Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

Z. Que et al. / Procedia Structural Integrity 13 (2018) 926–931

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Z. Que et al. / Structural Integrity Procedia 00 (2018) 000–000

crack-tip may act as strong hydrogen traps and thus quasi-cleavage micro-crack initiation sites. Furthermore, the MnS inclusions act as initiation sites for micro-void formation in ductile fracture. 4. Conclusions The effects of HTW and hydrogen on the fracture behaviour of RPV steels with different DSA and EAC susceptibility were evaluated by EPFM tests in air and various HTW environments. Exposure to HTW environments at 250 & 288  C only moderately reduces (< 25 %) the fracture initiation and tearing resistance of investigated low- alloy RPV steels (the toughness K JC remained > 200 MPa  m 1/2 ), accompanied with a moderate, but clear change in fracture morphology. The moderate effect is due to the low hydrogen availability in HTW in connection with high density of fine-dispersed hydrogen traps in RPV steels. The main results can be summarized as follows: In steels with low DSA susceptibility and low sulphur content, environmental effects on fracture resistance seem to be absent in HTW environments. In steels with low sulphur content and high DSA susceptibility, the synergy between DSA and hydrogen results in localization of plastic deformation and causes a moderate reduction in the fracture initiation resistance, which was most pronounced in hydrogenated HTW. In RPV steels with high sulphur content and moderate DSA susceptibility, there was a stronger environmental reduction of fracture resistance with aggressive occluded crevice environment and with preceding fast high-sulphur EAC crack growth in oxygenated HTW. The observed behavior suggests a combination of plastic strain localisation by the HELP mechanism, in synergy with DSA, and HESIV mechanism with additional minor contributions of HEDE (e.g., for micro-cracks formation at inclusions or intergranular cracking). Although the environmental effects appear as rather moderate, they could be critical for old LWR plants which are close to the allowed limits or for materials of low initial upper shelf toughness or with unfavourable combination of high S content, increased strength, high DSA and EAC susceptibility. Acknowledgements Funding for the “SAFE-II” and “LEAD” projects from the Swiss Federal Nuclear Safety Inspectorate (ENSI) is gratefully acknowledged. The authors would like to express their gratitude for the experimental contributions and helpful suggestions from H. Kottmann, B. Baumgartner, R. Schwenold, D. Stammbach, J. Holzer and E. Mueller from Paul Scherrer Institute. References Hänninen, H.,"Conjoint Actions of Hydrogen and Irradiation Embrittlement on the Pressure Vessel Steel of Nuclear Power Plants," VTT-¬MET C-¬209, Espoo, Finland, 1992. Krasikov, E.A., "Reactor pressure vessel steel embrittlement under the combined action of neutron field and hydrogen," in 19th European conference on fracture: Fracture mechanics for durability, reliability and safety (ECF19), 2012. Que, Z., "Effect of High-Temperature Water Environment on the Fracture Behaviour of Low-Alloy RPV Steels," in Proceedings of the 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors, Portland, USA, 2017. Roychowdhury, S., "Effect of high-temperature water and hydrogen on the fracture behavior of a low-alloy reactor pressure vessel steel," Journal of Nuclear Materials, vol. 478, pp. 343-364, 2016. Seifert, H.P., "Corrosion and environmentally-assisted cracking of carbon and low-alloy steels," in Comprehensive Nuclear Materials. Oxford, UK: Elsevier, 2012, pp. 105-142. Seifert, H.P., "Strain-induced corrosion cracking behaviour of low-alloy steels under boiling water reactor conditions," J. Nucl. Mater., vol. 378, pp. 312–326, 2008. Seifert, H.P., "Stress corrosion cracking of low-alloy reactor pressure vessel steels under boiling water reactor conditions," Journal of Nuclear Materials, vol. 372, no. 1, pp. 114-132, 2008. Soneda, N., Irradiation Embrittlement of Reactor Pressure Vessels (RPVs) in Nuclear Power, 26th ed. Cambridge, UK: Woodhead Pub., 2015. Šplíchal, K., "Combination of Radiation and Hydrogen Damage of Reactor Pressure Vessel Materials," International Journal of Pressure Vessel Piping, vol. 55, pp. 361-373, 1993. Schellenberger R., "JR curves of the low alloy steel 20 MnMoNi 5 5 with two different sulphur contents in oxygen-containing high temperature water at 240 °C," Nuclear Engineering and Design, vol. 151, pp. 449-461, 1994. Wu, X., "Hydrogen-involved tensile and cyclic deformation behavior of low-alloy pressure vessel steel," Metall. Mater. Trans A, vol. 35, pp. 1477- 1486, 2004. Yoon, J.H., "Effects of loading rate and temperature on J–R fracture resistance of an SA516-Gr.70 steel for nuclear piping," International Journal of Pressure Vessels and Piping, vol. 76, no. 9, pp. 663-670, August 1999.

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