Small flakes that consist primarily of magnetite have been discovered on the secondary side of the steam generator of the Three Mile Island Unit 1 plant. These iron oxide flakes are believed to cause significant increases in flow resistance, which in turn causes abnormal increases in steam generator water level. It is necessary to measure the physical properties of the tube scale so that the maximum amount of loose flakes can be generated prior to hydrodynamic cleaning (water slap). It is also important to study the flake properties to shed light on the flake formation and transport mechanisms. Once the physical properties of the tube scale are determined, the effects of hydrodynamic cleaning (water slap) can be optimized by preconditioning the scale. There are several preconditioning options including prewetting, predrying, and thermal cycling of the steam generator tubes. Understanding the physical properties of the scale would also be beneficial in optimizing the water slap technique itself. Elastic modulus, fracture stress, thermal expansion, and swelling of the flakes were measured. With one exception, all of the flakes studied were either one-or two-layered as judged by microstructural variation. The fracture stress of the flake materials tested was in the range of 20.0 to 113.8 MPa (2.9 to 16.5 ksi). There did not appear to be a substantial change in the range of stresses measured at elevated temperatures. There was no evidence of delamination during bend testing. The mean coefficient of linear thermal expansion was a factor of ∼2 larger than that of Fe3O4. The maximum amount of swelling measured was 0.0012%, which is consistent with earlier data on flakes from the Oconee-2 plant.