The complex scientific-research tasks on the structural strength of the weld joint node of the collector with the nozzle in the steam generator PGV-1000M (welded joint #111)
Damage in the zone of welded joint #111 (WJ #111) significantly reduces the lifetime of the steam generators of the nuclear power plant. Taking into account numerous studies in this direction, the problem of the initiation and propagation of cracks in the WJ #111 remains relevant to the present.
The research includes a numerical and experimental analysis of the possible reason of damage of WJ #111 zone. On the basis of the research, it can be concluded that the main factors of loading are: structural feature of zone of WJ #111 which is leading to the large stress concentration compared to other parts of the collector of SG; possibility of jamming of SG supports due to the uneven load distribution for SG rollers; out-of-roundness of cross section of pipe bends near SG which causes the manometric effect from the internal pressure of primary circuit.
Besides the factors of loading which affects for the crack initiation and growth are: the fluctuation of temperature of the coolant of the first and second circuits, even at the nominal operating mode and additional stresses in the WJ #111 with the presence of jamming of support of SG as a result; hydrotest of the second circuit which is lead to stresses higher than yield stress.
The possibility of supports jamming is studied in the current work and conclusion that such effect can actually occur at SG is drawn. So, the low value of coolant temperature fluctuations (3-5 degrees) leads to cyclic loading with the amplitude of about 20 MPa. Let’s note that cyclic loading acts with a total high stress state of WJ #111. Analysis of the results of monitoring of thermal-hydraulic parameters of the first circuit of unit #3 of SUNPP has shown that coolant temperature fluctuations value of about 3 ºC occur. During the given period of operation the oscillation frequency is about one oscillation per day. It can leads to propagation of cracks in WJ #111. The above statement is also confirmed by the fatigue striations which were found in specimen with crack.
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Fig. 1.
The SG with supports: a – general view (1 – SG; 2 – support; 3 – hydro-shock absorber); b – general view of support(4 – lodgment; 5 – base; 6 – roller support); c – upper plate with rollers; d – lower plate with rollers
