Development of an unloading machine for nuclear fuel pebbles

Abstract

The project goal is to develop a machine that can unload nuclear sphere fuel pebbles in a controlled sequence. The unloading machine will be operational in an environment filled with graphite dust and the gas medium is helium. Furthermore, the environment is radioactive and therefore maintenance activities must be reduced to a minimum. The unloading machine must fit in the bottom of a fuel storage tank. Access to the tank is only from the top, so as to control radioactive releases. The unloading machine must additionally be capable of unloading usable spheres and separate pieces of spheres to the bottom of the tank. A scale model was built to confirm the functionality of two unloading principles, gravity unloading and suction unloading. The gravity unloading concept was selected and further developed. Different improvements were made to the original concept used for the scale model. At first sequencing was not achieved, and therefore the concept was improved to separate spheres mechanically. This caused a possibility to damage spheres. The concept was further improved to remove the disadvantage of sphere damage. Spheres were now again unloaded through the centre of rotation of the unloading head. This mechanism achieved sphere separation, but not sequencing control. Another unloading level was added to the unloading head which proved theoretically to improve sequencing. At this stage of the design it proved theoretically possible to achieve sequentially controlled unloading of spheres, with separation of pieces of spheres to the bottom of the tank. The tests proved that the concept would work, but another improvement was required to prevent strung unloading of spheres. The tank unloading machine was not further tested with the required improvement. Therefore the tests should be repeated when the improvement has been incorporated. It is proposed to add bumps in the housing at the level of the second level of the unloading head. These bumps will prevent spheres from rolling towards the exit in the head. The spheres will then maintain their position in the second unloading level until the exit hole will pass the sphere to take it in. The conclusion is that the concept will probably be acceptable when the latest test was performed. It is however believed that this last improvement will be successful. The tank unloading machine is developed to be low on maintenance. It is believed that the bearings will not need replacement, thus no maintenance is foreseen. The tank unloading device can separate pieces of spheres before the usable spheres are delivered into the sphere transportation pipe.