Development of a machine based non-contact ID gauging system for zircaloy fuel tubes

Zircaloy 4 tube is used as fuel cladding material for Indian PHWR Nuclear reactors. The radial gap between cladding tube and uranium dioxide pellet is one of the major design parameter, and is maintained as 0.038 to 0.13 mm during manufacturing of fuel bundles. To maintain stringent radial gap, ground pellets with OD tolerance of +/- 0.012 mm are inserted in tubes with ID tolerance of +/- 0.012 mm. However, owing to difficulty in manufacture the tubes with this tolerance, tubes, after cutting to the length of 500 mm, are sorted to this tolerance based on ID measurement. Currently, ID sorting is being done manually. But with increasing production rate and to have better reliability, a machine based system for ID gauging and sorting was conceptualized. To fulfill this accuracy of sorting, a non-contact auto gauging system has been developed. The system works on the principle of air gauging i.e. back pressure is inversely proportional to clearance between the air jet nozzle and tube surface. A four jet air plug has been used to measure the internal diameter in two mutually orthogonal axis. During the controlled travel of the probe in the tube inside bore, more than 300 diameter measurements are made in length of 500 mm. Based on this measurement a database of all the measurements is available for minimum, maximum and average ID. A criterion of segregation of tubes is made on the basis of these measurements into 5 different categories. System can also measure the ovality at every point which is also used for sorting. Trials have been carried out at different probe insertion speeds. The cycle time is optimized at 15 secs for effective sorting of tubes. Based on these trials a machine based system is envisaged for ID gauging and sorting of these critical tubes in future. (author)