Effects of Fission Yield Data in the Calculation of Antineutrino Spectra for ${}^{235}\mathrm{U}(n,\text{fission})$ at Thermal and Fast Neutron Energies

Fission yields form an integral part of the prediction of antineutrino spectra generated by nuclear reactors, but little attention has been paid to the quality and reliability of the data used in current calculations. Following a critical review of the thermal and fast ENDF/B-VII.1 ${}^{235}\mathrm{U}$ fission yields, deficiencies are identified and improved yields are obtained, based on corrections of erroneous yields, consistency between decay and fission yield data, and updated isomeric ratios. These corrected yields are used to calculate antineutrino spectra using the summation method. An anomalous value for the thermal fission yield of ${}^{86}\mathrm{Ge}$ generates an excess of antineutrinos at 5–7 MeV, a feature which is no longer present when the corrected yields are used. Thermal spectra calculated with two distinct fission yield libraries (corrected ENDF/B and JEFF) differ by up to 6% in the 0–7 MeV energy window, allowing for a basic estimate of the uncertainty involved in the fission yield component of summation calculations. Finally, the fast neutron antineutrino spectrum is calculated, which at the moment can only be obtained with the summation method and may be relevant for short baseline reactor experiments using highly enriched uranium fuel.

Figure 1

Independent fission yields for germanium isotopes. Full (open) circles are the thermal (fast) ENDF/B-VII.1 values, the full line is a weighted Gaussian fit to the thermal values (excluding ${}^{86,87,88}\mathrm{Ge}$), triangles are the corrected thermal yields, and the dotted line is a GEF calculation.

Figure 2

(a) Thermal ${}^{235}\mathrm{U}$ antineutrino spectra calculated using the corrected ENDF/B yields divided by the one using the original ENDF/B yields. The dashed lines correspond to similar ratios when only the fission yield for a single relevant nucleus was modified. (b) Same as (a), but for fast neutrons.

Figure 3

(a) Ratio of ENDF/B-VII.1 to JEFF calculated ${}^{235}\mathrm{U}(n,\mathrm{F})$ antineutrino spectra for thermal and fast neutron energies. (b) Ratio of fast to thermal ${}^{235}\mathrm{U}(n,\mathrm{F})$ antineutrino spectra using corrected and original ENDF/B as well as JEFF yields.

Figure 4

Calculated thermal ${}^{235}\mathrm{U}(n,\mathrm{F})$ electron spectrum divided by the experimental one [9], using the corrected and original ENDF/B-VII.1 yields as well as the JEFF yields. Only experimental uncertainties are included.

Figure 5

${}^{235}\mathrm{U}(n,\mathrm{F})$ antineutrino spectra multiplied by the $\overline{\nu }+p\to n+e^{+}$ cross section for a combination of the ENDF/B yield and decay data choices: updated decay data with corrected fast (solid line) and thermal yields (dotted line), original ENDF/B-VII.1 decay data with corrected fast yields (dashed line), fast ENDF/B-V yields, and decay data (symbols) [12].