Reevaluation of the ¹⁵⁴Eu thermal capture cross-section based on spent fuel benchmarking studies

The ¹⁵⁴Eu(n,γ)¹⁵⁵ Eu radiative capture reaction is an important reaction to understanding both reactor dynamics (given the large capture cross-sections of both ¹⁵⁴Eu and ¹⁵⁵Eu) as well as for the characteristics of used nuclear fuel, where ¹⁵⁴Eu is both a strong gamma emitter and a key burnup indicator nuclide. In the most recent Evaluated Nuclear Data File (ENDF) releases (ENDF-B/VII.0, and subsequently ENDF-B/VII.1), the ¹⁵⁴Eu(n,γ)¹⁵⁵ Eu thermal capture cross-section (σ_th, reported for E = 0.0253 eV and extrapolated to lower energies via the ^l/v relationship) was updated based on evaluations compiled by Mughabghab, indicating an evaluated cross-section of 1348.5 barns, 36.6% lower than the prior reported evaluation of 1840 barns in ENDF/VI.8. Recent computational benchmarking studies based on the highly thermalized spectra of CANDU fuel assemblies suggest that this newer evaluation may in fact underestimate the true thermal capture cross-section, producing significant over-estimations of ¹⁵⁴Eu for highly thermalized systems (like CANDU reactors), while likewise producing a systematic (but more moderate) bias for light water reactor systems. In this paper, we will present the results of a series of used fuel depletion studies, evaluating the effect of employing the ENDF/VII thermal capture cross-section by calculating the effective onegroup capture cross-section for ¹⁵⁴Eu for both CANDU PHWR assemblies using ENDF/VI.8 and ENDF/VII.0 and comparing the results to destructive radiochemical assay on well-characterized assemblies. Our evaluations indicate a superior agreement for ¹⁵⁴Eu in highly thermalized systems, wherein the difference in ¹⁵⁴Eu inventories is up to 10% between the two evaluations.