90? C., where the deposition thickness occurs linearly over time so thinner films may be achieved with shorter contact time and thicker films with greater treatment time. These specific surface coverages correspond to 0.1 to about 2% of a 600 ?g/cm.sup.2 passive film. The treatment solution can be drained before it is depleted in available zinc.

[0110] According to various examples, the zinc oxide may be deposited in multiple steps by draining the solution between steps, replenishing it, or supplying a fresh solution of the same or different zinc concentration. In the same way, zinc oxide and manganese oxide may be deposited in alternate steps.

[0111] The specific molarity or concentration chosen depends on the surface to volume ratio of the system. For example the surface to volume ratio of a 12 inch diameter Schedule 80 stainless steel pipe is 138 cm.sup.2/liter of solution on the interior The surface to volume ratio of a 4 inch Schedule 80 stainless steel pipe is 411 cm.sup.2/liter Hence, one skilled in the art would see that a 4 inch pipe may require up to 4 times the concentration of zinc reagent as a 12 inch pipe (411/138?3) to achieve the same thickness of zinc oxide on the surface if the process is applied until zinc is depleted from the treatment solution.

[0112] According to various examples, the source of oxygen may be oxygen, ozone, water or hydrogen peroxide. The concentration of hydrogen peroxide in the treatment solution is at least (1) 0.01, 0.1. 0.2, 0.5, 1 percent by weight, (2) less than 2, 1 and 0.5 percent by weight and or (3) any peroxide concentration between any two such values (e.g., between 0.01 and 2 percent by weight such as between 0.1 and 1 percent by weight).

[0113] According to various examples, the pH is above pH 8, preferably above pH 11 and most preferably above pH 12. The pH is achieved by addition of a base such as ammonium hydroxide, potassium hydroxide, sodium hydroxide, EDA or TEA.

[0114] The treatment solution may contain other complexing agents such as a tartrate or a citrate to stabilize the solution.

[0115] Noble metal deposition may be performed before or after zinc compound deposition, or between zinc compound deposition steps. For example, deposition of platinum metal with a surface coverage of 0.1 to 1 ?g/cm.sup.2 which would correspond to about 1% of the passive film on a wetted surface at a nuclear plant, can be achieved from a solution containing 0.2 to 15 ppm and most preferably 0.5 ppm (parts per million or nearly equivalently mg/kg) sodium hexaplatinate at up to 90? C. with or without a reducing agent such as hydrazine at 1 to 1000 ppm, and most preferably about 60 ppm hydrazine when used.

Exemplary experimental results.

Example

[0116] FIG. 7 shows a polished metallographic cross-section of a zinc oxide deposited on a passivated 316L austenitic stainless steel test coupon. The zinc oxide layer is about 1 ?m thick, uniform and highly adherent. The coupon was pre-passivated in a 10% citric acid solution at 90? C. in four hours, a technique commonly used to accelerate passivate stainless steel parts and components. Other depositions were performed on clean bare and polished stainless-steel coupons and on metallic oxides as would occur on CRUD.

[0117] The treatment solution for this experiment contained a zinc salt at about 1?10.sup.?4 M, ammonium hydroxide to raise pH>11, and less than 1% hydrogen peroxide. The treatment temperature was 90? C. Thinner deposits as thin as 0.01 ?m were also achieved with less uniform surface coverage but still exhibiting excellent adhesion.

Example

[0118] FIG. 8 is an energy dispersive spectroscopy (EDS) result of the surface of the as deposited film in FIG. 7. The depth of penetration of the x-rays in the EDS technique is about 1 to 2 ?m. The presence of zinc is clear.

[0119] FIG. 9 is an EDS plot of a zinc oxide deposit that was subjected to 96 hours of treatment in simulated BWR reactor coolant at 290? C. The presence of iron, chromium, and nickel (from 316L coupon) as well as zinc to the admixing and incorporation of the zinc into the coupon surface. The treatment in the simulated BWR coolant resulted in no measurable change in the coupon or deposit mass further demonstrating the excellent adherence of the deposit.