## 5 Results and Discussion
### GBC-68 Cask Criticality Analysis
#### 5.2.2 The Impact of Targeted Discharged Burnup on Cask Criticality
Figure 12 shows that the burnup of the fuel assembly's upper layer (layer 4) from L\({}_{5\mathrm{vL}}\) load (5% LB yellow dash line) is around 15.8 GWd/tU. It corresponds to the part on criticality curve of the vanished layer (van) where gadolinium is still present, which means that fuel assemblies have not even reached the highest criticality level (peak reactivity). On the other hand, the upper layer burnup of the FAs from L\({}_{68}\) load is 22 GWd/tU (68 LB green dash line) which corresponds to the part of the criticality curve where Gd is burnt (after the peak reactivity). Thus, for the least optimal burnup distribution SP_C with modified target burnup, the least burnt fuel assemblies are strongly underburned on the upper layer and therefore will be called here underburned fuel assemblies. This effect can occur if fuel assemblies designed for a much higher target burnup are not burnt close enough to it. The considered here GE14 fuel assembly variation is intended for a higher average burnup than the one from the modified target burnup case. Thus, this economically unsustainable approach would have somewhat surprising additional implications on the storage of the fuel assemblies due to the insufficient reduction of the burnable poison caused by the too low burnup. The criticality inconsistency related to the underburned fuel assemblies disappears when considering the major actinides only set which is used for burnup criticality analysis on actinide-only level. This is due to the removal of gadolinium and fission products from the analysis. In the case of least optimal SP_C distribution obtained for the modified targeted burnup (mod SP_C), the ARD in comparison with fresh fuel approach is 13,400 and 18,700 pcm for L\({}_{5\mathrm{vL}}\) and L\({}_{68}\) load, respectively. Hence, the results repeat the expected pattern: lower ARD values for lower discharged burnups. Thus, the different approaches to the burnup credit analysis, such as the choice of the isotopic set can lead to different conclusions. Figure 11: Gd-155 concentration in FA layers for L\({}_{5\mathrm{vL}}\) and L\({}_{68}\) loads. To sum up, as expected, the reduction of target burnup of a fuel assembly without reducing the initial enrichment (under burning the FA) leads to a substantial change in final assembly reactivity (increase in most cases) and consequently increase the reactivity of the storage system. In the classical 2D lattice approach, the criticality of the FA decreases with time for UO\({}_{2}\) fuel, while in case of poisoned fuel, this behaviour is observed after the peak reactivity. This outcome will consequently be propagated to the criticality analysis of the systems with the spent fuel composition estimated from lattice calculations. The criticality analysis of the GBC-68 task performed here uses isotopic data of the fuel obtained from the nodal calculations. For this case, the described above trend can be reversed as shown in this section for actinides plus fission product set, while it will follow the expected trend for major actinides set.