#### Q1: Can you summarize the neutronic ramifications of using a chromium dopant in the fuel?
A1: The direct neutronic effect of the chromium dopant is minimal. The secondary effect related to the potential loss in fissile material due to adding the dopant is also relatively small because the dopant actually increases the fuel density.
This repository contains models from the following ORNL TM report [ORNL/TM-2021/1961](https://www.osti.gov/biblio/1783010). Please reference the following when using models from this repository.
> B. Hall, R. Sweet, R. Belles, and W. Wieselquist. "Extended Enrichment Accident Tolerant LWR Fuel Isotopic and Lattice Parameter Trends". ORNL/TM-2021/1961 (2021).
This repository contains models from the following ORNL TM report [ORNL/TM-2021/1961](https://www.osti.gov/biblio/1783010). Please reference the following when using models from this repository.
> B. Hall, R. Sweet, R. Belles, and W. Wieselquist. "Extended Enrichment Accident Tolerant LWR Fuel Isotopic and Lattice Parameter Trends". ORNL/TM-2021/1961 (2021).
* Files may be downloaded from the [releases page](https://code.ornl.gov/scale/analysis/atf_latt_phys/-/releases).
* This repository also contains frequently asked questions about the report, with answers in the [FAQ](https://code.ornl.gov/scale/analysis/atf_latt_phys/-/blob/master/FAQ.md).
For convenience, the conclusions from the report is included below.
## Conclusions from ORNL/TM-2021/1961
Calculations were performed using the pre-release version of SCALE 6.3 Polaris and ORIGEN codes with a 56-group ENDF/B-VII.1 neutron cross-section library. The effects of EE and HBU on lattice depletion characteristics were investigated using potential near-term PWR and BWR ATF concepts that included Cr fuel dopant, Cr-clad coating, and FeCrAl cladding. The report summarizes these near-term ATF concepts, and the expected range of burnup and enrichment based on historical data was determined for the PWR and correlated to the BWR. Previous ATF studies were reviewed and Polaris models introduced. The ramifications of dopants and coatings are straightforward. However, with FeCrAl, there are additional design choices owing to the thinner clad wall and significant reactivity penalty. Pellet size, clad OD, and enrichment all became design variables for FeCrAl; and especially for the BWR, where this design space is already large. Future work will perform additional studies at the core level.
Key quantities of interest included lattice physics quantities; isotopic inventory at various decay times; and their effects on decay heat, activity and shielding applications.
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 neutron data for depletion, lattice physics, and isotopic content calculations of the analyzed ATF fuel with enrichments approaching 9 wt % average for the PWR ATF and 10 wt % limit for a single pin in the BWR.
2. Chromium fuel dopants had a negligible effect on any investigated lattice parameter for both the PWR and BWR, e.g., reactivity effects on the order of 50 pcm. The dopant increased fuel density slightly and so the heavy metal loading decreased only slightly.
3. Chromium-clad coatings had a more significant effect than dopants on lattice reactivity (~300 pcm); therefore, additional enrichment would be required for the same fuel lifetime. For the PWR, the enrichment should be increased from 5.0 wt % to 5.1 wt % for nominal burnups and from 8.0 wt % to 8.15 wt % for EE and HBU.
4. FeCrAl cladding introduced additional design decisions regarding whether to keep the normal pellet size (and thereby reduce the OD of the fuel rod), or to increase the pellet size. Decreasing the OD may have ramifications for heat flux limits. Increasing the pellet size increased the amount of fuel in the assembly. We have assumed the specific power remains the same as for the baseline (40 MW/MTU for the PWR and 25 MW/MTU for the BWR), but an assembly with increased fuel mass might run at a lower specific power and still produce the same energy as required by a fixed core power. Thus, for some comparisons, it made more sense to consider lattice behavior as a function of energy release instead of burnup. However, for the majority of this work, which followed typical lattice code usage, the main results have been interpreted as a function of burnup at this stage.
* 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).
* The use of FeCrAl led to spectral hardening, which mainly impacted control element worth, reducing it by 5 to 10%.
5. Additional decay heat and activity due to ATF concepts was minimal. In all cases, including the nominal case, cladding contributed less than 0.1% to the total decay heat. FeCrAl clad had a smaller decay heat than did the baseline Zr-based cladding.
6. The results did not show significant changes when the lattice type and void fraction were changed. For the cases with the largest changes due to lattice type or void fraction changes, this assumption should be verified in the next phase.
Future work will investigate the core-level performance of ATF, using the PARCS core simulator (with Polaris cross sections) to investigate quantities such as at-power core MTC at beginning of cycle (high soluble boron) and reduced CRW. For the FeCrAl cases with a smaller fuel rod OD, a reduced clad surface area would increase the heat flux and might exacerbate crud buildup and axial offset anomalies and could challenge the departure from nucleate boiling limits.
## Acknowledgements
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 presentation in this report.
