Experimental and analytical investigation into boiling induced thermal stress: Its impact on the stress state of oxide scales of nuclear components

Abstract

Abstract This paper experimentally and theoretically investigates thermal stress of materials caused by non-uniform surface temperature fields due to bubble dynamics of nucleate boiling. The surface temperature of Calcium fluoride (CaF2) on a thin-film indium tin oxide (ITO) heater was measured by an Infrared high-speed camera. Then, the obtained temperature fields were used to calculate boiling induced thermal stresses. An analytical model was developed to predict thermal stress due to nucleate boiling. The localized cold spot under a growing bubble gives rise to the peak tensile stress. The magnitude of the peak stress is predominantly determined by liquid and solid properties, and applied heat flux to a limited extent. Nevertheless, higher heat flux is observed to result in a higher probability of material fracture due to an increased number of nucleation sites, which gives a larger effective surface area under stress. Newly discussing another cause of stresses for oxide layer mechanical integrity, the presented discovery of boiling induced stress implies potential effects on break-away of oxide scales under boiling conditions, including reactor fuel claddings and steam generator U-tubes. Moreover, the boiling stress can be potentially used to design self-cleaning surfaces by breaking unwanted deposits.