EP0673691B1 - Method of production of a Zircalloy-2 tubular blank internally clad with zirconium, adapted for ultrasonic measurement of the thickness of zirconium - Google Patents

Description

TECHNICAL AREA

The invention relates to a method of manufacturing a tubular zircaloy blank 2 clad internally with zirconium and intended for the production of composite cladding tubes for nuclear fuel. The internal zirconium coating constitutes a barrier to the fission products and to the hydrogen generated in the fuel which have an embrittling action on the external zircaloy cladding 2 and therefore the thickness of this coating must be precisely controlled and reproducible.

PRIOR ART

The regularity of the thickness of the internal zirconium layer of Zy2 / Zr composite cladding tubes for nuclear fuel is an essential characteristic which should therefore be checked with great precision and systematically on each cladding tube and by the made on each composite tubular blank from which it originated.

To this end, it is necessary to use industrial methods of non-destructive testing deemed to be precise and reliable but also easy to implement and rapid, as regards controlling the thickness of the internal zirconium cladding of each composite tubular blank. over its entire length.

Thus methods of thickness control by eddy current have been used, in particular by M. IWASAKI, N. SUZUKI, Y. NISHIMOTO, M. KOTAN and N. FUJII in their publication p. 447-452 of NUCLEAR ENGINEERING AND DESIGN 94 (1986). These methods, which are well suited to measuring the thickness of a few tenths of a millimeter of composite sheathing tubes, become imprecise and therefore unsuitable for thicknesses of 1 mm and more. In the present case, they do not cover the entire range of internal plating thicknesses in Zr of the blanks of composite sheath tubes which can vary from 0.5 mm to 1.5 or even 2 mm.

• soit en accentuant l'effet d'interface dans le matériau par création entre les 2 couches d'une couche intermédiaire à base de graphite et de méthylcellulose selon JP-58199139 mais cela conduit sauvent à modifier très sensiblement les conditions d'élaboration du produit avec le risque d'altérer certaines de ses propriétés ou simplement d'augmenter son coût de production
• soit en améliorant la précision et la reproductibilité des mesures ultrasonores, elles-mêmes, par la mise en oeuvre de moyens plus adaptés comme ceux décrits dans EP 0429854A, US 4991440 et surtout EP 335808B préconisant le contrôle ultrasonore d'épaisseur du placage interne par l'extérieur du tube au moyen d'un transducteur focalisant bien amorti, de fréquence principale de résonnance comprise entre 4 et 20 MHz, utilisé en émission réflexion et muni des moyens électroniques appropriés à la détection d'au moins un écho double de l'interface placage/âme du tube, de moyens indépendants de positionnement en distance et en orientation par rapport au tube à contrôler ainsi que de moyens de suivi de cette position pour des tubes successifs du même type.

If the ultrasonic thickness control methods of Zr / Zr composite sheath tubes have long been considered too imprecise due to the too small difference in acoustic impedance between pure zirconium and alloy zirconium, progress significant achievements have been made recently in this area,

• either by accentuating the interface effect in the material by creation between the 2 layers of an intermediate layer based on graphite and methylcellulose according to JP-58199139 but this saves very significantly to modify the conditions of production of the product with the risk of altering some of its properties or simply increasing its production cost
• either by improving the precision and the reproducibility of the ultrasonic measurements, themselves, by the use of more suitable means such as those described in EP 0429854A, US 4991440 and especially EP 335808B recommending the ultrasonic thickness control of the internal plating by l outside the tube by means of a well-damped focusing transducer, with a main resonance frequency of between 4 and 20 MHz, used in reflection transmission and provided with electronic means suitable for detecting at least one double echo of the interface plating / core of the tube, independent means of positioning in distance and orientation relative to the tube to be checked as well as means for monitoring this position for successive tubes of the same type.

These improvements, while increasing the sensitivity of the ultrasonic thickness measurements of the internal Zr cladding in the composite Zr / Zr alloy tubes, have proved to be still insufficient for a precise and reliable determination of the thickness of the Zr cladding in the tubes. Zircaloy 2 / Zr composites because the relative difference in acoustic impedances between Zircaloy 2 and Zr remains less than 2%. This is particularly the case for tubular blanks and composite tubes produced according to the method described in FR 2579122 which recommends refining the grain of the internal coating in Zr in order to improve its surface condition and this by adjusting the content of Zr ingot iron between 250 and 1000 ppm, in combination with a quenching of the billets from the wrought ingot of Zr from a temperature between 880 ° C and 1050 ° C.

The poor quality of the ultrasonic signal observed resulted, during dynamic circumferential exploration of the tubular blank or of the composite tube according to the method EP 335808B, by signal losses in different areas where the thickness measurement does not could therefore be realized.

OBJECT OF THE INVENTION

From a first experimental observation that the reliability of the ultrasonic thickness measurements of the plating Zr of the tubular blanks obtained in particular according to the process FR 2579122 previously described was a function of metallurgical parameters closely dependent on the method of production, the applicant sought and developed a process for manufacturing tubular zircaloy 2 blanks clad internally with zirconium which are suitable for ultrasonic control of the zirconium thickness.

• a) trempe à l'eau à partir du domaine bêta d'une barre corroyée de zircaloy 2, avant ou après découpe en billettes percées ou non, et suivie d'un éventuel recuit
• b) trempe à l'eau d'une billette en zirconium non allié de teneur en fer comprise entre 250 et 1000 ppm, avant ou après perçage, à partir d'une température comprise entre 880°C et 1050°C
• c) extrusion dans le domaine alpha sous forme de tube de la billette en zirconium
• d) positionnement du tube de zirconium à l'intérieur de la billette percée en zircaloy 2 et assemblage
• e) extrusion de l'assemblage ainsi obtenu dans le domaine alpha sous forme d'ébauche extrudée composite
• f) laminage à froid et traitement thermique de l'ébauche extrudée composite pour obtenir une ébauche tubulaire composite constituant le tube-gaine composite Zy2/Zr, caractérisé en ce que, au moyen d'un traitement thermomécanique approprié de la barre de zircaloy 2 corroyée dans le domaine alpha après trempe à l'eau à partir du domaine bêta, et/ou de la billette trempée de zircaloy 2, éventuellement recuite et percée, et/ou de la billette en zirconium non allié après extrusion sous forme de tube dans le domaine alpha, et/ou de l'ébauche tubulaire composite après laminage à froid, on règle la taille de grain selon l'indice ASTM entre 9 et 12 sur le zircaloy 2 et entre 6 et 10 sur le zirconium non allié de ladite ébauche composite, en conservant une différence de taille de grain ΔI entre le zircaloy 2 et le zirconium non allié d'au moins 2 indices ASTM.

More precisely, the invention relates to a method for manufacturing a composite tubular blank made of zircaloy 2 clad internally with zirconium comprising, according to the prior art, the following essential steps:

• a) water quenching from the beta domain of a wrought bar of zircaloy 2, before or after cutting into billets, pierced or not, and followed by possible annealing
• b) quenching with water a billet of unalloyed zirconium with an iron content between 250 and 1000 ppm, before or after drilling, from a temperature between 880 ° C and 1050 ° C
• c) extrusion in the alpha domain in the form of a zirconium billet tube
• d) positioning of the zirconium tube inside the billet pierced with zircaloy 2 and assembly
• e) extrusion of the assembly thus obtained in the alpha domain in the form of a composite extruded blank
• f) cold rolling and heat treatment of the composite extruded blank to obtain a composite tubular blank constituting the composite sheath tube Zy2 / Zr, characterized in that, by means of an appropriate thermomechanical treatment of the wrought zircaloy 2 bar in the alpha domain after quenching with water from the beta domain, and / or the billet quenched with zircaloy 2, optionally annealed and pierced, and / or the billet in unalloyed zirconium after extrusion in the form of a tube in the alpha domain, and / or of the composite tubular blank after cold rolling, the grain size is adjusted according to the ASTM index between 9 and 12 on the zircaloy 2 and between 6 and 10 on the unalloyed zirconium of said composite blank , keeping a grain size difference ΔI between zircaloy 2 and unalloyed zirconium of at least 2 ASTM indices.

Metallurgical examinations carried out on different samples subjected to the same ultrasonic control conditions have shown that the difficulties in obtaining a good quality ultrasonic signal were in particular a function of the regularity of the Zy2 / Zr interface but also and above all of the size difference of grain between the external zircaloy 2 component and the internal zirconium component of the tubular blank. Thus the Applicant has found on series of composite tubular blanks zircaloy 2 / zirconium (developed according to FR 2579122 already cited) whose internal zirconium plating contains in particular from 250 to 1000 ppm of iron only barely 30% thickness measurements of said internal cladding could be taken into account. Indeed, the numerous aberrant measurements generally corresponded to anomalies in the reflection of the interface, distributed randomly along each tubular blank, each of the points of the surface of which describes, during the control, a helix whose pitch is defined by the rotational speed and advance of the blank. Thus under the control conditions according to EP 335808B using a transducer excited at the frequency of 1 KHz, the tubular blanks being driven with a rotation speed of 250 rpm with an advance of 1 m / min for each propeller turn of pitch = 4 mm, theoretically 240 measurements had to be collected per turn of the propeller or only 72 measurements were found to be usable with an accuracy of ± 5 / µm on the measurement thick.

In addition, an ASTM grain index of 10 was determined for the external zircaloy 2 sheath and 10 also for the internal sheath or zirconium cladding, therefore corresponding to a difference in zero ASTM index, ie ΔI = 0.

• Figure 1 représentation schématique des étapes du procédé de l'art antérieur le plus proche.
• Figure 2 représentation schématique du même procédé incluant le mode de réalisation de l'invention consistant à effectuer après trempe de la barre de zircaloy 2 un forgeage complémentaire dans le domaine alpha.
• Figure 3 représentation schématique du même procédé incluant le 2ème mode de réalisation de l'invention consistant à effectuer après trempe le perçage de la billette de zircaloy 2 par filage à la presse selon par exemple le procédé décrit dans FR 2 685 881.
• Figure 4 représentation schématique du même procédé incluant le 3ème mode de réalisation de l'invention consistant à effectuer un traitement thermique de recristallisation sur billette en zirconium non allié après filage dans le domaine alpha.
• Figure 5 représentation schématique du même procédé incluant le 4ème mode de réalisation de l'invention consistant à effectuer au moins un traitement thermique de recristallisation sur l'ébauche tubulaire composite.

During the development of the Zy2 / Zr composite tubular blanks according to the prior art and in particular FR 2579122, the Applicant has determined 4 possible modifications of the operating mode, achievable independently of each other or in combination, all likely to change certain characteristics. metallurgical, in particular the grain size of the internal and external components of the composite blank to make said components suitable for ultrasonic thickness control. These modifications which implement new thermal and / or mechanical treatments without altering the properties of the final product in its use will be better understood by the following detailed description based on FIGS. 1 to 5, namely:

• Figure 1 schematic representation of the steps of the closest prior art method.
• Figure 2 schematic representation of the same process including the embodiment of the invention consisting in performing after quenching the zircaloy bar 2 additional forging in the alpha domain.
• Figure 3 schematic representation of the same process including the 2nd embodiment of the invention consisting in carrying out after quenching the drilling of the zircaloy billet 2 by press spinning according for example to the process described in FR 2 685 881.
• Figure 4 schematic representation of the same process including the 3rd embodiment of the invention consisting in carrying out a recrystallization heat treatment on a billet of unalloyed zirconium after spinning in the alpha domain.
• Figure 5 schematic representation of the same process including the 4th embodiment of the invention consisting in performing at least one recrystallization heat treatment on the composite tubular blank.

DESCRIPTION

According to the prior art (fig. 1) the external zircaloy component 2 is obtained after working A1 of a bar full of Zy2 up to the diameter of 177 mm (150 mm ≤ Φ ≤ 200 mm) which, after heating A2 for 1 hour at 1050 ° C, A3 is quenched with possible A4 annealing for 3 to 5 hours between 750 ° C and 780 ° C. After cutting the bar into billets, they are machined A5 and drilled (Φe 168 mm, Φi 78.8 mm).

In parallel, the internal component of unalloyed zirconium is obtained by working B2 of an ingot of zirconium melted under vacuum B1, the iron content of which is between 250 and 1000 ppm. After cutting the forged bar into 172 mm diameter billets (150 ≤ Φ ≤ 200 mm) these are heated between 880 ° C and 1050 ° C to be quenched B3 then machined B4 (Φe 168 mm - Φi 51 mm) before to be extruded B5 in the alpha domain with diameters Φe 82 mm and Φi 47 mm in the form of tubes; each tube being machined to 78e 78.8 mm and Φi 48 mm before positioning for assembly C1 inside the substantially coaxial hole of the zircaloy billet 2 machined and drilled.

The assembly C1 thus formed is spun C2 in the alpha domain, preferably around 600 ° C, to obtain a composite extruded blank (Φe 80 mm, Φi 48 mm) which is cold rolled C3 to Φe 63, 5 mm and Φi 41.5 mm to give a composite tubular blank subjected to a possible final heat treatment. Under these conditions and as previously indicated, only 30% of the thickness measurements of the internal zirconium cladding can be used.

A first solution, shown diagrammatically in FIG. 2, to improve the regularity of the Zy2 / Zr interface and above all to create an index difference of grain size ΔI at least equal to 2, consists in performing after quenching A3 of the zircaloy 2 bar a working in the alpha domain causing a very significant refinement of the grain size of zircaloy 2 at the roughing stage which is preserved after C1 assembly, C2 spinning and C3 rolling of the composite tubular blank, and allowing to significantly regularize the Zy2 / Zr interface.

More precisely after heating A2 for 1 hour at 1050 ° C (1030 to 1070 ° C), the alpha hardening of an already wrought bar A'1 is carried out by forging or rolling to a diameter of approximately 300 mm ( 250 to 350 mm) and not 177 mm (150 to 200 mm) according to the prior art. After heating for 3 to 5 hours between 750 ° C and 780 ° C, the bar diameter is reduced by forging A4 in alpha phase up to Φ 177 mm before cutting, machining and drilling the A5 billets. ready for C1 assembly and transformation according to the prior art.

• Pour le placage interne en Zr non allié I1 = 10
• pour la gaine externe en Zy2   I2 = 12
• soit un écart ΔI = 2 indices ASTM.

Grain size measurements in the ASTM index indicated:

• For internal plating in unalloyed Zr I1 = 10
• for the outer sheath in Zy2 I2 = 12
• or a difference ΔI = 2 ASTM indices.

In parallel, the ultrasonic thickness checks of the internal cladding on a series of 10 Zy2 / Zr composite tubular blanks thus produced indicated an average of 218 usable measurements with a dispersion of ± 5% out of a theoretical total of 240 per helix turn according to the previously described control conditions (see Table 1).

The second solution, shown schematically in fig. 3, makes it possible to improve the regularity of the Zy2 / Zr interface of composite tubular blanks just as significantly and above all to obtain ΔI ≥ 2. It consists in acting on the grain size of the full or pre-drilled billets of zircaloy 2 after quenching A3 and optionally annealed A4 by performing a drilling operation by spinning with the press A'4 between 400 ° C and 600 ° C according to the process described in patent FR 2,685,881 relating to the manufacture of zirconium-based duplex and triplex tubes. This process recommends using a conventional direct spinning press the spinning with discharge on a zirconium or zirconium alloy billet punch to improve and regularize the internal surface structure of the tubular element to be produced.

In the present case, the Applicant has found experimentally that the regularity of the interface closely depends on the micrographic structures of the 2 components of the assembly before spinning and that the interface resulting from said assembly was all the more irregular as the phase needles ex beta, originating from quenching from the beta domain of zircaloy 2 were wider. By spinning with delivery on a punch of the hardened or possibly annealed Zy2 billets, a particularly effective working of the internal face of the Zy2 blank is obtained which will be placed in contact with the external face of the tubular blank in unalloyed Zr. We notably obtain the breaking of the acicular structure of beta transformation into alpha quenching and consequently a refinement of the average size of the Zy2 grains with a smoothing of the interface irregularities.

Thus from machined hardened billets of Φe 168 mm and pre-drilled at Φi 25 mm we obtain after drilling A'4 the press at 500 ° C a blank of Zy2 of Φe 172 mm and Φi 70 mm which after re-machining A5 at Φe 168 mm and Φi 78 mm is assembled with the tubular blank of zirconium C1 with low iron content to be spun C2 and cold rolled C3 according to the prior art.

• pour le placage interne en Zr non allié I1 = 10
• pour la gaine externe en Zy2   I2 = 12
• soit un écart ΔI = 2 indices ASTM.

Grain size measurements in the ASTM index indicated:

• for internal plating in unalloyed Zr I1 = 10
• for the outer sheath in Zy2 I2 = 12
• or a difference ΔI = 2 ASTM indices.

In parallel, the ultrasonic thickness checks of the internal plating in Zr on a series of 10 composite blanks thus produced indicated an average of 204 usable measurements out of a theoretical total of 240 with a dispersion of ± 5% (see Table 1).

A third solution, shown diagrammatically in FIG. 4, also leads to perfectly acceptable precision and reproducibility of the measurements and consists in specifically favoring the magnification of the grain of the unalloyed zirconium blank during its preparation by a heat treatment of specific recrystallization B'5 of the tubular zirconium blank after spinning in the alpha B5 domain. This heat treatment is carried out at a sufficient temperature 500 ° C to 780 ° C for 1 hour to 4 hours and preferably at 730 ° C for 3 hours, to obtain a grain magnification whose ASTM index is between 4 and 6 .

During the subsequent operations of spinning C2 and cold forging C3 of the composite tubular blank according to the prior art, there is a significant refinement of the zirconium grains of the internal cladding whose grain size index reaches 7 while the grain size index of the external cladding of Zy2 remains stable at 10, i.e. an index difference ΔI = 3.

In parallel, the ultrasonic thickness checks of the internal plating in Zr on a series of 10 elaborate blanks indicated an average of 209 usable measurements for 240 theoretical with a dispersion of ± 5% around this average (see Table 1).

A 4th solution, a little less effective but easy to carry out indistrially, is shown diagrammatically in FIG. 5. It consists in performing on the composite tubular blank either a recrystallization annealing C'2 after assembly C1, spinning C2 according to the prior art, or a recrystallization annealing C4 after assembly C1, spinning C2, optionally a recrystallization annealing C'2 then rolling C3. This recrystallization annealing C'2 and / or C4 is carried out under such conditions, generally 1 to 3 hours between 700 ° C and 730 ° C, that on the internal zirconium cladding a grain size index of at least 7 and preferably 8 while keeping a grain size index of at least 9 and preferably 10 with an index difference Δ I ≥ 2 on the external zircaloy 2 sheath.

The ultrasonic thickness control carried out on a series of 10 composite tubular blanks thus developed indicated an average of 204 usable measurements for 240 theoretical with a dispersion of ± 5% (see table 1).

It should be noted that these different solutions occurring at different stages of the basic process can be combined together, each contributing to the increase in the number and therefore of the percentage of readable measurements and consequently to the improvement of the reliability of the ultrasonic measurements. of thickness of the internal cladding in unalloyed zirconium as shown in Table 1 below.

Note, however, that the most effective combinations are the binary combinations implementing a specific working of the external sheath in Zy2 (1st or 2nd solution) with a heat treatment of recrystallization (3rd or 4th solution). TABLE 1 SUMMARY OF RESULTS Type of process Number of readable measurements % measured readable I2 ASTM Zy2 I1 ASTM Zr ΔI (I2 - I1) Prior art 72 30 10 10 0 1st solution 218 91 12 10 2 2nd solution 204 85 12 10 2 3rd solution 209 87 10 7 3 4th solution 204 85 9 7 2 1st + 2nd 225 94 12 10 2 1st + 3rd 230 96 12 7 5 1st + 4th 225 94 12 8 4 2nd + 3rd 228 95 12 7 5 2nd + 4th 226 94 11 7 4 1st + 3rd + 4th 220 92 11 7 4 2nd + 3rd + 4th 218 91 10 7 3

Claims (10)

1. A process for the production of a composite tubular blank of zircaloy 2 internally plated with zirconium comprising in accordance with the prior art the following essential steps:

   a) quenching with water from the domain beta of a worked bar of zircaloy 2 prior to or after being cut into billets which are or are not pierced, with optional annealing,

   b) quenching with water of a worked billet of non-alloyed zirconium with an iron content of between 250 and 1000 ppm, prior to or after piercing, from a temperature of between 880°C and 1050°C,

   c) extrusion in the domain alpha in the form of a tube of the zirconium billet,

   d) positioning of the zirconium tube in the interior of the pierced billet of zircaloy 2 and assembly,

   e) extrusion of the assembly obtained in that way in the domain alpha in the form of a composite extruded blank,

   f) cold rolling and heat treatment of the composite extruded blank to obtain a composite tubular blank consituting the composite sheath-tube Zy2/Zr, characterised in that, by a suitable thermomechanical treatment of the bar of zircaloy 2 which is worked in the domain alpha, after quenching with water from the domain beta, and/or of the quenched billet of zircaloy 2 which is possibly annealed and pierced, and/or the billet of non-alloyed zirconium after extrusion in tube form in the domain alpha, and/or the composite tubular blank after cold rolling, the grain size is regulated in accordance with the ASTM index at between 9 and 12 on the zircaloy 2 and between 6 and 10 on the non-alloyed zirconium of said composite blank while retaining a grain size difference ΔI between the zircaloy 2 and the non-alloyed zirconium of at least 2 ASTM indices.
2. A process according to claim 1 characterised in that the grain size of the zircaloy 2 is preferably regulated to the indices 11 or 12 and the grain size of the non-alloyed zirconium is preferably regulated to the indices 7 or 8.
3. A process according to claim 1 or claim 2 characterised in that after quenching from the domain beta of the bar of zircaloy 2 an operation of working said bar in the domain alpha is effected.
4. A process according to claim 3 characterised in that the operation of working the bar in the domain alpha is a forging operation after heating for a period of 3 to 5 hours at between 750 and 780°C.
5. A process according to any one of claims 1 to 4 characterised in that a piercing operation by means of extrusion with a press using a known process is effected on the solid or pre-pierced billet of zircaloy 2, after quenching alpha from the domain beta.
6. A process according to claim 5 characterised in that said piercing operation using extrusion is effected at 400°C to 600°C on a direct extrusion press with extrusion over a mandrel of the billet of Zy2.
7. A process according to any one of claims 1 to 6 characterised in that after extrusion in the domain alpha in tube form of the billet of non-alloyed zirconium a recrystallisation heat treatment is effected at between 500°C and 780°C for from 1 to 4 hours.
8. A process according to claim 7 characterised in that the recyrstallisation heat treatment is preferably for 3 hours at 730°C.
9. A process according to any one of claims 1 to 8 characterised in that after extrusion and/or after rolling of the composite tubular blank a recyrstallisation heat treatment is effected under conditions such as to obtain a grain size at least of index 7 and preferably of index 8 on the internal sheathing of non-alloyed zirconium while retaining a grain size at least of index 10 and preferably index 11 on the external sheath of zircaloy 2.
10. A process according to claim 9 characterised in that the recrystallisation heat treatment of the composite tubular blank after extrusion is for 1 to 3 hours at between 700°C and 730°C.

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