Design and experimental assessment of thermal stratification influence on operational loading of surge line of Novovoronezh NPP, unit №5
One of the main tasks of NPP lifetime justification consists in detailed consideration of all factors affecting the residual life of critical NPP components. Particular attention should be paid to studying the phenomenon of thermal stratification (TS), which is the effect of coolant lamination to the “cold” and “hot” layer in a horizontal pipelines while flows having different temperatures stream with slow rates. The relevance of this issue is stipulated for the fact that cyclic loads caused by TS result in accumulation of metal damage due to thermal fatigue and can initiate formation and accelerated growth of defects.
The work is devoted to complex analysis of coolant TS observed in the horizontal runs of the surge line (SL) on WWER-1000 units. Comprehensive assessment of TS effects on a stress-strain state and cyclic damage accumulation of metal was also made. The experimental data regarding the distribution of temperature fields in horizontal SL sections, as well as cyclic loading history during several reactor campaigns were recorded by on-line monitoring system, put into operation on Novovoronezh NPP, Unit 5. Data analysis allowed to establish some distinctive features of TS in SL of Unit 5, depending on operational modes. The most significant TS effects were observed in the control section located in the first horizontal run nearby the pressurizer.
Experimental records were used as initial data for calculations which were provided in order to evaluate the stress-strain state of SL considering TS effects. Results of stress analysis allow to conclude that actually observed effects of thermal stratification and thermal fatigue have a significant impact on SL lifetime. Indicated by calculations areas, in which maximum accumulated damage is observed, coincide with places of operational defects being revealed during outage of Unit 5. As a compensatory measure aimed at increasing the SL residual life, it is proposed the procedure of treatment of SL weld joints using the method of surface plastic deformation.
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