When there is a change in power in a nuclear reactor, the concentration of the fission product 135Xe can increase in certain cases. Since it has a high absorption cross section for thermal neutrons, this can pose limitations on the reactor’s operation during power changes.
The purpose of the master’s thesis was to examine the impact of various parameters in the design of a nuclear reactor on reactivity changes due to the maximum possible variation in the concentration of 135Xe, as a function of burnup. Limitations on power changes due to the variation in 135Xe concentration were determined for a small modular reactor (SMR).
A model of a typical pressurized water reactor fuel rod with periodic boundary conditions was created in the Serpent neutron transport and material depletion code to assess the impacts of design parameters. To determine operational limitations, a two-dimensional model of a SMR based on the NuScale reactor design was developed. It was found that there is a smaller reactivity change in MOX fuel due to the variation in 135Xe concentration compared to UO2 fuel. Similarly, there is a smaller impact of 135Xe on reactivity in fuels with higher initial enrichment and faster neutron spectra.
Due to the lower power density, the impact of 135Xe on reactivity is smaller in SMRs compared to typical pressurized water reactors. From this perspective, SMRs are more flexible in power changes. Three different power change modes were tested. In the first, power changed stepwise, and limitations were obtained based on the initial and final power values. In the second, reactor power decreased from full to zero power and, after a certain time, increased to any desired power. Limitations were determined based on restart time and the height of the new power. In the third transition, power decreased to any level and then increased back to the original power when the concentration of 135Xe was at its maximum. It was found that limitations apply at most in the last 4 % of the fuel cycle.
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