@@ -15,8 +15,48 @@ For convenience, the conclusions from each report is included below.
## Conclusions from ORNL/TM-2020/1833 (PWR)
Calculations were performed using the SCALE Polaris, TSUNAMI, and ORIGEN computer codes to
evaluate the effects of EE and HBU on depletion characteristics of a representative commercial PWR fuel
assembly (Westinghouse 17×17 with 104 IFBA rods). The investigation focused on differences between
depletion to well-understood conditions (5 wt% 235U depleted to 60 GWd/MTU) and depletion with
enrichment up to 8 wt% and burnup up to 80 GWd/MTU.
Key quantities of interest include lattice physics parameters, isotopic inventory at various decay times,
neutronic similarity of fuel assemblies in SFP storage, and relative uncertainty in kinf due to cross section
uncertainty, including the effect of cross section uncertainty on isotopic content. Limited comparisons
between predictions using SCALE 56-group ENDF/B-VII.1 cross sections and SCALE 252-group
ENDF/B-VII.1 cross sections are also presented.
Conclusions from this evaluation 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 cross sections for depletion, lattice physics, and isotopic content calculations of the analyzed PWR fuel with enrichments up to 8 wt% and burnup up to 80 GWd/MTU.
2. Increased enrichment and higher burnup are positively correlated due to the requirements of commercial PWR 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 a Westinghouse 17×17 fuel assembly with 104 IFBA rods 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 235U enrichment).
* A first-order approximation shows that core average burnup is expected to increase 11 GWd/MTU for each 1.5 wt% increase in 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 temperature coefficients (MTC, DTC) and kinetics parameters (β-eff and delayed neutron decay constant) are not expected to change substantially due to the offsetting effects of increased enrichment and increased burnup.
* The soluble boron requirement increases strongly with increasing enrichment.
* Assembly pin peaking increases modestly with increasing enrichment.
* Power variation that occurs radially across fuel pellets (the “rim effect”) at the same fuel pin burnup declines with increasing enrichment.
4. The TSUNAMI-IP similarity index ck is >0.98 for assemblies of different enrichment / burnup combinations (5 wt% 60 GWd/MTU, 8 wt% 84 GWd/MTU, and 8 wt% 94 GWd/MTU) in a 60 representative SFP rack cell. This suggests that SFP burnup credit criticality code validation should not be strongly impacted by HALEU/HBU. Uncertainty in kinf due to cross section uncertainty is similar for the three rack cell cases.
5. The effect of cross section uncertainty on perturbed cross section depletion kinf increases slightly from 60 GWd/MTU to 80 GWd/MTU (~0.1% Δk/k). The lists of the top 25 nuclides most important to criticality for 5 wt% fuel at 60 and 80 GWd/MTU differ by only one nuclide.
6. Increasing enrichment from 5 to 8 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 for the 8 wt% case compared with the reference 5 wt% case.
7. Increasing burnup from 60 to 80 GWd/MTU for 8 wt% leads to a negligible change at time = 0 and a growing change from 5 days (5%) to 500 days (30%) for the 80 GWd/MTU case compared with the reference 60 GWd/MTU. At 500 days, the 80 GWd/MTU fuel has ~14 kW/MTU decay heat compared with ~11 kW/MTU for the base case.
8. Effects of increases in burnup and enrichment on decay heat are in opposite directions, so the combined effect is a 15% increase at 500 days for increased enrichment and burnup compared with a 30% increase for only burnup.
9. Activity shows similar trends to decay heat, but with less magnitude.
10. Isotopic results from the Polaris model depletion of a Westinghouse 17×17 fuel assembly with 104 IFBA rods overall showed no unusual, unexpected, or adverse code performance trends.
* No single isotope influenced decay heat by more than 10% in any case analyzed.
* No single isotope changed activity by more than 5% in any case analyzed.
* Curium-244 is the main isotope that changes the spontaneous neutron emission source substantially for the timescales in question.
* Of the criticality-related isotopes evaluated, only 243Am and 155Gd 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 11% for the 80 GWd/MTU, 8 wt% case.
## Conclusions from ORNL/TM-2020/1835 (BWR)
## Acknowledgements
Support for this work was provided by the NRC Office of Nuclear Regulatory Research and the Office of Nuclear Material Safety and Safeguards. The authors would like to thank many ORNL staff members for their feedback on the contents and presentation in this report.
Support for this work was provided by the US Nuclear Regulatory Commission Offices of Nuclear
Regulatory Research, Nuclear Reactor Regulation, and Nuclear Material Safety and Safeguards. The
authors would also like to thank many ORNL staff members for feedback on the contents and
