Internal and external heating are two methods commonly used in fuel element and bundle high-temperature deformation behaviour experiments. External heating is preferred in some cases because of its simplicity. However, the radial temperature gradient produced by this method in an element is in the opposite direction to that generated under in-reactor prototypical conditions where the maximum temperature occurs at the fuel centre. The impact of such a discrepancy upon the element deformation response is unknown. A study was undertaken to address this knowledge gap through a direct comparison of the element bowing behaviour under external and internal heating conditions. The experiments conducted on horizontal simply supported CANDU type non-uranium fuel element simulators showed a significant difference in the element sag rates during exposure to a steady temperature of 800°C as well as in the residual plastic bow measured after the tests. No appreciable difference in the response was found in the range below 700°C. To support these experiments, a finite element model of a CANDU fuel pin was created using the ANSYS finite element software. The model is fully three dimensional and includes the fuel pellets and sheathing as separate components, which interact via contact elements.
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