A cladding tube for a fuel rod for nuclear reactors

Abstract

A fuel assembly, a fuel rod and a cladding tube for a fuel rod for a nuclear reactor are disclosed. The cladding tube includes a tubular substrate defining an inner space for housing nuclear fuel pellets, and a surface layer applied on the tubular substrate. The tubular substrate is made of a zirconium base alloy and has a first thermal expansion coefficient. The surface layer has an alloy which consists of a major part of main elements comprising Cr and at least one of Nb and Fe, a minor part of zirconium, and possibly a residual part of interstitial elements. The alloy of the surface layer has a second thermal expansion coefficient. The concentrations of the main elements are selected so that the second thermal expansion coefficient is greater than the first thermal expansion coefficient from 20 to at least 1300° C.

Claims

1-14. (canceled)

15. A cladding tube for a fuel rod for a nuclear reactor, the cladding tube comprising a tubular substrate defining an inner space for housing nuclear fuel, and a surface layer applied on the tubular substrate, wherein the tubular substrate is made of a zirconium base alloy and has a first thermal expansion coefficient, wherein the surface layer consists an alloy, and wherein the alloy consists of: a major part of main elements comprising Cr and at least one of Nb and Fe, a minor part of zirconium, and possibly a residual part of interstitial elements, wherein: the alloy of the surface layer has a second thermal expansion coefficient and that the concentrations of the main elements are selected so that the second thermal expansion coefficient is greater than the first thermal expansion coefficient from 20 to at least 1300° C.

16. A cladding tube according to claim 15, wherein the second thermal expansion coefficient is at least 1% greater than the first thermal expansion coefficient from 20 to at least 1300° C.

17. A cladding tube according to claim 16, wherein the second thermal expansion coefficient is at least 2% greater than the first thermal expansion coefficient from 20 to at least 1300° C.

18. A cladding tube according to claim 15, wherein the minor part of zirconium of the surface layer constitutes 0.1-5 weight-% of the alloy of the surface layer.

19. A cladding tube according to claim 18, wherein the concentration of zirconium in the alloy of the surface layer increases towards the tubular substrate, and the concentration of the main elements in the alloy of the surface layer decreases towards the tubular substrate.

20. A cladding tube according to claim 15, wherein the major part of main elements of the surface layer consists of Cr and Nb.

21. A cladding tube according to claim 15, wherein the major part of main elements of the surface layer consists of Cr, Mo and Nb.

22. A cladding tube according to claim 15, wherein the major part of main elements of the surface layer consists of Cr, Mo and Fe.

23. A cladding tube according to claim 15, wherein the surface layer has a thickness of at most 0.1 mm.

24. A cladding tube according to claim 15, wherein the surface layer has a thickness of at least 0.003 mm.

25. A cladding tube according to claim 15, wherein the surface layer is laser deposited and joined to the tubular substrate by a fusion bonding.

26. A cladding tube according to claim 15, wherein the interstitial elements of the residual part of the surface layer are present in the alloy with a concentration at a level, As Low As Reasonably Achievable, (ALARA—principle).

27. A fuel rod comprising a cladding tube according to claim 15, and nuclear fuel enclosed in the cladding tube.

28. A fuel assembly comprising a plurality of fuel rods according to claim 27.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The invention is now to be explained more closely through a description of various embodiments and with reference to the drawings attached hereto.

[0040] FIG. 1 discloses schematically a longitudinal sectional view of a fuel assembly for a nuclear reactor.

[0041] FIG. 2 discloses schematically a longitudinal sectional view of a fuel rod of the fuel assembly in FIG. 1.

[0042] FIG. 3 discloses schematically an enlarged longitudinal sectional view of a part of the fuel rod in FIG. 2 comprising a tubular substrate and a surface layer.

[0043] FIG. 4 discloses a diagram schematically indicating the thermal expansion coefficients of the tubular substrate and the surface layer.

[0044] FIGS. 5A-C disclose a respective diagram schematically indicating the concentration of the elements of the surface layer as a function of the distance from the outer surface of the surface layer.

DETAILED DESCRIPTION

[0045] FIG. 1 discloses a fuel assembly 1 configured for being used in a nuclear fission reactor, in particular in a Light Water Reactor, LWR, such as a Boiling Water Reactor, BWR, or a Pressurized Water Reactor, PWR.

[0046] The fuel assembly 1 comprises a bottom member 2, a top member 3 and a plurality of elongated fuel rods 4 extending between the bottom member 2 and the top member 3. The fuel rods 4 are maintained in their positions by means of a plurality of spacers 5.

[0047] Furthermore, the fuel assembly 1 may, for instance when to be used in a BWR, comprise a flow channel or fuel box indicated by dashed lines 6 and surrounding the fuel rods 4.

[0048] FIG. 2 discloses one of the fuel rods 4 of the fuel assembly 1 of FIG. 1. The fuel rod 4 comprises a nuclear fuel, for instance in the form of a plurality of sintered nuclear fuel pellets 10, and a cladding tube 11 enclosing the nuclear fuel, in this case the nuclear fuel pellets 10. The fuel rod 4 comprises a bottom plug 12 sealing a lower end of the cladding tube 11, and a top plug 13 sealing an upper end of the cladding tube 11. The nuclear fuel pellets 10 are arranged in a pile in an inner space 14 of the cladding tube 11. The cladding tube 11 encloses the fuel pellets 10 and a gas in the inner space 14.

[0049] A spring 15 is arranged in an upper plenum 16 of the inner space 14 between the pile of nuclear fuel pellets 10 and the top plug 13. The spring 15 presses the pile of nuclear fuel pellets 10 against the bottom plug 12.

[0050] As can be seen in FIG. 3, the cladding tube 11 comprises a tubular substrate 20 and a surface layer 21 applied on the tubular substrate 20. Preferably, the surface layer 21 forms an outer surface layer 4′ of the fuel rod 4 extending circumferentially around the fuel rod 4.

[0051] The tubular substrate 20 defines the inner space 14 housing nuclear fuel pellets 10. The tubular substrate 20 is made of a zirconium base alloy, which may comprise at least 98 weight-% of Zr, such as Zircaloy-2, Zircaloy-4, ZIRLO, ZrSn, E110, and M5.

[0052] The tubular substrate 20 has a first thermal expansion coefficient Cl, schematically illustrated in FIG. 4. As can be seen, the first thermal expansion coefficient C.sub.1 is not perfectly linear. This is due to the phase transformation from alpha phase to beta phase. The temperature range, at which the phase transformation occurs; may vary between various zirconium base alloys.

[0053] The surface layer 21 consists an alloy which alloy consists of a major part of main elements, a minor part of zirconium, and possibly a residual part of interstitial elements. The alloy of the surface layer 21 has a second thermal expansion coefficient C.sub.2.

[0054] The major part of main elements of the alloy of the surface layer 21 comprises Cr and at least one of Nb and Fe, or consists of Cr, Mo and at least one of Nb and Fe.

[0055] The minor part of zirconium of the surface layer 21 may constitute 0.1-5 weight-% of the alloy of the surface layer 21. This concentration may be an average concentration of Zr in the alloy of the surface layer 21. Preferably, the concentration of zirconium in the alloy of the surface layer 21 may increase towards the tubular substrate 20, and thus the concentration of the main elements in the alloy of the surface layer 21 may decrease towards the tubular substrate 20. This is schematically illustrated in FIGS. 5A-5C.

[0056] The possible interstitial elements of the residual part of the surface layer 21 are present in the alloy of the surface layer 21 with a concentration at a level that is, As Low As Reasonably Achievable, (ALARA principle). The total concentration of the interstitial elements may thus be less than 0.5 weight-%, preferably less than 0.4 weight-%, more preferably less than 0.3 weight-%, even more preferably less than 0.2 weight-%, and most preferably less than 0.1 weight-%.

[0057] The interstitial elements may thus comprise small or very small quantities of impurities and traces of further elements and substances than those defined above, i.e. than Cr, Fe, Nb, Mo and Zr. For instance, other elements than Zr, such as Sn, C, N, Si, O, etc., may migrate from the zirconium base alloy of the substrate 20 into the surface layer 21.

[0058] The concentrations of the main elements of the alloy of the surface layer 21 are selected so that the second thermal expansion coefficient C.sub.2 is greater than the first thermal expansion coefficient C.sub.1 from 20 to at least 1300° C.

[0059] Preferably, the second thermal expansion coefficient C.sub.2 may be at least 1% greater than the first thermal expansion coefficient C.sub.1 from 20 to at least 1300° C.

[0060] More preferably, the second thermal expansion coefficient C.sub.2 is at least 2% greater than the first thermal expansion coefficient C.sub.1 from 20 to at least 1300° C.

[0061] As is illustrated in FIG. 4, the second thermal expansion coefficient C.sub.2 may vary within the range defined by the dashed lines depending of the concentrations of the selected main elements. The second thermal expansion coefficient C.sub.2 is linear or approximately linear.

[0062] The surface layer 10 may have a thickness of at most 0.1 mm, and at least 0.003 mm, at least 0.005 mm or at least 0.01 mm.

[0063] The surface layer 21 may be laser deposited and joined to the tubular substrate 20 by a fusion bonding. The main elements, and possibly Zr, to be comprised by the surface layer 21 may be provided in powder form. A mixture of powders of the main elements, and possibly Zr, may be applied to the tubular substrate 20 and form the surface layer 21 by means of a laser.

[0064] By means of this laser deposit of the main elements, and possibly Zr, the above mentioned increase of the concentration of zirconium towards the tubular substrate 20, and decrease of the concentration of the main elements towards the tubular substrate 20 may be achieved.

[0065] The fusion bonding of the surface layer 21 is defined by a fusion zone 22 in which the concentration of the main elements decreases and the concentration of Zr increases. The fusion zone 22 is approximately illustrated in FIGS. 5A-5C, and located between the dashed lines in FIGS. 5A-5C.

[0066] The increase of the concentration of Zr in surface layer 21 and the fusion zone 22 will principally follow the lines in FIGS. 5A-5C irrespective of how Zr has been added, i.e. as a component of the powder to be laser deposited or if Zr atoms have migrated from the tubular substrate 20 towards the outer surface 4′.

[0067] In the following three different examples of suitable combinations of main elements of the major part of the alloy are presented. These three examples shall not be interpreted as excluding other examples of combinations of suitable main elements.

EXAMPLE 1

[0068] In example 1, the major part of main elements of the surface layer 21 consists of Cr and Nb. Cr may be present in the alloy with a concentration of 51 weight-%, and Nb may be present in the alloy with a concentration of 47 weight-%. Zr may be present in the alloy with a concentration of 2 weight-%.

[0069] The alloy of this example has neutron cross section of 2.1 Barns, or substantially 2.1 Barns, and linear thermal expansion of 6.7×10.sup.−6/° C., or substantially 6.7×10.sup.−6/° C.

[0070] The diagram of FIG. 5A discloses schematically the variations of the concentrations of Cr, Nb and Zr in the surface layer 21 from the outer surface 4′ to the tubular substrate 20.

[0071] Without deviating substantially from example 1, the concentration of Cr may lie in the range 48-54 weight-% and the concentration of Nb in the range 44-50weight-%.

EXAMPLE 2

[0072] In example 2, the major part of main elements of the surface layer consists of Cr, Mo and Nb. Cr may then be present in the alloy with a concentration of 23 weight-%, Mo may be present in the alloy with a concentration of Mo may be present in the alloy with a concentration weight-% 19 weight-% and Nb may be present in the alloy with a concentration of 55 weight-%.

[0073] The alloy of example 2 may have neutron cross section of 1.9 Barns, or substantially 1.9 Barns, and linear thermal expansion of 6.6×10.sup.−6/° C., or substantially 6.6×10.sup.−6/° C.

[0074] The diagram of FIG. 5B discloses schematically the variations of the concentrations of Cr, Mo, Nb and Zr in the surface layer 21 from the outer surface 4′ to the tubular substrate 20.

[0075] Without deviating substantially from example 2, the concentration of Cr may lie in the range 20-26 weight-%, the concentration of Mo in the range 16-22 weight-%, and the concentration of Nb in the range 52-58 weight-%.

EXAMPLE 3

[0076] In example 3, the major part of main elements of the surface layer consists of Cr, Mo and Fe. Cr may then be present in the alloy with a concentration of 29 weight-%, Mo may be present in the alloy with a concentration of 48 weight-%, and Fe may be present in the alloy with a concentration of 20 weight-%.

[0077] The alloy of example 2 may have neutron cross section of 2.7 Barns, or substantially 2.7 Barns, and linear thermal expansion of 6.7×10.sup.−6/° C., or substantially 6.7×10.sup.6 /° C.

[0078] The diagram of FIG. 5C discloses schematically the variations of the concentrations of Cr, Mo, Fe and Zr in the surface layer 21 from the outer surface 4′ to the tubular substrate 20.

[0079] Without deviating substantially from example 2, the concentration of Cr may lie in the range 26-32 weight-%, the concentration of Mo in the range 45-51 weight-%, and the concentration of Fe in the range 17-23 weight-%.

[0080] The present invention is not limited to the embodiments disclosed but may be varied and modified within the scope of the following claims.