Integration of the findings on the impact of irradiation, dry density and particle size on the microbial community: EURAD-CONCORD Deliverable 15.9

The work performed in WP ConCorD’s Subtask 4.1 focused on the effect of irradiation and other repository-relevant stressors on indigenous microorganisms in bentonite. The experiment of UGR + CIEMAT aimed to explore the impact of gamma radiation on the survival of microbial communities within compacted bentonites. To achieve this objective, FEBEX bentonite was compacted at a dry density of 1.6 g/cm3 and fully saturated with artificial bentonite pore water. One group of samples underwent gamma irradiation at a dose rate 66 Gy/h, accumulating either a total dose of 14 kGy or 28 kGy. Additionally, certain samples were enriched with a sulphate-reducing bacteria (SRB) consortium to stimulate bacterial activity. Following 6-months and 1-year anaerobic incubation periods, DNA extraction, amplification and sequencing of the 16S rRNA gene were conducted along with quantitative PCR (qPCR) to analyse the microbial diversity across various treatments. The Most Probable Number of SRB was determined using anoxic Postgate’s Medium. Furthermore, heterotrophic aerobic colony-forming units were computed to evaluate bacterial viability. The effect of gamma radiation on bacterial communities in bentonite depended on the cumulative radiation dose (14 or 28 kGy). The diversity data revealed that an incubation period before and after the radiation dose allowed the microbial communities to adapt and recover since the results were quite similar to those control treatments that had not been irradiated. The corrosion analyses of the copper coupons over the 6-month study period aligned with the microbial diversity results. The coupons from control treatments, where greater diversity and bacterial viability were observed, exhibited increased corrosion in the form of copper oxides and possible copper sulphides. Moreover, the addition of the SRB consortium may have played a role in the corrosion of the coupons, as a sulphide signal, potentially associated with copper sulphide precipitates, was detected only in the treatments enriched in SRB.