Impact of Modular Total Absorption Spectrometer measurements of β decay of fission products on the decay heat and reactor (v)over-bare flux calculation

We report the results of a beta-decay study of fission products Br-86, Kr-89, Rb-89, (90)gsRb, (90)mRb, Kr-90, Rb-92, Xe-139, and Cs-142 performed with the Modular Total Absorption Spectrometer (MTAS) and on-line mass-separated ion beams. These radioactivities were assessed by the Nuclear Energy Agency as having high priority for decay heat analysis during a nuclear fuel cycle. We observe a substantial increase in beta feeding to high excited states in all daughter isotopes in comparison to earlier data. This increases the average gamma-ray energy emitted by the decay of fission fragments during the first 10 000 s after fission of U-235 and Pu-239 by approximately 2% and 1%, respectively, improving agreement between results of calculations and direct observations. New MTAS results reduce the reference reactor (v) over bar (e) flux used to analyze reactor (v) over bar (e) interaction with detector matter. The reduction determined by the ab initio method for the four nuclear fuel components, U-235, U-238, Pu-239, and Pu-241, amounts to 0.976, 0.986, 0.983, and 0.984, respectively.