Impact of isomeric ratios on reactor antineutrino spectra: a sensitivity study*

A recent re-evalation of the available experimental data provided updated values of Isomeric Yield Ratios (IYRs) for low-energy neutron-induced fission of 42 fission products, and highlighted the limitation of the theoretical model currently used for all evaluated fission yield libraries.

Following this re-evaluation, we performed a sensitivity study to investigate the impact of the updated yields on antineutrino spectra predicted using the summation method for all major actinides (²³⁵U, ²³⁹Pu, ²⁴¹Pu and ²³⁸U). The contribution of individual fission products was studied in detail, and a few were identified as the main contributors to the discrepancies observed between the antineutrino spectra produced with the new IYRs vs those obtained using the values tabulated in JEFF-3.3, the most recent fission yield evaluated library.

For this work, the sensitivity study was also extended to include the over 200 fission products with a known long-lived isomer, and whose IYRs have never been experimentally determined. In this case, the IYRs were varied over the entire range of physically acceptable values, and their individual effect on antineutrino spectra was recorded.