Degradation assessment of nuclear power plant extraction steam piping after long term service

Degradation due to ageing in a Nuclear Power Plant’s Extraction Steam Piping has been assessed. Samples of twelve years old seamless carbon steel SA 106B pipe have been taken and subjected to metallographic examination, hardness testing, radiographic examination and ultrasonic thickness measurement to investigate and analyze ageing in piping material. Metallographic examination of aged sample revealed irregularly distributed graphite nodules in ferrite grains and on grain boundaries as well, the reduction in pearlite phase has also been observed from 40% to 25%. This Transformation in microstructure has reduced hardness of steel. Reduction in hardness has found 20.4% and 0.7% on inner and outer layer of the pipe material respectively. Corrosion pits having average depth of 0.07 mm have been found on external surface. Radiograph of aged pipe revealed material removal and wall thinning due to erosion on inner surface of pipe. The extent of erosion had been checked, which has found 8.7%

Root cause failure analysis of a domestic pressure cooker through metallurgical characterization and computational simulation

Pressure cooker is a closed domestic pressure cooking vessel for use with external heat source and capable of maintaining nominal cooking pressure up to 1.0 kgf/cm2 (100 kN/m2 approximately) gauge nominal. In pressure cookers, despite the development of electronic controllers, the basic edition of such vessels are still equipped with fewer advanced safety functions due to economic constraints. Although the provision of pressure relief valves (PRV) is considered as one of the major protective features, however, pressure cooker failure accidents have been reported frequently. This paper describes the analysis of pressure cooker failure that failed prematurely after 1 year of service considering the design life of > 5 years. It was hypothesized that the root cause of pipe failure was either material degradation from exposure to an aggressive environment or an inherent defect in the pressure cooker. To test this hypothesis, a thorough visual examination of the exhumed failed section and the fracture surface was undertaken, followed by liquid penetrant testing, material identification, hardness testing, and metallographic analysis. Computational models of static and transient loading were also used to determine the stress distribution along the actual geometry of the failed cooker and to understand the main causes of recurrent failures. Visual and macroscale examination revealed significant body deformation at the lower dish-ended shell showing distorted locking grooves. It was also noticed that dirt and food particle, from the earlier cooking, were stuck in the pressure valves orifice. In addition, no evidence of metallurgical defect was observed. The inspection indicated that the cause of failure is primarily due to the choking of pressure relief value (PRV) and overpressure safety valves. Consequently, the pressure release occurred from the sealing side of the top lid, which resulted in its ejection