SE436047B - THERMOMECHANICAL PROCEDURE FOR MANUFACTURING COMPOSITION COATING PIPES AND COMPOSITION COATING PIPES - Google Patents

Description

In the conventional process for manufacturing casing tubes with a barrier layer bonded to the inner surface, a hollow zirconium alloy blank is provided with a zirconium metal sleeve for the barrier layer and the composite article is coextruded.

The composite body is then reduced to the final diameter by cold working a plurality of steps through a reduction device, for example a step rolling mill.

After each reduction step, the composite product is conventionally subjected to annealing by heat treatment at a temperature and for a period of time sufficient for substantially complete recrystallization of the zirconium alloy.

However, it has been found that the annealing temperature and annealing time required for complete recrystallization of the zirconium alloy results in an undesired grain growth in the zirconium metal barrier layer.

According to one aspect of the invention, therefore, the heat treatment of the composite product is carried out after the final dimension reduction step at a temperature and for a period of time which allows substantially complete recrystallization of the zirconium metal layer and gives a fine-grained microstructure therein while heat treatment gives stress release but not complete recovery. of the zirconium alloy.

According to another aspect of the invention, the crystallographic texture of the zirconium metal layer can, if desired, be improved by compression deformation of the surface of the layer, for example by ball blasting, without deformation of the zirconium alloy in the composite tube.

The invention is described in more detail below with reference to the accompanying drawing.

Figure 1 is a side view, partly in section, through a nuclear fuel element. 8000838-6 3 Figure 2 is a cross section through the fuel element according to figure 1.

Figure 3 is a table layout of an example of pipe reduction and treatment according to the invention.

A nuclear fuel element 11 shown in Figures 1 and 2 comprises an elongate composite housing tube 12 containing a column of fuel pellets 13 and is closed at both ends with lower and upper end plugs 14 and 16.

A plenum space 17 is designed to allow longitudinal expansion of the nuclear fuel and to provide a space for gases released from the fuel during operation of the reactor. A spring 18 between the top of the fuel column and the upper end plug 16 holds the fuel column in position. As best shown in Figure 2, the composite housing pipe 11 is dimensioned relative to the diameter of the fuel pellets so that an annular gap 19 is formed between the fuel pieces and the inner surface of the housing pipe.

According to a preferred embodiment of the invention, the composite housing tube 11 comprises a housing tube 21 made of a zirconium alloy and a barrier layer 22 of zirconium metal, which is metallurgically bonded to the inner surface of the pipe 211.

Zirconium alloys suitable for tube 21 include Zircaloy-2 and Zircaloy-4. Zircaloy-2 contains, by weight, about 1.5% tin, 0.12% iron, 0.09% chromium, 0.005% nickel and the rest zirconium. Zircaloy-4 contains less nickel than Zircaloy-2 but contains slightly more iron. Both of these alloys contain elements other than zirconium in an amount exceeding 5,000 parts per million.

The barrier layer 22, which may comprise from about 1 to about 30% of the thickness of the composite enclosure, is made of zirconium metal with limited impurity content ranging from high purity or substantially pure zirconium with less than 500 million parts (ppm) of impurities. to a contaminant content of up to 5000 ppm but preferably a contaminant content of less than about 4200 ppm.

Of contaminants, the oxygen content should be minimized and kept within a range of 200 ppm or less to a maximum of about 1200 ppm. Other pollutants may be within the normal range for commercial reactor grade zirconium sponges and are listed below: Aluminum 75 ppm or less, boron 0.4 ppm or less, cadmium 0.4 ppm or less, carbon 270 ppm or less, chromium 200 ppm or less, cobalt 20 ppm or less, copper 50 ppm or less, hafnium 100 ppm or less, hydrogen 25 ppm or less, iron 1500 ppm or less, magnesium 20 ppm or less, manganese 50 ppm or less, molybdenum 50 ppm or less , nickel 70 ppm or less, niobium 100 ppm or less, nitrogen 80 ppm or less, silicon 120 ppm or less, tin 50 ppm or less, tungsten 100 ppm or less, titanium 50 ppm or less and uranium 3.5 ppm or less .

The barrier layer 22 of zirconium metal is metallurgically bonded to the zirconium alloy tube 21 with sufficient transverse diffusion between these layers to provide a strong bond but not so great a diffusion that the barrier layer 22 is contaminated to more than about 1.2.7-25.4 μm from the bonding interface.

It has been found that a barrier layer of zirconium metal with a thickness of the order of 5-15% of the composite enclosure and with a particularly preferred thickness of about 10% prevents the zirconium alloy in the encapsulation tube 21 from being exposed to the corrosive fission products.

The barrier layer also separates the zirconium alloy housing tube from direct mechanical action with the fuel pellets and reduces the stresses that can be caused by this. The barrier layer is found to retain its desirable structural properties, such as yield strength and hardness at levels that are significantly lower than conventional zirconium alloys. Thus, the metal barrier layer does not cure as much as conventional zirconium alloys upon exposure to radiation, and this together with the initially low yield strength allows the metal barrier layer to be plastically deformed and yield to the stresses induced by the pellets in the power peak elements or power transients. . Voltages induced by fuel pellets or fuel pieces in the fuel element can arise, for example, by swelling the nuclear fuel pieces at the working temperature of the reactor, so that the fuel pieces come into contact with the housing.

The composite housing according to the invention can be manufactured by any of the methods set forth below.

According to one method, a hollow tube of zirconium metal selected as the barrier layer is introduced into a hollow blank of zirconium alloy selected for the cladding tube. This unit is then subjected to explosion bonding of the pipe to the substance.

The composite body is extruded at an elevated temperature of about 538-760 ° C using conventional tubular extrusion methods. The extruded composite tube is then subjected to a process involving conventional tube reduction until the desired size of the composite housing is reached.

According to another method, a hollow tube of zirconium metal selected for the barrier layer is inserted into a hollow blank of zirconium alloy selected for the manufacture of the casing tube. The unit is then subjected to a heating step, for example at 760 ° C for about 8 hours, to effect diffusion bonding between the zirconium metal tube and the blank. The composite body is then extruded using conventional tube extrusion technology and the extruded composite tube is subjected to a process involving conventional tube reduction until the desired size of the enclosure is reached.

According to yet another method, a hollow tube of the zirconium metal material selected as the barrier layer is inserted into a hollow blank of the zirconium alloy selected for making the casing tube. This unit is then extruded using conventional tube extruding technology. The extruded composite body is then subjected to a process involving conventional pipe reduction until the desired size of the enclosure is reached.

The dimensions of the starting materials are determined by the ratios between the cross-sectional areas of the barrier layer and the zirconium alloy parts of the desired composite encapsulation product.

By way of example, the total cross-sectional area of the finished enclosure is given by ATP '77 / 4 (ODTF 'IDTF)' in which ATF is the area of the finished product, ODTF is the outer diameter of the final product and IDTF is the inner diameter of the final product. The cross-sectional area of the desired barrier layer is indicated by _ 2 _a I? ABF '77/4 (ODBF IDBF)' in which ABF is the cross-sectional area of the metal barrier layer, OD BF the inner diameter of the metal barrier layer. The total cross-section of the original blank for the casing tube is indicated by is the outer diameter of the metal barrier layer and IDBF is ATI: 77/4 (ODTI2 - IDTI2) 'in which ATI is the total cross-sectional area of the blank comprising the metal barrier layer, the ODT is the ODT is the inside diameter of the starting material.

The desired cross-sectional area of the output barrier layer is indicated by A: A ABF (-).

BI TI ATF 8000838-6 Example.

An example of the manufacture of a composite housing tube 11 according to the invention is as follows.

A blank for the zirconium alloy encapsulation tube and the insert for the zirconium metal barrier layer are machined, cleaned and combined by standard methods, the dimensions being selected for extruding the composite unit in a hot extruder. The material for the encapsulation pipe consists of normal Zircaloy-2 alloy according to ASTM B353, Grade RA-1, and the insert for the barrier layer consists of zirconium metal with impurity contents within the limits specified above. The holes in the blank and the insert are designed with a conicity of 8 thousandths and are compressed to ensure good contact between the connecting surfaces.

Examples of dimensions of the machined parts are the following: for the casing blank, length 229 mm, outer diameter 146 mm, inner diameter 62.0 mm; for the barrier layer insert, outer diameter 62.0 mm, inner diameter 42.2 mm.

Prior to joining, the adjacent surfaces of the blank and insert are subjected to careful etching to remove traces of contaminants. A suitable etching solution is 70 ml H 2 O, 30 ml HNO 3 (70% aq) and 5 ml HF (48% aq).

To ensure satisfactory bonding during extrusion, the unit can be bonded by pressing the conical insert into the conical hole in the blank in a vacuum in 20_pm mercury while maintaining a temperature of about 760 ° C for 8 hours with an initial pressing force of 13,600-20,400 kp. This has been shown to provide binding over 20-25% of the interface area.

To reduce the end losses during extrusion, pieces of Zircaloy-2 with a length of 5 cm can be welded at each end of the connected unit and machined for connection. 8000838-6 8 The extrusion of the bonded blank unit into an encapsulation tube shell is accomplished using the following parameters: extrusion speed l5 cm / minute, reduction ratio 6: 1, temperature 595 ° C and extrusion force 3500 tons.

All blanks with the exception of the hole and the liquid mandrel can be lubricated with a water-soluble lubricant, which is fired at 70 ° C for one hour. After extrusion, both ends of the pipe are trimmed to remove the added end pieces and the inner surface is honed to remove any surface scratches and to improve the surface finish.

The final reduction of the composite pipe to a pipe of suitable dimension for fuel element enclosure is achieved by cold working in three steps by a per se well-known step rolling mill for pipe reduction with heat treatment and cleaning between the steps. The steps of a representative reduction process are given in Table l. Nn »aoooazs-6> m ~ OH Nw.O fl mw.mH m @ \ m ~ ww.a« MWBWEQHQ IZHZZH EE wßOO ~ OI mw0.0 ~ ßw ¥ M fl MmHHmmm m0Q MmHxUO fi uQ fl Hm mN.NH Nm ~ NH ~ m_o ~ m.mm m.mm MMBNZGHQ IMWBBN .T Hæ.o Am H fifl u mmpm n «_uo ~ w« .m: fl Høamnwn @ a »m> mc fl H © cmnmnwEw> Anu m> Anu .ÜGHHNMWH QHÜM «æ_O uwmwummco al vxdwwu wwwwu f n H .uo al mw _m = f cm @@ Hw ma al Amum fl m www uwmcmm H '. ~ uwmøummco al vxøøwu muwcm QH .oo fi ww .mc al cmøw f o mc al nm al etc. m mmm uwmswm mm.m uwmwummøo fi vxdumu mumußm QH .ooßßw .ma fi cmon f w A fl mwmå um f u al ms al mxonwmß umummmQHm> v. w: Hauuwu> mV mc flfl must fl m mmm uwmcwm Nm.OH w flfi whßh UGE m fi nwm ZMBMDQOmm Um fi m | BHmOmZOM WOW MWQMUOÜH OZHQQZ H QAWNÅH Åmav Am fl v AHHV AHHV A.

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MZ OWBW 8000838-6 The reduction process is conventional with the exception of modifications according to the invention. The basis for these modifications and the beneficial results obtained with them are discussed below.

The heavy cold working that takes place during the pipe reduction steps causes deformation of the shape of the metal crystallites and causes many crystal defects in the crystallites. Cold-worked metals are thus present in a relatively high energy state which is not thermally stable. In annealing of metals, heat is used to give the atoms of the metal mobility and allow them to redistribute to a lower energy configuration, and this annealing is a function of both temperature and time, with temperature being the more sensitive parameter. In general, annealing temperature and annealing time are selected so that they are sufficient to provide substantially complete recrystallization but insufficient to allow excessive crystal or grain growth.

Thus, for the annealing steps 5 and 8 in the reduction process according to Table 1, the temperature and time are selected so that substantially complete recrystallization of the zirconium alloy in the tube 21 is obtained.

However, the relatively purer metal in the barrier layer 22 recrystallizes at a lower temperature and it is found that conventional annealing temperatures and annealing times suitable for the zirconium alloy, such as in steps 5 and 8, cause grain growth in the barrier metal to a degree unsuitable in the barrier. finished product.

According to one aspect of the invention, therefore, the composite tube is subjected to heat treatment at a lower temperature after the final reduction step as shown in step 12.

The temperature and time of the heat treatment at step 12 are therefore chosen so that the zirconium metal in the barrier layer 22 eoooaza-611 is substantially completely recrystallized without grain growth.

This provides a barrier layer with fine-grained uniaxial microstructure with improved strength and ductility, increased resistance to stress corrosion cracking and high plastic stability.

The temperature and time at the heat treatment step 12 are also selected to provide complete stress release but not complete recrystallization of the zirconium alloy in the tube 21.

This has the additional advantage that the zirconium alloy maintains the elongated grain structure produced by the reduction process and has higher strength at high elongation rate or degree of elongation but is still released from internal stresses.

Suitable temperatures and times for annealing steps 2, 5 and 8 are in the range from about 538 to 704 ° C for about 1 to 15 hours and preferably for about 1 to 4 hours.

Suitable temperatures and times for the heat treatment step 12 are in the range from about 440 to 5 ° C for about 1-4 hours.

According to another aspect of the invention, if desired, the crystallographic texture (ie the degree of preferred crystallographic orientation) of the zirconium metal barrier layer can be improved by mechanical, compressive deformation of the surface.

Thus, for example, prior to the final heat treatment step 12, the barrier layer may be ball blasted from the inside of the unit so that compressive deformation of this layer is accomplished without significant deformation of the zirconium alloy tube.

Such mechanical treatment prior to the final heat treatment, shown as step 10 in Table 1, provides improved crystallographic structure with the base poles (0002) strongly aligned in the radial direction of the composite housing tube.

Claims (7)

8000838-6 \ l PATENT REQUIREMENTS 1. A method of making an elongate composite cladding tube intended to contain nuclear fuel as a fuel element for a nuclear reactor, the cladding tube being made of a cladding tube shell comprising a zirconium alloy tube containing components other than zirconium containing about 5000 million zirconium. impurities in an amount of less than 500 million parts metallurgically bonded to the inner surface, characterized in that the following steps are performed: (l) reduces the diameter of the enclosure pipe by cold working in a series of reduction steps to the desired inner diameter and wall thickness, (2) heat treats the enclosure pipe between each such reduction step at a temperature and for a period of time sufficient to substantially completely recrystallize the zirconium alloy, (3) heat treating the cladding tube after the last of these reduction steps at a lower temperature and for a period of time such as to again essentially complete recrystallization of the zirconium metal layer and gives a fine-grained microstructure therein with the release of stresses but with the prevention of complete recrystallization of the zirconium alloy. 2. A method according to claim 1, characterized in that the temperature and time at step (2) is from about sas to about 7 ° C and from about 1 to about 15 hours and that the temperature and time at step (3) is from about 440 to about 51 ° C and from about 1 to about 4 hours. 3. A method according to claim 1 or 2, characterized in that a substantially uniform compressive deformation of the surface of the zirconium metal layer is carried out before heat treatment step (3). u) 8000838-6 13 4. A method according to claim 3, characterized in that the deformation is effected by ball blasting. 5. An elongate composite cladding tube intended to contain nuclear fuel as a fuel element for a nuclear reactor, characterized in that the composite cladding tube comprises a zirconium alloy tube containing constituents other than zirconium in an amount exceeding about 5000 million a layer of zirconium metal containing impurities in an amount of less than 500 million parts metallurgically bonded to the inner surface of the tube, the zirconium metal layer being substantially completely recrystallized to fine-grained microstructure and the zirconium alloy tube being substantially completely stress-free-release release. 6. A composite encapsulation tube according to claim 5, characterized in that the surface of the zirconium metal layer is pressure deformed. 7. A composite encapsulation tube according to claim 6, characterized in that the deformation is effected by ball blasting.