Abstract
The present article presents the mechanical characterization of the fuel rods oscillator developed for the purposes of the COLIBRI experimental program in CROCUS. COLIBRI aims at investigating the radiation noise related to fuel vibrations. The main motivation is the increased amplitudes in the neutron noise distributions recorded in ex- and in-core detectors that have been observed in recent years in Siemens pre-Konvoi type of pressurized water reactors. Several potential explanations have been put forward, but no definitive conclusions could yet be drawn. Among others, changes in fuel assembly or pin vibration patterns, due to recent modifications of assembly structural designs, were pointed out as a possible cause. Computational dynamic tools are currently developed within the Horizon 2020 European project CORTEX, to help with understanding the additional noise amplitude. The COLIBRI program is used for their validation. An in-core device was designed, tested, and licensed between 2015 and 2019 for fuel rods oscillation in CROCUS, in successive steps from out-of-pile tests with dummy fuel rods to critical in-core tests. The characterization of its mechanical behavior is presented, in air and in water, and as a function of the load, for safety and experimental purposes. The device allows simultaneously oscillating up to 18 fuel rods. The maximum oscillation amplitude is 5 mm, while the maximum allowed frequency is 2 Hz, i.e. in the frequency range in which the induced neutron flux fluctuations are most pronounced in nuclear power plants.
Highlights
INTRODUCTIONIncrease Between Umetal Rods and Inner zone) experimental program in CROCUS is to investigate the radiation noise related to fuel vibrations
HE aim of the COLIBRI (CROCUS Oscillator for Lateral TIncrease Between Umetal Rods and Inner zone) experimental program in CROCUS is to investigate the radiation noise related to fuel vibrations
The main motivation for this investigation is the increased amplitudes in the neutron noise distributions recorded in ex- and in-core detectors that have been observed in recent years in Siemens pre-Konvoi type of pressurized water reactors
Summary
Increase Between Umetal Rods and Inner zone) experimental program in CROCUS is to investigate the radiation noise related to fuel vibrations. It consists in carrying out experiments on rod displacement (static) and oscillation (dynamic) with a variation of both number and configuration of oscillated rods at relevant ranges of amplitudes and frequencies. Changes in fuel assembly or pin vibration patterns, due to recent modifications of assembly structural designs, were pointed out as a possible cause. Such mechanical noise is suspected to arise from vibration of groups of fuel assemblies. The tests of the fuel rods oscillator are presented, focusing on the mechanical characterization for safety and experimental purposes
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