## 5 Results and Discussion  
### Depletion Analysis of BWR GE14 and SVEA100 Fuel Assemblies  
To estimate the difference in BWR GE14 and SVEA100 fuel assemblies' behaviour during the irradiation, the depletion analysis has been performed for burnups up to  
Figure 4: Flowchart of the modelling sequence from the full core nodal analysis to the cask criticality evaluation.  
Figure 5: GBC-68 cask model in KENO V.a and the transformation of the layer structure form the core simulation to the cask model (NE—natural uranium blanket, LAYER—the FA layer).  
60 GWd/tU [27] for both assembly types. The study considered the following FA models used in first part of the study [22]:  
* 2D lattice model of the fuel assembly without burnable poison (M1).
* 2D lattice model of the fuel assembly with Gd burnable poison (M2). M1 and M2 configurations estimate criticality of the FA in reflective boundary conditions with constant fuel temperature, coolant density and coolant temperature.
* 3D nodal model of realistic fuel assembly with thermal-hydraulics coupling (M3). The FA configuration includes natural uranium blankets and has reflector from top and bottom. The FA is modelled with reflective boundary conditions in a radial direction, and vacuum at the assembly's ends. The coolant density along with fuel and coolant temperatures are non-uniformly distributed and obtained by the nodal code.  
The so-called leading bottom layer of the FA mainly defines the final criticality [22], so the lattice models had coolant density as average for that layer of 0.6 g/cm\({}^{3}\). Fuel and coolant temperatures were set as average for BWRs, 900 K and 560 K, respectively [27; 31]. The criticality curves for both FA types and all considered models are presented in Figure 6.  
The SVEA100 fuel assembly has lower average initial enrichment than GE14 fuel assembly. Consequently, the SVEA100 criticality curve obtained with M1 model (UO\({}_{2}\) fuel only) is lower than of GE14. The difference between infinite multiplication factors (k\({}_{\text{inf}}\)) at the beginning of life (BoL) is 6%. At the same time, the criticality at the reactivity peak obtained with the M2 model, which contains gadolinium, is 3% higher and occurs \(\sim\)10 GWd/tU earlier in the burnup for the SVEA100 fuel assembly due to lower initial gadolinium content compared to the GE14 fuel assembly. For the realistic nodal model M3, the difference in k\({}_{\text{eff}}\) between first reactivity peaks decreased to 2% with SVEA100 peak being higher and occurring 5 GWd/tU earlier in comparison with the GE14 FA. It is seen that for both FA types the peak reactivity appears earlier for the 3D realistic model M3 than for simplified 2D lattice model M2. This is due to the non-uniform coolant density profile considered in the M3 model. In this case, the power is mostly concentrated at the bottom part of the FA [22] resulting in intensive depletion of the burnable poison there and consequently, in the earlier peak reactivity appearance compared to the lattice approach. A similar outcome was also demonstrated in the work summarised in Ref. [15]. Overall, the SVEA100 FA criticality reduces faster than for GE14 due to the lower initial fissile content, so its average discharged burnup should also be lower. The typical discharged burnups for  
Figure 6: k\({}_{\text{eff}}\) vs. burnup for three different models BWR GE14 and SVEA100 fuel assemblies.  
FAs enriched up to 3% is less than 40 GWd/tU according to [27], and around 50 GWd/tU for higher enriched FAs (up to 5%) [32].