SUMMARY OF THE SESSIONS ON IASCC – Chaired by Ulla Ehrnstèn and Anna Hojná

Part I of the IASCC session comprised only one talk (just before the tea break on Monday afternoon).

Gary Was (University of Michigan, USA), in a paper with the title “The role of dislocation channeling in IASCC initiation of neutron irradiated stainless steel”, showed that stress to initiate channels and IASCC drop with dpa, and dislocation channel initiation precedes IASCC, specifically at BWR normal water chemistry (NWC). The preference for cracking at discontinuous channels implicates stress localization as a key factor. Results of experiments (4-point bending tests) on neutron irradiated stainless steels have identified dislocation channels and MnS dissolution as key factors in intergranular cracking. Formation of dislocation channels prior to cracking at MnS inclusions supports a mechanism in which discontinuous channels provide the local stress increase required to initiate intergranular cracks in an aggressive local environment.

After the tea break the IASCC session continued with five papers.

The first talk was given by Terumitsu Miura (INSS, Japan). He summarized results from investigations on oxidation behavior of cold-worked 316 stainless steel in simulated PWR water as a function of dose (3, 19 and 73 dpa). The results show the typical two-layered oxide structure on the surface as well as grain boundary oxidation, the depth of which increased with dose. Enrichment of Cr was observed in the grain boundary oxide and of Ni ahead of this. These were similar in the specimens with different dose, indicating that the oxidation mechanism does not change due to neutron irradiation.

The second paper was presented by Yasuhiro Chimi (JAEA, Japan) and dealt with the question on how irradiation-induced deformation localisation affect crack growth in BWR NWC and HWC environments. The investigations were performed on 316 material with a dose of about 14 dpa. Investigations were performed on C(T) specimens after crack growth tests on material near the crack flange and on secondary crack-tips. Twins and dislocation bands were observed near the crack flange. The oxide in the secondary crack-tip area (which is possibly dormant) under NWC conditions was oxide filled and the oxide was Cr-rich, while the area ahead of the tip was rich in Ni. Such was not observed under the HWC conditions.

The third presentation, held by Masato Koshiishi (Nippon Nuclear Fuel Development Co., Japan), dealt with the role of cold-work and He-bubbles in a possible repair situation of irradiated 304 stainless steel components such as core shroud. The investigations were performed on material which was peened to achieve the cold-work on the surface. The results show that recrystallization from the peening suppressed the He-induced cracking from welding. Furthermore, the grain size affects the He bubble diameter, being smaller in the recrystallized area. Also the bubble growth was smaller than in the bulk material.

The fourth presentation was about development of oxide dispersed strengthened stainless steels for BWR control rods and was presented by Yusaku Maruno (Hitachi, Japan). The results presented showed a material with a grain size of ≤5 µm, containing Zr oxides. Zr had been chosen based on knowledge that oversized elements will suppress RIS. The material had been produced by mechanical alloying, vacuum sealing, and consolidation by hipping and extrusion. The strength of the material is about 1.7 times that of conventional stainless steel, but the elongation to fracture is lower, only about 4% (so far). The SCC resistance was also higher and the irradiation-induced hardening lower than conventional stainless steel materials, thus showing that this material could be considered promising. In the discussion it was noted that the irradiation time was small, which may result in that the probable grain growth was yet not seen.

The final, fifth presentation was given by Peter Chou (EPRI, USA). The presentation summarized literature data on APT investigations on neutron-irradiated stainless steel, aiming at possible answering of some open questions on how material characteristics evolve by dose. He concluded that clusters may be important, although not taken into account so much earlier, and that more data is needed to fulfil the overall picture. In the discussion, the known challenges with APT technique and data analysis was brought up as well as that the dataset consists of materials with large and important differences which may affect the conclusions.

Please note that three additional IASCC papers were presented during one of the summary/poster presentation sessions:

Katsuhiko Fujii (INSS, Japan): “APT measurement of stainless steel welds after long time aging and irradiation – Mo effect on microstructure –”

Koji Fukuya (INSS, Japan): “APT analysis of solute clustering in neutron-irradiated stainless steels”

Mi Wang (University of Michigan, USA): “IASCC behavior of nickel base alloys in light water reactor environments”