Fuel pins of UO2-Zr were irradiated to a burnup of 5600 MWd/MT UO2 in the OECD Halden Boiling Water Reactor. As a result, the pellet stacks shortened by ∼0.7%. A mathematical model, based on a ratcheting pellet movement resulting from mechanical fuel-cladding interaction during successive irradiation periods, accounted well for the observed decrease in length. Examination of the inside of the cladding at several axial locations confirmed the ratcheting mechanism and indicated no contribution to pellet-stack shortening from isotropic fuel densification.