Burst Ductility of Zirconium Clads: The Defining Role of Residual Stress
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IN a thermal nuclear reactor, the appropriate fuel is typically enclosed in Zirconium (Zr) clads.[1–6] The thickness of the fuel clads must be kept small (few hundred microns) to maintain neutron economy and efficient heat transfer.[3,5] As radioactive gas, a by-product of the controlled fission also needs to be contained; the fuel clads must exhibit sufficient burst ductility: an important safety requirement.[7,8] Burst ductility is measured, by open or closed end burst tests, under internal pressurization.[7–13] Although such tests involve both GULSHAN KUMAR, ARIJIT LODH, and JAIVEER SINGH, Graduate Students, and INDRADEV SAMAJDAR, Professor, are with the Department of Metallurgical Engineering and Materials Science, IIT Bombay, Mumbai 400076, India. Contact e-mail: [email protected], [email protected] A.K. KANJARLA, Assistant Professor, is with the Department of Metallurgical and Materials Engineering, IIT Madras, Chennai 600036, India. RAMESH SINGH, Associate Professor, is with the Department of Mechanical Engineering, IIT Bombay, Powai, Mumbai 400076, India. D. SRIVASTAVA, Outstanding Scientist, Head of Physical Metallurgy Section, and G.K. DEY, Head of Materials Science Division, are with the Materials Science Division, Bhabha Atomic Research Center, Trombay, Mumbai 400085, India. N. SAIBABA, Chief Executive, is with the Department of Atomic Energy, Nuclear Fuel Complex, Hyderabad 500062, India. R.D. DOHERTY, Emeritus Professor, is with the Department of Materials Science and Engineering, Drexel University, Philadelphia, PA 19104, and also Visiting Professor with the Department of Metallurgical Engineering and Materials Science, IIT Bombay. Manuscript submitted June 13, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
hoop (or circumferential) and axial stresses (at an approximate ratio of 2:1),[8–11] hoop stresses are expected to cause failure.[14] The burst ductility is usually represented as Total Circumferential Elongation or TCE: a standard quality parameter for the Zr fuel clads.[7,11,15] Ductility in metallic materials depends on normal aniso and strain hardening exponent (n).[2] The triaxiality tropy (R) of the stress state[2,10] and/or the imposed hydrostatic pressure[2,16,17] may also affect the ductility. For example, pilgering with complex tri-axial stress state is expected to offer significantly higher ductility than uniaxial tension.[2,18] The role of imposed hydrostatic pressure on the ductility is well documented.[16,17,19,20] Although original observations of Bridgman[19,20] were contested by Hu,[21] studies of Pugh[22] are widely regarded as conclusive. A manifold increase in tensile ductility was achieved[2,16,17,19,20,22] through imposition of hydrostatic pressure. It is important to point out, at this stage, that hydrostatic pressure can be provided externally or intrinsically. The latter may come through hydrostatic component of the residual stress tensors.[23–25] Results on ductility improvements through residual hydrostatic stress are, however, not currently found in the domain of p
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