## 5 Results and Discussion
### GBC-68 Cask Criticality Analysis
#### 5.2.3 The Impact of the FA Type on the Cask Criticality  
This section will explore the effect of the different FAs on burnup credit. For this part of the study, the additional BWR SVEA-100 fuel assembly design was considered. The four-cycle core map used for the full-core simulations is unsuitable for FAs of this type since the fuel is low enriched and normally its average burnup does not go above 40 GWd/tU. However, for the comparison, the current core layout with the already established reshuffling schemes but reduce the cycle length was used. To make sure that both reactor cores, one with GE14 fuel assembly type and another with SVEA-100, are burnt to the same extent, the cycle length was chosen, so the core criticality after the last cycle is similar for both cases. The cycle length for SVEA-100 FA was estimated as 8.5 GWd/tU, which is 33% lower than for the reference GE14 assembly. Figure 13 shows a comparison between the burnup distributions for the reactor core with the different FA types for SP_A and SP_C shuffling procedures. The target burnup in the case with SVEA-100 assembly is 32.5 GWd/tU and 50 GWd/tU for GE14. The burnup range decreases by 30% in the case with SVEA-100 for optimal and least optimal shuffling procedures, while the height of the distribution increases. This narrower burnup distribution suggests that the loading scheme is more optimal for the SVEA fuel assemblies with the lower target burnup. However, there are two things to be kept in mind. First, the very short cycle time for the SVEA-100 would not be attractive; thus, in reality, the FA would not be operated in four cycles, but more probably in three cycles. Second, the short cycle time created by the four-cycle scheme and the relatively low burnup increase per cycle has also helped to achieve this very favourable result. Thus, the four-cycle scheme  
Figure 12: Burnup profile (BP) for FAs from L\({}_{5\%\text{L}}\) load and L\({}_{68}\) loads and k\({}_{\text{inf}}\) as a function of the average burnup of layer 4 of the GE14 FA model (van). designed for the GE14 fuel assembly seems to favour the SVEA-100 FA, which has a much shorter lifetime than a higher enriched GE14 configuration. Figure 14 compares burnup credit from applying realistic FA data (Equation (1)) between the two different assembly types. It is seen that ARD in comparison with fresh fuel approach (Figure 14a) for both shuffling procedures SP_A and SP_C is slightly higher for GE14 assembly than for SVEA in case of L\({}_{68}\) load (4300 pcm on average) and slightly lower (2200 pcm on average), for L\({}_{5\%\text{L}}\). The L\({}_{68}\) load could be better in the case with GE14 FA since its design is much more sophisticated than the SVEA-100 fuel assembly design and is optimised to achieve higher burnup and improve the economic performance of the fuel: part length rods, higher initial enrichment and heavy Gd load. The SVEA100 assembly is better in the case of L\({}_{5\%\text{L}}\) load due to narrower structure of the burnup distribution, which makes the burnup of the 5% least burnt FAs slightly closer to the average discharge burnup. Figure 14: BUC in comparison with fresh fuel (**a**) and peak reactivity (**b**) approach. Figure 13: Burnup distribution for two FA types SVEA100 and GE14 and SP_A and SP_C shuffling procedures. Compared to the peak reactivity approach, the ARD is lower for GE14 assembly than for SVEA-100 (Figure 14b). The difference in ARDs between GE14 and SVEA-100 FAs is 9600 pcm on average in the L\({}_{68}\) load, and 16,200 pcm for L\({}_{5\%\text{LL}}\). This is due to the higher peak reactivity value of the SVEA100 criticality curve (Figure 6). Thus, the benefit from applying realistic assembly conditions instead of the peak reactivity approach is lower for GE14 assembly, which represents here a modern BWR fuel assembly design, than for the previous generation SVEA100 assembly design. On the other hand, the ARD for fresh fuel approach is approximately at the same level for both FA types with a slight advantage of GE14 assembly.