-NETNUC- SCC Properties and Oxidation Behaviour of Candidate Materials at SCW conditions

1. -NETNUC-SCC Properties and Oxidation Behaviour of Candidate Materials at SCW conditions Sami Penttilä NETNUC/GEN4FIN meeting 03.04.2009, VTT, Espoo

2. Content • Weight gain tests • SCC susceptibility • Status of oxidation studies using bellows system • Summary

3. Weight gain tests • Sixteen materials from the four different alloy groups for screening stage: Chemical compositions of the studied alloys (wt-%)

4. Weight gains at different temperatures for selected alloysafter ~600 h exposure to SCW conditions (25 MPa)

5. Weight gain test Weight gains of 316L pipe with different surface treatments T = 650°C, t = 1000 h, p = 25 MPa and D02 = 125 ppb

6. 316L pipe samples with different surface finishes after 1000 h exposure to SCW at 650°C machined as received grit #600 grit #1200

7. Optical images of the 316L pipe samples after 1000 h exposure to SCW at 650°C grit #1200 surface finish machined surface oxide thickness ~ 40 – 50 µm oxide thickness ~ 2 µm

8. Improving corrosion and oxidation behavior of materials at SCW conditions • Chemical composition, Cr content > 18% • GB modification → thermomechanical processing • Application of surface treatment, e.g. CW • Effect of CW • An enhanced Cr diffusivity • increased defect density • Results to a more compact and continuous (uniform diffusivity) Cr-oxide layer • Dense Cr-oxide layer decrease the outward/inward transport of Fe and O Babcock-Hitachi Europe Gmbh, Advanced materials for AD700 boilers, Milano, Italy, 2005

9. SCC susceptibility - SSRT • T = 500°C and 650°C, p = 25 MPa and DO2 = 125 ppb • Strain rate of 3x10-7 s-1 • Materials • 316NG • 347H • 1.4970 • BGA4 • PM2000

10. Stress-strain curves for the studied alloys under SCW conditions at 500°C and 650°C 500°C 650°C

11. SEM images of the fracture surface of alloy 1.4970 after SSRT at 500°C

12. SEM images of the fracture surface of alloy BGA4 after SSRT at 500°C

13. Summary of SCC susceptibility after SSRT at 500°C and 650°C under SCW conditions

14. Oxidation kinetic studies (CER/CEI) using pneumatic servo-controlled bellows system

15. Status of DB-system • Problems with pressure tube material (316L) • Problems with welding • New miniature DB-system has been designed by P. Moilanen • Purpose is to perform mechanical loading (3-PB etc.) tests for selected materials at SCW conditions • Oxidation kinetic studies using CER/CEI methods • CEI and CER analysis have showed that most of the commercial materials behave like semiconductors under SCW conditions

16. Summary • The oxidation rate of F/M steels is too high for SCWR core components even at the temperatures below 500°C • Austenitic stainless steels, which have a good enough oxidation resistance up to 500 – 550°C have been selected as a candidate materials for most HPLWR core components • 20% Cr ODS steel was selected for the fuel rod cladding material because of its excellent oxidation resistance even up to 650°C, its SCC resistance and its good creep specifications • However, the SSRT results in this work are not in line with the creep strength specifications • More studies are needed in the behaviour of high Cr F/M ODS steels in supercritical water • Other problems with the ODS materials are the difficulties in welding and high prices • Therefore these materials are recommended to applications where high strength and oxidation resistance are needed, but only little additional joining is required • A possible road for the fuel cladding development may be coating of austenitic stainless steels or F/M steels • Processes that have been studied are diffusion coatings (i.e. slurry application, CVD) and overlay coatings (thermal spray, PVD, electroless coatings)