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Prospects of using electron beam melting for production of zirconium alloys in Ukraine.

At present at NPP of Ukraine operate 15 power units, 13 of which are of WWER-1000 type, which produce about half electric energy of the country. Nuclear fuel for these units is supplied from Russia, that's why absolute dependence of their serviceability upon external supplies threatens energy safety of the state. Attempt to find alternative sources of nuclear fuel, in particular adapt to national nuclear units fuel of the company <<Westinghouse>> (USA) for the time being did not bring results. So problem of establishment of the national nuclear-fuel cycle (NFC) relates to the most important ones, which determine energy independence of the country. Integral part of this problem is production of zirconium of nuclear purity and organization of production from it of the alloys for manufacturing of protection tubes-shells for fuel rods (FR), channel and directing tubes, cassettes, spacer grids, and other parts of the nuclear reactor core [1].

In this connection national specialists face the task of choosing the most rational from economic viewpoint method of zirconium production, process of the refining melting and formation of ingots of alloys of the required composition with necessary complex of properties of the metal, technologies of production of tube billets, and manufacturing from them of the FR protection shells and other necessary items.

In this article the authors suggest their vision of these tasks, taking into account experience, accumulated in the course of fulfillment of the Complex Program of development of national NFC [2] and subsequent works in this direction, carried out jointly with National Scientific Center <<Kharkov Institute of Physics and Technology>> of the NAS of Ukraine and State Scientific-Industrial Enterprise <<Zirco-nium>> (Dneprodzerzhinsk) mainly on assignment of Ministry of Fuel and Energy of Ukraine.

In world practice for production of zirconium most widely method of magnesium-thermal reduction from zirconium tetrachloride (Kroll method) is used [3, 4]. In Russia for this purpose method of electrolytic reduction from the [K.sub.2]Zr[F.sub.6], KCl and KF salts is used [1, 3, 5], and in Ukraine in SSIE <<Zirconium>> method of calcium-thermal reduction of zirconium tetrafluoride was mastered [3, 6]. In the magnesium-thermal method the initial metal is produced in the form of a sponge, in electrolytic one--in the form of a powder, and in calcium-thermal method--in the form of a cylindrical billet with small ratio of height to diameter.

In first two cases for subsequent metallurgical conversion with production of ingots of the alloys vacuum-arc remelting (VAR) is used, for which purpose from the sponge and the powder consumable electrodes are made, into which respective alloying elements are introduced.

Process of production of ingots of the alloys consists of two sequential remelting operations, in first of which problem of consolidation and refining of the metal, and in second one of additional refining and homogenization of the metal and formation of the required crystalline structure of the ingot are solved. Produced ingots are subjected to forging and other methods of machining and heat treatment with production of tube billets.

A hollow tube billet is produced by different methods: machining on turning lathes and drilling machines, piercing of a solid or a preliminarily drilled forged piece with subsequent machining.

Preparation of the billets for hot extrusion in majority of cases terminates with their closure in a copper shell or application on their surface by means of an electrochemical method of a copper layer with subsequent etching away or removal of the latter by machining. In a number of cases it is left as a sub-lubricant layer for further deformation. Next technological operation in production of tubes-shells is a multiple cold rolling with intermediate heat treatments in vacuum.


Using hot pressing method so called TREX-tubes (hollow tube billets for subsequent processing) are produced, and then from them the FR tubes-shells are made using cold rolling [7]. This technological scheme is used at present by all producers of nuclear fuel in the world.

It is impossible to refine cylindrical billets of zirconium, produced by calcium-thermal reduction, using the VAR method, because it is rather difficult to produce from them a consumable electrode. For refining of these billets it is convenient to use electron beam cold hearth melting (EBCHM). This process is mastered at SSIE <<Zirconium>>, where in the EDP-07 furnace the billets, produced by calcium-thermal reduction, are remelted into ingots of up to 230 mm diameter [8]. Long-term practice of producing zirconium ingots using EBCHM and subsequent mastering of electron beam skull melting (EBSM) with refining of the metal in crucibles with electromagnetic mixing (EMM) of the melt on transformed into the casting installation the EMO-250 furnace [9] stipulated accumulation in this country of big experience of electron beam melting of zirconium in contrast to other producers of this metal in the world, whereby produced ingots and billets from zirconium of nuclear purity could be used for melting of alloys and production from them of protection shells for FR and other necessary items.

Investigations in manufacturing from the KTTs-100 zirconium, produced by calcium-thermal reduction, of tube billets were carried out according to two technological schemes (Figure 1). The first one shows technology of melting of the ingots using double VAR, the second one--technology of EBSM with production of tube billets using casting methods. First scheme was implemented at Kharkov IPhT [10], second--in PhTIMA of the NASU jointly with SSIE <<Zirconium>> [11]. In both cases alloy of the Zr-1Nb composition was melted, from which experimental lots of the FR tubes-shells were manufactured.

Ingots and cast billets were produced with application of a triple charge, consisting of the KTTs-100 zirconium, zirconium iodide (up to 30% of this element were introduced into the charge for reduction of oxygen content in the alloy down to the required level according to the technical requirements) and niobium [12]. The TREX-tubes were manufactured in both cases according to the developed at State Enterprise <<Scientific-Research Tube and Pipe Institute>> scheme of production of hot-pressed pipes [13]. This technology allows using instead of traditional forging a high-temperature pressing of the cast tube billets in (P-area with high degrees of deformation and subsequent hardening after rolling heating. High-temperature pressing (elongation ratio [mu] = 25--48) enables re-crystallization of the billet cast structure and formation of homogeneous structure of martensite type. Metal of the billets, melted using electron beam melting with application of the EMM melt, had, after pressing according to the developed technology, sufficient technological ductility for subsequent cold processing.

Manufacturing from this metal of the FR tubes-shells showed that quality parameters of the tubes met all requirements (technological conditions, the ASTM standards), established for the tubes-shells. Comprehensive estimation of quality of these tubes confirmed possibility of using this technology [10, 11, 14].

It should be noted that raising of practical issue on production of tube billets using casting methods became possible after development of mentioned technology for production of hot-pressed tubes. Advantages of casting technologies in comparison with the traditional one consist in reduction of the number of technological operations and wastes, absence of the need in expensive forging and mechanical equipment, and reduction of the metal saturation with oxygen.

As a whole results of carried out investigations give basis to state that developed technologies for production of zirconium using calcium-thermal reduction, melting on its basis of alloys, and manufacturing from them of the FR protection shells and other parts of the nuclear reactor core allow solving tasks of national NFC or its elements. Main difficulty consists in production of high-purity calcium with sufficiently low content of oxygen, necessary for implementation of the process of calcium-thermal reduction of zirconium, which was previously supplied from Russia.

In recent time in discussion of prospects and directions of zirconium production in Ukraine many specialists give preference to Kroll method as more economical in comparison with other reductions of zirconium, whereby main arguments is experience of other countries-producers of zirconium and intensive works in this direction in Russia [15].

In case of taking this position national metallurgists will face the problem of choice of the most rational method for melting and refining of zirconium sponge and production of billets for subsequent processing into necessary items, first of all into FR protection shells, because VAR technology of zirconium is absent in Ukraine. For its development not just time is needed, but also big financial investments. At the same time in the country unique experience of EBM of this metal is accumulated, which it is advisable to use. Unconditional advantage of EBM is more efficient refining, including from interstitial impurities [16], removal of which from zirconium in VAR is connected with serious difficulties.


In Figure 2 possible options of technological schemes for production of the TREX-tubes are presented, which may be implemented in practice in development of national NFC. These schemes take into account possibility of producing tube billets both from ingots and using method of casting technology.

Scheme in Figure 2, a represents a traditional technological process with application for production of the VAR ingots of consumable electrodes, manufactured from sponge. As a rule two remelting operations are carried out--refining and homogenization ones, but in practice sometimes three and more remelting operations are carried out [17]. Such process is used at present in all countries, which have zirconium production. In Russia initial zirconium is produced by method of electrolytic reduction in the form of a powder, for refining and homogenization of which by means of formation of the ingots also double VAR is used.

All other schemes in Figure 2 describe options with application of EBM. The authors are supporters of implementation in Ukraine of one of these schemes, but at present it is rather difficult to establish the most promising one. This is connected with the fact that in the presented schemes certain technical solutions are suggested, which did not pass yet experimental check or passed it on a limited scale. This stipulated need in substantiation of advisability of their application, taking into account experience existing in melting of other metals.

All these schemes envisage performance of first remelting with formation of the ingot in the EBCHM installations, which seems advisable taking into account high refining capacity of this process. Sponge, pressed into briquettes, is used as the charge.

In practice of zirconium production experience of the EBM sponge does not exist yet, but such experience is accumulated in Ukraine in production of titanium. The data obtained confirm good prospects of this approach [18]. Also deserves attention possibility of melting not the sponge, but directly the bloom, formed in magnesium-thermal reduction. Melting of the latter in electron beam installations has been intensively studied in the E.O. Paton EWI of the NAS of Ukraine in production of titanium [19].

As far as melting of sponge is accompanied by intensive gas release, important becomes the issue of selection of the most rational sources of heating, i.e. the electron beam guns. This issue was always paid great attention, which is proved by consideration of designs of the guns practically in all monographs concerning EBM, including those published in recent time [11, 18, 20].

Among investigations in this directions should be singled out the work [21], devoted to analysis of state of development and application of guns of different types for EBM, including compact axial electron beam heaters of new generation. In the considered case it is advisable to use as a heating source high-voltage glow discharge (HVGD) guns, which were developed some time ago specially for melting of the gas-saturated materials [22] and serviceability of which in this country was checked as well in melting of spongy titanium [23,24].

Another principle peculiarity of the suggested first remelting is carrying out of the process with EMM of the melt during formation of the ingot in the mould, which is used in this case for the purpose of intensifying refining processes. In the EBM practice application of the melt EMM for this purpose is known only in refining of niobium [11]. Obtained positive results prove advisability of carrying out special investigations in this direction also at EBM of zirconium sponge.

Scheme in Figure 2, b shows the option, in which second remelting of the EBM ingot of first remelting is carried out in the vacuum-arc furnace, i.e. according to traditional for production of the zirconium ingot technology. Exactly this scheme was checked in investigations of Kharkov IPhT [10].

Scheme in Figure 2, c envisages in both remelting operations EBM with EMM of the melt in the mould, whereby main task of using EMM in second remelting consists in improvement of the metal crystalline structure. Similar to the considered case of using mixing for intensification of the refining processes, available at present experimental material relating to superposition of magnetic fields during formation of the zirconium ingot is rather limited [11], which also shows the need in performance of additional investigations in this direction. Efficiency of using electromagnetic action for improvement of the structure is proved by experience of production of steel, aluminium and magnesium ingots [25, 26].

Advantage of this system is exclusion from the technological chain of VAR, which is inferior to EBM in regard to efficiency of refining.

In all three considered options of technological schemes the produced ingots were subsequently subjected to identical operations of forging and other methods of processing for manufacturing of the TREX-tubes.

Schemes in Figure 2, d-f show options of production of tube billets with application of the methods of the chill mould casting technology and metallurgical mould with superposition of electromagnetic fields on the melt in the mould and centrifugal casting. In all these options the metal passes double EBM: first melting--during remelting of the sponge briquettes into the ingot, second--in the skull unit with EMM of the melt and subsequent casting of tube billets. Advantage of all these schemes is exclusion from the technological process of forging, which is accompanied not just by noticeable losses of metal, but also with its saturation with oxygen. Application of the melt EMM in the skull crucible guarantees accumulation of required mass of the melt in the crucible, ensures possibility of melting of multicompo-nent alloys, and reduces specific consumption of electric energy and evaporation losses of the metal.

Results of the experimental check of mentioned schemes are presented in [11]. But as far as at present zirconium sponge is not produced in the country, zirconium of calcium-thermal reduction was used as the initial metal in the carried out experiments, first remelting of which was carried out in EBCHM furnace [6]. The same relates to the scheme in Figure 2, b, tested in works of Kharkov IPhT [10].

Schemes in Figure 2, g, h relate to the developments, which should be considered as promising ones. For their experimental check it is necessary to carry out special investigations with application of just sponge. Peculiarity of both schemes consists in the fact that ingot for production of tube billets is produced by a single electron beam remelting of the sponge (scheme g), or casting of the billets is performed also after single melting of the sponge (scheme h). Such approach seems possible, because EBCHM allows performing refining process at any necessary rate. Good prospects of work in this direction may be confirmed by carried out in the USA investigations in production of ingots of the Ti-6Al-4V titanium alloy with single electron beam remelting of titanium sponge [27]. It is probable that it is preferable to use the HVGD guns as heating sources for implementation of the considered schemes.

As far as at present experience of zirconium sponge melting in electron beam furnaces is absent in the world practice, for experimental check of the presented suggestions it is expedient to acquire before establishment of its national production necessary for such investigations quantity of the sponge in the countries, in which commercial production of this material is mastered (the USA, France, India, etc.). The investigations themselves may be carried out in PhTIMA and at SSIE <<Zirconium>>. These organizations have necessary equipment and experience in EBM of zirconium.

One more issue, which concerns production and refining of niobium, deserves attention. This metal is main alloying element in zirconium alloys in Ukraine, which the country purchases abroad. At the same time the country has stocks of niobium raw materials, and need in organization of high-purity niobium production occurs not just in production of the alloys for NFC, but also in connection with the fact that in near future the country will face problem of production of superconducting materials, in manufacture of which niobium of high purity plays determining part [28].

In conclusion we wish to note that the authors give preference to the technologies, in which tube billets from zirconium alloys are produced by casting methods in electron beam installations, and manufacturing of the TREX-tubes is performed according to the scheme of high-temperature pressing in (3-area using deformation of high degrees in pressing and hardening after the rolling heating, which excludes forging from the technological process. However, taking into account the fact that univocal opinion in regard to these issues does not exist among the specialists, it seems advisable to carry out their comprehensive discussion in order to work out recommendations concerning the most rational way of solution of this problem.

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[23.] Tikhonovsky, A.L.. Laschuk, N.K., Tur, A.A. et al. (1993) Electron beam melting titanium sponge using high-voltage glow discharge guns. Advances in Spec. Electrometallurgy, 10, 70-73

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[25.] Samojlovich, Yu.A. (1986) Crystallization of ingot in electromagnetic field. Moscow: Metallurgiya.

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[27.] Wood, J.R. (2002) Producing Ti-6A1-4V plate from single-melt EBCHM ingot. JOM, 54(2), 56-58.

[28.] Zalikman, A.N., Korshunov, B.G., Elyutin, A.V. et al. (1990) Niobium and tantalum. Moscow: Metallurgiya.


(1) Physical-and-Technological Institute of Metals and Alloys, NASU, Kiev, Ukraine

(2) SE <<Ya.E. Osada Scientific-Research and Design-Technological Institute of Pipe Industry>>, Dnepropetrovsk, Ukraine
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Author:Ladokhin, S.V.; Vakhrusheva, V.S.
Publication:Advances in Electrometallurgy
Article Type:Report
Date:Oct 1, 2008
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