SUMMARY OF THE SPECIAL SESSION ON ALTERNATIVE AND ADVANCED MATERIALS FOR NPPs (2014) – Chaired by Gary Was and Peter Andresen

The 2014 special session was on “Alternative and advanced materials for existing and future nuclear power plants” and was held on Wednesday morning. Part I consisted of four talks.

Koji Arioka (INSS, Japan) presented the first talk entitled “One of the Key Processes to Assess Long Term SCC Initiation”. This talk focused on long-term reliability, where “long-term” refers to beyond 60 years. He stressed that innovative concepts and quantitative methodologies are indispensable to assess the current material degradation and development of future alternative SCC resistant materials considering such extended life. The talk focused on the effect of change in grain boundary bonding strength of materials used for components and piping in LWR as one of processes occurred during such long terms operation. Emphasis was on the formation of cavities along grain boundaries ahead of a notch after long times at low temperature and driven by a stress gradient. The kinetics of this phenomenon depends on the degree of cold-work, material structure, carbide precipitation, and temperature. Diffusivity was measured using 20% cold-worked steel in the range of temperature between 320 and 450 °C. Cold-work induced diffusivity 10,000 times faster at 320 °C compared with the extrapolated value of the literature data of annealed iron at higher temperature. He postulated that super abundant vacancies were produced by super abundant absorbed hydrogen in the material. A similar slope for 1/T type dependence (~150 KJ/mol) was observed between crack initiation and diffusivity of cold-worked carbon steel, suggesting that the rate limiting process of crack initiation of cold-worked carbon steel is assumed to be diffusion of cold-work-induced vacancy diffusion in steel.

The second talk was on “Advanced Materials for IASCC Resistance” and was presented by Larry Nelson (Honorary Member, USA). This talk began with a review of major degradation modes in reactor cores and the main concerns for long-term operation. A summary of the key attributes of stainless steel alloys that exhibited better resistance to environmentally-induced crack initiation was presented, and these included high Ni or Ni+Cr content, low Si content, high CSL fraction, presence of grain boundary carbides, high stacking fault energies, small grain size and cold-work. Characteristics of swelling-resistant alloys were also reviewed. A list of candidate alloys spanning several alloy systems was presented, that resulted from an EPRI led study to identify alloys with potentially better resistance to radiation than those in use today. These alloys came from the following alloy classes: austenitic, Ni-base, ferritic-martensitic, oxide dispersion strengthened (ODS) ferritic and refractory.

The third talk in Part I was given by Martin Morra (GE-GRC, USA) and dealt with”Microstructural and SCC Characteristics of Alternative High Strength Nickel Alloys”. The talk focused on high strength Alloys 725 and Hicoroy 11 that have been evaluated as replacements for Alloys X-750 and 718. Both alloys show very low SCC growth rates when tested under a range of BWR water chemistries, compared to Alloys X-750 and 718. Microstructurally and compositionally similar, Alloys 725 and Hicoroy 11 share features such as gamma double precipitation strengthening and nearly precipitate free grain boundaries. Results showed that both alloys have greater resistance to intergranular EAC than either alloys X-750 or 718. SCC growth rate tests performed on Alloy 725 heat treated to two different microstructures and strength levels showed similar low growth rates. There were no negative effects of yield strength on SCC growth rates in Alloy 725. Lastly, chemistry-modified Hicoroy 11 had similar SCC growth rates with higher yield strength.

Peter Andresen (GE-GRC, USA) presented the fourth talk in Part I on “SCC of Irradiated and Unirradiated High Cr Ferritic Steel”. Results of SCC tests on Alloys T91, HT-9, nanoferritic, and APMT were presented. For all alloys studied, under aggressive water chemistry (2 ppm O2, 30 ppb SO4), K and cycling conditions, as the cycling frequency decreased, crack arrest occurred. Crack arrest was even observed for Alloy HT-9 irradiated to 38.7 dpa in HFIR and tested under the same conditions as the unirradiated samples. Peter concluded that the greatly superior SCC and corrosion fatigue crack growth rate response of ferritics make them a very promising improvement over austenitic stainless steel.

After the coffee break and Group photograph the special session continued with four more talks.

Part II of this session began with a presentation by David Tice (AMEC FW, UK) on “SCC Crack Growth Rate Testing of Alloy 625”. David discussed the crack growth rate response of Alloy 625 in various heat treatments. Alloy 625, and especially Alloy 625Plus, can be precipitation hardened and, of course, cold-worked to a high yield strength of >1000 MPa from precipitation hardening and ~1800 MPa with a combination of precipitation and cold-work. They tested as-received material (920 °C for 4.5 h + water quench, which was a hot-rolled annealed condition). Materials with 0, 5, 10, 15 and 20% cold-work by rolling were tested in the S-L orientation. Testing was performed in 325 and 350 °C water containing 2 ppm Li as LiOH and 15 to 20 cc/kg H2. They noted significant banding in the microstructure, and observed growth rates in the range of 2.5 – 4.5×10-8 mm/s for 20% cold-rolled materials, dropping to ~1 x 10-9 mm/s for non-cold-rolled materials. All specimens exhibited an IG cracking morphology. A good correlation between %-cold-work or hardness and crack growth rate was observed.

The second talk was given by Mychailo Toloczko (PNNL, USA) on “Irradiation Resistance and Mechanical Properties of Ferritic-Martensitic Steels”. He reviewed the microstructures of 9-12Cr F-M steels and ODS ferritics, and then summarized the effects of irradiation on microstructure, on tensile property changes, on fracture toughness changes, on swelling and creep, and on grain boundary chemistry. The IASCC response was addressed during the talks by Peter Andresen and Gary Was.

Eliska Křečanova (CVR, Czech Republic) presented the third talk entitled “Corrosion Testing of Candidate Materials for Supercritical Water-Cooled Fossil Fueled and Gen IV Nuclear Power Plants”. Eliska summarized some of the practical applications of supercritical water, along with its properties. They have tested eight austenitic and ferritic-martensitic steels at 550 and 600 °C at 25 MPa pressure for 1000 h, as well as in steam at 570 °C at 17 MPa. Flat, 3-point bend specimens were tested with and without welds. Large differences in surface oxide were observed for specimens heated through the subcritical water regime vs. heated in Ar, both followed by supercritical water exposure. The oxide layer that forms without the subcritical to supercritical water transition contains more Cr and less crystal defects. By comparison, the oxide layers on samples that weren’t in contact with water at subcritical temperature (samples heated up in Ar) are similar to the oxides which form under hot gas conditions (e.g., flue gas).

The last talk was given by Gary Was (University of Michigan, USA) entitled “Corrosion and SCC of Candidate Ferritic-Martensitic Alloys in High Temperature Sub-Critical and Supercritical Water”. Gary presented slow strain rate tensile and exposure (mass change) tests of various ferritic-martensitic steels exposed to high-temperature supercritical water and steam. The effect of proton irradiation was also studied. APMT showed good corrosion resistance in high-temperature steam and at 360 °C. Of the F-M steels tested, only HT-9 exhibited IG cracking in supercritical water. Crack depths were less than 40 um at 500 °C. Irradiation enhanced both crack depth and density. Thermo-mechanical treatment enhancement improved ductility of both irradiated and unirradiated T91 in supercritical water.

The session ended with a panel discussion in which the eight presenters were asked to respond to questions and comments from the audience. There was excellent participation in which all presenters were engaged.