Impact of Approximations in Operating History Data on Spent Fuel Properties With Serpent 2

Abstract

In this work, the effect of averaging operating history parameters such as power history, boron concentration and coolant density, and temperature on spent nuclear fuel properties was investigated. The examined properties were assembly activity, decay heat, photon emission rate, spontaneous fission rate, and the concentration of some mobile nuclides and fissile nuclides. Calculations were performed on two similar VVER-440 fuel assemblies irradiated in different positions of the core using Serpent 2. Averaging power history over the entire irradiation history had a significant effect on assembly activity, decay heat, and photon emission rate overestimating these properties approximately 70% right after irradiation. However, the effect quickly died out and after 10 years of cooling, the effect was less than 1%. If the last cycle (third cycle) was modeled accurately and the power density of only the first two cycles was averaged, the differences remained always below 1%. The effect of operating history approximations on spontaneous fission rate and the nuclide concentrations was much smaller remaining mostly below 1.5%. The sensitivity of nuclide concentrations to approximations in individual operating history parameters was dependent on the nuclide in question and no trend applying to all studied nuclides could be observed.