RU2145739C1 - Tubular can for nuclear fuel rods and its manufacturing process - Google Patents

Abstract

FIELD: nuclear power engineering. SUBSTANCE: zirconium-alloy tubular can that also includes 0.18-0.25% of iron, 0.07-0.13% of chromium, 0.35-1.7 mass percent of tin, 900-2800 ppm of oxygen, 80 to 200 ppm of carbon, 50 to 120 ppm of silicon with mass proportion of iron and chromium in alloy being 1.6 to 3 is subjected to post-rolling heat treatment including several annealing procedures in inert atmosphere where ΣA is higher than 1,5•10^-17. ΣA means here total complex of annealing procedures, products of terms t, h, and Q/RT exponential function, where T is temperature, K. EFFECT: improved corrosion resistance in water at high temperature with adequate stability of thermal yield and on-load corrosion resistance.

Description

 The present invention relates to methods for manufacturing a pipe from an alloy of zirconium, intended for the preparation of a protective shell (hermetic casing) of the rod for nuclear fuel. It finds particular significant application, although not exclusive, in the manufacture of pipes for sealing nuclear fuel rods for nuclear pressurized water reactors.

To date, most often used shells made of an alloy based on zirconium, called "Circular 4", which, in particular, contains by weight
- 1.20 - 1.70% tin
- 0.18 - 0.24% iron
- 0.07 - 0.13% chromium,
while the total content of iron and chromium was in the range between 0.28 and 0.37%. In the classical case, the ratio between the iron and chromium content was between 1.38 and 3.42 approximately
The mechanical stability of the Zirkaloy 4 shells reaches a satisfactory level, but corrosion due to high-pressure water at a high temperature limited the duration of a possible stay in the reactor.

 Pipes serving as a protective shell have already been proposed, respectively, including an inner layer of Zirconium 4 and an outer layer of an alloy based on zirconium having a reduced tin content, but in contrast, an increased oxygen content from 1900 to 2300 ppm (EP-A-0552098). Such pipes provide satisfactory results provided that the shell is in a state of metallurgically relieved stress, but their manufacture is more complicated than the manufacture of pipes of a homogeneous structure.

Finally, a method was proposed (EP-A-0196286) for manufacturing pipes serving as a protective sheath from an alloy based on zirconium containing 1-5% by weight of alloy elements such as Sn, Fe, Cr, and Ni, by which the billet is annealed obtained from drawing, before it is subjected to cold rolling in phase, which can be separated by annealing such that the sum Σ _A is higher than 2.3 • 10 ¹⁴ , taking into account the contribution of annealing after drawing.

 The first rolling is carried out on a pipe obtained from drawing without intermediate annealing.

The term Σ _A denotes the sum for a complex of anneals, products of terms t (in hours) and exponential function Q / RT, and T is the temperature in K.

When calculating Σ _A in this document, the hypothesis is that Q is approximately 65,000 cal / mol.

 The present invention aims to provide a method of manufacturing a pipe that serves as a protective sheath that meets better than previously known requirements, practices, in particular in that it allows combining the high resistance to general corrosion in an aqueous medium at high temperature with satisfactory thermal stability and good flow resistance resistance to stress corrosion.

To this end, the invention provides, in particular, a method of manufacturing a pipe for a protective shell of a nuclear fuel rod from a zirconium-based alloy containing also 0.18 - 0.25% iron, 0.07 - 0.13% chromium, 0.35 - 1.70% by weight of tin, 900-2300 ppm of oxygen, between 80 and 200 ppm of carbon and between 50 and 120 ppm of silicon, the method comprising several sequential metallurgical operations in the heat treatment of a workpiece obtained by drawing, characterized in that the relationship between iron and chromium in the alloy is between 1.6 and 3, and the thermal treatment comprises, after at least one rolling several annealing in an inert atmosphere, such that the sum Σ _A was more than 1.5 • 10 ^-17, and is advantageous if it lies between 2 • 10 ^-17 and 2 • 10 ^-16 .

In the case of application of the considered Q / R alloys (the ratio between the activation energy Q and the total gas constant is close to 2) is approximately 40,000 K ^-1 .

The complex of thermal treatments carried out under these conditions allows one to obtain a suitable precipitation size and balance the phases with the last annealing, carried out mainly at a temperature between 450 ^o C and 500 ^o C (most favorably 470 ^o C) so as to distribute iron intelligently between different precipitates Zr (Cr / Fe) ₂ - Zr ₂ Fe ₂ Si - Zr ₂ Fe ₂ Ni and Zr ₃ Fe in such a way as to obtain the ratio Fe / Cr in Zr (Cr / Fe) ₂ , concluded between 2.2 and 2.6, the ratio that can be obtained with a weight content between iron and chromium in the alloy up to 3.

 The limit above 3 for the Fe / Cr ratio is the boundary above which the fluidity increases and the resistance to corrosion under stress significantly deteriorates. A ratio of less than 1.6 leads to a high spread of stability values for the same composition.

 The range between 1.6 and 3 seems particularly interesting, since the scatter of the results (for generalized corrosion resistance) is relatively small around the average and the corrosion resistance is especially high. Corrosion is minimal for a ratio of approximately 2.4 (the exact ratio depends on the selected quenching rate) and slowly increases only above 2.4. In practice, the best compromise (low corrosion, optimal character and size of precipitation, flow resistance) corresponds to a Fe / Cr ratio between 2.2 and 2.6.

The full sequence of manufacture of an alloy ingot may include in general terms the steps of:
- forging the structure of round billets or bar materials (beams), for example, 180 mm in diameter for end pipes of 9.5 mm in diameter:
- hardening in phase β;
- internal machining, for example, with a diameter of 70 mm;
- hardening in phase β, followed by burning;
- hot drawing, at the output of which the tube stock in phase α;
- thermodynamic sequence consisting of rolling with intermediate annealing.

 The product obtained from drawing (a tubular billet obtained by drawing with a thickness greater than the final thickness) is called "traces". It is subjected to successive rolling-annealing sequences.

A round billet or bar is obtained by successive melting and solidification, then by deformation in a hot state: it or it undergoes hardening, followed, if necessary, by annealing. The quenching rate, concluded between 5 ^o C and 30 ^o C per second, starting from a temperature above 1000 ^o C, for the Fe / Cr contents defined above determines the nature and size of the precipitations, which ultimately contribute to a high resistance to generalized corrosion.

 This intermediate size will be predominantly the same as it generates at the end of the complete sequence of treatments, the final precipitation size of more than 0.18 microns, which seems to be especially favorable.

A possible explanation for the adverse effect on the fluidity of the Fe / Cr ratio is higher than defined here (the accuracy of this hypothesis should be considered without consequences for patent validity), is that an increase in the ratio Fe / Cr excessively increases the amount of precipitation (Fe, Cr) ₂ and changes the nature of the phases, causing the precipitating phase of Zr ₂ Fe to appear between the others. As for the presence of the minimum kinetics of generalized corrosion, it is likely that it is associated with the joint use of several mechanisms of the formation of precipitating phases.

 On the other hand, it was found that the Fe / Cr ratio in the above range has no effect on the hydrogen fraction absorbed in the high temperature aqueous medium.

The favorable results obtained using the invention depend not only on the appropriate choice of Fe / Cr, but also on the application of the appropriate sequence of heat and metallurgical treatments, ending with annealing, bringing the pipe into a state of stress relief. This annealing may allow the temperature to be maintained for at least four hours between 470 ^° C. and 500 ^° C. The sequence may, in particular, have some or a combination of the following thermal and metallurgical steps:
- initially manufacturing the bar by successive melting and solidification of the ingot in vacuum, then forging with the final hardening in region β with a cooling rate between 5 ^o C per second and 30 ^o C per second, starting from a temperature above 1000 ^o C and up to approximately 800 ^o C;
- bringing the beam into phase α by annealing, then quenching β with a temperature between 700 ^° C and 750 ^° C, mainly about 715 ^° C, for four to six hours, most preferably about five hours;
- drawing in the form of a workpiece in phase α;
at least one cold rolling sequence without preliminary annealing and annealing between 700 ^° C and 750 ^° C (mainly 720 ^° C - 740 ^° C, most advantageously about 730 ^° C) for one to three hours;
- successive rolling in the form of tubes of decreasing thickness and intermediate annealing in argon with a temperature between 640 ^o C and 740 ^o C, most favorably about 700 ^o C;
- final annealing to relieve stress with a temperature between 470 ^o C and 500 ^o C again in an inert atmosphere, usually argon, while the pipe remains in phase α.

 The pipe thus obtained is no longer subjected to heat treatments that alter its metallurgical structure. But it still receives surface treatments and is tested before being used as a protective sheath.

The surface treatment may, in particular, include sandblasting or chemical cleaning using a mixture of HF-HNO ₃ , followed by washing. Then it is polished with a moving tape or wheel. Control can be carried out in a classical way using ultrasound, Foucault currents (eddy currents) and / or visually.

 Among the alloy compositions defined above, some are particularly interesting. In particular, he can choose a composition in which the content of tin, iron and chromium corresponds to the compounds corresponding to the definition of "Circular 4" above, with 0.075-0.10% chromium and this with the ratio Fe / Cr concluded between 1.9 and 2 , 5 and, in addition, 50 - 120 ppm of silicon and 80 - 200 ppm of carbon.

 Another composition of particular interest to the structure of pipes serving as a protective shell for pressurized water reactors can be determined and evaluated as "low tin content doped with impurities in oxygen." It contains 0.18-0.25% iron, 0.075-0.10% chromium, 0.45-0.75% tin, 50-120 ppm silicon, 80-200 ppm carbon and 1900-2300 ppm oxygen.

 Thus obtained pipes have high resistance to various types of corrosion, including in the presence of iodine.

 Now we describe as an example the range of manufacturing used.

The initial alloy is presented as an ingot. By melting and solidifying, successively performed in a vacuum, for example, in an amount of three, it takes the form of a bar, which is quenched in water at a controlled speed, to bring in region β with a cooling rate comprised between 5 ^o C per second and 30 ^o C in second from at least 1000 ^o C to at least 800 ^o C. All the annealing after hardening is carried out at a temperature of less than 800 ^o C in order to remain in phase α and not fall into the two-phase region. Then the bar is annealed to increase precipitation at a temperature of about 715 ^o C for four hours and thirty minutes. The beam is then drawn in a standard manner in the form of a billet, most advantageously if it is at a temperature of about 650 ^o C. A billet that can withstand random, possible annealing at 730 ^o C is then subjected to two passes of cold rolling, with annealing followed by temperature of about 730 ^o C to provide a suitable Σ _A.

The resulting track is subjected to several successive rolling, gradually providing a pipe with a given, defined size. Each pass through the rolls alternates with annealing in an inert atmosphere, usually argon. Advantageously, favorably, if intermediate annealing is carried out at a temperature of about 700 ^° C., while transverse annealing to relieve stress occurs usually at temperatures between 470 ^° C. and 500 ^° C.

Claims (13)

1. A method of manufacturing a pipe from an alloy based on zirconium containing Sn, Fe, Cr, according to which the billet in the α-phase obtained by drawing the beam is subjected to several passes of sequential rolling in the α-phase in the form of tubes of decreasing thickness, with intermediate annealing in α -phase at a temperature of less than 800 ^o C in an inert atmosphere, characterized in that they create a bar of alloy, the alloying elements of which are by weight iron 0.18 - 0.25%, chromium 0.07 - 0.13%, tin 0, 35 - 1.70%, oxygen of 900 - 2300 million ^-1, carbon 80 - 200 million silicon ^-1 and 50 - 120 million ^-1, wherein the ratio of Fe / C r is 1.6 - 3, the final annealing is performed to relieve stresses so that the alloy of the final pipe is mainly in the α-phase, and the combination of annealing is processed after the first rolling so that ΣA is more than 1.5 • 10 ^-17 , moreover, ΣA denotes the sum of the products of time t (in hours) and the exponential function, Q / RT, for the set of anneals, where T is the temperature in K and Q / R is 40,000 K ^-1 . 2. The method according to claim 1, characterized in that the last annealing is carried out at a temperature of 450 - 500 ^o C for a time sufficient to ensure that the alloy was predominantly in the α-phase. 3. The method according to claim 2, characterized in that the last heat treatment is carried out in argon and hold for at least 4 hours a temperature of 470 and 500 ^o C. 4. The method according to any one of claims 1 to 3, characterized in that the annealing or annealing is carried out between passes through the rolls at a temperature of 700 - 750 ^o C for ΣA less than 2 • 10 ^-16 . 5. The method according to claim 4, characterized in that the preform is obtained by dragging a bar from the alloy in the α-phase and intermediate annealing between the passes of cold rolling is carried out at a temperature of 700 - 750 ^o C for 3 hours 6. The method according to claim 5, characterized in that the preliminary stage of supplying the alloy bar in the α phase, intended to be subjected to drawing in the form of a workpiece, is annealed, based on the β phase, with a temperature of 700 - 730 ^o C for 4 - 6 hours, mostly about 5 hours 7. The method according to claim 6, characterized in that the beam in the β-phase is fed to hardening with an initial cooling rate of 5 - 30 ^o C / s, starting from a temperature above 1000 ^o C and up to about 800 ^o C.  8. The method according to any one of the preceding paragraphs, characterized in that the Fe / Cr ratio is maintained in the range of 2.2 - 2.6. 9. The method according to any one of claims 1 to 8, characterized in that the alloy contains 1.20 - 1.70% tin, 0.18 - 0.24 iron and 0.075 - 0.10% chromium, with the total amount of content iron and chromium is 0.28 - 0.37% as well as 50 - 120 million ^-1 silicon, 80 - 200 million carbon ^-1. 10. The method according to any one of claims 1 to 9, characterized in that the alloy contains 0.18 - 0.24% iron and 0.075 - 0.10% chromium, 0.45 - 0.75% tin and 1300 - 2300 million ^-1 oxygen, as well as 50 - 120 million ^-1 silicon, 80 - 200 million ^-1 carbon. 11. The pipe serving as the containment of nuclear fuel rod, of zirconium-based alloy also containing at least 0.18% iron, 0.07 - 0.13% chromium, tin, at least 900 million oxygen and less than ^-1 200 million ^-1 carbon, characterized in that the alloy contains up to 0.25% iron, 0.35 - 1.70% tin, up to 2300 million ^-1 oxygen, at least 80 million ^-1 carbon and 50 - 120 million ^{- 1} silicon, while the weight ratio of Fe / Cr is 1.9 - 2.5 and the alloy is mainly in the α phase and contains Zr (Fe / Cr) ₂ precipitates, which in most sizes are larger than 0.18 μm. 12. The pipe according to claim 11, characterized in that it has a distribution of iron between precipitation
Zr (Cr / Fe) ₂ -Zr ₂ FeSi-Zr ₂ FeNi-Zr ₂ Fe or Zr ₃ Fe
such that the ratio Fe / Cr in Zr (Cr / Fe) ₂ is in the range 2.2 - 2.6.  13. The pipe according to claim 11 or 12, characterized in that the chromium content is 0.075 - 0.10%.