The growing importance of variable renewable power sources results in coal-fired boilers being operated with increased thermal cycles than envisioned when designed. This may result in increased oxide spallation within boiler tubing, potentially leading to downstream hot short failures or turbine erosion. T91 tube sections were pre-oxidized with two thick oxide scale levels at 710 °C, 200 bar steam. These tube sections (plus fresh tubes) were cycled under steam between 600 °C, 200 bar and 200 °C, 10 bar in exposure intervals of 10 cycles. The tubes were exposed for 60 cycles maximum, or until significant oxide spallation failure occurred. Similarly, pre-oxidized T91 flat coupons were exposed as a control. Differences between the oxide scale spallation of both the inner and outer tube sections were examined, and these observations, incorporating oxide morphologies, were used to develop an oxide spallation model during thermal supercritical steam cycling to improve power plant performance.