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Although necessary for PWR type nuclear reactors, Zr alloys are not required for many other major technological endeavours (Zr alloys are sometimes used in some chemical engineering applications). This means that the amount of Zircaloy produced each year (on the order of thousands of metric tonnes) is far below the amount of other engineering alloys such as titanium alloys or steels (respectively, on the order of millions or billions of metric tonnes)[24]. The methods used to make Zircaloy variants in the nuclear industry are proprietary information and thus cannot be revealed accurately in this document, however a rough overview of the general process can be described.

2.1.2.1 Renement of Ore

Zirconium is normally found in deposits of zircon (Zr silicate), along with other elements such as hafnium [25]. For nuclear grade Zr, it needs to be separated from the hafnium and this can be achieved by processes such as liquid-liquid extraction. Simply put, liquid-liquid extraction, or counter-current exchange is the process of bringing two immiscible liquids together, whereby one has the desired element dissolved in it (normally a strong acid) and the other is an organic solvent with a preferential solubility for the desired element. A quantity of the element will become absorbed by the organic solvent, from which it can be extracted at higher purity [26]. Multiple stages of mixing and extraction are normally necessary. This technique is used throughout the nuclear industry for the purication of various substances, such as extracting uranium from ore, or separating ssion products out of radioactive waste [26]. The counter-current exchange process ultimately produces ZrO2of the necessary purity (<100 ppm Hf) [27]. This oxide is then used as a reactant for the Kroll process to produce Zr metal. The metal can then be melted, mixed with the necessary alloying agents and cast into ingots. As this is ultimately an industrial process, there are a number of dierent techniques which may accomplish the same goal. The one detailed above is one of the more straightforward methods.

2.1.2.2 Fabrication of tubes

The general process of producing tubes of Zr alloy is done by a series of anneals and cold drawing stages, known as pilgering. First, the Zircaloy ingot is heated to a temperature of around 1050

◦_{C and then quench cooled. This is to make sure that the material structure is locked in the}

beta BCC phase and remove existing alloy texture [28]. The texture of the alloy is critical to its corrosion resistance and mechanical properties, meaning it is important that the metal have no strong initial texture before the forming process starts. After the β quenching, the tubing normally undergoes a hot extrusion step (information on the temperature of this step tends to be commercially sensetive). Considering that the pre-extrusion billets are on the order of 200 mm across and the nal tubes are around 9.5 mm outside diameter, with a wall thickness of less than a millimetre, the amount of strain required to process the material is signicant. This means that the entire extrusion process cannot be done in a single step and a hot extrusion process is

Initial cross section Diameter reduction Wall thinning

Mandrel Zirconium alloy

Rollers

Figure 2.1: Schematic of the Pilgering process.

necessary to start o with to allow an initial high strain extrusion. After this initial process, annealing may be carried out to remove any cold work, caused by the previous step. Following this rst annealing, the main forming process can begin. First, the metal is rolled through a pilgering mill around a mandrel. This reduces the thickness of the wall and reduces the tube diameter. (Fig. 2.1)[24].

The process needs to be done cold, as cold working techniques can produce dimensions to more accurate tolerances than hot working methods. After the pilgering, the metal is annealed and the pilgering is repeated. This process of pilger/anneal is repeated a number of times, until the tube is of appropriate dimensions to be lled with fuel pellets, back lled with helium and assembled for insertion into a reactor.

2.1.2.3 Texture Evolution

Texture is of critical importance for two reasons. Firstly, as described on page 38, Zr crystals have preferred orientations for deformation, meaning the mechanical properties of a textured tube are anisotropic. Secondly, impurities produced by ssion, can diuse into the Zr lattice and weaken the structure, perhaps the most notable of these being iodine [29]. These impurities have preferential diusion paths relating to the crystallographic orientations. As the impurities diuse faster along the basal planes than along prismatic planes, it is important that the crystals are orientated with normal of the basal planes in the tube's radial direction, or it will be subject

to an increased rate of iodine induced stress corrosion cracking [30].The pilgering process starts with a thick walled, large diameter tube and nishes with a thin walled, small diameter tube. This means that there are two reduction processes working during pilgering: wall thinning and diameter reduction (See Fig.2.1). If wall thinning has been the dominant forming operation, then the tube wall is strained primarily in the circumferential direction, thus there will be a tendency for the crystals to be orientated with their c-axis in the circumferential direction. If diameter reduction has been the dominant process, then the strain will have primarily been in the radial direction and thus the crystals will be more commonly orientated with their c-axis along the tube radii.