Analysis of Corrosion Fatigue for the Deaerator Heater Tanks in Nuclear Power Plants

Throughout the world, failures of deaerators and associated storage vessels in thermal power plants have been reported. In 2004 a through wall crack was discovered at one of the Ontario Power Generation (OPG) thermal stations raising whether CANDU nuclear plant deaerators are at risk to similar failure mechanisms. The cause of cracking in the thermal plant was identified as corrosion fatigue cracking. The cracking area of the DA heater tank in the plant was located at the vicinity of the weld connecting the splash plate to the vessel wall. This plate divides the vessel into two compartments. The upper compartment of the tank is filled with condensate water coming from Low Pressure (LP) Heaters. The lower compartment contains steam fed by the LP turbine. In addition, condensate spray nozzles are mounted on this plate. To analyze this phenomenon and assess its potential risk for the nuclear plants, a Finite Element model of the DA heater tank cross section was created. Using the design conditions and operating transients measured during DA cycling, stress and fatigue analyses for the area susceptible for corrosion fatigue cracking were carried out. The criteria of ASME Sec VIII, Div. 2 were used for the analysis. It was concluded that CANDU nuclear plant deaerators are not susceptible to corrosion fatigue cracking during their life cycles because of their low operating cycles and less harsh working environment.