Formation patterns of hydrodynamic loads on the fuel rod bundles in the turbulent coolant flow in WWER fuel assemblies

Flow induced vibrations of fuel rods in fuel assemblies promote fuel rod deterioration and can lead to the release of nuclear fission fragments into the fuel assembly. One of the ways to reduce vibration intensity is to decrease hydrodynamic loads on the fuel rod bundle through redistribution of the inlet coolant flow. Simultaneous multi-point measurements of pressure pulsations in the turbulent water flow in a full-scale model of fuel assembly WWER-440 are used to determine hydrodynamic loads that cause fuel rod vibrations. It is shown that amplitude and frequency of hydrodynamic loads depend not only on the flow velocity but also on the flow configuration at the inlet to the nuclear reactor fuel assembly. High levels of hydrodynamic loads are characteristic of the initial hydrodynamic area of the rod bundle near the lower supporting grid where high levels of flow turbulence at the fuel assembly inlet are observed, in particular when throttling spacers are used. At the initial hydrodynamic area where flow turbulence is high, hydrodynamic loads in the low-frequency range (up to 10 Hz) contribute up to 40–60% of the total hydrodynamic loads. When flow disturbance at the inlet to the fuel assembly is low, the low-frequency response is absent. High levels of hydrodynamic loads in the low-frequency region produce intense vibration of the fuel rod bundle, leading to the formation of defects of the cladding and the release of nuclear fission products into the coolant. The results of the investigations have shown that high levels of flow disturbance at the inlet to the fuel assembly promotes the formation of appreciable hydrodynamic loads on the fuel rod bundles, causing their intense vibration and fuel rod damage.

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