The measurement of the rocking behaviour of graphite bricks using ultrasound

Abstract

Assemblies of graphite bricks are found in graphite cored nuclear reactors throughout the world. In such reactors the graphite bricks, which are used to moderate the nuclear fission reaction, are assembled in arrays of the order of 10 m in height and 10m in width. The bricks themselves come in a range of shapes and sizes. The static and dynamic response of such structures is dependent on both the properties of the bulk material and the nature of the interface between individual bricks. Typical real surfaces exhibit some form of macroscopic waviness in addition to the microscopic roughness and therefore only a small fraction of the surfaces are in contact. This interfacial behaviour has been found to be one of the most significant factors affecting the rocking behaviour of dry-jointed structures, such as graphite cored reactors, and has proved one of the most difficult to explore experimentally. This paper describes a novel application of the ultrasonic measurement technique for the investigation of the effect of microscopic and macroscopic surface features on the rocking behaviour of graphite bricks. An ultrasonic scanning system has been combined with a loading rig to enable factors such as the size of the contact area and its position relative to the edge of a brick to be measured in situ as the brick is loaded. The ultrasound is used to measure the contact pressure distribution between a brick and a rigid foundation. This information has been used in a model to predict the rocking stiffness of the brick-foundation system and good comparisons with rocking experiments achieved. This measurement system could also be used to study the rocking behaviour of other systems of dry contacting blocks such as those found in costal defences and ancient buildings.