## 4 Computational Methods  
### Reactor Core Analysis  
Two-group homogenised cross-section sets for the fuel assemblies and reflectors were prepared using Polaris with 56-group ENDF/B-VII.1 master library of the SCALE system. In the case of the GE14 fuel assembly, cross-section sets were produced for dominant and vanished lattices, while in the case of SVEA-100, for a single lattice. The branch conditions were applied to the coolant density and fuel temperature, while the coolant temperature remained constant due to its little change during the normal reactor operation. The nodal data obtained with Polaris was converted to the DYN3D cross-section library format for further nodal reactor analysis. The full-core analysis results, such as coolant density and burnup profiles, were used further for the lattice depletion and criticality calculations using Polaris and KENO modules, respectively (see Section 4.2).  
The vital information about reactor operation such as refuelling process is limited in the open literature. Hence, the authors developed their own refuelling model [22]. After each cycle, the fuel assemblies were arranged from low to high burnup using a Python script. They were then moved according to the specific pattern described by one of four hypothetical shuffling procedures (SP)--SP_A, SP_B, SP_C and SP_D [22]. The different strategies are based on:  
* SP_A shuffling procedure which moves FAs with the highest burnup from cycle \(i\) (\(i=0\), N where N--is the number of cycles) to the positions of the lowest burnt FAs from the cycle (\(i+1\)) and so on, is an optimal one.
* SP_B shuffling procedure follows the opposite pattern and moves FAs with the lowest burnup of the cycle \(i\) to the positions of the FAs with highest burnup of the cycle (\(i+1\)).
* SP_C shuffling procedure, which is also the least optimal strategy, moves the FAs with the lowest burnup from cycle \(i\) to the position of the FAs with lowest burnup of the cycle (\(i+1\)) and so on or, in other words, the FA saves its burnup position.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value, cm** \\ \hline Outer diameter & 215 \\ Inner diameter & 175 \\ Inside height & 410.76 \\ Lid thickness & 20 \\ Cell pitch & 16.8 \\ Boral panel thickness & 0.2565 \\ \hline \hline \end{tabular}
\end{table}
Table 2: GBC-68 cask parameters [27].  
Figure 3: SCALE model of GBC-68 storage cask filled with BWR GE14 spent fuel assemblies.  
* SP_D procedure repeats the pattern of the SP_C, but at the (N \(-\) 1) cycle it follows the SP_B pattern by moving least burnt FAs to the positions of highest burnt ones from the cycle N.  
The above shuffling procedures but for 4 cycle reactor operation are described in more details in the first part of the research [22]. Five scenarios of the reactor operation were considered based on single or mix of two shuffling procedures: four of them are based on SPs with the same name and SP_E case, is on altering SP_A and SP_B after each cycle. The nodal core model consisted of 27 axial layers in total, 25 of which represented the FA and 2 represented the top and bottom reflectors. The nodal analysis was performed with 5-day burnup steps for four cycles of reactor operation. The neutron flux distribution over the core was evaluated after each burnup step.