@@ -54,6 +54,35 @@ Conclusions from this evaluation are as follows.
## Conclusions from ORNL/TM-2020/1835 (BWR)
Calculations were performed using the pre-release version of SCALE 6.3 Polaris and ORIGEN codes with 56 -group ENDF/B-VII.1 cross section library. The effects of EE and HBU on lattice depletion characteristics of a representative commercial BWR assembly (10×10 GNF-2) were evaluated. Similar to the first volume on PWR lattice behavior, the investigation focused on differences between depletion to conventional conditions with existing fuel (5max-4.5av wt% U-235 enrichment depleted to 60 GWd/MTU) and depletion with enrichments up to 10max-7.4av wt% and burnup up to 80 GWd/MTU. Unlike the PWR volume, here a new lattice enrichment map was developed for each different EE case with limited optimization on gadolinia loading, pin peaking and depletion curves.
Key quantities of interest include lattice physics quantities, isotopic inventory at various decay times and their effect on decay heat, activity and shielding applications. Limited comparisons between predictions using SCALE 56-group ENDF/B-VII.1 cross sections and SCALE 252- group ENDF/B-VII.1 cross section are also presented. Conclusions from this evaluation are in general, very similar to the ones found the PWR Volume 1 report and are as follows.
1. No unexpected or anomalous trends were found that would call into question the accuracy of the Polaris code using SCALE 56-group ENDF/B-VII.1 data for depletion, lattice physics, and isotopic content calculations of the analyzed BWR fuel with enrichments up to 10max7.4av wt% and burnup up to 80 GWd/MTU.
2. Increased enrichment and higher burnup are positively correlated due to the requirements of commercial BWR fuel management (fuel economics and reactor physics). This correlation tends to result in offsetting lattice physics effects when combined with single-assembly results to estimate core average characteristics.
3. Lattice physics results from the Polaris model depletion of GNF-2 10×10 DOM and VAN lattices with void fractions varying from 10% to 70% overall showed no unusual, unexpected, or adverse code performance trends.
* Calculated fuel kinf, peaking factors, and reactivity coefficients are smooth and continuous as a function of enrichment and burnup.
* Lattice physics trends were predictable from first principles (e.g., spectral hardening resulting from increased U-235 enrichment).
* A first-order approximation shows that lattice average burnup is expected to 10 GWd/MTU for each 1 wt% increase in lattice average fuel enrichment above 5 wt%. This approximation can be used to extend the results of single-assembly lattice physics calculations to expected core average behavior.
* Core average fuel temperature coefficient (DTC) and β-eff kinetics parameter are not expected to change substantially due to the offsetting effects of increased enrichment and increased burnup. However, moderator void coefficient depends on initial gadolinium loading, and the moderator void coefficient for a lattice at beginning of life was only slightly negative for some lattices in this study. Thus special attention should be paid when optimizing enrichment maps.
4. Uncertainty in depletion kinf due to cross section uncertainties changes negligibly for EE and HBU compared to the reference case. Increasing enrichment, increases kinf uncertainty initially (~50 pcm); however, the uncertainty decreases with burnup and becomes lower than the reference case after 50 GWd/MTU.
5. Increasing enrichment from 5max-4.5av wt% to 10max-7.4av wt% at 60 GWd/MTU leads to minor changes in decay heat. At time = 0, decay heat increased by 3% and then decreased to -10% at 500 days and reaches to -5% at 1000 days relatively compared with the reference 5max-4.5av wt% case.
6. Increasing burnup from 60 to 80 GWd/MTU for 5max-4.5av wt% leads to a negligible change at time = 0 and a growing change from 10 days (7%) to 1000 days (43%) relatively compared with the reference 60 GWd/MTU. However absolute difference is negligible at 1000 days, the 80 GWd/MTU fuel has less than 1.5 kW/MTU decay heat difference compared with the reference 60 GWd/MTU case.
7. Effects of increases in burnup and enrichment on decay heat are in opposite directions, so the combined effect is a 18% increase at 500 days for increased enrichment and burnup compared with a 35% increase for only burnup. However, the absolute difference only makes 1 kW/MTU difference.
8. Decay heat calculations for VAN lattice show similar trends with DOM lattice. Increasing void fraction has negligible effect on decay heat compared to burnup and enrichment increase.
9. Activity shows similar trends to decay heat, but with less magnitude.
10. Isotopic results from the Polaris model depletion of GNF-2 10×10 DOM and VAN fuel lattices overall showed no unusual, unexpected, or adverse code performance trends.
* No single isotope influenced decay heat by more than 11% in any case analyzed.
* No single isotope changed activity by more than 5.2% in any case analyzed.
* Cm-244 is the main isotope that changes the spontaneous neutron emission source substantially for the timescales in question.
* Of the criticality-related isotopes evaluated, when increasing burnup to 80 GWd/MTU and enrichment to 10max-7.4av wt%, only Am-243 changed in composition by over a factor of 2 for the cases analyzed.
* When changing from the 252- to the 56-group library, no isotope changed in mass by more than 10% for the 80 GWd/MTU, 10max-7.4av wt% VAN 70% void fraction case.
11. Each EE lattice requires a new enrichment loading pattern to be designed to match the reference case lattice design constraints. The loading patterns designed in for this study are adequate for the first phase. However, a more rigorous gadolinia loading and enrichment pattern optimization is needed for more realistic MVC and pin power distribution comparisons. Based on loading pattern trends in the industry, higher gadolinia loadings are expected. Increased gadolinia loading is also expected to further reduce the differences observed in the gadolinia depletion region for EE cases.
12. The results do not show significant changes when lattice type was changed, and void fraction was increased from the reference case. These inconsistent comparisons (DOM to VAN, 40% to 70% void fractions) show the bounding, conservative changes that could be expected for consistent comparisons (e.g., using 5max-4.5av wt% VAN lattice at 70% as the reference case and performing the same EE and HBU analyses). For the cases with the largest changes due to lattice type or void fraction change, this assumption should be verified in the next phase.
13. Higher than expected differences were observed in isotope contents calculated at 80 GWd/MTU with 56g vs 252g cross section libraries for some isotopes. However, these differences are negligible when compared to the magnitude of the change in isotopic contents when the reference case was compared to EE and HBU cases. Furthermore, comparison of 252g library results to the reference case results calculated from 56g library is an inconsistent bounding comparison to show that conclusions in this report would be valid when cross section library is changed. Reactivity coefficients calculated using 252g library in Appendix A confirms this conclusion. However, this assumption will be further verified in the next phase by repeating selected isotopic analyses with 252g library and confirming the findings.
14. Although no unexpected behavior was observed, verification basis for 56g and 252g cross section libraries will be extended to 80 GWd/MTU using continuous energy Monte Carlo depletion calculations in the next phase of this study.
15. Changes in pin power distributions were not analyzed in this phase because of their dependency to enrichment loading patterns which will be optimized in the next phase.
## Acknowledgements
Support for this work was provided by the US Nuclear Regulatory Commission Offices of Nuclear
