# Burnup Credit Evaluation for BWR Spent Fuel from Full Core Calculations
## 4 Full-Core Modelling
### Refuelling Simulations  
The BWR FA average discharge burnup varies for different reactor generations, FA lattice sizes and fuel enrichments. For the modern BWR fuel, it lies in a range of 45-50 GWd/tU [36; 37]. For this study, burnup limitations described in the generic design assessment of UK ABWR [38] were used. The data is summarised in Table 5.  
In LWRs, a fraction of the core (usually the assemblies from the final cycle) is discharged after each cycle to the storage pond. The other FAs change their positions in the reactor core according to the specific shuffling procedure, and the new fresh fuel assemblies are loaded to the empty spaces. This process is called refuelling. The cycle length for modern BWRs is 12-18 months, while a single FA stays in the reactor core for approximately four years [39] over 3 to 4 cycles. The open data about refuelling patterns are limited, so four shuffling procedures (SP) have been developed in this study to cover the possible scope of the discharged fuel burnups of the individual fuel assemblies. Considered SPs are shown in Figure 5.  
The refuelling process/shuffling procedure was programmed using Python script, which determined the new position for FAs after each cycle according to the SP algorithm from Figure 5 and then passed these data to the DYN3D code for further calculations. The script updates all DYN3D input files and launches the code, so the user only needs to create the initial DYN3D input and choose the appropriate shuffling procedure and cycle length, while further burnup and shuffling calculations will be performed automatically for an unlimited number of cycles.  
In all considered SPs, fuel burnups of the FAs were sorted from minimum to maximum at each cycle and then moved according to the pattern showed in Figure 5. In the SP_A procedure, the FA coming from a maximum burnt position of the cycle C0 moves to the position of the least burnt FA of cycle C1, then it goes to the most burnt position of cycle C2, and finally, it is located to the least burnt position of the C3 cycle, while assemblies from the cycle C3 are removed from the core and the fresh FAs are loaded to the empty C0 cycle positions. In other words, maximally burnt fuel assemblies from one cycle move to the positions where the fuel assemblies of the next cycle accumulated the lowest  
Figure 5: Shuffling procedures developed and tested to determine the discharge burnup of the individual fuel assemblies.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Burnup, GWd/tU** \\ \hline Average discharge burnup & 50 \\ Maximum assembly average burnup & 60 \\ Maximum pin burnup & 65 \\ \hline \hline \end{tabular}
\end{table}
Table 5: Summarised data about burnups for BWR fuel as used in this study [38].  
increase in burnup. The SP_B procedure has a reversed-to-SP_A order of FAs movements, so the least burnt fuel assemblies from the cycle C0 go to the highly burnt FA positions of the cycle C1 and so on. The SP_C shuffling procedure implies that the FAs remain on the same burnup positions at each cycle, or in other words, the least burnt fuel assemblies from C0 go to similar positions of C1, and the process repeats until cycle C3. The shuffling procedure SP_D follow the SP_C shuffling procedure up to the cycle C2. Then, the FAs from C2 with the smallest burnups are moved to the maximally burnt FAs positions of the C3, and C3 FAs are discharged from the reactor core.