ENGINEERING FORMULAS FOR ESTIMATING THE INFLUENCE OF NATURAL CONVECTION ALONG THE SURFACE OF FUEL ASSEMBLY OF WWER-1000 ON THE RESULTS OF MEASURING ITS SIZES BY THE ULTRASONIC METHOD IN COOLING POND OF NUCLEAR POWER PLANT

Engineering formulas are presented for calculating the value of methodological error in measuring the distances from the ultrasonic sensor to the surface of the irradiated fuel assembly of WWER-1000 caused by decay heat from the fuel assembly. The methodical error arises due to the water temperature gradient along ultrasonic wave path. This is due to the presence of natural convection at the surface of the fuel assembly discharged from the reactor into cooling pool at nuclear power plant. The paper presents a methodology for calculating the methodical error. It is assumed that the water temperature between the sensor and the surface of the fuel assembly is determined by convective heat transfer between the fuel assembly and the water in cooling pool at nuclear power plant. The surface of VVER-1000 fuel assemblies is modeled by a flat vertical plate with a uniform surface heat flux. The propagation of an ultrasonic wave in a medium between the surface of a fuel assembly and an ultrasonic sensor is described in the approximation of geometric acoustics. The results of numerical calculations according to the presented method are executed in the form of nomograms. Engineering formulas for calculating the value of methodological error are obtained by processing an array of calculated data. The obtained formulas are used in the development of measuring systems for monitoring the deformation of fuel assemblies of WWER-1000 in cooling pond of nuclear power plant.