# Lifetimes of Used Nuclear Fuel Containers Affected by
## 6 Discussion and Conclusions  
This analysis, the first of its kind on the Canadian deep geological repository (DGR) design, has concentrated on the production of sulphide due to sulphate-reducing bacteria (SRB) activity and the subsequent effect on the time to failure or lifetime of the used fuel containers (UFCs). A first-order kinetics was approximated to describe the reduction of sulphate into sulphide carried out by the SRB. The reaction rate constant is then exponentially adjusted to reflect the high-pressure conditions inside the DGR. The two scenarios created yielded similar sulphide fluxes to the UFC and their lifetimes are above one million years. The concentration of sulphide at the LHS boundary eventually reaches a steady-state value of between 1 and 4 ppm in both scenarios. This result is in agreement and supports previous studies that have assumed a constant sulphide concentration of 3 ppm [6]. Figure 5: Histogram of UFC lifetimes under scenario B.  
Lifetimes of UFC are inversely proportional to the groundwater sulphate concentration in both scenarios. This relation is expected for scenario A which may, roughly speaking, be characterized as a diffusion system with a source term at the LHS boundary because its steady-state determines the rate of corrosion. However, this relation is somewhat unexpected for scenario B which has a source term throughout the bentonite clay. Despite that the scenarios entail different assumptions on the behavior of SRB, their results are of the same order of magnitude, which bounds the uncertainty of the effect of having active SRB inside the DGR under the conditions tested. The assumption for scenario A is that SRB would be active only at the rock interface and the region closest to it. While scenario B assumes a more conservative situation, namely, that bacterial activity would be present across the bentonite layers. In both cases, the suppressing effect of high-density bentonite significantly reduces the mass transport of sulphide and corrosion to the UFCs. The sensitivity analysis shows that more important than the true groundwater chemical concentration of corrosive species, the dry density of the gapfill bentonite or the HCB has a much more significant impact on the mean lifetime of the nuclear waste containers (Figures 6 and 7). Equally significant is an adequate description of the SRB activity at the walls of the replacement rooms and inside the bentonite clay. The sensitivity analysis on the parameter \(C_{f}\), that controls the sulphide production inside the bentonite clays, shows that lifetimes  
Figure 6: Lifetimes for different gapfill dry densities or \(C_{f}\) under scenario A, vertical line shows the baseline value. Figure 7: Lifetimes for different gapfill dry densities or \(C_{f}\) under scenario B, vertical line shows the baseline value. are quite sensitive to changes in this value. However, in order to have a more accurate description of the sulphide production, additional experiments on microbial growth and sulphide production would be needed, but this is hindered by the fact that carrying out these physical experiments takes months or longer periods of time. Equally important is the characterization of the rock type, groundwater, and microbial communities of the prospective location of the DGR so that the modeling parameters reflect the particularities of the chosen site. Future work will focus on including temperature-dependent coefficients to account for the effect of the thermal energy output coming from the UFCs. Conceptualization, K.P. and J.A.G.-H.; methodology, J.A.G.-H., K.P. and M.S. ; software, J.A.G.-H.; validation, J.A.G.-H. and M.S. ; investigation, M.S. and J.A.G.-H.; writing, J.A.G.-H., K.P. and M.S. ; supervision and funding acquisition, K.P. All authors have read and agreed to the published version of the manuscript. This research was funded by NWMO and NSERC CRD grants, and JAG was funded by CONACYT. Not applicable. Data available upon request from authors. [PERSON] of University of Waterloo, [PERSON], [PERSON] and [PERSON] of NWMO.