TECHN ICAL FI ELD OF TH E INVENTION

The present i nvention refers generally to doping of tubular ceramic SiC and SiC-SiC components, such as flow channels and cladding tubes in fuel assemblies , for nuclear reactors, especially water reactors, such as Boiling Water Reactors, BW R, and Pressurized Water Reactors, PW R. The i nvention could also be applicable to fast reactors , such as lead fast reactors .

In particular, the present invention refers to a tubular ceramic component suitable for being used in a nuclear reactor, comprising an i nner layer of silicon carbide, an intermediate layer of silicon carbide fibres in a fill material of silicon carbide, the intermediate layer adjoining the i nner layer, and an outer layer of silicon carbide, the outer layer adjoining the i ntermediate layer.

BACKG ROUND OF TH E INVENTION AND PRIOR ART

It is known to use silicon carbide or silicon carbide composites in nuclear components, such as fuel assemblies and flow channels.

US 2006/0039524 discloses a multi-layered cladding tube comprising an inner layer of monolithic silicon carbide, a central layer of silicon carbide fibres surrounded by a silicon carbide matrix, and an outer layer of silicon carbide.

WO 201 1 /1 34757 discloses a flow channel for a fuel assembly. The flow channel comprises an i nner layer of silicon carbide, a central layer of silicon carbide fibres surrounded by a filler material of silicon carbide, and an outer layer of silicon carbide .

Pure silicon carbide, or substantially pure silicon carbide, grows isotropically when exposed to irradiation and high temperatures. The growth is due to impurities (secondary phases) in the crystalline silicon carbide, and to the formation of defects in the crystalline silicon carbide. In a nuclear reactor, a relatively rapid growth of a silicon carbide component will occur during an initial phase up to a certai n level which then remai ns relatively constant duri ng the lifetime of the component. This is a problem in cladding tubes of silicon carbide. Since the fuel in the cladding tubes swells conti nuously during t he lifetime of the fuel , it is difficult to maintai n a constant pellet- cladding gap.

The growth due to impurities may be avoided by securing a small amount of secondary phases, which may be possible by choosi ng a suitable manufacturi ng method .

The growth due to the formation of defects occurs in the temperature interval 250-400 °C through the formation of point defects, i .e. atoms of Si or C are moved to interstitial positions in the crystalline structure.

SUMMARY OF THE INVENTION

The object of the present invention is to overcome the problems discussed above. I n particular, the invention aims at a reduced and more uniform growth , or swelling , of the tubular ceramic component upon exposure to a neutron fl ux duri ng operation in a nuclear reactor. This object is achieved by the tubular ceramic component initially defined, which is characterized i n that the silicon carbide of the inner layer, the fill material and the outer layer is doped and comprises at least one dopant in solid solution withi n crystals of the silicon carbide. The silicon carbide of the inner layer, the fill material and the outer layer may thus comprise pure crystalline silicon carbide, or substantially pure crystalline silicon carbide, with the dopant or dopants in solid solution i n the silicon carbide crystals and with very small quantities of secondary phases, for instance less than 1 % of secondary phases.

By adding one or more dopants to the silicon carbide of the inner layer, the fill material and the outer layer, the growth of the cladding tube duri ng operation in the nuclear reactor may be reduced and modified to be more uniform . Especially during the initial phase of the operation, the relatively rapid growth of non- doped silicon carbide components may be significantly reduced . The dopant or dopants will provide a pre-swelling or growth of the silicon carbide, before the operation of the tubular ceramic component in the nuclear reactor. The change i n connectivity due to the presence of the dopant or dopants in solid solution in the crystal structure of the silicon carbide will mean that a population of defects will exist within the structure that will enhance mobility of certain defects and promote additional defects to recombine, preventing further swelling or growth.

The dopant or dopants will provide a possibility to control the defect-related growth and the formation of point defects. The main part of the defect-related growth is due to the displacement of C-atoms. This displacement creates internal stresses and deformation. At sufficiently high i nternal stresses (which i ncrease with reduced temperature), the growth stops (since new point defects are not any longer stable), i .e. the saturation growth is highest at low temperatures.

Accordi ng to an embodiment of the invention, the dopant comprises at least one of the substances B, N, Al , P, O, Be, Li , S, Ti , Ge, P2O3, P2O5, AI2O3, AI N , AUC3 and TiC-i-x. Doping of the silicon carbide may thus be achieved by addi ng one or more of the elements B, N, Al , P, O, Be, Li , S, Ti , Ge, and/or one or more of the compounds P2O3, P2O5, AI2O3, AI N , AUC3 and TiCi- _x duri ng the manufacturing of the tubular ceramic component.

These dopants may have following properties maki ng them suitable in the silicon carbide of the tubular ceramic component.

- Low neutron cross-section minimizing the absorption of neutrons.

- Larger size of the element than C increasi ng the formation of internal stresses. The smaller of the dopants may replace the C- atoms in the silicon carbide, whereas the larger of the elements, e.g. S and Ge, may replace the Si -atoms in the silicon carbide. Some of the dopants may replace both Si and C to different degrees.

- Strong repulsive interaction to interstitials leading to saturation at a lower degree of growth. Accordi ng to an embodiment of the invention, the concentration of the dopant in the silicon carbide is 1 -1 000 ppm.

Accordi ng to an embodiment of the invention, the concentration of the dopant in the silicon carbide is 1 0-1 000 ppm.

Accordi ng to an embodiment of the invention, the concentration of the dopant in the silicon carbide is 50-1 000 ppm.

Accordi ng to an embodiment of the invention, the dopant comprises at least N, wherei n the nitrogen is enriched to contain a higher percentage of the isotope ¹⁵ N than natural N.

Accordi ng to an embodiment of the invention, the dopant comprises at least B, wherein the boron is enriched to contain a higher percentage of the isotope ^{1 1} B than natural B. Accordi ng to an embodiment of the invention, the silicon carbide of the i nner layer, the fill material and the outer layer has a concentration of secondary phases that is less than 1 %, preferably less than 0,8%, more preferably less than 0,6%, and most preferably less than 0,4%.

Accordi ng to an embodiment of the invention, the tubular ceramic component forms a cladding tube of a fuel rod and encloses a pile of nuclear fuel pellets.

Accordi ng to an embodiment of the invention, the tubular ceramic component forms flow channel of a fuel assembly and encloses a plurality of fuel rods. BREI F DESCRIPTION OF TH E DRAWINGS

The i nvention is now to be explained more closely through a description of various embodiments and with reference to the drawings attached hereto.

Fig 1 discloses schematically a longitudinal sectional view of a fuel assembly for a nuclear reactor.

Fig 2 discloses schematically a longitudinal sectional view of a fuel rod of the fuel assembly in Fig 1 .

Fig 3 discloses schematically a partly sectional view of a part of the fuel rod i n Fig 2.

Fig 4 discloses schematically a partly sectional view of a part of the fuel assembly in Fig 1 . DETAI LED DESCRI PTION OF VARIOUS EMBODIM ENTS

Fig. 1 discloses a fuel assembly 1 for use i n nuclear reactors, in particular in water cooled light water reactors, LW R, such as a Boiling Water Reactor, BW R, or a Pressurized Water reactor, PW R. The fuel assembly 1 comprises a bottom member 2, a top member 3 and a plurality of elongated fuel rods 4 extendi ng between the bottom member 2 and the top member 3. The fuel rods 4 are maintai ned in their positions by means of a plurality of spacers 5. Furthermore, the fuel assembly 1 comprises, when i ntended to be used in a BW R, a flow channel 6 that surrounds and encloses the fuel rods 4.

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 in the form of a plurality of si ntered nuclear fuel pellets 1 0, and a cladding tube 1 1 enclosing the nuclear fuel pellets 1 0. The fuel rod 4 comprises a bottom pl ug 1 2 sealing a lower end of the cladding tube 1 1 , and a top plug 13 sealing an upper end of the fuel rod 4. The nuclear fuel pellets 1 0 are arranged in a pile in the cladding tube 1 1 . The cladding tube 1 1 thus encloses the fuel pellets 1 0 and a gas. A spring 1 4 is arranged in an upper plenum 1 5 between the pile of nuclear fuel pellets 1 0 and the top pl ug 1 3. The spri ng 1 4 presses the pile of nuclear fuel pellets 1 0 against the bottom pl ug 1 2.

Fig 3 discloses a tubular ceramic component 20 of a first embodiment according to which the tubular ceramic component 20 forms the cladding tube 1 1 of the fuel rod. The tubular ceramic component 20 comprises an inner layer 21 , an i ntermediate layer 22 adjoining the inner layer 21 , and an outer layer 23 adjoining the intermediate layer 22.

Fig 4 discloses a tubular ceramic component 20 of a second embodiment according to which the tubular ceramic component 20 forms the flow channel 6 of the fuel assembly 1 . Also in the second embodiment, the tubular ceramic component 20 comprises an i nner layer 21 , an i ntermediate layer 22 adjoini ng the i nner layer 21 , and an outer layer 23 adjoining the intermediate layer 22. The inner layer 21 consists of homogeneous, preferably monolithic, silicon carbide. The i ntermediate layer 22 consists of silicon carbide fibres 25, 26 in a fill material 27 of homogeneous silicon carbide. The outer layer 23 consists of homogeneous, preferably monolithic, silicon carbide.

As can be seen in Fig 3, the silicon carbide fibres 25, 26 of the intermediate layer 22 are wound in two sublayers , wherein the silicon carbide fibres 25, 26 of the two layers run crosswise, i.e. the fibre direction of the silicon carbide fibres 26, 27 of the two sublayers crosses each other.

It should be noted that the intermediate layer 22 also may comprise only one sublayer with silicon carbide fibres 25, 26, or more than two sublayers with silicon carbide fibres 25, 26.

The silicon carbide of the inner layer 21 , of the fill material 27 and of the outer layer 23 is crystalline and doped with one or more dopants.

The dopants are present in solid solution within crystals of the crystalline silicon carbide of the i nner layer 21 , of the fill material 27 and of the outer layer 23. The dopant, or dopants, may be added to the silicon carbide i n various ways. For instance the dopants can be added duri ng the process of depositing the silicon carbide onto the silicon carbide fibres 25, 26 and onto the intermediate layer 22. In a first step, silicon carbide fibres 25, 26 may be wound in one or more sublayers to a tubular shape, for i nstance on a suitable form.

In a second step, silicon carbide may be deposited on the silicon fibres 25, 26 of the intermediate layer 22 to form the fill material 27. During the deposition process, the silicon carbide will penetrate the interspaces between the silicon carbide fibres 25, 26. The silicon carbide may be deposited by any suitable method such as sputteri ng, physical vapour deposition, PVD, chemical vapour deposition, CVD, etc. The dopant may then be added in advance to the silicon carbide to be deposited , or be mixed with the silicon carbide during the depositing process.

In a third step, the silicon carbide may be deposited to the intermediate layer 22 to form the inner layer 21 onto the intermediate layer 22 by any of the depositing methods mentioned above. The dopant may then be added in the same way as to the silicon carbide of intermediate layer 22.

In a fourth step, the silicon carbide may be deposited to the intermediate layer 22 to form the outer layer 23 onto the intermediate layer 22 by any of the depositing methods mentioned above. The dopant may then be added in the same way as to the silicon carbide of i ntermediate layer 22. The outer layer 23 may be deposited after or before the deposition of the inner layer 21 .

Accordi ng to another method , the dopant or dopants may be supplied during the manufacturing of the silicon carbide , for instance by adding the dopant or dopants to S1O2 and C i n a so called Acheson furnace.

The concentration of the dopants in the silicon carbide of the i nner layer 21 , of the fill material 27 and of the outer layer 23 may be 1 -1 000 ppm, preferably 1 0-1 000 ppm, more preferably 50-1 000 ppm, and most preferably 50-500 ppm.

The silicon carbide of the inner layer 21 , of the fill material 27 and of the outer layer 23 may contai n a balance of possible residual substances i n addition to the dopant or dopants. The silicon carbide of the inner layer 21 , of the fill material 27 and of the outer layer 23 has a concentration of secondary phases that is less than 1 %. The silicon carbide fibres 25, 26 are made of pure, or substantially pure, silicon carbide bei ng free of dopants. A balance of possible residual substances may be present in the silicon carbide fibres 25, 26. The dopants to be added to and comprised by the silicon carbide comprise at least one of the substances B, N, Al , P, O, Be, Li , S, Ti , Ge, P2O3, P2O5, AI2O3, AI N , AUC3 and TiC-i-x.

The silicon carbide may be doped by the addition of one of these substances, or with a combi nation of two or more of these substances.

B, boron

B is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. Preferably, the boron is enriched to contain a higher percentage of the isotope ^{1 1} B than natural B in order to reduce the neutron absorption cross-section. B may be added as an element, for i nstance by sputteri ng, PVD or CVD. N, nitrogen

N is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. Preferably, the nitrogen is enriched to contain a higher percentage of th e isotope ¹⁵ N than natural N in order to reduce the neutron absorption cross-section. N may be added as an element, for instance by sputtering, PVD or CVD. The element N is larger than C, and thus N may be effective to replace C-atoms in the silicon carbide.

Al . aluminium

Al is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. Al may be added as an element, for instance by sputtering, PVD or CVD. Al may also be added as one of the compounds A I 2 O3 , AIN and AUC3. Also i n these cases, Al will be contained as an element in solid solution in the crystals of the silicon carbide. The element Al is larger than C, and thus Al may be effective to replace C-atoms in the silicon carbide.

P, phosphorous

P is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. P may be added as an element, for instance by sputtering, PVD or CVD. P may also be added as one of the compounds P 2O 3 and P2 O5 . Also in these cases, P will be contained as an element in solid solution in the crystals of the silicon carbide. The element P is larger than C, and thus P may be effective to replace C-atoms in the silicon carbide.

O, oxygen

O is a possible dopant which may be contai ned i n sol id solution in crystals of the silicon carbide. O may be added as an element, for instance by sputteri ng, PVD or CVD. O may also be added as one of the compounds P 2O 3 , P 2O5 and A I 2O3. Also in these cases, O will be contained as an element in solid solution in the crystals of the silicon carbide. The element O is larger than C, and thus O may be effective to replace C-atoms i n the silicon carbide. Be, beryllium

Be is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. Be may be added as an element, for instance by sputtering , PVD or CVD. Li , lithium

Li is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. Li may be added as an element, for instance by sputtering , PVD or CVD. S, sulphur

S is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. S may be added as an element, for instance by sputtering, PVD or CVD. The element S is larger than both C and Si , and thus S may be effective to replace C- atoms and Si-atoms in the silicon carbide.

Ti , titanium

Ti is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. Ti may be added as an element, for instance by sputtering, PVD or CVD. Ti may also be added as the compound TiCi _-x . Also i n this case, Ti will be contained as an element in solid solution in the crystals of the silicon carbide. The element Ti is larger than both C and Si , and thus Ti may be effective to replace C-atoms and Si-atoms in the silicon carbide.

Ge, germani um

Ge is a possible dopant which may be contained in solid solution in crystals of the silicon carbide. Ge may be added as an element, for i nstance by sputtering, PVD or CVD. The element Ge is larger than both C and Si , and thus Ge may be effective to replace C- atoms and Si-atoms in the silicon carbide.

The present i nvention is not limited to the embodiments disclosed and discussed above, but may be varied and modified within the scope of the following claims.