BARC/2011/E/016 BARC/2011/E/016

Transcription

1 BARC/2011/E/016 BARC/2011/E/016 INFLUENCE OF DEUTERIUM CONTENT ON TENSILE BEHAVIOR OF Zr-2.5Nb PRESSURE TUBE MATERIAL IN THE TEMPERATURE RANGE OF AMBIENT TO 300 C by A.K. Bind, R.N. Singh and J.K. Chakravartty Mechanical Metallurgy Division and Priyesh Dhandharia, Agnish Ghosh, Nitin More, A.G. Chhatre and S. Vijayakumar Engineering Directorate, Nuclear Power Corporation of India Ltd. 2011

2 BARC/2011/E/016 GOVERNMENT OF INDIA ATOMIC ENERGY COMMISSION BARC/2011/E/016 INFLUENCE OF DEUTERIUM CONTENT ON TENSILE BEHAVIOR OF Zr-2.5Nb PRESSURE TUBE MATERIAL IN THE TEMPERATURE RANGE OF AMBIENT TO 300 C by A.K. Bind, R.N. Singh and J.K. Chakravartty Mechanical Metallurgy Division and Priyesh Dhandharia, Agnish Ghosh, Nitin More, A.G. Chhatre and S. Vijayakumar Engineering Directorate, Nuclear Power Corporation of India Ltd. BHABHA ATOMIC RESEARCH CENTRE MUMBAI, INDIA 2011

3 BARC/2011/E/016 BIBLIOGRAPHIC DESCRIPTION SHEET FOR TECHNICAL REPORT (as per IS : ) 01 Security classification : Unclassified 02 Distribution : External 03 Report status : New 04 Series : BARC External 05 Report type : Technical Report 06 Report No. : BARC/2011/E/ Part No. or Volume No. : 08 Contract No. : 10 Title and subtitle : Influence of deuterium content on tensile behavior of Zr-2.5Nb pressure tube material in the temperature range of ambient to 300 C 11 Collation : 22 p., 7 figs., 2 tabs. 13 Project No. : 20 Personal author(s) : 1) A.K. Bind; R.N. Singh; J.K. Chakravartty 2) Priyesh Dhandharia; Agnish Ghosh; Nitin S. More; A.G. Chhatre; S. Vijayakumar 21 Affiliation of author(s) : 1) Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Mumbai 2) Engineering Directorate, Nuclear Power Corporation of India Ltd., NUB, Anushaktinagar, Mumbai 22 Corporate author(s) : Bhabha Atomic Research Centre, Mumbai Originating unit : Mechanical Metallurgy Division, BARC, Mumbai 24 Sponsor(s) Name : Department of Atomic Energy Type : Government Contd...

4 BARC/2011/E/ Date of submission : July Publication/Issue date : August Publisher/Distributor : Head, Scientific Information Resource Division, Bhabha Atomic Research Centre, Mumbai 42 Form of distribution : Hard copy 50 Language of text : English 51 Language of summary : English 52 No. of references : 12 refs. 53 Gives data on : 60 Abstract : Tensile properties of autoclaved zirconium-2.5 wt. % niobium pressure tube material were evaluated by uniaxial tension tests at temperatures between 25 and 300 C and under strain-rates of x 10-4 /s. Six number of Zr-2.5Nb alloy pressure tube spools of length 130 mm were obtained from pressure tube number Five spools were polished with abrasive paper to remove the oxide layer. These spools were gaseously charged with controlled amount of deuterium. The target deuterium concentrations were 25, 50, 75, 100 and 200 wppm of hydrogen equivalent. Ten samples were machined by EDM wire cutting from every spool. The tensile specimen axis was oriented along longitudinal direction of the tube. Metallographic examination of the deuterium charged samples suggested that the deuterides were predominantly circumferential deuterides. Analysis of tensile results showed that both yield and ultimate tensile strengths of this alloy decreased monotonically with increasing test temperatures. The tensile ductility decreased marginally with increase in test temperature from ambient to 300 C. It was also observed that both strength and ductility appear to be unaffected by deuterium content at all temperatures, thereby suggesting that at least up to 200 wppm (Heq.) of deuterium tensile properties are not influenced by deuterium. 70 Keywords/Descriptors : ZIRCONIUM ALLOYS; PRESSURE TUBES; DUCTILITY; DEUTERIUM; PHWR TYPE REACTORS; YIELD STRENGTH; STRAIN HARDENING 71 INIS Subject Category : S36 99 Supplementary elements :

5 Contents Page No. Abstract 1 Nomenclature 2 1. Introduction 3 2. Experimental 4 3. Results 5 4. Discussion 6 5. Conclusions 7 Acknowledgements 7 Reference 8 List of tables 9 List of figures 9 Tables 11 Figures 12 iv

6 Influence of Deuterium content on tensile behavior of Zr- 2.5Nb pressure tube material in the temperature range of ambient to 300 C A. K. Bind 1, Priyesh Dhandharia 2, Agnish Ghosh 2, Nitin S. More 2, R.N. Singh 1, J.K. Chakravartty 1, A. G. Chhatre 2, S. Vijayakumar 2 1 Mechanical Metallurgy Division, Bhabha Atomic Research Centre, Trombay, Mumbai Engineering Directorate, Nuclear power Corporation of India Ltd., NUB, Anushaktinagar, Mumbai Abstract Tensile properties of autoclaved Zirconium-2.5 wt. % Niobium pressure tube material were evaluated by uniaxial tension tests at temperatures between 25 and 300 C and under strainrates of x 10-4 /s. Six number of Zr-2.5Nb alloy pressure tube spools of length 130 mm were obtained from pressure tube number Five spools were polished with abrasive paper to remove the oxide layer. These spools were gaseously charged with controlled amount of deuterium. The target deuterium concentrations were 25, 50, 75, 100 and 200 wppm of hydrogen equivalent. Ten samples were machined by EDM wire cutting from every spool. The tensile specimen axis was oriented along longitudinal direction of the tube. Metallographic examination of the deuterium charged samples suggested that the deuterides were predominantly circumferential deuterides. Analysis of tensile results showed that both yield and ultimate tensile strengths of this alloy decreased monotonically with increasing test temperatures. The tensile ductility decreased marginally with increase in test temperature from ambient to 300 C. It was also observed that both strength and ductility appear to be unaffected by deuterium content at all temperatures, thereby suggesting that at least up to 200 wppm (Heq.) of deuterium tensile properties are not influenced by deuterium. Keywords: PHWR Type Reactors, Zr-2.5Nb alloy, Pressure tubes, autoclaved, Deuterium content, temperature, tensile properties, Yield strength, Ultimate Tensile Strength, elongation 1

7 Nomenclature AR ASME ASTM BCC CA CT CWSR EDM HCP Heq. ID K L K H L LOCA MPa Nb n L n H NPCIL O PHWR ppm PT RC S Axial-Radial American Society of Mechanical Engineers American Society for Testing and Materials Body Centered Cubic Circumferential-axial Calandria Tube Cold Worked and Stress Relieved Electro Discharge Machining Hexagonal Close Packed Hydrogen equivalent Internal Diameter Ludwik s strain hardening coefficient Holloman s strength coefficient Longitudinal orientation Loss Of Coolant Accident Megapascal Niobium Ludwik s strain hardening exponent Holloman s strain hardening exponent Nuclear Power Corporation of India Ltd. Oxygen Pressurised Heavy Water Reactor Parts per million Pressure Tube Radial-circumferential Engineering stress 2

8 UTS YS Zr -Zr -Zr pl Ultimate Tensile Strength Yield Strength Zirconium Alpha zirconium having HCP crystal structure Beta zirconium having BCC crystal structure True plastic strain True stress Ludwik s strength coefficient 1. Introduction Cold-worked and stress relieved (CWSR) Zr-2.5Nb tubes is being used as pressure tubes for Pressurized Heavy Water Reactors (PHWR) [1-4]. The pressure tubes serve as miniature pressure vessels operating at about 300 C with a coolant pressure of ~ 10 MPa. The design of the pressure tube is based on section III of the ASME pressure vessel code, which specifies the criteria of maximum design stress on the basis of ultimate tensile strength, yield strength, creep and stress-rupture strengths at the operating temperature. For pressure tube alloys (both Zircaloy-2 and Zr-2.5Nb alloy) one third of the ultimate tensile strength has been found to be the limiting property [5]. Zr-2.5Nb pressure tube material is subjected to aqueous corrosion during service. One of the consequences of corrosion is the release of nascent deuterium. Part of the deuterium thus released is picked up by the pressure tube. Deuterium in excess of solid solubility precipitates out as deuteride (hydride). Being brittle deuterides make the host matrix brittle. However, degree of embrittlement is significantly influenced by the orientation of deuterides, as the deuterides oriented normal to the tensile stress provide easy crack propagation path [6]. Zr-2.5Nb pressure tube alloy picks up less than 1 wppm of deuterium (Heq.) per year. For the quadruple melted alloy the specification limit for hydrogen is 5 ppm. Thus for the design life of 30 years, under normal operating condition end of life deuterium content will be less than 40 wppm of Heq.. However, hydrogen/deuterium is known to migrate down the concentration and thermal gradient, and up the hydrostatic stress gradient. Hence, it may be possible to encounter a situation under accidental condition where hydrogen or deuterium content could be as high as 100 wppm [7]. 3

9 Most of the studies on the influence of hydrogen isotopes on the mechanical properties of Zr-alloys are carried out by adding hydrogen. However, during service deuterium is picked up by the pressure tube material. It is has been reported by Singh et. al [8] that the stress free transformation strains of the deuteride is different from that of hydrides, and hence the former is expected to affect the mechanical properties of Zr-alloys differently. However, no study has been carried out on the influence of deuterium on the mechanical properties of Zr-2.5Nb alloys. Thus the objective of this work was to determine the influence of deuterium content on the tensile properties of the Zr-2.5Nb pressure tube material in the temperature range of ambient to 300 C as a function of deuterium content. 2. Experimental A full length Zr-2.5 wt. % Nb pressure tube bearing No was supplied by Nuclear Power Corporation of India Ltd. (NPCIL) for this work. The outer diameter of tube was about 90 mm with wall thickness of ~3.5mm. The details like ingot number, chemical composition and tensile properties have been provided in table 1. The pressure tube was cut into three parts of length about 2 m. Six spools of length 130 mm were machined from the middle section of the tube. Five spools were successively polished up to 1200 grit silicon carbide abrasive paper to obtain a fresh, contamination free surface. The polished spools were gaseously charged with controlled amount of deuterium in a modified Seivert s apparatus [9]. The target deuterium concentration was 25, 50, 75, 100 and 200 pmm of hydrogen equivalent. The average deuterium content of the samples was estimated from the difference between the initial and the final pressure readings recorded during deuterium charging process. Throughout this manuscript, the values of deuterium content have been reported in wppm of hydrogen equivalent (Heq.). Standard metallographic technique was followed to reveal the hydride microstructure, its morphology and distribution using optical microscopy with the specimens etched in a solution of HF:HNO 3 :H 2 O::2:9:9 for 30 seconds. In order to observe the hydride/deuteride morphology its size and distribution, the samples were prepared with faces parallel to the radial-circumferential and radial-axial plane of the pressure tube. Curved tensile specimens (gage dimensions 31 x 6.0x 3.5 mm) with their axes parallel to the longitudinal direction of the pressure tubes were machined from spool pieces using EDM wire cutting. The tensile tests were carried out in the temperature range of C under a nominal strain-rate of x 10-4 /s. The test matrix is described in table 2. 4

10 All tension tests were conducted using an Instron machine fitted with a resistance heated furnace with temperature control of 1 C. For elevated temperature tests the specimens were soaked for one hour prior to the load application. The tests were carried out following the guidelines of ASTM Standard. 3. Results During fabrication of cold worked and stress-relieved (CWSR) Zr-2.5Nb pressure tube, majority of the -Zr grains acquire an orientation with their basal poles along the circumferential or radial direction and -Zr phase is present along the grain-boundaries [10-11]. Thus, the texture of the pressure tube material is such that in the as fabricated condition, crystallographically only two orientations are permissible. These are along the circumferential-axial plane and along the radial-axial plane [6]. Hydride platelets oriented along the circumferential-axial plane are called circumferential hydride and those oriented along radial-axial plane are called radial hydride. These orientations of hydrides have been illustrated by the schematics in Fig. 1 [12]. The microstructural features of hydrides observed under optical microscope on AR planes of the Zr-2.5Nb pressure tube material, which were charged with about (a) 25, (b) 50, (c) 75, (d) 100 and (e) 200 ppm of deuterium, are shown in Fig. 2. This figure shows that in the as-hydrided condition, hydride plates (dark lines) are oriented along the circumferentialaxial plane only. These hydrides are called circumferential hydrides [6]. Fig. 3 shows the engineering stress (S) vs plastic strain (e pl ) curves for Zr-2.5Nb pressure tube material tested at (a) 25, (b) 100, (c) 200, (d) 250 and (e) 300 C. The plots for sample containing up to 200 wppm of deuterium are almost superimposed over each other. It is evident from these curves that the flow behavior of this alloy in the temperature range of C is not affected up to 200 wppm of deuterium. Influence of deuterium content on tensile properties of Zr-2.5Nb pressure tube material in the temperature range of C has been depicted in fig. 4. At all test temperature the (a) yield strength (YS), (b) ultimate tensile strength (UTS), (c) percent uniform elongation (e u ) and (d) percent total tensile elongation (e t ) seem to be unaffected by deuterium content up to 200 ppm (Fig. 4). Thus up to 200 wppm of deuterium is not affecting the tensile properties of this alloy. Fig. 5 shows the true stress ( ) vs true plastic strain ( pl ) curves for Zr-2.5Nb pressure tube material tested at (a) 25, (b) 100, (c) 200, (d) 250 and (e) 300 C. Similar to engineering stress-strain curves true stress curves containing different deuterium content are superimposed 5

11 over each other. Hence, it is evident from these curves that variation in deuterium content up to 200 wppm of is not affecting the flow behavior of this alloy in the temperature range of C. The true stress-strain data was fitted using Holloman relationship of the form: n H K H pl (1) Influence of deuterium content on Holloman parameters (a) Strength co-efficient (K H ) and (b) strain hardening exponent (n H ) of Zr-2.5Nb pressure tube material in the temperature range of C is shown in fig. 6. As is evident from these curves up to 200 wppm of deuterium is not affecting the K H and n H values of this alloy. An attempt was made to fit the true stress-strain data using Ludwik relationship of the form: n L o K L pl (2) Influence of deuterium content on (a) strength coefficient ( o ), (b) strain hardening coefficient (K L ) and (c) strain hardening exponent (n L ) is shown in fig 7. Similar to Holloman parameters, Ludwik parameters also seem not to be affected by deuterium content up to 200 wppm in the temperature range of C. 4. Discussion The micrograph in fig. 2 shows the traces of deuterides (dark lines). The deuterides exhibit plate shaped morphology with broad face oriented along circumferential-axial plane of the tube and hence its trace on axial-radial plane is line segment. The observed morphology of deuterides under optical microscope is similar to that reported by Singh et al. [12] for hydrides. Deuterides/hydrides are brittle in nature and presence of brittle phases makes the host matrix brittle resulting in reduction in tensile ductility, impact and fracture toughness. However, for a significant reduction in these properties certain minimum volume fraction of these phases is required. For plate shaped precipitates the critical volume fraction to cause significant reduction in mechanical properties depends on the orientation of the plates with respect to tensile load direction. Hydride/deuteride plates oriented normal tensile load are most deleterious as brittle plates oriented normal to tensile load provide easy crack propagation path. In the present investigation the deuterides were predominatly oriented along circumferential-axial plane of the tube. Since the tensile loading direction was along axial (Longitudinal) direction the major dimension of deuteride plate was parallel to the former. It has been reported by Northwood and Kosasih [6] that tensile behavior of Zr-alloy pressure 6

12 tube material is not affected up to 500 ppm of hydrogen with hydrides oriented along circumferential-axial plane. However, Zr-alloy samples containing only 50 wppm of hydrogen with hydrides oriented as radial hydrides show nil reduction in area at ambient temperature [6,12]. In the present investigation the deuterides were predominantly oriented along circumferential-axial plane and hence are not expected to influence the tensile behavior. However, recent work [7] has shown that impact behavior is affected even by circumferential hydrides/deuterides. The study on the influence of deuterium content on the fracture behavior of Zr-2.5Nb pressure tube material is under progress. 5. Conclusions Influence of deuterium content on tensile properties of CWSR Zr-2.5Nb pressure tube material was investigated in the temperature range of C. Metallographic examination revealed that deuterides were predominantly oriented along circumferential-axial plane of the pressure tube and morphology of deuterides as observed under optical microscope was similar to those hydrides. It was observed that flow behavior, tensile properties such as yield strength, ultimate tensile strength, uniform elongation and total elongation, Holloman parameters and Ludwik parameters were not affected by deuterium content up to 200 ppm (Heq.) in the temperature range of C. Acknowledgement Constant encouragement and invaluable support provided by Dr. S. Banerjee, Chairman, Department of Atomic Energy & Secretary, Atomic Energy Commission, Government of India, Dr. R. K. Sinha, Director, BARC and Dr. A. K. Suri, Director, Materials Group, BARC, Mumbai is acknowledged. Technical assistance provided by Shri P. S. Shembe in tensile testing and Shri K. C. Mazumdar in deuterium charging is thankfully acknowledged. 7

13 References 1. S. A. Chaedle, C. E. Coleman and H. Light, Nuclear Technology, 57, (1982) P. A. Ross-Ross, Atomic Energy of Canada Limited Publication 3126 (1968). 3. L. G. Bell, J. Nuclear Materials, 57, (1975) E. F. Ibrahim and B. A. Cheadle, Canadian Metallurgical Quarterly, 24 (1985) R. N. Singh, R. Kishore, T. K. Sinha and S. Banerjee, (2000): Tensile Properties of Zr- 2.5Nb Pressure Tube Alloy between 25 and 800 C Materials Science Division, BARC Report No. 2000/E/ D. O. Northwood and U. Kosasih, Hydrides and delayed hydrogen cracking in zirconium and its alloys, International Metals Reviews, 28(2) (1983) R. N. Singh, U. K. Viswanathan, Sunil Kumar, P. M. Satheesh, S. Anantharaman, Per Stahle (2011): Influence of hydrogen content on impact toughness of Zr-2.5Nb pressure tube Alloy Nucl. Engg. and Design 241 (2011) R. N. Singh, P. Ståhle, A. R. Massih and A. A. Shmakov (2007): Temperature dependence of misfit strains of delta-hydrides of zirconium Journal of Alloys and Compounds R. N. Singh, R. Kishore, S. Mukherjee, S. Roychowdhury, D. Srivastava, B. Gopalan *, R. Kameswaran #, Smita S. Sheelvantra #, T. K. Sinha, P. K. De and S. Banerjee, (2003): Hydrogen charging, hydrogen content analysis and metallographic examination of hydride in Zirconium alloys, BARC report No. BARC/2003/E/034, pp D. Srivastava, G. K. Dey, and S. Banerjee, (1995): Evolution of microstructure during fabrication of Zr-2.5 wt. Pct. Nb alloy pressure tubes, Metall. Trans. A, 26A 2707 (1995). 11. D. Srivastava, S. Neogi, G. K. Dey, S. Banerjee, E. Ramadasan and S. Anantharaman (2011): Microstructural examination of Zr-2.5Nb pressure tube S-07 from Kakrapar Atomic Power Station Unit -2 BARC report No. BARC/E/2011/ R. N. Singh, R. Kishore, S. S. Singh, T. K. Sinha and B. P. Kashyap, (2004): Stressreorientation of hydrides and hydride embrittlement of Zr-2.5 wt. % Nb pressure tube alloy Journal of Nuclear Materials 325 (2004) pp

14 List of tables Table 1: Ingot number, chemical composition and tensile properties of Zr-2.5Nb pressure tube used in this investigation Table 2: Test temperature and deuterium content used for this investigation List of figures Fig. 1 Schematic of a section of pressure tube showing the orientation of circumferential and radial hydrides and typical phase grain [12] observed in Zr-2.5Nb pressure tube alloy. A Axial, C Circumferential and R Radial directions. AR Axial-Radial, RC Radial- Circumferential and AC Axial-Circumferential planes. Both circumferential and radial hydride orientations are also illustrated in this figure. d A, d R and d C are dimensions of -Zr grains along axial, radial and circumferential directions, respectively. Fig. 2: Optical micrograph along axial-radial plane of Zr-2.5Nb pressure tube material charged with (a) 25, (b) 50, (c) 100 and (d) 200 ppm of deuterium (Heq.). Fig. 3: Engineering stress vs plastic strain curves for Zr-2.5Nb pressure tube material tested at (a) 25, (b) 100, (c) 200, (d) 250 and (e) 300 C. As is evident from these curves up to 200 wppm of deuterium is not affecting the flow behavior of this alloy in the temperature range of C. Fig. 4: Influence of deuterium content on tensile properties of Zr-2.5Nb pressure tube material in the temperature range of C (a) yield strength (YS), (b) ultimate tensile strength (UTS), (c) % uniform elongation (e u ) and (d) % total tensile elongation (e t ). As is evident from these curves up to 200 wppm of deuterium is not affecting the tensile properties of this alloy. Fig. 5: True stress ( ) vs true plastic strain ( pl ) curves for Zr-2.5Nb pressure tube material tested at (a) 25, (b) 100, (c) 200, (d) 250 and (e) 300 C. As is evident from these curves up to 200 wppm of deuterium is not affecting the flow behavior of this alloy in the temperature range of C. 9

15 Fig. 6: Influence of deuterium content on Holloman parameters (a) Strength co-efficient (K) and (b) strain hardening coefficient (n) of Zr-2.5Nb pressure tube material in the temperature range of C. As is evident from these curves up to 200 wppm of deuterium is not affecting the K and n values of this alloy. Fig. 7: Influence of deuterium content on Ludwik parameters (a) Strength co-efficient ( o ), (b) strain hardening coefficient (K L ) and (c) strain hardening exponent (n L ) of Zr-2.5Nb pressure tube material in the temperature range of C. As is evident from these curves up to 200 wppm of deuterium is not affecting the o, K L and n L values of this alloy. 10

16 Table 1: Ingot number, chemical composition and tensile properties of Zr-2.5Nb pressure tube used in this investigation Chemical composition Tensile Properties Tube No. Ingot No. Nb O H N Fe UTS- 300 YS- 300 %e- 300 UTS- RT YS- RT %e- RT QNB989B % ppm ppm ppm ppm KSI KSI KSI KSI KSI KSI Table 2: Test temperature and deuterium content used for this investigation 1. Deuterium contents AR, 25, 50, 75, 100 & 200 ppm of Heq. 2. Test temperatures 25, 100, 200, 250 & 300 C 11

17 Circumferential hydride AC RC C AR A Radial hydride R Elongated grain d R d C A d A Fig. 1 Schematic of a section of pressure tube showing the orientation of circumferential and radial hydrides and typical phase grain [12] observed in Zr-2.5Nb pressure tube alloy. A Axial, C Circumferential and R Radial directions. AR Axial-Radial, RC Radial- Circumferential and AC Axial-Circumferential planes. Both circumferential and radial hydride orientations are also illustrated in this figure. d A, d R and d C are dimensions of -Zr grains along axial, radial and circumferential directions, respectively. 12

18 (a) (b) (c) (d) Fig. 2: Optical micrograph along axial-radial plane of Zr-2.5Nb pressure tube material charged with (a) 25, (b) 50, (c) 100 and (d) 200 ppm of deuterium (Heq.). 13

19 (a) (b) (c) (d) (e) Fig. 3: Engineering stress vs plastic strain curves for Zr-2.5Nb pressure tube material tested at (a) 25, (b) 100, (c) 200, (d) 250 and (e) 300 C. As is evident from these curves up to 200 wppm of deuterium is not affecting the flow behavior of this alloy in the temperature range of C. 14

20 (a) (b) (c) (d) Fig. 4: Influence of deuterium content on tensile properties of Zr-2.5Nb pressure tube material in the temperature range of C (a) yield strength (YS), (b) ultimate tensile strength (UTS), (c) % uniform elongation (e u ) and (d) % total tensile elongation (e t ). As is evident from these curves up to 200 wppm of deuterium is not affecting the tensile properties of this alloy. 15

21 (a) (b) (c) (d) (e) Fig. 5: True stress ( ) vs true plastic strain ( pl ) curves for Zr-2.5Nb pressure tube material tested at (a) 25, (b) 100, (c) 200, (d) 250 and (e) 300 C. As is evident from these curves up to 200 wppm of deuterium is not affecting the flow behavior of this alloy in the temperature range of C. 16

22 (a) (b) Fig. 6: Influence of deuterium content on Holloman parameters (a) Strength co-efficient (K H ) and (b) strain hardening coefficient (n H ) of Zr-2.5Nb pressure tube material in the temperature range of C. As is evident from these curves up to 200 wppm of deuterium is not affecting the K and n values of this alloy. 17

23 (a) (b) (c) Fig. 7: Influence of deuterium content on Ludwik parameters (a) Strength co-efficient ( o ), (b) strain hardening coefficient (K L ) and (c) strain hardening exponent (n L ) of Zr-2.5Nb pressure tube material in the temperature range of C. As is evident from these curves up to 200 wppm of deuterium is not affecting the o, K L and n L values of this alloy. 18