Cladding ballooning and burst testing capability with fragmented-pellet simulators and post-test wall thickness mapping

Ballooning and burst separate-effects tests are widely used to reproduce cladding behavior under loss-of-coolant accident (LOCA) conditions and to validate transient fuel-performance models. A high-temperature burst rig was developed at the HUN-REN Centre for Energy Research to perform isothermal tests up to 1100 °C under controlled linear pressurization rates (1 kPa/s to 1 MPa/s) in inert atmosphere. To explore fuel relocation and potential dispersal pathways at burst, selected tests used simplified fragmented-pellet simulators assembled from precision ceramic grinding balls with two representative size fractions. High-speed imaging (up to 62,500 fps) captured the rapid ejection of fragments through the burst opening. After testing, the geometry and wall-thickness distribution of ballooned and burst specimens were quantified using three complementary techniques: (i) metallography-based thickness measurements from stitched optical micrographs, (ii) X-ray computed tomography (CT) with surface extraction to point clouds followed by automated binning into axial slices and angular sectors, and (iii) 3D optical profilometry of both outer-surface and inner-surface scans enabled by a custom automated actuator, using negatives produced by flexible replica casting. The techniques are complementary and generally consistent in overlapping regions, and together they reveal pronounced asymmetry of deformation and localized wall thinning before the burst. The resulting experimental and metrology workflow provides code-facing metrics (e.g., minimum remaining wall thickness and its axial/circumferential location) to support improved burst criteria and uncertainty quantification in fuel-performance modelling during LOCA.