Feasibility Study of Supercritical Light Water Cooled Fast Reactors for Actinide Burning and Electric Power Production NERI program funded by the U.S. Department of Energy Idaho National Energy Engineering Laboratory Massachusetts Institute of Technology University of Michigan Westinghouse Corporation

Task 2: Fuel Cladding and Structural Material Corrosion and Stress Corrosion Cracking Studies S. TeysseyreJ. McKinley, Y. Yi, B. Krieger, and G. S.Was, UM B. Mitton and R. Latanision, MIT Identification of most promising candidate alloys for fuel cladding and core internal structures. Design and construction of an out-of-pile SCW loop for SCC experiments. Assess corrosion (MIT) and SCC (UM) of candidate alloys. Assess radiation stability and SCC susceptibility of candidate alloys. Modeling of corrosion and SCC in supercritical water.

U-M SCW System Capability Oxygen content and conductivity are monitored at room temperature and atmospheric pressure in both the inlet and outlet line Temperature and Pressure: maximum 550°C at 34.5 MPa Conductivity: Lower than 0.1  S/cm Dissolved oxygen: Below 10 ppb - above 20 ppm Flow rate: ml/min

Elemental Analysis of Alloys Used in SCC Experiments AlloysCMnFeSSiNiCrMoCuNCoNbP 316L Bal ND L Bal ND0.068ND 0.02Materials Austenitic Stainless Steels: 316L and 304L 316L: 1100°C:20min + WQ; grain size 44  m 304L: as-received condition: grain size 40  m

Constant Extension Rate Tensile (CERT) Experiments Environment:SCW (500°C or 550°C and 25 MPa) or Ar Conductivity: <0.1  S/cm Oxygen:non-deaerated or deaerated to <10 ppb Strain rate:~3 x s -1 Flow rate: ml/min

Alloy304L316L EnvironmentSCW ArgonSCW Temperature (°C) Pressure (MPa) O 2 (ppb) < 10--< 10 Inlet conductivity (µS/cm)<0.5< 0.1--< 0.1 Flow rate (ml/min)10 Strain rate (s -1 )Variable * 3 x Test duration (hr) Failure mode Ductile rupture initiated by intergranular fracture Intergranular cracks initiated Ductile Strain to failure (%)3225 ** 3633 Stress at failure (MPa) ** Yield strength (MPa) Results summary * The strain rate was 1 x s -1 for the first 4.25% strain and 5 x s -1 for the balance of the experiment. ** Test was stopped prior to failure

500ºC, Argon Strain rate: 3 x s ºC, 25.5 MPa Deaerated water Conductivity: <0.1  S/cm Strain rate: 3 x s L 550ºC, 25.5 MPa Non-deaerated water Conductivity: <0.4  S/cm: Strain rate: 5 x s ºC, 25.5 MPa, deaerated water Conductivity: <0.1  S/cm Strain rate: 3 x s L Stress-strain behavior

304L in non-deaerated SCW Ductile rupture initiated by intergranular fracture Minimal necking Intergranular surface cracks Alloy 304L is susceptible to intergranular stress corrosion cracking in 550°C non-deaerated supercritical water Intergranular fracture Ductile Rupture

304L sample strained to failure in non-deaerated SCW Cracks density  20 cracks/mm 2 Influence of oxygen content on cracks density in 304L 304L sample strained to 25% in deaerated SCW Cracks density  7 cracks/mm 2 The lower oxygen content used for the deaerated sample resulted in a less oxidizing environment and may be the cause of the lower crack density. Intergranular cracks in both Non-deaerated and deaerated SCW conditions

Behavior of 304L The heat of 304L used in these experiments was also tested in: BWR normal water chemistry (~30 samples) 288°C, pH RT ~6.0, cond. <0.2  S/cm, 2 ppm O 2, 160 mV SHE PWR water chemistry (~8 samples) 320°C, pH RT ~6.5, cond. 20  S/cm, <5 ppb O 2, 32.5 cc/kg H 2, 1000 ppm B, 2 ppm Li, -770 mV SHE Under neither of these conditions was IG cracking ever observed

Influence of alloy type in deaerated SCW 304L 25% strained Intergranular cracks appear on the gages 316L Strained to failure Necking similar to that in argon Cracks in the oxide layer Ductile rupture Alloy 316L does not display evidence of intergranular cracking

316L in deaerated SCW Significant necking in the fracture surface Completely ductile fracture

Surface of a 316 sample after 5 days in deaerated SCW at 500°C Oxide formation in deaerated SCW Surface of a 316 sample after 12 days in deaerated SCW and CERT test Both qualitative measurements indicate iron oxide

Oxide formation in non-deaerated and deaerated SCW OCCrNiFe wt%at%wt%at%wt%at%wt%at%wt%at% 304-non deaerated SCW deaerated SCW deaerated SCW Alloy composition (wt%): CCrNiFe 304L-alloy L-alloy Oxide composition (wt% and at%):

Future Work Analysis of oxide on four CERT samples - thickness (SEM of cross sections) - composition (EDS, XPS) - phase identification (XPS, XRD) CERT tests on nickel-base alloys (Inconel 690 and 625) Irradiation of stainless steels and nickel-base alloys to ~5 dpa Analysis of radiation damage microstructure SCC tests on irradiated 304L SS, 316L SS, 625 and 690 in 500°C, deaerated SCW Ferritic-martensitic alloys - T-91 and HT9