Structural Integrity Confirmation of a Once-through Steam Generator from the Viewpoint of Flow Instability

Abstract

Helically-coiled once-through steam generators have been utilized for an integral type reactor showing several benefits such as high quality steam generation, geometric compactness, and compensation for a thermal expansion. Steam generator operations with unstable two-phase flow conditions on the tube-side may cause degradation of the tube materials and curtail the lifetime of the component. Based on existing experimental results for a once-through steam generator, its structural integrity was confirmed from the viewpoint of flow instability. The work was composed of three items, the prevention of static instability between the module steam/feedwater pipes, tube inlet orifice sizing against a dynamic instability between the heated coils, and a thermal-cyclic stress analysis for an overall component lifetime evaluation. The static thermo-hydraulic calculation for the steam generator cassette showed that while the prevention of the static instability was satisfied for the power operational mode, special care should be taken during the startup/cooling operational modes. The tube inlet orifice size was determined based on the orifice coefficient concept and existing experimental data for once-through steam generators. The thermal-cyclic stress evaluation for the heated tube revealed that the maximum alternating stress intensity was lower than the allowable fatigue limit value of the tube material.