https://info.ornl.gov/sites/publications/Files/Pub154717.pdf...

https://info.ornl.gov/sites/publications/Files/Pub154717.pdf


SUMMARY
Commercial light water reactor (LWR) operators and fuel vendors in the United States (US) are pursuing changes to the reactor fuel that include increased enrichment and accident-tolerant fuel (ATF) designs.
Enrichments under consideration are between 5 wt % and 10 wt % 235 U, which are a subset of high-assay low-enriched uranium fuels.

ATF features are designed to improve fuel system performance under
accident conditions. With increased enrichment, fuel cycle economics can be improved if fuel can be licensed for higher burnup (HBU) than typical current limits (e.g., 62 GWd/MTU maximum fuel pin).

To prepare for and support these potential changes, the effects of ATF and HBU are being assessed for selected representative LWR fuel designs. Lattice physics parameter and used fuel isotopic changes are investigated for a conventional 17×17 pressurized water reactor (PWR) design and a conventional 10×10 boiling water reactor (BWR) design.

This study focuses on effects related to the introduction of ATF
features including coated clad, doped UO 2 , and FeCrAl clad. Calculations were performed using the pre-release SCALE 6.3 Polaris and ORIGEN computer codes.
The SCALE/Polaris code using the 56-group ENDF/B-VII.1 neutron cross-section library is the primary investigation tool. Previous work has determined best acceptable ATF modeling practices and confirmed the fidelity of Polaris ATF calculations to higher-order calculations.

Near-term ATF concepts include doped pellets, chromium-coated cladding, and FeCrAl cladding (with FeCrAl channel boxes for BWR assemblies). Batch average enrichments of 6 wt % (PWR) and 5.4 wt %
(BWR) are estimated to be adequate to support increased maximum pin burnup to the equivalent of 75 GWd/MTU.
Additional enrichments needed to cover variations within a batch (split batch) and variations within an assembly (axial blankets and radial zoning) are estimated to be 0.6 wt % for PWRs and 1.0 wt % for BWRs. Enrichments required to offset the negative reactivity of PWR ATF materials range from
0.1 to 0.15 wt % for chromium-clad coating to 0.4 to 0.8 wt % for FeCrAl.

Due to the complexity of BWR design, an equivalent enrichment penalty was not determined in this work.

A delta approach is used to evaluate the effects ATF concepts compared with non-ATF fuels.

Key quantities of interest include reactivity effects, enrichment requirements, lattice physics parameters (reactivity coefficients and kinetics parameters), and isotopic inventories at various decay times. In
general, doped pellets and chromium-coated clad have only minor effects on the reactivity, reactivity coefficients, and isotopic content of used fuel.

Larger effects are seen for FeCrAl with a large reactivity penalty (~5000 pcm), minor MTC changes, and reduced control element worth (5–10%) due to the
slightly harder spectrum. There is no significant impact of any ATF design on delayed neutron fraction, decay heat, or isotopics predictions