1st Ukrainian-Hungarian Seminar “Safety, Reliability and Risk of Engineering Plants and Components”, Miskolc 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

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

1. Introduction 2. 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 aspects 4. Statistics of crack evolution before and after inactivation 5. Entire stress at welding joint before and after inactivation  Software complex “StrengthMaster”  Flaw assessment module “WFMaster”  Crack shape prediction 6. Remaining life of WJ prediction

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

3D model of Dy300 down-flow piping

Chemical composition and mechanical properties of steel 08X18H10T (similar to steel 321) and weld metal As received (base metal) Т, 0 СR T m, МPаR T p0,2, МPаА,%Z, %R T p0,2 / R T m , ,43 After 18 years (specimen with welding) , ,58 After 18 years (weld metal) , ,65 ElementCMnSiCrNiSPTiCuFeMo Steel 08X18H10T (accord. ГОСТ ) < 0.08 < 1.5 <  0.02  C  0.30 bal - weld metal (accord. ГОСТ )   bal

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

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

Chemical composition of water before and after inactivation of Chernobil NPP Parameter, (1988 – 1997) Maximum observed Averaged for 10 years Allowed Minimum observed pH Conductivity, S/cm < Chloride-ion 3.7 <2.1 <100 <2.0 CaCO 3 -equivalent, g/kg < Copper, Cu, g/kg < Iron, Fe, g/kg < Oil, g/kg <100 <100 <200 <100 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

Metalography features of IGSCC process

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

Templets of welding joint with the “bending opened” crack

Metalography features of IGSCC process

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

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

Intergranuliar cracks length evolution during Pipeline type Year of NDT Average current month of NDT Pressurized Overall crack length, mm Relative crack length, % Down-flow Overall crack length, mm Relative crack length, % 4,894,814,895,716,399,610,812,4 Emergency cooling Overall crack length, mm Relative crack length, % 1,9 2,99 4,755,026,11 Altogether Overall crack length, mm Relative crack length, % 4,34,474,966,618,6810,512,513,6

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

Schematic of welding joint containing seven IGSCC cracks

Actual measured crack depth versus crack length (1998)

3D calculation model of Dy300 down-flow piping

Universal software complex “Strength 3D”

Computer version of national Code of Ukraine ВБН В

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

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

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

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

Stabilized crack depth versus crack length evolution

Remaining life of welding joint versus half-length of crack

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.