Neutronic Simulation of Fuel Assembly Vibrations in a Nuclear Reactor

Abstract

The mechanical vibrations of core internals such as fuel assemblies (FAs) cause oscillations in the neutron flux that require in some circumstances nuclear power plants to operate at a reduced power level. This work simulates and analyzes the changes of the neutron flux throughout a nuclear core due to the oscillation of a single FA without considering thermal-hydraulic feedback. The amplitude of the FA vibration is bounded to a few millimeters, and this implies the use of fine meshes and accurate numerical solvers due to the different scales of the problem. The results of the simulations show a main oscillation of the neutron flux with the same frequency as the FA vibration along with other harmonics at multiples of the vibration frequency much smaller in amplitude. Also, this work compares time domain analysis and frequency domain analysis of the mechanical vibrations. Numerical results show a close match between these two approaches for the fundamental frequency.

Keywords:

• Neutron noise
• fuel assembly vibrations
• neutron diffusion approximation
• time domain modeling
• frequency domain modeling

Acknowledgments

This project has received funding from the Euratom research and training programme 2014–2018 under grant agreement number 754316. Also, this work has been partially supported by Spanish Ministerio de Economía y Competitividad under project BES-2015-072901 and financed with the help of Primeros Proyectos de Investigación (PAID-06-18), Vicerrectorado de Investigación, Innovación y Transferencia of the Universitat Politècnica de València (UPV).