Measurements on UO2 dimensional stability on Oskarshamn 1 fuel

Fuel from the 440 MWe Oskarshamn I reactor, irradiated up to about 6000 MWd/t, has been examined by various methods with respect to fuel densification and axial gaps in the fuel stack. The conclusions are: fuel densification has occurred to a low but significant extent as measured on fuel stack length. The measured densification is within the extent suggested by the USAEC. The fuel stack length decrease is partly caused by pellet-pellet wear and adjustment. There exists a mechanism for pellet axial hang-up in the rods at thermal contraction at reactor shutdown. There were no indications of axial gaps in operating conditions. The gaps detected after reactor shutdown are filled out at power operation. Based on the results of the examination it can be statistically shown that the remainder of the core can not contain any large fraction of rods with gaps during operation and with a high level of confidence not coinciding gaps in adjacent rods