PSI - Issue 71

A. Syed et al. / Procedia Structural Integrity 71 (2025) 82–89

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transferring heat to the secondary via steam generators. During reactor operation, the uranium oxide pellets deform into an hourglass shape due to the high temperatures, causing mechanical contact between the pellets and the cladding. This contact generates stress, which may exacerbate any existing cracks in the cladding, posing a potential risk of radioactive material leakage. Studying the fracture toughness of fuel cladding at different temperatures is crucial for ensuring the structural integrity and optimal lifetime of fuel bundles in reactors during operational transients. Fracture toughness varies with specimen thickness unless a plane strain condition is met, where it remains constant and can be used for safety assessments (Lao et al. (1986)). However, due to the low thickness of fuel cladding (only 0.4 mm), standard specimens can't be used to measure plane strain fracture toughness. Instead, a crack is directly introduced into the cladding, with a custom loading setup (Fig. 1(a)) designed to simulate real operational conditions. This approach is called non standard fracture specimen design.

(a)

(b)

Fig. 1: (a) Schematic of conical mandrel test setup used (b) Specimen design used for the tests (All dimensions are in mm) Nomenclature a Crack length PHWR Pressurized heavy water reactor A pl Plastic deformation energy W Specimen length b Remaining ligament size σ m Hydrostatic stress CT Compact Tension σ e von-Mises Stress h Triaxiality factor σ Yield strength of material J J-Integral r Distance along crack-tip K I Stress intensity factor t Specimen thickness μ Coefficient of friction Various non-standard methods have been proposed to estimate the fracture toughness of thin-walled tubular components. The pin-loading tension (PLT) technique was used to evaluate fracture toughness of thin walled specimens by Grigoriev et al. (1995, 1996, 1997, 2000 and 2005). Using the PLT method, specimens with axial cracks are machined from tubular components, and the cracks are opened and propagated along the specimen's longitudinal axis using two split pins. They evaluated the mechanical properties of irradiated and un-irradiated properties of Zr-2 clad tubes and measured the delayed hydride cracking velocity in claddings. Fracture toughness of Zircaloy-4 material was measured using CT specimens obtained from a rolled plate by Bertolino et al. (2003) and Samal et al. (2020). A segmented expanded mandrel was used by Foster et al.(1989), Nobrega et al. (1985) and Nilsson et al. (2011) to characterize the material and evaluate the structural integrity. Expanding segments are positioned radially inside a cladding tube to imitate thermally expanding fuel. The internal conical mandrel technique was used by Syed et al. (2020) to determine the fracture toughness of Zircaloy-4 clad tube at room temperature by using a cracked clad tube of different a/W ratios. They used finite element technique to evaluate the friction between the clad tubes and mandrel. Using these friction values, they further evaluated the crack initiation toughness values. They have shown that even for different values of a/W ratio, mandrel angle and specimen length, the crack initiation toughness lies in a range of 60-100 kJ/m 2 . In this work, internal conical mandrel technique is further extended to evaluation of high temperature fracture toughness of thin walled clad tubes. The fracture toughness is evaluated at room temperature and 300°C. Finite element analysis has been carried out to evaluate the effect of friction between the clad tube and conical mandrel. Crack initiation toughness has been evaluated at room temperature and 300°C that can be used for design and life assessment of Zircaloy-4 clad tubes.