Segregation in Vacuum Arc Remelted Zirconium Alloy Ingots

16 th International Symposium on Zirconium in the Nuclear Industry Chengdu, China Segregation in Vacuum Arc Remelted Zirconium Alloy Ingots A. Jardy 1, F. Leclerc 2, M. Revil-Baudard 1-2, P. Guerin 2, H. Combeau 1, V. Rebeyrolle 2 1 Institut Jean Lamour - UMR 7198, Département SI2M, Ecole des Mines, Parc de Saurupt, F-54042 NANCY Cedex, France 2 AREVA, CEZUS Research Centre, Avenue Paul Girod, F-73403 UGINE, FRANCE CEZUS Proprietary - CEZUS - All rights reserved - p.2

Framework Manufacturing mastering highly contributes to nuclear fuel quality : u Implementation at Cezus Research Center (CRC) of the simulation of the various processes used along Zirconium component manufacturing, to provide Cezus plants with a support regarding process performance and quality improvements VAR -Vacuum Arc Remelting- of ingots is a key process regarding the chemical homogeneity and the soundness of Zirconium components : u Development for twenty years of the code SOLAR SOLidification during Arc Remelting- by the Nancy School of Mines, through a partnership with several companies (Aubert&Duval Timet - Cezus) u Application of Solar to zirconium alloys, to get a deep understanding of physical phenomena, to monitor melting condition improvements and to develop new products Purpose : Illustrate the benefits offered by SOLAR regarding the mastering of the VAR melting process, through a comparison between predicted and measured macro-segregation in Zircaloy-4 and M5 TM ingots, in the case of both test and industrial ingots CEZUS Proprietary - CEZUS - All rights reserved - p.3

Contents 1. Basic principles of VAR melting and related simulation 2. Validation of SOLAR for Zircaloy-4 3. Application to M5 TM remelting u A specific full-scale experiment u Niobium segregation in M5 TM industrial ingots CEZUS Proprietary - CEZUS - All rights reserved - p.4

VAR MELTING Basic Principles electrode drive rod electrode electric arc horizontal coil cooling water vacuum pumps liquid pool ingot mould Vacuum Arc Remelting : Arc with a consumable electrode under vacuum, specific for reactive metals (Ti, Zr) and superalloys. The electric arc induced by the induction provides the needed energy to melt the electrode (cathode) The liquid metal falls into the liquid pool and builds up the ingot, cooled in the contact of a Cu Crucible The affinity of alloying elements within the liquid or the solid and the movements into the liquid induce segregation during solidification CEZUS Proprietary - CEZUS - All rights reserved - p.6

SOLAR code Basic phenomena Modelling of the ingot growth in 2D Solving nine coupled partial differential equations Using a finite volume method Furnace design Input Operating conditions umelting current / voltage and melt rate ustirring data (coil current and period) Thermal-physical properties uliquidus and solidus temperatures depending on the alloying elements usolidification path upartition ratios for each alloying element Chemical composition maps Solidification data (Liquid pool geometry, cooling rate, local solidification time) Temperature map Output Current density in the ingot Velocity field in the melt pool Intensity of flow turbulence Shrinkage location CEZUS Proprietary - CEZUS - All rights reserved - p.7

SOLAR code Basic phenomena (1/3) A- Heat Transfer (Heat equation) u Heat transport (conduction in the ingot, turbulent convection in the liquid pool) u Heat input at the ingot top / Heat loss by radiation at the ingot top u Latent heat effects related to solidification u Heat loss by contact with the mold wall (conduction) u Heat loss in the shrinkage gap (radiation) CEZUS Proprietary - CEZUS - All rights reserved - p.8 Temperature map

SOLAR code Basic phenomena (2/3) B- Momentum Transfer Azimutal velocity (m/s) Turbulence intensity Temperature ( C) (Navier Stokes equation) u Flow in the liquid driven by Thermal and solute convection Electromagnetic forces Turbulence model u Liquid/solid interaction in the mushy zone (porous medium) CEZUS Proprietary - CEZUS - All rights reserved - p.9

SOLAR Basic phenomena (3/3) C- Solute Transfer (Mass equation) u Continuous material input at the ingot top u Solute transport (diffusion, convection) u Evaporation of some alloying elements u Segregation : Solute partitioning during solidification T L T liq β β + L ws wl Partition ratio - k w s = wl w (%) T sol Liquid fraction 0 1 Solidification path g l = f(t) g l According to the Lever Rule : w l = w 1 + - ( 1 - )( k 1) g l CEZUS Proprietary - CEZUS - All rights reserved - p.10

Validation of SOLAR for Zircaloy-4 Melt pool profile Purpose : Check the validity of liquid pool geometry simulation Conditions : u Small Ingot (120 kg) with : constant melting parameters Alternating stirring cycle u Electrode marked with iron additions u Melt pool depth is given for a liquid fraction equal to 0,5 in the mushy zone. The melt pool profile is well predicted along the ingot Comparison between calculated (dots) and measured (continuous lines) melt pool profiles CEZUS Proprietary - CEZUS - All rights reserved - p.13

Validation of SOLAR for Zircaloy-4 Chemical segregation Purpose : Check the validity of macro-segregation simulation Conditions : u Industrial ingots (6 tons) u Welding of two primary ingots, Two additional meltings u Radially averaged iron content measured on 22 ingots With appropriate thermodynamic data for Zircaloy-4, the iron profile is well predicted along the ingot C - C C (C-C0) / C0 11% 9% 0 7% 0 5% 3% 1% -1% -3% -5% Axial segregation C : Iron content (ppm) Axial iron distribution : Comparison between calculated (continuous line) and measured (dots and dotted line) iron contents 0 0.2 0.4 0.6 0.8 1 Reduced height C 0 : Average iron content (ppm) CEZUS Proprietary - CEZUS - All rights reserved - p.14

A specific full-scale experiment on M5 TM Purpose : Study macrosegregation and solidification in M5 TM Conditions : u 2.6 tons ingot Melting current (% of m aximum current) 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% u Single melt u Electrode manufactured only from recycled material (same lot) u Two melting rates applied High rate in the first part Low rate in the second u Marking with a strong and unidirectional magnetic stirring Stirring current (% of m aximum cu rren t) 0% 300% 200% 100% 0% -100% -200% -300% 15% 30% 45% 60% Time (% of melting time) 75% 90% Liquid pool marking CEZUS Proprietary - CEZUS - All rights reserved - p.16

A specific full-scale experiment on M5 TM : Niobium radial segregation at various heights h = h m h = ¾ h m 1.10 1.10 Nb content [ wt % ] 1.05 1.00 0.95 Nb content [ wt % ] 1.05 1.00 0.95 0.90 Experimental data SOLAR 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 r/r m 0.90 Experimental data SOLAR 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 r/r m h = ½ h m h = ¼ h m 1.10 1.10 Nb content [ wt % ] 1.05 1.00 0.95 Nb content [ wt % ] 1.05 1.00 0.95 0.90 Experimental data SOLAR 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 r/r m CEZUS Proprietary - CEZUS - All rights reserved - p.17 0.90 Experimental data SOLAR 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 r/r m High segregation in the bottom part linked to melting conditions Reasonable agreement between measurements and predictions

A specific full-scale experiment on M5 TM : Niobium axial segregation 1.0 0.9 At the bottom of the ingot : 0.8 0.7 Negative segregation due to the high melting rate and strong stirring. 0.6 In the central and upper part : h/h m 0.5 0.4 0.3 0.2 0.1 Measurements SOLAR Positive low segregation, decreasing with the height. At the top of the ingot : Positive segregation corresponding to the enrichment of the final melt pool. 0.0 0.90 0.95 1.00 1.05 1.10 Nb content [ wt % ] CEZUS Proprietary - CEZUS - All rights reserved - p.18

Macro / Micro-structure Examinations (a) longitudinal XS (b) radial XS (c1) microstructures at the ingot centre (c2) microstructures near the ingot periphery 5cm Ingot top 20µm 20µm 0.4mm 0.4mm Ingot middle 20µm 20µm 0.4mm 0.4mm Ingot bottom 20µm 20µm 0.4mm 0.4mm CEZUS Proprietary - CEZUS - All rights reserved - p.19

Application to industrial M5 TM ingots 1.0 0.9 Purpose : Determine the macrosegregation of Niobium in industrial ingots h/hm 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 Measurements SOLAR Conditions : u 6 tons ingots u Three meltings u Measurements in 130 ingots. The Niobium content in 6-tons ingots after 3 meltings is very homogeneous Predicted and measured Niobium contents are in rather good agreement 0.0 0.95 0.96 0.97 0.98 0.99 1.00 1.01 1.02 Nb content (wt %) 1.03 1.04 1.05 CEZUS Proprietary - CEZUS - All rights reserved - p.20

Summary The VAR process involves various complex phenomena coupled together : the simulation via SOLAR provides a better understanding of those phenomena and of their interactions. Applying the simulation to test ingots for various zirconium alloys allowed to calibrate SOLAR, which is used with the appropriate confidence level for industrial ingots. SOLAR is currently used to optimize the melting conditions, minimizing the macro-segregation of the main alloying elements. CEZUS Proprietary - CEZUS - All rights reserved - p.21

Acknowledgments The authors wish to thank the people at CEZUS Ugine plant who helped with the M5 TM experiment reported in this paper. The present study also relies largely on the work achieved by several graduate students at Nancy School of Mines during the course of their PhD theses, as well as on the valuable assistance from all the SOLAR Club partners in the development of the codes and validation trials. Furthermore, as part of the OPERAS (Optimizing Processes based on Electrode Remelting with Arc or Slag) Project, this work has been partly supported by the French National Research Agency (ANR-08-MAPR-0006-04). CEZUS Proprietary - CEZUS - All rights reserved - p.22

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