# Burnup Credit Evaluation for BWR Spent Fuel from Full Core Calculations
## 5 Results and Discussion
### BWR Full-Core Analysis
#### 5.1.2 Optimisation of the Discharged Fuel Burnup Distribution  
As was discussed earlier, due to the limited open information about reloading patterns used in modern BWRs, the authors applied four different models of the shuffling procedures (Figure 5) to deliver a full core analysis in order to evaluate the possible range of burnups of the discharged fuel assemblies. Seven different cases of fuel reloading were generated based on four shuffling procedures and two core layouts, described above. SP_A_new (SP_C_new) and SP_A_old (SP_C_old) cases use SP_A (SP_C) shuffling procedure and adjusted (Figure 7) and reference (Figure 2) full core layouts, respectively. The SP_B, SP_D and SP_E cases are based on the more promising, adjusted core layout. SP_B and SP_D utilise the shuffling procedures with the same name, given above in Figure 5, while SP_E is an  
Figure 6: Discharged burnup distributions of two different shuffling procedures based on the reference core map. Figure 7: BWR full core layout adjustment. additional case where the shuffling procedures SP_A and SP_B were altered with each other for each new fuel cycle, starting with SP_A in the first cycle followed by SP_B in the second cycle and so on. Figure 8 depicts the burnup distributions of the fuel assemblies discharged from the reactor core during four cycles of operation for the seven different cases with identical cycle lengths. The reactor core has a 90-degree reflective symmetry; hence, results are presented for a quarter of the core which contains 218 FAs in total (Figure 7). Fifty FAs of the third cycle were removed from the core after each operational cycle which, after four cycles, resulted in a total number of 200 discharged fuel assemblies. Figure 8: Burnup distribution of the discharged fuel assemblies after four cycles from one quarter of the core for the cases with different shuffling procedures and core layouts described above: **(a)** SP_A_new/SP_A_old, **(b)** SP_B, **(c)** SP_C_new/SP_C_old, **(d)** SP_D and **(e)** SP_E (NEW—adjusted layout; OLD—reference layout). The quality of the burnup distributions was analysed using the following parameters:  
* Average burnup of the discharged fuel assemblies (B\({}_{\text{AVFB}}\)). * Average burnup of the 5% least burnt fuel assemblies (B\({}_{5\%\text{L}}\)). * Average burnup of the 5% highest burnt fuel assemblies (B\({}_{5\%\text{H}}\)). * Average burnup of the 68 fuel assemblies from the centre of the distribution (B\({}_{68}\)). The number of FAs is equal to the capacity of the BWR storage cask. * Burnup range (BR)--the difference between the minimum and maximum fuel assemblies' burnups. Table 6 summarises the change in the developed above parameters for the produced burnup distributions in comparison with SP_A_new which represents the most optimal distribution due to its smallest burnup range. The analysis shows that the shuffling procedure and core layout adjustment do not significantly affect B\({}_{\text{AVRG}}\) and B\({}_{\text{68}}\) burnups. However, it has to be kept in mind here that SP_C_old would not be permitted due to exceeding the suggested burnup limit in some fuel assemblies. At the same time, the B\({}_{5\%\text{L}}\), B\({}_{5\%\text{H}}\) and BR parameters are more sensitive to the choice of the shuffling procedure, with SP_C_new/SP_C_old cases being significantly away from the SP_A_new. The modification of the core layout significantly improved burnup range of the distributions (by 3 GWd/tU) which was produced with the optimal SP_A shuffling procedure, while BR did not change noticeably for the SP_C case. This indicates that the adjustment has taken away the assembly-burnup outliers into lower flux positions and thereby limited the burnup accumulation. It opens new optimisation potential since the whole core could now be burnt in a slightly longer cycle, thus achieving an even higher averaged burnup. The analysis of the deltas from Table 6 shows that SP_B and SP_E scenarios are closed to the optimal SP_A_new.