Corrosion of Zircaloy-4 and its welds in nitric acid medium
Introduction
Reprocessing of spent nuclear fuel used in fast breeder reactors (FBRs) involves use of nitric acid of high concentrations and temperatures for dissolvers and evaporators which are highly corrosive. The materials chosen for the fabrication of such reprocessing plant equipment should possess excellent corrosion resistance, ease of fabricability and reliability [1], [2]. Conventional austenitic stainless steels are not preferable in such highly oxidizing conditions as they undergo severe intergranular corrosion [3]. Extensive studies have been carried out world wide for selecting suitable materials for fabrication of equipments for highly oxidizing nitric acid applications in spent nuclear fuel reprocessing plants [1], [4], [5]. Studies carried out earlier on American Iron and Steel Institute (AISI) type 304L SS, nitric acid grade SS, CP-Ti, Ti–5%Ta indicated good corrosion resistance of CP-Ti and Ti–5%Ta compared to AISI type 304L SS in highly oxidizing nitric acid [3], [6]. Titanium was chosen for fabricating electrolytic dissolver with a dissimilar joint by explosive joining process for linking to type 304L SS equipments in the reprocessing of spent fuel from Fast Breeder Test reactor (FBTR) at Kalpakkam [7]. For future reprocessing plants, based on research and international experiences, developmental efforts have been made for the fabrication of dissolvers by using Ti–5%Ta–1.8%Nb alloy [1], [8], [9], [10].
The excellent corrosion resistance of Zirconium in nitric acid has been known for over 50 years [11]. Zirconium is highly resistant to nitric acid environments [11], [12] and is considered as candidate material for various applications in spent nuclear fuel reprocessing plants involving highly concentrated nitric acid medium. In France, 80 tons of zirconium and 5500 m of piping were employed in La Hague reprocessing plant for the manufacturing of various components like dissolvers, oxalic mother liquor evaporator and heat exchangers, vitrification dust scrubber, and liquid waste treatment reactors [5], [13].
The corrosion resistance of unalloyed titanium improves dramatically as the impurity levels in hot nitric acid solutions increases. Titanium exhibits excellent resistance to recirculating nitric acid process streams [12]. However, in hot and pure nitric acid solutions and vapour condensates of nitric acid, significant corrosion was observed for titanium [12], [13]. Unlike titanium [13], [14], Zirconium is unaffected by vapour condensates in boiling nitric acid [13]. Zirconium is much more corrosion resistant to nitric acid than titanium, but the only concern with the use of Zirconium is its susceptibility to corrosion fatigue and stress corrosion cracking (SCC) in highly concentrated nitric acid [1], [15], [16]. Particularly, the heat affected zone of Zirconium weldments was considered to be more sensitive to SCC. However, Chauve et al. [13] concluded that the tests carried out in nitric acid up to 65 wt% proved that the risk of stress corrosion cracking was extremely low and does not have to be taken into account for industrial application.
Zirconium and its alloys are extensively used as fuel cladding and for coolant channels in water cooled power reactors due to the unique combination of properties like low neutron absorption, good ductility, strength and corrosion resistance in high temperature water. Controlled additions of Fe, Cr and Sn to Zircaloy-2 lead to a new alloy Zircaloy-4, which has good corrosion resistance in water/steam, and low hydrogen absorption rates [17]. Zircaloy-4 has been proposed in the present work as a candidate material for the fabrication of dissolver and evaporator in future fast breeder reactor reprocessing plants. The present paper deals with corrosion behaviour of Zircaloy-4 in wrought and welded forms (Manual TIG welding and Electron Beam welding) in boiling 11.5 M nitric acid solution. The results are compared with other candidate materials like CP-Ti, Ti–5%Ta and Ti–5%Ta–1.8%Nb.
Section snippets
Specimen preparation and characterisation
Ti–5%Ta–1.8%Nb alloy of 25 mm diameter rod and Zircaloy-4 (Zr-4) plate was received from the Nuclear Fuel Complex (NFC), Hyderabad. Ti–5%Ta–1.8%Nb alloy rod was rolled into plate and stress relieved at 923 K for 1 h. The nominal composition of the materials used in the present study is given in Table 1. Manual TIG welding (TIG) and Electron Beam welding (EB) was carried out at NFC, Hyderabad on Zircaloy-4 pipes of 5 mm thick and 120 mm diameter. The welding parameters for the TIG weld and EB Weld
Microstructure and microhardness
Zirconium and its alloys in nitric acid display excellent corrosion resistance and are insensitive to intergranular corrosion unlike stainless steels. Zirconium is usually welded in a glove box by the TIG weld process, or under vaccum by Electron beam process [13]. Zirconium welds often display brilliant tints ranging from pale yellow to intense blue [19]. The EB welding of Zircaloy-4 pipe displayed an intense blue coloured weld region. These stains are caused due to interference tints caused
Conclusions
- 1.
Zircaloy-4 exhibited superior corrosion resistance in both wrought and welded condition in comparison to CP-Ti, Ti–5%Ta, and Ti–5%Ta–1.8%Nb in 11.5 M HNO3.
- 2.
The corrosion rates of CP-Ti, Ti–5%Ta, and, Ti–5%Ta–1.8%Nb in condensate phase are more than those in vapour and liquid phases, while for Zircaloy-4 in wrought and weld forms such behaviour was not noticed.
- 3.
SEM examination revealed insignificant corrosion attack of Zircaloy-4 and its welds in all three phases in comparison to CP-Ti, Ti–5Ta, and
Acknowledgements
The authors would like to acknowledge Smt. M. Radhika and Shri A.K. Balamurugan of Indira Gandhi Centre for Atomic Research, Kalpakkam, and Dr. Santanu Bera of BARC Facilities, Kalpakkam, for SEM, SIMS and XPS analyses, respectively.
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