Comprehensive Revision of the Summation Method for the Prediction of Reactor E Fluxes and Spectra

The summation method for the calculation of reactor nu over bar e fluxes and spectra is methodically revised and improved. For the first time, a complete uncertainty budget accounting for all known effects likely to impact these calculations is proposed. Uncertainties of a few percent at low energies and ranging up to 20% at high energies are obtained on the calculation of a typical reactor nu over bar e spectrum. Although huge improvements have been achieved over the past decade, the quality and incompleteness of the present day evaluated nuclear decay data still limit the accuracy of the calculations and therefore dominate by far these uncertainties. Pushing the beta-decay modeling of the thousands of branches making a reactor nu over bar e spectrum to a high level of details comparatively brings modest changes. In particular, including nuclear structure calculations in the evaluation of the nonunique forbidden transitions gives a smaller impact than anticipated in past studies. Finally, this new modeling is challenged against state-of-the-art predictions and measurements. While a good agreement is observed with the most recent inverse beta decay measurements of reactor nu over bar e fluxes and spectra, it is unable to properly describe the reference aggregate beta spectra measured at the Institut Laue-Langevin High -Flux Reactor in the 1980s. This result adds to recent suspicions about the reliability of these data and preferentially points toward a misprediction of the 235U nu over bar e spectrum.