Neutron irradiation induced microstructural changes in NBG-18 and IG-110 nuclear graphites

Abstract

A B S T R A C TThis paper reports the neutron-irradiation-induced effects on the microstructure of NBG-18 and IG-110 nuclear graphites. The high-temperature neutron irradiation at two different irradiation conditions was carried out at the Advanced Test Reactor National User Facility at the Idaho National Laboratory. NBG-18 samples were irradiated to 1.54 dpa and 6.78 dpa at 430°C and 678°C respectively. IG-110 samples were irradiated to 1.91 dpa and 6.70 dpa at 451°C and 674°C respectively. Bright-field transmission electron microscopy imaging was used to study the changes in different microstructural components such as filler particles, microcracks, binder and quinoline-insoluble (QI) particles. Significant changes have been observed in samples irradiated to about 6.7 dpa. The closing of pre-existing microcracks was observed in both the filler and the binder phases. The binder phase exhibited substantial densification with near complete elimination of the microcracks. The QI particles embedded in the binder phase exhibited a complete microstructural transformation from rosettes to highly crystalline solid spheres. The lattice images indicate the formation of edge dislocations as well as extended line defects bridging the adjacent basal planes.The positive climb of these dislocations has been identified as the main contributor to the irradiation-induced swelling of the graphite lattice.

On the nature of cracks and voids in nuclear graphite

“…The small levels of Ga visible in Figure 4 (a) are most likely due to an unavoidable Ga implantation outer layer associated with all FIB prepared samples [23]. Specifically we note that, firstly, all samples studied (including unirradiated material) contained both filled and empty cracks in agreement with the findings of Wen et al [6] for unirradiated material and Karthik et al [9] for neutron irradiated material -in both cases samples having been prepared by conventional ion beam thinning (and also microtomy in the case of Wen et al [6]). …”

“…Neutron irradiated nuclear graphite was also investigated by Karthik et al [9], who used TEM to observe changes in both the microcracks and the nanostructure. In both filler and binder phases of NBG-18 and IG-110 graphites irradiated to 1.42 dpa and 1.91 dpa respectively, there were no significant changes in the size distribution of microcracks as compared to non-irradiated specimens.…”

An understanding of lattice strain, defects and disorder in nuclear graphite

“…The results of TEM are important in correlating with the asymmetrical microstructural changes observed in graphite after neutron irradiation [15,16].…”

“…However, despite this, very few in-depth microstructural studies have been conducted on irradiated graphite [10,11,12,13,14,15,16,17]. The majority of those that have been performed have employed highresolution transmission electron microscopy (HRTEM) to examine irradiation-induced defect formations [15,16] and micro-crack evolution [18] in various nuclear graphite grades. This work looks to expand upon the current knowledge by examining the influence of irradiation dose and temperature on the bulk crystal structure of graphite.…”

Micro to nanostructural observations in neutron irradiated nuclear graphites PCEA and PCIB

“…This is particularly useful because the availability of data for neutron irradiated specimens is limited. Recent work by Karthik et al presented the microstructural changes occurring in graphite grades NBG-18 and IG-110 as a result of neutron irradiation [8]. No significant changes were reported for low dose specimens (1.54 dpa at 430 C and 1.91 dpa at 451 C) of both grades.…”