Dynamic Processes of Fuel Assemblies Lifting in the VVER-440 Reactor

With the loss of contact in the “ball-cone” type of support structure when the fuel assembly lifts off in the upward coolant flow, its vibrational behavior can change considerably. Therefore, it is important to define the conditions of lifting (water rate, lifting height of the fuel assembly) and the vibrational characteristics of the fuel assembly rods. Experimental studies are conducted using a full-size mock-up of the fuel assembly for the VVER-440 of the second generation, and a model is developed simulating fuel assembly movement when lifting. It is established that the mock-up lifts when water rate is ~192 m³/h, which corresponds to the velocity of flow in the fuel assembly rods of about 6 m/s. The return of the mock-up to the standard position occurs when the water rates decreases from 183 to 171 m³/h (velocity of flow in the fuel assembly rods of 5.7 to 5.4 m/s). Based on the results of studies under field conditions (coolant pressure 12.5 MPa, temperature 285 °C) it is established that lifting of the VVER-400 fuel assembly is possible when the water rate is 296 — 315 m³/h (velocity of flow in the fuel assembly rods of 9.3 — 9.9 m/s).

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