## 2 Disposal Scenario and Monte Carlo Model
### Model Development and Assumptions  
For the criticality calculations of this study, MCNP6 has been employed as one of the most validated Monte-Carlo (MC) codes for this purpose [12]. The fully 3-dimensional MCNP6 model consists in the disposal canister loaded with 4 FAs and surrounded by a 35 cm layer of water saturated bentonite clay, albeit the bentonite's effect on the criticality of the scenario is negligible. The remaining volume of the bentonite backfilling and the host rock formation are not anymore part of the MCNP model, i.e., modeled as void.  
The Swiss regulations for a deep geological repository of high-level waste (HLW) require a proof of safety of the repository over a time span (\"Nachweisezitraum\") of 1,000,000 years, explicitly including the criticality safety of the installation [13]. On the other hand, safe containment of the HLW by the disposal canisters has to be shown only for at least 1000 years. Although it is intended to exceed this minimum lifetime of the canisters by a suitable margin setting the lifetime design target to 10,000 years [10], and although it is not excluded that the canister sealing remains intact for many years longer, the containment of the HLW cannot be guaranteed by the canisters over the entire period of 1,000,000 years. For this reason, a breach of the canister is postulated and a full immersion of the FAs with water from the surrounding host rock is assumed by the MC model. Furthermore, the water properties such as composition, density and temperature were modelled as optimal for neutron moderation:  
* The water is modelled as pure H\({}_{2}\)O, ignoring the presence of diluted minerals. This leads to a better simulated neutron moderation and lower neutron absorption eventually resulting in higher keff values;
* In previous preliminary assessments for canisters loaded with UO2 FAs, higher water densities have been found to correspond with higher keff values [2]. In view of the prevailing pressure and temperature regimes in the envisioned deep geological repository, a conservative water density of 1.005 g/cm\({}^{3}\) has, therefore, been set in the MCNP model;
* The material temperatures of all model components are an item of required information in the neutron transport calculations, i.e., for the selection of ND libraries employed by the MCNP model. In the present model, these temperatures were assumed to be all equal to 293 K. This is a rather low temperature leading to a better neutron moderation in the MC-simulations.  
A comparison of these conservative assumptions for the properties of water with more realistic conditions regarding their effects on the system criticality is currently in work atPSI. Criticality calculations are carried out for Opalinus Clay and bentonite porewaters instead of pure H\({}_{2}\)O, investigating their influence on keff simulation output.  
Despite the postulated canister breach, the geometry of the canister is considered as intact in all simulations of this work. Also degradation issues and damages of the disposal canister and the FAs, which are inevitable over longer post-closure time scales, and corresponding changes in the geometry or the materials are not taken into account by the MC model.