On-line measurement of oxide fuel pellets for process control

An important aspect of quality of nuclear fuel pellets is dimensional stability. Close dimensional tolerances are required to ensure both easy pin filling and good heat transfer in the reactor. This paper describes a machine for measuring pellets in the production line (100% sampling) and automatically rejecting any outside the specification. The functional components of the machine are mechanical handling, diameter measurement and computer control: (i) The mechanical handling system allows pellets to be presented to the measurement head and disposed to the accept/reject stations at a rate sufficiently high for 100% inspection of production. (ii) Diameter measurement uses a capacitance technique which allows fast and accurate measurement without contact. The measurement head consists of a ring of sixteen radial capacitance gauges. This allows the determination of eight diameters on a section through a pellet. The technique used to ensure drift-free operation is described. (iii) The controlling computer repeatedly scans the measurement head as the pellet is drawn through it and converts the capacitance measurements into diameters. In this way the dimensions of the pellet are determined on several axial sections. From this information the computer detects any out of tolerance due to under/over size, ovality, barrelling, chipping, etc and diverts the pellet into an accept or reject hopper. These functions are described in some detail together with an assessment of measurement accuracy and estimate of reliability of defect detection. (orig.)