FR2916002A1 - REMOVAL OF SECOND PHASE DEPOSITS FROM NIOBIUM ON ZIRCONIUM-NIOBIUM ALLOYS - Google Patents

Abstract

The invention is a method for removing niobium-rich second phase particle (PSP) deposits from the surface of zirconium-niobium alloy parts. This process involves washing a zirconium-niobium alloy piece, just pickled and rinsed, with an acidified oxalic acid wash solution or ammonium oxalate. The process of the present invention results in a rapid, efficient and complete removal of PSP surface deposits from the zirconium-niobium alloy part, without causing pitting, resulting in a clean, shiny surface without must use operations of projection of water under pressure or mechanical wiping.

Description

B08-0684EN WESTINGHOUSE ELECTRIC COMPANY LLC Elimination of

  Deposition of Niobium Second Phase Particles on Pickled Zirconium-Niobium Alloys Invention of: MCLAUGHLIN David F. Priority of a Patent Application Filed in the United States of America March 7, 2007 under No. 11 / 682,981

  2 Elimination of second-phase niobium particle deposits on etched zirconium-niobium alloys

  The present invention relates to a method for cleaning zirconium-niobium alloys for cladding fuel in nuclear reactors. More particularly, the present invention relates to a rapid, efficient and complete chemical process for the removal of second-phase particle deposits present on the surface of zirconium-niobium alloys, in which a clean, shiny, non-porous surface is obtained. pierced without the use of pressurized water spraying or mechanical wiping operations.

  Zirconium (Zr) has many useful properties, including good mechanical strength and high resistance to corrosion. In its purified, hafnium-free form, zirconium is commonly used as a building material for nuclear reactor cores because of its low neutron absorption cross section. Alloys commonly used in nuclear grade zirconium applications typically contain tin, iron and sometimes nickel; more recent alloying compositions such as the "Zirlo" alloy (Westinghouse Electric Co., LLC, Pittsburgh, PA) also contain a certain percentage of niobium (Nb), which serves to improve the corrosion resistance in the nuclear reactor environment. As in most metal object applications, the fabrication of fuel tubes and nuclear reactor core bodies leaves undesirable traces on the metal surface such as scratches, oxidation spots and chemical impurities. from lubricants. As a result, zirconium parts are usually chemically stripped before use, and the components of the fuel assemblies can undergo many chemical etching operations during the manufacturing process, to ensure the quality of the surface and the surface. to eliminate foreign bodies. A conventional pickling bath for zirconium contains 10 to 40% by weight of nitric acid and 1 to 5% of hydrofluoric acid, which is a very aggressive solution. A specific problem occurs when etching zirconium alloys containing niobium. Niobium segregation tends to take place inside the alloy, resulting in very small particles, called second-phase particles (PSPs). Ordinarily, the PSPs have a Zr-Nb binary composition or

  3 ternary Zr-Nb-Fe. When stripping the alloy, the dissolution of the Zr matrix occurs more rapidly than that of the PSP, so very large quantities of extremely fine black particles are released in the etching acid during the chemical etching process . Unfortunately, when the alloy is removed from the etching acid, even after extensive rinsing, the surface is usually matt black due to a dense coating of adhering particles that do not detach from the surface of the metal during rinsing. This substance is called "coal" in industry, with reference to the similarity of its appearance to that of black masses of fine fungal spores bearing the same name.

  Before any niobium-zirconium alloy can be used in a nuclear reactor, all of the "charcoal" deposit must be removed, partly to obtain luminous and bright surfaces on the parts, and to prevent any subsequent release of such materials. particles in the cooling water of a reactor and their deposit inside the reactor. On easily accessible external surfaces, it is not difficult to remove PSP deposits, and this can be done by splashing water under pressure or by mechanical wiping with rags or sponges. However, many reactor parts ultimately have internal surfaces that are not readily accessible, such as the fuel tube interior for pressurized water reactors (PWRs) as well as boiling water reactors (BWRs), and the inside the channel boxes in the BWRs. Some interior surfaces, such as smooth cylindrical tubes, can be chemically cleaned by scrubbing the interior of the room with cleaning pads ("scrapers"), but other small channels can not be cleaned effectively, and small crevasses are not accessible at all.

  An ideal solution to the problem would be a chemical wash during which the part would be immersed, which should result either in the dissolution of the PSP deposit or its detachment from the surface of the metal. The dissolution of the Zr-Nb and Zr-Nb-Fe second-phase particles is probably not a plausible solution, since any solvent capable of dissolving them would undoubtedly attack the zirconium substrate in an even more aggressive manner. causing both surface damage and release of an even greater amount of PSP from the alloy. Therefore, a process is needed to rapidly and completely remove the Zr-Nb and Zr-Nb-Fe second phase particles from the entire surface of zirconium-niobium alloys without damaging the surface of the alloys.

  The present invention fills this need by providing a method for removing niobium-rich second phase particle (PSP) deposits from the surface of a just-stripped and rinsed zirconium-niobium alloy piece comprising a wash the piece of zirconium-niobium alloy just pickled and rinsed, using an acidified washing solution with oxalic acid or ammonium oxalate. The present invention aims to provide a method by which PSP deposits are entirely removed by a chemical action, requiring neither mechanical wiping nor projection of water under pressure, and which is therefore adaptable to remove PSP deposits from interior surfaces complex inaccessible for cleaning by wiping and splashing water under pressure. It is another object of the present invention to provide a method in which PSP deposits can be easily removed even after the zirconium-niobium coin has been etched and rinsed with water to remove fluoride and stop stripping action, by increasing the solubilization capacity of oxalate to hydrated zirconia, obtained by combining the oxalate with a second mineral acid. The present invention further aims to provide a method that can be performed quickly without increasing the temperature beyond the ambient temperature, and thus eliminating the risk of damage to the surface of the zirconium-niobium alloy piece. The present invention further aims to provide a method giving a metal surface which can not be distinguished from that obtained by the current industrial pickling zirconium-niobium alloys, which is usually followed by a projection of water under pressure or mechanical wiping.

  The present invention further aims to provide a method in which the spent solutions can be treated in the same manner as the spent pickling acid and the current pickling rinse solutions, where a calcium hydroxide treatment is used to precipitate calcium oxalate instead of calcium fluoride. These and other objects of the present invention will become more apparent upon reading the detailed description hereinafter.

  The present invention provides a method for removing niobium-rich second phase particle (PSP) deposits from the surface of a just-etched, rinsed zirconium-niobium alloy part, comprising washing the alloy part. of zirconium-niobium just pickled and rinsed, using an acidified washing solution with oxalic acid or ammonium oxalate. In a particular embodiment of the invention, the solution of oxalic acid or ammonium oxalate is acidified with nitric acid. The concentration of oxalic acid or ammonium oxalate in the wash solution may range from about 1.0 to 8.0% by weight, preferably from about 2.5 to 5, 0% by weight, and more preferably about 2.5% by weight. The concentration of nitric acid in the wash solution may range from about 1.0 to 40% by weight, preferably from about 5 to 10% by weight, and most preferably about 5% by weight.

  The zirconium-niobium alloy piece just scoured and rinsed is washed in the wash solution used according to the present invention for about 1 to 10 minutes, preferably about 2 to 5 minutes and more preferably about 4 minutes, at room temperature ( that is, about 26 C). Higher temperatures, especially 35 C to 50 C, allow an acceleration of the washing process, but increase the risk of pitting of the surface of the piece of zirconium-niobium alloy and are therefore not recommended. The process for removing PSP deposits from the surface of a zirconium-niobium alloy piece just pickled and rinsed can easily be integrated into ongoing industrial work while respecting the environment. In particular, the spent pickling acid and / or contaminated pickling rinse solutions are usually treated with calcium hydroxide to neutralize the acidity, to fix the toxic fluoride in the form of CaF2, which is very little soluble, and precipitating calcium oxalate, which is also very sparingly soluble and thus can be easily separated, as a solid filter cake, from spent solutions resulting from the washing process of the present invention. As a result, the same environmentally safe processing methods as those currently used for pickling zirconium alloys can easily be adapted to treat the oxalate wash solutions employed in the present invention. As used herein, the term "coal" refers to a dense coating of adherent matte black particles that do not detach from the metal surface of a zirconium-niobium alloy during rinsing. The terms "coal" and "second-phase particle deposition (PSP)" are used interchangeably herein. As used herein, the term "Zirlo sample" refers to a sample, the size of a postage stamp, a sheet metal alloy composition

  6) (Westinghouse Electric Co., LLC, Pittsburgh, PA) used for experimental studies, which composition contains tin, iron and about 1% niobium.

  The present invention is described more particularly in the non-limiting example below, which is given for illustration only and many variations will be obvious to those skilled in the art. Example 1 Introduction In order to develop a washing solution for removing PSP deposits from a zirconium-niobium alloy part without dissolving the PSP deposits (and thus without dissolving the zirconium substrate either), it was necessary to identify how PSPs attach to the surface of a zirconium-niobium alloy. To this end, preliminary studies have been conducted in the Science and Technology Department of Westinghouse Electric Co., LLC (Pittsburgh, PA). These studies have shown that the freshly etched metal zirconium is coated with a very thin layer of hydrated zirconia (ZrO2 • x H2O), as a result of the electrochemical balance between zirconium metal and water at low pH. In the particular case where x is 2, the hydrated zirconia is stoichiometrically equivalent to Zr (OH) 4 zirconium tetrahydroxide. Although in reality the compound of formula Zr (OH) 4 may not exist, it is convenient to refer to it as such. This zirconium "hydroxide" layer is very thin, i.e. of nanometric thickness, but it is adherent and behaves like an "adhesive", so that relatively small PSPs adhere to the metal surface of the zirconium-niobium alloy.

  From these findings, studies have been conducted to determine the optimal wash solution capable of removing the zirconium "hydroxide" glue from the surface of zirconium-niobium alloys. This "hydroxide" zirconium is very slightly soluble in water and it is known that it is soluble in only a few substances such as sodium hydroxide and / or potassium in highly concentrated solution, hydrogen peroxide in alkaline solution, concentrated hydrofluoric and sulfuric acids, oxalic acid and optionally ammonium carbonate (Blumenthal, WB, The Chemical Behavior of Zirconium, D. van Nostrand Co. Inc., Princeton, NJ, 1958, pp. 191-193) .35

  Initial Testing Initially, seven Zirlo samples underwent an ordinary pickling process known in the art. This process involved immersing the samples for 20 minutes in a pickling liquid consisting of 40% by weight concentrated nitric acid and 5% by weight concentrated hydrofluoric acid. Then, each Zirlo sample was immediately placed for 4 minutes in a different wash solution. The washing solutions contained concentrated sodium and / or potassium hydroxide, alkaline hydrogen peroxide, concentrated hydrofluoric and sulfuric acids, oxalic acid and ammonium carbonate. The different wash solutions and the ability of these wash solutions to remove PSP deposits from the samples are shown in Table 1.

  Table 1 Efficacy of various washing solutions for removing "coal" from Zirlo samples Hydroxide Peroxide Hydroxide Acid Acid Sodium carbonate Sulfuric hydrogen sulfide oxalic acid / concentrated ammonium oxalate concentrated concentrated ammonium alkali Ineffective; Ineffective; Effective Elimination; but effective inefficient. slow reaction slow and incomplete reaction causes quickly without and eliminates elimination "coal". Incomplete damage damage to the unacceptable surface of the "coal" surface. on the surface "coal". Of the wash solutions tested, it was found that only oxalate was able to rapidly remove "coal" without damaging the surface of the Zirlo samples. Thus, these tests demonstrated that just stripped Zirlo samples, immediately immersed in oxalic acid or ammonium oxalate, had a tendency to immediately release the surface deposits of PSP, leaving a surface area on the coupons. clean and bright. It was found that the PSP deposits did not dissolve in the wash solution, but rather accumulated at the bottom of the reaction vessel.

  8 Other tests The industrial stripping work is always immediately followed by an abundant rinsing, preferably in demineralised water, to stop the pickling reaction and maintain a good quality of the surface.

  In a first series of experiments, it was found that, if freshly stripped Zirlo samples were first rinsed in deionized water, neither oxalic acid nor ammonium oxalate eliminated "coal". Apparently, exposure to water has the effect of "fixing" the ZrO 2 • x H2O layer and rendering it insoluble (or much less soluble) in the oxalate solution. In another series of experiments, it was found that if freshly stripped Zirlo samples were first rinsed with oxalate instead of demineralized water, there was an accumulation of oxalate in the oxalate bath. fluoride contaminating the oxalate bath, which also "fixed" the surface deposits of PSP, so that bath efficiency gradually decreased with time. In a final series of tests, it was discovered that the key factor in successfully using oxalate to remove PSP deposits from a pickled and rinsed zirconium alloy was to acidify the solution of the solution. oxalate with another mineral acid. Nitric acid was chosen because of the fact that it does not interact directly with zirconium, an interaction that could cause damage to the surface of the alloy. In particular, a solution of 2.5 to 5% by weight of oxalic acid and 5% by weight of nitric acid was used as washing solution, at room temperature, ie approximately 26 ° C., after pickling Zirlo samples, as described above, and after extensive rinsing in deionized water for 20 minutes. This resulted in a rapid and efficient removal of surface deposits of PSP from the samples. The method described above has the advantage that there is virtually no accumulation of PSP deposits in the oxalate solution. In contrast, it was found that the PSP deposits only became detached after the samples were removed from the oxalate-nitric acid wash solution and placed in a second rinse bath. In addition, it has been found that this method is effective for a wide range of oxalic acid concentrations, i.e. 1% to 8% by weight. We also found

  Nitric acid concentrations in a very wide range, ie 5 to 40% by weight, are also effective. Higher concentrations of nitric acid, greater than about 40% by weight, were found to cause pitting of the metal surface of the samples.

  Finally, if the temperature of the nitric acid oxalate wash solution is increased, that is to say if it is brought to a value between approximately 35 ° C. and 50 ° C., this makes it possible to shorten the time required for disposal of the "coal" deposit. However, this also causes damage to the metal surface of the samples, in the form of many small punctures. Conclusion By subjecting Zirlo samples to a conventional pickling process known in the art, to rinsing abundantly in deionized water and then washing with a 5 wt% wash solution. oxalic acid and 5% by weight of nitric acid at room temperature, about 26 ° C., it was possible to rapidly and efficiently remove PSP surface deposits from the samples, without causing pitting, which resulted in a clean and bright surface.

Claims (9)

claims   A method for removing niobium-rich second phase particle (PSP) deposits from a just-pickled and rinsed zirconium-niobium alloy piece comprising washing the zirconium-niobium alloy just scoured and rinsed at using an acidified washing solution with oxalic acid or ammonium oxalate.   2. Method according to claim 1, characterized in that the solution of oxalic acid or ammonium oxalate is acidified with nitric acid.   3. Process according to claim 2, characterized in that the concentration of oxalic acid or ammonium oxalate in the wash solution is approximately 1.0 to 8.0% by weight and the concentration of the nitric acid in the wash solution is from about 1.0 to 40.0% by weight.   4. Method according to claim 2, characterized in that the concentration of oxalic acid or ammonium oxalate in the wash solution is about 2.5 to 5.0% by weight and the concentration of the nitric acid in the wash solution is from about 5.0 to 10.0% by weight.   5. Method according to claim 2, characterized in that the concentration of oxalic acid or ammonium oxalate in the wash solution is about 2.5% by weight and the concentration of nitric acid in the Wash solution is about 5% by weight.   6. Method according to claim 1, characterized in that the piece of zirconium-niobium alloy is washed for about 1 to 10 minutes in the washing solution.   7. Method according to claim 1, characterized in that the piece of zirconium-niobium alloy is washed for about 2 to 5 minutes in the washing solution.   8. Method according to claim 1, characterized in that the piece of zirconium-niobium alloy is washed for about 4 minutes in the washing solution at a temperature of about 26 C.   9. The method of claim 1, characterized in that the spent washing solution is treated with calcium hydroxide to precipitate calcium oxalate.