## 5 Discussion  
This study considered a rather limited number of FA-averaged discharge burnups. This circumstance can be of relevance, especially regarding the quantification of ND-related uncertainties in depletion calculations. For low burnups, the few data points of \(\sigma_{ND}^{CASMO}\) at 0 GWd/tHM BU and the next discharge BU of 17.61 GWd/tHM in Figure 5 do not offer a complete picture of the ND-related uncertainties. In fact, assembly burnup calculations performed for UO2 PWR-FAs show the highest ND-related uncertainties in calculated assembly BUs at low exposures of about 10 GWd/tHM, as reported in [28]. Of course, those uncertainties cannot be translated directly into uncertainties in keff. Furthermore, with a maximum of about 2% they are rather moderate. Nevertheless, those findings indicate a possibly poor resolution of the present ND-UQ in this BU-range. The UQ and further analyses in this study concentrated on a disposal canister loading with fuel enriched to 4.94 \({}^{\text{w}}/_{0}\) 235U. The conservatism of this approach in terms of ND uncertainties in the criticality calculations for the underlying SNF disposal scenario has been examined by performing ND-UQs also for lower enriched fuel and could eventually be verified. Compared to fuel with an enrichment of 3.2 \({}^{\text{w}}/_{0}\) 235U, for example, \(\sigma_{ND}^{TOTAL}\) is about 30 pcm (fresh fuel) to 90 pcm (at \(\sim\)32 GWd/tHM BU) higher in case of 4.94 \({}^{\text{w}}/_{0}\). ND play an essential role already in the first step of the BUCSS-R calculation scheme, the reconstruction of the operation history and corresponding depletion calculations based on CASMO-SIMULATE core models. ND uncertainties affecting the calculation parameters, however, were not considered by the current ND-UQ method, leaving room for its further development. Knowledge of the precise reasons for the observed anti-correlations may be of general value for a better understanding of the underlying nuclear data effects. A follow up of this study with an investigation targeting specifically the anti-correlations from depletion and criticality calculations would thus be worthwhile. Furthermore, it shall be noted that the question of how exactly to integrate the ND-related uncertainties corresponding to the criticality calculations in the overall CSE methodology goes beyond the given paper. It can be just mentioned here in passing that normally the nuclear criticality safety criteria involve the so-called upper subcritical keff limits (USL), which are derived on the basis of associated validation studies using criticality safety benchmarks. Thus, by their nature the USL values shall be correlated with the application case keff values. The stronger such correlation, the stronger can be the cancellation effect for the net ND-related uncertainties affecting the CSE and, for instance, for the loading curves in the context of the given study. More detailed explanation of this phenomenon and some quantitative examples can be found in [23; 43]. Figure 6: Pearson correlation coefficients between ND-uncertainties from depletion and criticality calculations. Finally, as pointed out in [11], besides the BUCSS-R calculation scheme \"an alternative way to derive the loading curves recently became available based on the exploitation of the advanced features of the additional CMS code \"SNF\", which can be seamlessly integrated into the PSI CMSYS system. A description of the new methodology developed at PSI outside the BUCSS-R project, which is based on the CASMO/SIMULATE/SNF/COMPLINK/MCNP system of codes and which was named the CS2M method has been presented in Reference [47]\". The CS2M calculation scheme consists of two specific couplings:  
* CMSYS with SNF to reconstruct isotopics and calculate their evolution during decay;
* SNF to MCNP using the in-house COMPLINK methodology to initialize the material compositions of all fuel rods in the MCNP models.