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.

Performance Analysis and Tube Inlet Orifice Length Evaluation of a Once–Through Steam Generator

“…The minimum orifice length ranges from 183.8 mm to 198.9 mm for 100% power level, and from 285.1 mm to 288.0 mm for 20% power level. As expected, the low power level results in a higher minimum orifice length because flow instability tends to be more severe at a low feedwater flow rate [6] and this accordingly brings forth a higher orifice loss coefficient [4]. Thus, the secondary coolant flow rate for the minimum power level should be chosen as the orifice design condition; this implies that the orifice length is determined by the lowest power level considered in plant power operation.…”