“ Materials like persons are interesting with their imperfections ” (F.C. Frank, W.T. Read)

1. 1st Ukrainian-Hungarian Seminar “Safety, Reliability and Risk of Engineering Plants and Components”, Miskolc 11-12 April 2006 Intergranular stress corrosion cracking of the piping Dy300 welding joints of unit #3 before and after inactivation of Chernobyl NPP A.J. Krasowsky Pisarenko Institute for Problems of Strength, National Academy of Sciences of Ukraine

2. “Materials like persons are interesting with their imperfections” (F.C. Frank, W.T. Read)

3. 1.      Introduction2.      Main factors governing the IGSCC of welding joints·       Chemical composition of water·       Sensibilisation of austenitic steel 08X18H10T·       Tensile stress (residual + service)3.      Main features of IGSCC·       Metallurgical aspects·       Fracture mechanics aspects4.      Statistics of crack evolution before and after inactivation5.      Entire stress at welding joint before and after inactivation·       Software complex “StrengthMaster” ·       Flaw assessment module “WFMaster”·       Crack shape prediction6. Remaining life of WJ prediction

4. General view of unit #3 RBMK-1000, Chernobyl NPP

5. 3Dmodel of Dy300 down-flow piping

6. Element As received (base metal) C Mn Si Cr Ni S P Ti Cu Fe Mo Steel  08X18H10T (accord. ГОСТ 5632-72) <0.08 <1.5 <0.8 17-19 10.0- 11.0 0.02 0.035 5C-0.06 0.30 bal - Т,0С RTm, МPа RTp0,2, МPа А,% Z, % RTp0,2/ RTm 20 510 216 35 55 0,42 weld metal (accord. ГОСТ 5632-72)  0.06 0.9-2.0  0.60 17.8- 20.0 9.8 - 12.0 - - - - bal 1.88- 3.0 350 412 177 26 51 0,43 After 18 years (specimen with welding) 20 618 320 46 72 0,52 350 432 250 32 77 0,58 After 18 years (weld metal) 20 654 422 44 63 0,65 350 465 303 22 43 0,65 Chemical composition and mechanical properties of steel 08X18H10T (similar to steel 321) and weld metal

7. History: Gamma-NDT results of pipes Dy300 welding joints revealed on unit No 3 ChNPP during 1997

8. History: Ultrasonic-NDT results of pipes Dy300 welding joints revealed on unit No 3 ChNPP during 1997-1998.

9. Parameter, (1988 – 1997) Maximum observed Averaged for 10 years Allowed Minimum observed pH 7.6 7.2 6.5-8.0 6.7 Conductivity, S/cm 0.35 0.21 <1.0 0.1 Chloride-ion 3.7 <2.1 <100 <2.0 CaCO3-equivalent, g/kg 3.0 3.0 <5.0 3.0 Copper, Cu, g/kg 8.7 5.1 <20.0 2.6 Iron, Fe, g/kg 36.0 13.3 <50 5.0 Oil, g/kg <100 <100 <200 <100 Chemical composition of water before and after inactivation of Chernobil NPP Conductivity after inactivation: 2001 – 2002: 0.55 – 1.2, average 0.88 micro-S/cm allowed 2.0 micro-S/cm 2004 – 2005: 0.53 – 1.01, average 0.56 micro-S/cm

10. Metalography features of IGSCC process

11. Axial stress distribution at the root of welding joint for different service stress (courtesy Prof. Makhnenko)

12. Templets of welding joint with the “bending opened” crack

13. Metalography features of IGSCC process

14. IGSCC rate versus SIF of steel 10X18H10T at different corrosive environment (courtesy Prof. M. Speidel)

15. Predicted regions of IGSCC (courtesy Prof. M. Speidel)

16. Pipeline type Year of NDT 1998 1999 1999 2001 2002 2003 2004 2005 Average current month of NDT 2 14 20 40 58 70 82 93 Pressurized Overall crack length, mm 1065 1250 1565 2520 3825 3755 4735 4865 Relative crack length, % 3.62 4.24 5.31 8.56 12.99 12.7 16.0 16.5 Down-flow Overall crack length, mm 2385 2345 2385 2755 3115 4685 5280 6060 Relative crack length, % 4,89 4,81 4,89 5,71 6,39 9,6 10,8 12,4 Emergency cooling Overall crack length, mm 70 70 110 110 175 185 225 225 Relative crack length, % 1,9 1,9 2,99 2,99 4,75 5,02 6,11 6,11 Altogether Overall crack length, mm 3520 3665 4060 5415 7115 8625 10240 11150 Relative crack length, % 4,3 4,47 4,96 6,61 8,68 10,5 12,5 13,6 Intergranuliar cracks length evolution during 1997 - 2005

17. Intergranular cracks length evolution during 1997 – 2005: Relative crack length , l % :

18. Schematic of welding joint containing seven IGSCC cracks

19. Actual measured crack depth versus crack length (1998)

20. 3D calculation model of Dy300 down-flow piping

21. Computer version ofnational Code of Ukraine ВБН В.2.3...

22. Window of input data for remaining life calculation, «WFMaster»software

23. Axial stress distribution along the wall thickness from the root of welding joint (courtesy Prof. Makhnenko)

24. Axial stress distribution along the wall thickness, H, at different service conditions

25. Stabilized crack shape (a – crack depth, L – crack half length) (single nucleus approximation)

26. Stabilized crack depth versus crack length evolution

27. Remaining life of welding joint versus half-length of crack

28. CONCLUSIONS 1.Three main factors are responsible for IGSCC process in welding joins of pipeline Dy300: a.Tensile stress of necessary level b.Corrosion environment (oxygen in cooling water) c.Grain boundaries sensibilisation of steel at HAZ 2.Effect of these factors was considered at remaining life prediction. 3.Evolution of IGSCC cracks relative length within 89 welding joints during 1997 – 2005 demonstrates similar rate before and after inactivation of unit #3. These data of in-service NDT are unique. 4.Due to the fact that crack depth is governing parameter of IGSCC process whereas NDT inspection provides the crack length, the conservative model of single nucleus was developed for remaining life assessment of welding joints.