SCWR materials research at VTT

Transcription

1 VTT TECHNICAL RESEARCH CENTRE OF FINLAND LTD SCWR materials research at VTT 10 th of October, 2016, CVR/Rez, Czech Republic IAEA Technical meeting on SCWRs Sami Penttilä & Aki Toivonen VTT Technical Research Centre of Finland Ltd.

2 Content Introduction VTT projects Results EU projects Summary on Slow Strain Rate tests SCC susceptibility Academy of Finland project IDEA Effect of CW (shot peening) on oxidation resistance Exposure tests at 700 C Mixed Conduction Model GIF Round Robin exercise on corrosion Miniature autoclave tests (Crack Growth Rate) VTT Centre for Nuclear Safety (CNS) 07/10/2016 2

3 Introduction One of the critical technological issues in SCWR is the materials for fuel cladding and core components that must be used in high pressure (25 MPa) and high temperature (up to ~650 C) SCW conditions According to the requirements of the thermal hydraulic design and structural stability, the assembly has to retain its original configuration with high accuracy fuel cladding, water rod etc. An increased metal temperature will lead to increased corrosion and creep deformation of the reactor components 1. Wall thickness loss of the component increased stress 2. Insulation of the material increased metal temperature 3. Spallation of oxide spalled oxide can erode components Ni-based alloys are less favorable for use in European SCWR concept due to their high neutron absorption and associated swelling and embrittlement ODS alloys are considered for many applications in the nuclear industry and they are also target material for fuel cladding in an SCWR but many challenges are need to be overcome before their commercial use 07/10/2016 3

4 EU-projects SCWR R&D activity at VTT HPLWR HPLWR Phase GETMAT SCWR-FQT EU proposal E-SMART, 5 th of Oct EU proposal MEACTOS, 5 th of Oct Academy of Finland project IDEA Networks & Forums GIF SCWR M&C PMB Canada, EU, China, Russia IAEA technical meetings on SCWRs 07/10/2016 4

5 Background Materials studies at VTT in SCWR related EU projects HPLWR -literature survey of applicable materials HPLWR Phase 2 -General corrosion -SCC (SSRT) -Creep in SCW of in-core candidate materials for European SCWR GETMAT -General corrosion and SCC of oxide dispersion strengthened (ODS) steels SCWR-FQT -General corrosion & SCC of materials for in-core SCW test loop E-SMART HPLWR High Performance Light Water Reactor GETMAT GEneration 4 and Transmutation MATerials SCWR-FQT - Supercritical Water Reactor Fuel Qualification Test 5

6 Experimental methods General corrosion tests SCC susceptibility tests - SSRT tests Creep tests SCW at o C/25 MPa with 150 ppb / 8 ppm O 2 Post test SEM characterization for coupon samples & fracture surfaces 07/10/2016 6

7 EU projects - Materials on SCC susceptibility tests GETMAT HPLWR Phase 2 SCWR-FQT 07/10/2016 7

8 SCC suspectibility results of HPLWR Phase 2 07/10/2016 8

9 Results HPLWR Phase 2 Alloy Maximum stress (MPa) Strain to failure (%) Fract. Mode 347H@ 500 C Duct. 316NG@ 500 C Interrupted at 325 Interrupted at 37 NA 316NG@ 650 C TG+IG C ,5 Duct @ 650 C Badly oxidized BGA4 500 C TG+IG BGA4 650 C NA NA NA PM2000@ 500 C 320 Interrupted at 52 NA PM2000@ 650 C Duct. 07/10/2016 9

10 Results SCWR-FQT (initial materials) 347H SCWR-FQT (1x10-7 1/s) Rp 0.2 Max. stress (MPa) Plastic strain to failure (%) Fract. Mode 550 C Duct. 550 C TG+IG 550 C TG 316L 07/10/

11 Results SCWR-FQT (316L tube, tests in SCW/air) As-received, air, 550 o C As-received, SCW, 550 o C Welded, SCW, 550 o C Material Air / SCW R p1.0%, MPa 07/10/ R m, MPa A p, % Fracture mode Plate Air Ductile Plate SCW Ductile Tube, mill-annealed Air Ductile Tube, mill-annealed SCW Intergranular Tube, welded Air Ductile Tube, welded SCW Intergranular/ interdendritic Tube, shot-peened Air Ductile Tube, shot-peened SCW Intergranular

12 SCWR-FQT SSRT tests Effect of manufacturing process on cracking: 316Ti tube (8x1 mm) SSRT test in SCW at 450 C with 150 ppb O 2, 1x10-7 1/s outer surface inner surface SCC tests will be needed with specimens that are cut from real cladding tube 07/10/

13 Results GETMAT 12Cr/550 o C: ductile (similar secondary cracks observed in F/M steels, ref 1) 12Cr/650 o C 14Cr/650 o C 14Cr/550 o C: intergranular decohesion similar observations in ref 2 after tests in air 07/10/2016 1) JNM Vol 372, Issues 2 3, 2008, pp ) JNM Vol 405, Issue 2, 2010, pp

14 Results GETMAT Specimen / temperature Rp 1.0% Rm, MPa Ap, % at max stress Ap, % at failure Fracture mode 12Cr 550 C 12Cr 650 C Ductile Ductile 14Cr 550 C Macroscopically resembles IGSCC 14Cr 650 C Macroscopically resembles IGSCC 550 C Ductile 550 C in air interrupted at The strength values are below those given by the original materials supplier 07/10/

15 SCC susceptibility - Discussion The results indicate that the ODS steels are possibly less susceptible to SCC than austenitic stainless in SCW at o C. However, ODS steels have been tested very little in SCW and more data is required in order to make any final conclusions. The austenitic stainless steels with 15-18% Cr showed a range of behaviours with respect to their SCC resistances. According to literature there are many possible mechanisms affecting the sensitisation process in the temperature range relevant to supercritical water reactors. The best performers of austenitic stainless steels in this study were and 316L However, another heat of 316L (plate vs. tube spec.) was clearly the worst performer. If austenitic stainless steels with 15-18% Cr are used in this temperature range in SCW, the sensitisation behaviour of the specific heat should be studied before the heat is used. 07/10/

16 SCC susceptibility - Summary of the SSRT tests in SCW ODS steels: + no clear SCC + high strength - low strain hardening capasity at all temperatures Austenitic SSs: + Generally good strain hardening capasity up to o C - relatively low strength at 550 o C - some grades/heats very susceptible to SCC - large heat to heat variation in SCC susceptibility 16

17 General summary on EU projects HPLWR, GETMAT & SCWR-FQT Material screening: P91, P92, HCM12, 316L, 347H, 321, , San28, 800H, 625, 690 and 9-20Cr ODSs Main candidate materials were austenitic stainless steels and ODS steels Short term solution already licensed nuclear grade materials Long term solution e.g. ODS steels or coatings ODS steels still strugle with joining and manufacturing issues, however, progress has been achieved in friction stir welding Cr and/or Ni contents must be high in order to have sufficient corrosion resistant at estimated peak cladding temperature of EU type SCWR, ~ C SCW environment seems to increase creep rate of thin walled components compared to inert environment or air Ni-based alloys are less favorable due to their high neutron absorption and associated swelling and embrittlement Beneficial effect of surface modification on corrosion resistance has been seen but it might decrease SCC resistance When considering EU type SCWR in short term, either lower T or compromise with higher Ni content and higher U-235 enrichment 07/10/

18 Academy of Finland project IDEA 347HFG commercially shot peened inner surface at 550 C/25MPa (8 ppm DO 2 ) after 1000 h of exposure outer inner A shot peened inner surface showed an improved oxidation resistance at 550 C compared to the outer surface. 07/10/

19 347HFG outer & inner surface (shot peened) at 550 C/25 MPa, 1000h (8 ppm DO 2 ) ~25 µm Inner surface outer surface 07/10/

20 347HFG shot peened C/25MPa, 150 ppb DO 2, 2000 h Inner surface shot peened Outer surface < 1µm ~ 75 µm The results indicate that the material surface condition can have a stronger influence than temperature or material composition within the tested ranges 07/10/

21 Academy of Finland project IDEA: Weight change test under SCW at 700 C 2 Weight gain [mg/cm 2 ] 1,8 1,6 1,4 1,2 300 h 600 h 1000 h 1 0,8 0,6 0,4 * 0, H San Alloys 07/10/

22 Exposure test at 700 C/25MPa in SCW Coated (CrN) 316L coupon sample and SEM cross-section after 1000 h of exposure at 700 C/25MPa SEM cross-section of 800H after 2000 h of exposure at 700 C/25MPa 07/10/

23 A simplified scheme of the growth of the inner and outer layers according to the Mixed Conduction Model 07/10/

24 Modeling of oxide structure and composition (1/3) The present model approach does not consider electronic conduction to be a rate-limiting step. In the case of SCW, the large number of defects in the oxide films at high temperatures is not likely to support the high electric field conditions. The transport equation in the low-field approximation (which is analogous to Nernst-Planck equation for the flux): r cj(,) xt XFE J j (,) xt =-Di, j - Di, jcj(,) xt x RT 07/10/

25 Modeling of oxide structure and composition (2/3) It is assumed that ionic point defects play the role of electron donors or acceptors, the electronic conduction being coupled to the transport of ionic defects. The profiles of the molar fractions of main metallic elements (Fe, Cr, Ni, Mn,etc) are normalized to the total metallic content of the film. The concentration cj of a metallic oxide constituent j can be expressed as the dependence of its molar fraction y j = c jvm, MO Predominant transported defects are O and Cr vacancies è the non-stationary transport equations for a given component of the film (in this case, Fe, Cr, Ni, Mo, Cu, Si, Al and Mn) are: 07/10/

26 Modeling of oxide structure and composition (3/3) In order to find the concentration profiles for different charge carriers, the boundary conditions for the transport of point defects have to be defined. The relevant boundary conditions at the alloy/film and film/electrolyte interfaces, as well as the initial conditions are given as: k1 Fe yfe, av æ m, MO 1 RT ö yfe( Li,) t =, ycr ( Li,) t = 2- k3 CrVm, MO + r k ç 4 k1 Cr ycr, a 3FED è Cr ø k y V k y V k y V y L t y L t y L t 1 Ni Ni, a m, MO 1 Mn Mn, a m, MO 1 Ti Ti, a m, MO Ni( i,) =, Mn( i,) =, Ti( i,) =, k4 k4 k4 k y V k y ( L,) t =, y ( L,) t = 1 Cu Cu, a m, MO 1Al Cu i Al i k4 y V k y V, y ( L,) t = Al, a m, MO 1 Nb Nb, a m, MO Nb i k4 k4 07/10/

27 Estimation of oxide film thickness A growth law employed to predict oxide film growth on austenitic materials in nuclear power plant coolants adopted for the inner layer: 1 ( 0) 3 2FE L( ) ( 0) ln 1 bl, 2 i t i t Li t Vm MOk be t, b a ur = = + é + - = ù = b ë û RT For the outer layer, a growth law was derived under the assumption that it is formed via the reaction of interstitial cations transported through the inner layer with water: L () t = o ( k1, Fe yfe, a + k1, Ni yni, a + k1,s i ysi, a + k1, Mn ymn, a + k1,mo ymo, a + k1, Cu ycu, a + k1, Al yal, a ) ( Li ( t ) - Li ( t = 0)) k2 07/10/

28 Results normalized Fe,Al,Cr Mn Si Ni Nb Ti Al Fe Cr normalized Mn,Si,Ni,Nb,Ti normalized Fe,Al,Cr Mn Si Ni Nb Ti Al Fe Cr normalized Mn,Si,Ni,Nb,Ti distance from the M/F interface / nm distance from the M/F interface / nm Experimental (points) and calculated according to the model (solid lines) fractions of metallic constituents at 650 C up to 600 h (Fe, Cr, Al, Ni, Mn, Si, Ti and Nb) in the oxide formed on MA956 (left) and PM2000 (right) 07/10/

29 Sensitivity study norm.fe,cr,al Fe Cr Al Mn Ti Si Nb norm. Mn,Si,Ti,Nb MA956 / 1000 h distance from the M/F interface / nm /10/

30 Rate constants and diffusion coefficients Oxide film thickness as a function of time Rate constant of inner layer growth k 2 / mol cm -2 s k 1Al / mol cm -2 s k 3Cr / mol cm -2 s PM2000 MA time / h time / h PM2000 MA956 PM2000 MA D / cm 2 s D / cm 2 s Diffusion coefficients of point defects in the inner layer D ife D ial D O DD CrV time / h D ife D ial D O D CrV time / h PM 2000 MA 956 thickness / µm Oxide thicknesses calculated from the model (open symbols) and the thicknesses estimated from the experimental GDOES depth profiles (closed symbols) PM 2000 MA time / h time / h 07/10/

31 Summary - MCM A model for the growth of bi-layer oxide was successfully employed in SCW conditios to reproduce compositional profiles for both major and minor constituents of the corrosion layers on candidate materials at 650 C within Academy of Finland project IDEA. In general, the diffusion coefficients of interstitial cations in the inner layer are the largest, whereas the diffusion coefficients of oxygen and Cr vacancies the smallest, which is in accordance with the fact that the outer layer of oxide grows by the transport of cations through the inner layer most probably via an interstitialcy mechanism. Most of the rate constants and also the diffusion coefficients tend to decrease with time of oxidation. Concerning the values of the rate constants, this might be related to the fact that the steady-state approximation that is used for the boundary conditions at the interfaces is not entirely valid for 600 h of oxidation. Another possibility is that not all experimental features are taken into account in the present version of the model, such as the role of Al in the inner layer growth and the effect of solvent on the outer layer since these properties may have a significant impact on the rate constants at the inner layer/scw interface and hence on the structure and composition of the outer layer. Concerning the diffusion coefficients, in order to explain the effect of film aging on its properties, the effect of microstructure should be considered. 07/10/

32 GIF SCWR M&C PMB round robin exercise on corrosion (CAN, EU, JPN) As an example Russian type AISI 321 at 550 C after 1000h of exposure AISI 321 at 550 C/25MPa, 500h, 8 ppm DO2 20x Scale: mm 3,5mm 11mm Differences in test facilities Scale: mm It is evident that weight gain is insufficient to quantify the corrosion rate of materials due to possible oxide exfoliation which will reduce the measured weight or material originally from the loop etc. can precipitate on the samples surface 07/10/

33 Miniature SC-autoclave with belows based loading device The main advantages of pneumatic testing system include high sensitivity and accuracy. The load control with pneumatic loading unit is more accurate than with the conventional servo-hydraulic and step-motor driven devices. This enables using of small size specimens, which is important when considering activated specimens. Within the Academy of Finland project IDEA, first tests were performed successfully for the double bellows miniature autoclave at 288ºC and 500ºC in SCW under the pressure of 25 MPa. Max. 650 C / 35MPa Constant load tests (with R=0.8), 0.1Hz 5DCT specimen (AISI 316NG) Feedback controlled Load accuracy +/-2N The maximum K level 9 Mpa(m)*1/2 Water flow ~1.0l/h DO ppb 07/10/

34 Load, displacement and PD-signals under SCW conditions (500 C/25 MPa) on 316NG 5DC(T) specimen 07/10/

35 VTT Centre for Nuclear Safety (CNS) 2,360 m 2 lab wing: basement two floors of lab space high-bay with hot cells 3,300 m 2 office wing: conference centre three floors of office space 150 people Office wing intended to serve nuclear sector researchers: o computerized fluid dynamics o process modelling (APROS) o fusion plasma computations o severe accidents o core-computations o waste-management o safety assessments o staff working in the laboratory wing. Lab space includes facilities for various aspects:: o radiochemistry o nuclear waste o dosimetry o failure analysis o mechanical and microstructural characterisation of materials Basement primarily intended for storage and handling of radioactive materials and waste. 07/10/

36 TECHNOLOGY FOR BUSINESS