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An extensive study of fuel swelling in oxide fuel has been performed on fuel which has been ramp-tested in the Halden Heavy Boiling Water Reactor. The ramp-tests were performed to study the mechanisms of PCI in Advanced Gas-cooled Reactor fuel (AGR) but the initial clad deformation measurements were later supplemented by the use of Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM).
The driving force for PCI arises from a combination of pellet thermal expansion and the creation of fission gas porosity during a temperature/rating transient. Fuel specimens from thirteen ramped rods and two control-reference rods have been examined using TEM and SEM. Swelling measurements have been made at four or five radial locations in the fuel. At least six full grain boundaries were used for the inter-granular study at each location and three complete trans-granular fractures employed for the intra-granular bubbles. In the latter case, the trans-granular regions were examined under very high magnifications to reveal pores as small as 20-25 nm diameter. The SEM study comprises nearly three thousand micrographs.
The microscope study was augmented by use of the ENIGMA fuel modelling code to obtain estimates of the local temperatures and conditions from which the SEM/TEM samples were obtained.
NEA-1705/02: The analysis from the 4135, 4136 and 4140 data has been incorporated. Data 4000, 4004, 4064, 4065, 4162, and 4163 have been revised. Many of the tables have been improved. The main report has been extensively modified from the earlier version.
Keywords: fission gas, fuels, pellet clad interaction (pci), power ramp, swelling.