Development of New Structural Materials for Advanced Fission and Fusion Reactor Systems

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1 Stress Corrosion Cracking Susceptibility of Austenitic Stainless Steels in Supercritical Water Conditions R. Novotny 1), P. Hähner 1), J. Siegl 2), S. Ripplinger 1), Sami Penttilä 3), Aki Toivonen 3) 1) JRC-IE, Petten, Westerduinveg, 1755 LE Petten, the Netherlands 2) Czech Technical University in Prague, Zikova 4, Prague 6, Czech Republic 1) Materials and Building, Technical Research Centre of Finland, Espoo, Finland

2 FP6 project: HPLWR Phase 2 HPLWR 2: High Performance Light Water Reactor Phase 2 Start: Sept. 1st, 2006 Duration: 42 months Partners: 12 WP4 on SCWR Materials and water chemistry European Contribution to GIF

3 FP6 project: HPLWR Phase 2 Light Water Reactor with supercritical coolant (25MPa) and more than 500 C core exit temperature Advantages: Direct steam cycle like BWR No main coolant pump in PL No recirculation pumps No steam separators in RPV 40% higher turbine power 44% net plant efficiency Major cost reductions envisaged

4 FP6 project: HPLWR Phase 2

5 HPLWR Plant target data

6 HPLWR Plant target data

7 HPLWR-2 Objectives Working on critical scientific issues to assess the feasibility of a HPLWR concept to determine its future potential in the electricity market. Critical Scientific Issues: Elaborate the nuclear island and balance of plant Design and analysis of a core and reactor internals Assess the safety of the HPLWR concept Find a selection of materials for in-vessel components Model the relevant heat transfer phenomena

8 HPLWR-2 Objectives

9 WP4-Materials Objective: Investigate materials behavior in supercritical water and to select optimal in-core and out-of-core materials with respect to: Stress Corrosion Cracking (SCC) resistance Oxidation resistance Creep resistance Irradiation resistance Tasks: Autoclave experiments: Oxidation mechanisms of ferritic/martensitic and austenitic steels, Ni-based alloys Combined mechanism of creep and oxidation Stress corrosion cracking tests Materials Data Base and Models for uniform corrosion, stress corrosion cracking, etc. Construction of Supercritical Water Loop for in-pile materials testing

10 WP4-Materials Corrosion Test Facilities VTT autoclaves 2 x (695 C / 35 MPa) with option for mechanical testing JRC-IE autoclaves 2 x (650 C / 35 MPa) with different loading systems On-line corrosion monitoring (electrochemical potential, el.chem. noise, contact electrical impedance, acoustic emission) Reference electrode Ag/AgCl development (VTT) In-pile SCWL development at Rez (NRI) Parallel corrosion tests at CEA using tubular specimens and furnace for SCC testing >> reporting to GIF

11 WP4-Corrosion tests

12 Summary of oxidation tests (VTT, JRC)

13 Summary of oxidation tests (VTT, JRC) Oxide thicknesses on the studied alloys after 1 year exposure to SCW (125 ppb O 2 ) at 400, 500 and 650 C - linear extrapolation from 600 or 300 h results Alloy 400 C (mm/year) 500 C (mm/year) 600 C (mm/year) 650 C (mm/year) P P ODS(1) ODS(2) PM NG * * H * = no measurements at lower temperatures - = too thin to measure BGA F/M steels: high oxide growth rate, ~1.5 mm/year , AISI 316, AISI 347: >0.2 mm/year oxide growth rate -9%Cr ODS: mm/year oxide growth Low corrosion resistance for these components Problems with assembly box, moderator box, fuel cladding: thickness mm, T = o C

14 SCC Susceptibility SSRT VTT

15 SCC Susceptibility SSRT VTT All strength values have decreased considerably as the test temperature has been increased from 500 C to 650 o C (strain rate was the same, 3x10-7 s -1, in both cases). Remarkable decrease has taken place in the yield stress of PM2000, on which the yield stress decreased to ~1/3 of the value at 500 o C

16 SCC Susceptibility SSRT VTT BGA

17 SCC Susceptibility SSRT JRC Slow Strain Rate Test (SSRT) in SCW Autoclave Material: 316L austenitic stainless steel Pt 316 SS AE Pull rod Nuts/Screws 316 SS Insulation Case Ceramic holders Pt + S1 + S2 Autoclave lid Heater Insulation Autoclave body Preheater/Cooler Inlet/Outlet

18 Results SSRT JRC Strain Rate Stress (MPa) Strain (%)

19 Results SSRT JRC Strain Rate

20 Results SSRT JRC Oxygen Content Stress (MPa) Strain (%)

21 Results SSRT JRC Oxygen Content The main features - ductile dimples. An occurrence of intergranular facets was found sporadically in the central part of fracture surface. c) 6-01 no significant influence on fracture micromorphology in areas corresponding to the stress corrosion cracking

22 Results SSRT JRC Temp. Difference

23 Results SSRT Temp. Difference Failure of the specimen was initiated by stress corrosion cracks propagated from specimen surfaces. specimen 8-01 stress corrosion cracks were found neither on the fracture surface nor on the surface of specimen. Fracture morphology corresponds to the static rupture.

24 Conclusions Corrosion and SSRT s Corrosion: For the thin-walled components in the design of an SCWR, corrosion, stress corrosion cracking and creep resistance are anticipated to be important degradation modes that need to be understood and controlled. The oxidation rates have to be lower than what is acceptable for materials in supercritical fossil power plants because of smaller wall thicknesses in the SCWR core designs. the oxidation rate of F/M steels is too high for SCWR core components even at the temperatures below 500 o C. austenitic stainless steels have a good enough oxidation resistance up to o C 20% Cr ODS steel was selected for the fuel cladding because of its excellent oxidation resistance even up to 650 o C, its SCC resistance and its good creep specifications Stress Corrosion Cracking: No clear SCC was observed on the fracture surfaces, but on side surfaces there were small cracks of which morphology, however, could not be identified except in the case of 316NG (which had both inter- and transgranular cracks, IGSCC and TGSCC). On the other hand, the experimental creep resistant steel BGA4 specimen contained IGSCC both on the fracture surface and side surfaces. At 500 o C, PM2000 did not show any susceptibility to SCC at all.

25 Conclusions Corrosion and SSRT s The SCC occurrence is favored by high oxygen content and slow strain rate. With decreasing test temperature, higher oxygen content and/or slower strain rate should be used to induce SCC. The repeatability of SCC occurrence for given SSR test parameters should be verified by subsequent experiments. Fractographic findings confirmed that failure processes are combination of transgranular stress corrosion cracking and transgranular ductile fracture. The proportion of SSC and ductile fracture on the failure process of individual specimens is predetermined by the parameters of slow strain rate test (i.e., oxygen content in test water solution, strain rate and test temperature). Influence of individual parameters of SSR tests on stress corrosion cracking was estimated.

26 FM CGR Tests Bellows based loading system Application for Crack Growth Rate SCC tests

27 FM CGR Tests Bellows based loading system Bellows based Loading System Pressure Adjusting Loop

28 FM CGR Tests Materials and Environment Ti-stabilized austenitic stainless steels was tested: 08Cr18Ni10Ti Material C Si Mn S P Cr Ni Ti Mo 08Cr18Ni10Ti SEN(B) specimens pre-cracked in air, a/w = 0.5 T-L Orientation Simulated BWR, SCWR water Temperature [ o C] 288; 550 Pressure [bar] 88; 230 Inlet Conductivity [ S.cm -1 ] 0.09 Outlet Conductivity [ S.cm -1 ] Inlet Dissolved O 2 [ppb] ; 2000 Outlet Dissolved O 2 [ppb]

29 FM CGR Tests Materials and Environment four tests carried out: 1.specimen AC125 (t = 550degC, p = 230 bar, Diss. Oxygen = 2000 ppb, ultra-pure water) Load (N) 300 DCPD (V) Time (h) Time (h)

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