Design and experimental assessment of thermal stratification effects on operational loading of surge line of Unit 5, Novovoronezh NPP

• Main Authors: V.P. Povarov, O.V. Urazov, M.B. Bakirov, V.I. Levchuk
• Format: Article
• Language: English
• Published: National Research Nuclear University (MEPhI) 2017-06-01
• Series: Nuclear Energy and Technology
• Subjects: Thermal stratification; Surge line; Thermal fatigue; Thermomechanical load; On-line monitoring; Stressed-deformed state
• Online Access: http://www.sciencedirect.com/science/article/pii/S2452303817300286

One of the key tasks in substantiating NPP service life extension consists in detailed examination of all factors affecting the remaining lifespan of critical NPP components. Particular attention must be paid to studying the phenomenon of thermal stratification (TS) which is the effect of coolant lamination into the “cold” and “hot” layers in horizontal pipelines when flows with different temperatures propagate at slow velocities. Special importance of this issue is explained by the fact that cyclic loads caused by TS contribute in accumulation of metal damage due to thermal fatigue and can provoke formation and accelerated growth of defects. This study is dedicated to implementing complex analysis of coolant TS observed in horizontal sections of the surge line (SL) on WWER-1000 units from the viewpoint of assessment of TS effects on the stressed-deformed conditions of metal and accumulation of cyclic damages. The experimental data on the distribution of temperature fields in horizontal SL sections, as well as cyclic loading history during several reactor fuel residence campaigns were recorded by the on-line diagnostic monitoring system put into operation on Unit 5 Novovoronezh NPP. Certain distinguishing features of TS in SL of Unit 5 were identified as the result of data analysis depending on operational modes. The most significant TS effects were observed in the control cross-section located within the first horizontal section from the pressurizer. Calculation and experimental assessment of effects of thermal loading factors on the stress-deformed state of SL demonstrated that effects of thermal stratification and thermal fatigue significantly affect the operational loading of the pipeline. It is noted that zones with maximum accumulated damage determined according to the results of calculations coincide with places where factual operational defects were detected. Procedure involving treatment of SL welded joints by the method of surface plastic deformation is suggested as the compensatory measure aimed at extending the SL residual lifespan.