## 2 Finite Element Modeling of the Deep Geological Repository  
The finite element analysis software COMSOL Multiphysics (COMSOL) was used to determine the maximum temperature that would occur for various DGR designs specified by the three design variables. The model built was based on the description of NWMO's conceptual DGR in crystalline rock in [4], and it is a modification of the model developed in [7]. To consider UFC age, a unit cell that represents one repeating section inside a placement room could not be modeled, as was done in prior studies. Instead, half the length of a placement room (140.75 m) was modeled as a unit cell (Figure 2a); thus, UFC age was assumed to be symmetric from the room center. Further dimensional details of the conceptual DGR in crystalline rock by NWMO are illustrated in [12]. The model components were UFCs, buffer boxes, spacer blocks, gap-fill, and the host rock. Buffer boxes encased UFCs, spacer blocks distanced the boxes, and gap-fill lined the placement room walls (Figure 2b). Like buffer boxes, the spacer blocks and gap-fill material were bentonite-based components. Since the number of UFCs that could be placed in a placement room depends on the UFC spacing, the model placed an enlarged spacer block and highly compacted bentonite block at the end of the room to fill any voids due to unused space (Figure 2c). In this way, the model only considered conductive heat transfer. The depth of the modeled host rock was 5000 m based on the observation in [7] in which  
Figure 2: Model components, geometry, and boundary conditions. (**a**) Overall unit cell, (**b**) Middle of placement room, (**c**) Start/end of placement room. the temperature rise at this depth was negligible. The main dimensions of the model are shown in Figure 2, with the room spacing and UFC spacing design variables identified in Figure 2a,b, respectively. An infinite repository was modeled through the application of adiabatic boundary conditions on the unit cell's boundaries perpendicular to the horizontal dimensions, which act as symmetry axes. The top and bottom boundaries of the model were based on assumptions for a constant ground surface temperature of 5 \({}^{\circ}\)C and a geothermal gradient of 12 \({}^{\circ}\)C/km, respectively [13]. These adiabatic and isothermal boundaries are marked in Figure 2a. The initial temperature of the model was specified using the geothermal gradient. The thermal properties of the model components were those specified in [7; 12], which are summarized in Table 1. All modeled UFCs were assumed to be discharged from a CANDU reactor for a minimum of 30 years by the time of repository placement, as per NWMO's specifications [12]. Additionally, every UFC was assumed to contain used fuel that had been irradiated at a burnup of 220 MWh/kg. The heat output of one container under this assumption was estimated in [14] and is shown in Table 2.  
Using the described infinite repository model, a simulation for one million years using COMSOL would result in a maximum temperature profile that exhibits two peaks. The profile would represent the maximum temperature within the DGR at each specific time. The first peak in the maximum temperature would occur within 100 years after UFC placement into the DGR, while the second peak would occur at approximately 1550 years [7]. The occurrence of a second peak would be due to the use of adiabatic boundary conditions, and for a finite repository, it would be an overestimation of the DGR's temperature. A method was developed in [7] to correct a temperature profile resulting from the use of an infinite repository to model a finite repository. The results of the method indicated that the first peak in the temperature profile for an infinite repository is representative of the peak in the temperature profile for the finite repository it represents.