Corrosion and wear resistant coating on zirconium alloy cladding

Abstract

The invention relates to compositions and methods for coating a zirconium alloy cladding of a fuel element for a nuclear water reactor. The composition includes a master alloy including one or more alloying elements selected from chromium, silicon and aluminum, a chemical activator and an inert filler. The alloying element(s) is deposited or are co-deposited on the cladding using a pack cementation process. When the coated zirconium alloy cladding is exposed to and contacted with water in a nuclear reactor, a protective oxide layer can form on the coated surface of the cladding.

Claims

1. A coated composite, comprising: a zirconium alloy substrate comprising elemental zirconium, having a surface; and a coating composition, which comprises: master alloy comprising an alloying element selected from the group consisting of chromium, silicon, aluminum, and mixtures thereof; chemical activator; and inert filler powder, wherein, the coating composition is deposited on the surface of the zirconium alloy substrate, the alloying element of the master alloy being diffused into the surface of the zirconium alloy substrate, to form a diffusion coating layer, which comprises an alloy phase of the elemental zirconium and the alloying element.

2. The coated composite of claim 1, wherein the alloy phase of the elemental zirconium and the alloying element is selected from the group consisting of ZrCr.sub.2, ZrSi.sub.2, and ZrAl.sub.3.

3. The coated composite of claim 1, wherein the substrate is a fuel rod cladding.

4. The coated composite of claim 1, wherein the chemical activator is salt activator.

5. The coated composite of claim 4, wherein the salt activator is halide salt activator.

6. The coated composite of claim 1, wherein said coated composite is configured for contact with water in a nuclear reactor and when in contact with the water, an oxide layer is formed on the diffusion coating.

7. The coated composite of claim 1, wherein the diffusion coating layer is formed using a method of pack cementation.

8. A coated fuel rod cladding, comprising: a zirconium alloy substrate comprising elemental zirconium, having a surface; and a diffusion coating deposited on the surface of the zirconium alloy substrate, the diffusion coating comprising: a master alloy comprising at least one alloying element selected from the group consisting of silicon, chromium and aluminum; and an alloy phase, comprising: the elemental zirconium of the zirconium alloy substrate; and the at least one alloying element of the master alloy, wherein the at least one alloying element of the master alloy is diffused into the surface of the zirconium alloy substrate to form the diffusion coating.

9. The coated fuel rod cladding of claim 8, wherein the alloy comprises an alloy selected from the group consisting of chromium-silicon alloy, chromium-aluminum alloy and silicon-aluminum alloy.

10. The coated fuel rod cladding of claim 8, wherein the alloy phase comprises an alloy selected from the group consisting of ZrCr.sub.2, ZrSi.sub.2 and ZrAl.sub.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:

(2) FIG. 1 is an elevational view, partially in section of a nuclear reactor vessel and internal components, in accordance with the prior art;

(3) FIG. 2 is an elevational view, partially in section of a fuel assembly illustrated as shown in FIG. 1 in vertically shortened form, with parts broken away for clarity, in accordance with the prior art;

(4) FIG. 3 is a cross-section view, partially in section of a fuel rod, in accordance with the prior art;

(5) FIG. 4 is a cross-sectional view of a fuel rod cladding having a coating deposited on an exterior surface, in accordance with certain embodiments of the invention; and

(6) FIG. 5 is a cross-sectional view of the coated fuel rod cladding shown in FIG. 4 and as a result of being exposed to water, having a second layer, i.e., protective oxide layer, formed on a first layer, in accordance with certain embodiments of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

(7) The invention relates generally to fuel rod elements for use in nuclear water reactors, such as pressurized water reactors and boiling water reactors. The fuel rod elements include cladding. The cladding may be composed and constructed of a variety of conventional materials known in the art. For example, as previously described herein, it is known to construct fuel rod cladding for a nuclear water reactor from zirconium (Zr) alloy containing a majority amount of Zr and a minority amount, e.g., up to about 2% by weight based on total weight of the composition, of other metals, such as but not limited to niobium (Nb), tin (Sn), iron (Fe), chromium (Cr) and combinations thereof. Non-limiting examples of conventional Zr alloys for use in a nuclear water reactor include, but are not limited to, Zircaloy-2, Zircaloy-4, and ZIRLO.

(8) The fuel rod cladding is positioned in the core of a nuclear water reactor and therefore, is in contact and reacts with water to produce hydrogen according to the following reaction:
Zr+2H.sub.2O.fwdarw.ZrO.sub.2+2H.sub.2.

(9) Without intending to be bound by any particular theory, it is believed that as oxidation proceeds, hydrogen enters the cladding surface and precipitates as zirconium hydride. The formation of an external oxide layer and hydride precipitators cause embrittlement of the cladding, which lowers the safety margin and may potentially lead to failure under accident conditions. Furthermore, in accordance with the invention, it is believed that application of a coating to the surface of the cladding can significantly reduce hydrogen uptake and therefore, improve reliability and safety margins.

(10) The invention includes applying to, e.g., depositing on, the cladding surface a substantially uniform coating layer. Further, the invention includes depositing, e.g., co-depositing, onto the cladding surface one or more alloying elements including chromium, silicon, aluminum and mixtures thereof, which can be in elemental form or alloy form. In certain embodiments, the coating can be applied to either the interior surface or exterior surface of the cladding and in other embodiments, the coating can be applied to both the interior and exterior surfaces of the cladding. The coating can be applied using a variety of methods. The coating is effective to at least reduce or prevent waterside corrosion and wear of the cladding.

(11) In accordance with the invention, a coating mixture is formed by combining a master alloy, a chemical activator and an inert filler powder. The master alloy is selected based on the elements that are desired to be deposited, e.g., co-deposited, on the cladding surface. The master alloy includes one or more of chromium, silicon and aluminum. In certain embodiments, the master alloy includes chromium or silicon or a mixture of chromium and silicon, or a mixture of chromium and aluminum, or a mixture of silicon and aluminum. Suitable chemical activators for use in the invention include those that are generally known in the art. In certain embodiments, salt activator, such as, halide salt activator, is used. Non-limiting examples of suitable halide salt activators include, but are not limited to, NaF, NaCl, NH.sub.4Cl, NH.sub.4F, as well as dual activators, such as, NaF/NaCl, and mixtures thereof. A variety of inert filler powders are also generally known in the art and they are suitable inert filler powders for use in the invention. Non-limiting examples include, but are not limited to, Al.sub.2O.sub.3, SiO.sub.2 and mixtures thereof.

(12) The coating layer can be applied to the Zr alloy surface by employing conventional methods and apparatus known in the art. Alternatively, in accordance with the invention, a pack cementation method can be employed for applying the coating to the Zr alloy cladding surface for use in a light water nuclear reactor. Traditionally, the pack cementation method is employed for coating various alloys used in gas turbines and fossil fuel burning power plants. An advantage of employing the pack cementation process is the uniformity of the resulting coating. Such uniformity can be accomplished even on complex shapes and configurations.

(13) In accordance with the invention, the coating is applied on zirconium tubes of various thickness and in certain embodiments, the thickness varies from 1 micron to 200 microns. For example, the coating can be applied to thicker zirconium tubes, e.g., called TREX (Tube Reduced Extrusion). The dimension of the TREX is typically 2.5 OD1.64 ID (0.43 wall) and the length can be as long as 12 feet. The coated tube or TREX may be subjected to cold work to reduce the overall thickness of the tube and to achieve the final dimension. Intermediate annealing may be used to release residual stress in the coating and zirconium tubes. In certain embodiments, the coated Zr alloy cladding can be subjected to cold work using conventional methods, such as, but not limited to, pilgering, to reduce the overall thickness of the coating and/or cladding. Multiple cold work steps may be conducted to achieve a final, e.g., desired, dimension of the cladding.

(14) The pack cementation process is a batch vapor deposition process that includes simultaneous heat treatment. The entire cladding surface or select portions of the cladding surface can be coated. The cladding surface or the portions of the cladding surface that are to be coated are surrounded, e.g., packed, in the coating mixture. For example, in certain embodiments, the coating mixture forms a powder bed composition and the cladding or portions thereof are packed, e.g., buried, within the bed. The powder bed composition is formed by thoroughly mixing together the master alloy, chemical activator and inert filler components of the chemical mixture. The master alloy and inert filler components are typically provided in dry form, e.g., a powder.

(15) The cladding is placed and sealed in a chamber, e.g., retort or furnace, surrounded by the pack. The chamber has a heating zone and an inert atmosphere. The chamber is heated to an elevated temperature. The temperature in the chamber can vary and may depend on the components selected for the coating mixture. In certain embodiments, the chamber temperature can be in a range from 600 C. to 1100 C. The temperature is maintained within this range for a period of time that is sufficient to deposit a coating on the surface of the cladding. In general, the chamber temperature is selected such that it is sufficiently high for the master alloy to react with the chemical activator to form a gaseous compound. The gaseous compound serves as a transfer medium that carries the master alloy to the cladding surface. The gaseous compound contacts the cladding surface and decomposes at the surface to deposit or co-deposit the master alloy element(s), e.g., one or more of chromium, silicon and aluminum, on the surface of the cladding. As a result, a diffusion coating layer is formed thereon. The chemical activator is released, returned to the pack, e.g., powder bed composition, and continues to react with the master alloy. The transfer process continues until the master alloy in the pack is depleted, e.g., used, or the temperature in the chamber is decreased, e.g., cooled.

(16) The diffusion coating layer consists of a phase that includes elements of the cladding material, e.g., zirconium (Zr), and elements deposited from the master alloy. In certain embodiments, wherein the cladding material is zirconium alloy and the master alloy deposited is chromium, the diffusion coating layer includes a ZrCr phase or alloy having elements of zirconium and chromium, such as, but not limited to, ZrCr.sub.2. Similarly, for a zirconium alloy cladding and silicon master alloy, the diffusion coating layer includes a ZrSi phase or alloy having elements of zirconium and silicon, such as, but not limited to, ZrSi.sub.2 and, for a zirconium alloy cladding and an aluminum master alloy, the diffusion coating layer includes a ZrAl phase or alloy having elements of zirconium and aluminum, such as, but not limited to ZrAl.sub.3.

(17) FIG. 4 is a schematic showing a coated cladding tube that illustrates a diffusion coating layer applied to an exterior surface of the cladding tube. FIG. 4 shows a coated cladding 80 having an interior surface 82 and an exterior surface 84. The exterior surface 84 has deposited thereon a diffusion coating 86.

(18) In certain embodiments, an additional heat treatment may be performed to convert the diffusion layer into a coating having improved rigidity.

(19) The cladding having deposited thereon the diffusion coating layer is installed in a nuclear reactor core during plant operation. As the coated cladding is exposed to and contacted with water, a protective oxide layer is formed. Wherein the element deposited in the diffusion coating layer is chromium, the oxide layer includes Cr.sub.2O.sub.3, wherein the master alloy element deposited in the diffusion coating layer is silicon, the oxide layer includes SiO.sub.2, and wherein the master alloy element deposited is aluminum, the oxide layer includes Al.sub.2O.sub.3.

(20) FIG. 5 is a schematic showing a coated cladding tube that illustrates a protective oxide layer coating applied to an exterior surface of a coated cladding tube. FIG. 5 shows the coated cladding 80, the interior surface 82, the exterior surface 84 and the diffusion coating 86, as shown in FIG. 4. In addition, FIG. 5 includes a protective oxide layer 88 formed on the diffusion coating 86. The protective oxide layer 88 is effective to at least reduce or preclude hydrogen diffusion. The diffusion coating 86, that is underlying the protective oxide layer 88, provides an additional barrier to hydrogen diffusion. Thus, the diffusion coating 86 and the protective oxide layer 88 cause oxidation and hydrogen uptake in the cladding material, e.g., Zr, to be at least reduced or precluded.

(21) While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.