MECHANICAL BEHAVIOUR OF HIGH BURNUP PHWR CLADS
13th International Conference on CANDU Fuel - 2016 Aug. 15-18


Presented at:
13th International Conference on CANDU Fuel
2016 Aug. 15-18
Location:
Kingston, ON Canada
Session Title:
Session 6: Design and Development - II

Authors:
P.K. Shah (Bhabha Atomic Research Centre, Mumbai, India)
J.S. Dubey (Bhabha Atomic Research Centre, Mumbai, India)
A. Kumar (Bhabha Atomic Research Centre, Mumbai, India)
R.S. Shriwastaw (Bhabha Atomic Research Centre, Mumbai, India)
B.N. Rath (Bhabha Atomic Research Centre, Mumbai, India)
S. Kumar (Bhabha Atomic Research Centre, Mumbai, India)
P. Mishra (Bhabha Atomic Research Centre, Mumbai, India)
  

Abstract

The clad tubes being the primary barrier between the irradiated fuel and the hot heavy water coolant, their integrity is essential for good fuel and plant performance. During their in-reactor residence the clad tubes are exposed to severe operating conditions i.e., high temperature, corrosion, erosion, irradiation, and stresses. Therefore, the structure and properties of the clad tube change significantly from that of as manufactured unirradiated cladding. Due to the corrosion reaction with hot water, Zircaloy picks-up hydrogen/deuterium over the operating time. In addition, in the case of clad tube defects, the coolant enters inside the tube and leads to a large localized clad oxidation and high up-take of hydrogen in the clad alloy. This hydrogen/deuterium in excess of its maximum solubility precipitates as zirconium hydride which may cause severe embrittlement of the clad tubes. Ensuring a minimum amount of strength and ductility in the clad along the circumferential direction is one of the most important ways of ensuring its integrity. These properties are evaluated using burst test, ring tension test and several other non-conventional test techniques. The present work involves study of mechanical properties and fracture morphology of transverse ring tension specimens taken from PHWR fuel clads, from different fuel bundles with burnup up to 20,000 Mwd/TU. One of the defective bundles had an end-cap weld defect, in one of the outer pins, that lead to clad tube punctures and was discharged at a lower burnup level, was also studied. The test results indicate an increase in transverse strength and decrease in ductility of the clad due to in-reactor operation. This study showed that the low and high burnup clads, in absence of zirconium hydride precipitates at reactor operating temperature, had higher strengths and retained adequate ductility levels. The circumferential tensile loading resulted in three very distinct type of failure and fracture edges, depending on the hydride platelet density and burnup levels; (a) ?at cleavage type rupture, on radial-axial plane, (b) shear band failure at 45 degree to the tensile loading axis, and (c) a more gross deformation along the specimen width and thickness with cup-cone type ductile rupture. Fracture in the irradiated clad is by the shallow ductile rupture of the ligaments between the cracked hydride platelets. The failure in the irradiated fuel clad, occurs with formation of shear band and the fracture surface is inclined nearly 45 degrees to the tensile loading axis. Defective fuel clad, with high hydride density, is severely embrittled at lower test temperature but its ductility improves signi?cantly at higher test temperature.

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