Longitudinal Displacement of a Fuel Rod Nonfixed in the Support Grid under the Conditions of Hydromechanical Vibration of VVER-440 Reactor Fuel Assemblies

Experiments aimed at studying the effect of fuel rod vibration in a turbulent coolant flow has on the effort of moving and longitudinal displacement of a fuel rod in the system of VVER-440 reactor fuel assembly (FA) spacer grids (SGs) for the case when the fuel rod is not fixed in the lower support are carried out. It has been found that the fuel rod longitudinal displacement force becomes essentially (by approximately 30%) higher in a coolant flow than it is in stagnant water. The fuel rod's longitudinal displacement force decreases with an increase in the water flow velocity and, hence, with an increase in the fuel rod bundle vibration intensity. At the water flow velocity equal to approximately 3.8 m/s, which is close to the its nominal value in the VVER-440 reactor FAs, the displacement force decreases by approximately 10% as compared with its value at the velocity equal to 0.8 m/s. The regularities relating to the change with time in the fuel rod's longitudinal displacement speed and acceleration testify that the force resisting its motion in the spacer grid system has a periodic pattern. During vibration (flexural oscillations) of a fuel rod bundle, the stiffness in the fuel rod-SG cell system changes its value, and the axes of SG cells shift relative to one another for the characteristic period of one vibration cycle. It is particularly the above-mentioned effects connected with fuel rod bundle vibration that lead to a growth in the fuel rod's longitudinal displacement force. The results from the accomplished calculated assessments show that the algebraic sum of the Archimedean force, gravity force, force due to pressure loss across the fuel bundle, and inertial forces with the water velocity values up to 4.81 m/s do not exceed 6% of the minimal fuel rod longitudinal displacement force. In turn, this means that the system of honeycomb-type spacer grids reliably secures the VVER-440 reactor fuel rods that are not fixed in the FA lower support grid.