## 5 Results and Discussion
### GBC-68 Cask Criticality Analysis
#### 5.2.2 The Impact of Targeted Discharged Burnup on Cask Criticality  
In the first part of this study, the burnup distributions for discharged fuel assemblies of several different reshuffling strategies were provided for an identical target burnup of approximately 50 GWd/tU for all cases or as we call it here, the reference target burnup. To evaluate the effect of the target burnup on the final burnup distribution and the cask criticality, the full core analysis was performed for the most (SP_A) and least optimal (SP_C) shuffling procedures and BWR GE14 fuel assembly type but with the reduced cycle length 10.3 GWd/tU instead of 11.75 GWd/tU for the reference case leading to a lower target burnup. In this case, the target burnup was decreased to approximately 44.5 GWd/tU burnup, which corresponds to the maximum assembly average burnup limit of 53 GWd/tU used in Germany for BWRs [35]. This new target burnup is called the modified target burnup. Figure 9 represents four burnup distributions obtained for SP_A and SP_C shuffling procedures with reference and modified target burnups. First, it indicates that the shape of burnup distribution does not change significantly with the target burnup decrease. However, the burnup range, the difference between the minimum and maximum burnups of the distribution, becomes slightly lower. At the same time, the number of FAs in the highest bin increases by 30% for optimal SP_A distribution, and by 18% for the least optimal SP_C in the modified target burnup case. The average, maximum and minimum burnups along with FA number in the highest bin of the SP_  
Figure 10 represents burnup credit from applying realistic FA data (ARD) in comparison with fresh fuel (a) and peak reactivity (b) approaches for L68 and L5%L task loads, produced using optimal and least optimal shuffling procedures and reference and modified target burnups at one-year cooling time. The ARD is defined as:  
\[\text{ARD}=\left(\frac{k_{j}-k_{i}}{k_{j}k_{i}}\right)\times 10^{5} \tag{1}\]  
where \(k_{j}\) is k\({}_{\text{eff}}\) of the GBC-68 cask filled with fresh or at their peak reactivity BWR fuel assemblies, \(k_{i}\) is of the GBC-68 cask filled with BWR fuel assemblies from L68 or L5%L load of specific burnup distribution. It is seen that in the case with modified target burnup and least optimal shuffling procedure (mod SP_C), the burnup credit from ARD for both fresh fuel and peak reactivity approaches is higher for the L5%L task load (labelled as 5% in Figure 10) in comparison with L68 load (68 in Figure 10). However, it was expected that all ARDs would be lower for L5%L than for L68 load as in the case with reference target burnup (ref SP_C) since the fuel burnups of L5%L load are lower. Further analysis revealed that the presence of significant gadolinium concentrations in the upper parts of the fuel assemblies from L5%L load causes this issue (see Figure 11). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Distribution Parameter**} & \multicolumn{4}{c}{**Distribution**} \\ \cline{2-5}  & **SP\_A\_ref** & **SP\_A\_mod** & **SP\_C\_ref** & **SP\_C\_mod** \\ \hline Average burnup, GWd/tU & 50.7 & 44.6 & 49.7 & 43.7 \\ \hline Minimum burnup, GWd/tU & 45.7 & 40.9 & 34.5 & 30.1 \\ \hline Maximum burnup, GWd/tU & 54.5 & 47.8 & 58.7 & 52.5 \\ \hline FA number in the highest bin & 59 & 77 & 17 & 20 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The summary of parameters of the SP_A and SP_C distributions with reference and modified target burnups. Figure 10: BUC in comparison with fresh fuel approach (**a**) and peak reactivity approach (**b**). To understand the reason of the residual Gd presence, the axial burnup profiles of the FAs from L\({}_{68}\) and L\({}_{5\mathrm{vL}}\) loads (green and yellow lines, respectively, in Figure 12) and criticality curve (red line) for the vanished layer of the GE14 fuel assembly were investigated.