Professor V.I. Makhnenko, scientist A.S. Milenin E.O. Paton Electric Welding Institute of National Academy of Sciences of Ukraine 1 Remaining Time Assessment of the Steam Generator Welding Joint №111-1 of NPP VVER-1000 with Respect to Detected Cracks.

2 Fig. 1. Scheme of location of the welded joint № 111: 1.body of steam generator; 2.collector; 3.nipple  1200; 4.pipe conduit DU-850; 5.pocket; A discontinuity (defect).

3 Fig. 2. Defectogram of the welded joint №111-1 of “hot” collector 1PG-1 of Uzhno-Ukrainskaya nuclear power-plant in ~1 year after first repair.

4 Fig. 3. Defectogram of ultrasonic inspection of the steam generator № 3, power unit № 4 of Zaporozhskaya nuclear power- plant.

5 Fig. 4. Cartogram of discontinuity №3 of the welded joint №111-1 (“hot” collector) PG-3 of power unit №4 Zaporozhskaya nuclear power-plant according to the data of the Expert's report “UkrTsNIITMASh” of

6 Main Problem: permissibility of exploitation of the steam generator 4PG-3 of Zaporozhskaya nuclear power-plant with a detected defect in the region of the welded joint №111-1 until at least the next planned repair (about 1 year). Questions are to be answered:1. What is the degree of risk of the generation of the through-wall crack within the mentioned period of the exploitation, i.e. the leakage in the region of the growing defect, that will lead to the emergency stop of the power unit? 2. What is the degree of risk of the spontaneous destruction with the unpredictable consequences?

7 Table 1. Chemical composition and mechanical properties of steel 10GN2MFA. Chemical composition, weight percent.Mechanical properties. СSiMnCrNiMoV σ В, MPa (20 0 С) σ В, MPa (350 0 С) σ 0.2, MPa (20 0 С) σ 0.2, MPa (380 0 С)

8 Fig. 5. Calculation data about the distribution of working stresses  zz in the region of the pocket depending on pressure in the steam generator P=6.4 MPa and in the collector P C =16 MPa.

9 Table 2. Stresses  zz in the region of the defect depending on working pressure 6.4 MPa.

10 Table 3. Residual stresses   (MPa) in the region of the welded joint № r, mm z, mm Seam №

r, mm z, mm Seam № Table 4. Residual stresses  zz (MPa) in the region of the welded joint №111.

12 Fig. 6. Total stresses  zz at the plane of crack z=const.

13 Fig. 7. Scheme of the diagram of the static corrosive crack growth resistance of constructional material: 1.diagram lg v – K I according to experimental data; 2.idealized diagram.

14 first stage (K I <K ISCC ), when the mechanism of the electrochemical corrosion in the growth of crack prevails, here the values of the crack growth rates are rather small and with reference to the case under consideration don't exceed 1-2 mm/year; second stage (K ISCC <K I <K IC ), when the mechanism of the hydrogen embrittlement in the growth of the corrosion crack prevails, here the values of crack growth rates are sufficient enough and for constructional steels in the environment of the feedwater they can mount to the values 50 mm/year; third stage (K I >K IC ) corresponds to the state of high risk of the spontaneous growth of crack. Three main stages of the diagram of the static corrosive crack growth resistance:

15 Table 6. Sizes of defects and equivalent crack, values of K I (G) and K I (D). Steam generator Number of defect Size of defect, mmEquivalent crack Scheme of semi-elliptical crack Depth, H, mm Width, 2L, mm а, mm с, mm K I (G), MPa  m 1/2 K I (D), MPa  m 1/2 1PG-1 UUNPP PG-2 UUNPP a = H, c = L, if H  L; if H > L;

16 Table 5. Total stresses   (MPa) in case of T=300 0 C and working pressure in the region of the welded joint № r, mm z, mm Seam №

17 Calculation of the coefficient of stress intensity K I (D), K I (G) and  ref for the semi-elliptical crack a×2c. where S j is an equivalent stress Here  zz (i) is total stress  zz according to the diagram in Fig. 6 at the depth (a/20)·i (i=0,1,2,…,20); C i, D i, E i, F i are tabulated weighting functions, If (a/c)<1.0 and (a/  )<0.7

18 If (a/c)>1.0 and 0.2<(a/  )<1.0 where ;; ; ; ;;;

19, if L r < L r max K r =0, if L r ≥ L r max where  ref is the stress that is determined with external power load without taking into account the residual stresses and that is able at some level of the loading to cause the plastic collapse in the region of the defect under consideration. Mathematical formulation of R6 approach.

20 To calculate  ref it was used where

21 Fig. 8. Time dependences of a, c (a) and K I (D), K I (G) (b). (a)(b)

22 Fig. 9. Kinetics of reducing of the safety factor against spontaneous growth of crack a 0 =40 mm, c 0 =45 mm in time: K ISCC =10 MPa·m 1/2, v m =44 mm/year.

23 Conclusions: 1. Numerical analysis of state of the steam generator 4PG-3 of Zaporozhskaya nuclear power-plant showed that the risk of leakage (through defect) in the course of year of its exploitation (with double time reserve) is obviously absent. 2. The kinetics of the changing of safety factor against the spontaneous propagation of crack showed that during a year of the exploitation the risk of such a spontaneous growth is minimal.