An Ultrasonic Trepanning Technique for Radial Sampling of Ceramic Fuel Pellets

An ultrasonic trepanning technique for use in the radial sampling of small-diameter ceramic fuel pellets takes an initial sample in the form of a small-diameter solid cylinder from the center of the pellet. Subsequent samples are thin-wall concentric hollow cylinders. Radial samples 1.56 to 6.80 mm in diameter and 5.35 to 6.80 mm long have been obtained from 93% dense UO₂ pellets and 91% dense mixed-oxide (UO₂-PuO₂) fuel pellets. Cylindrical samples with wall thicknesses of 0.25 to 0.44 mm can be obtained without difficulty. Diameters and wall thicknesses of individual samples and concentricity between samples can be held within ±0.01 mm, at a sample cutting rate of 0.02 mm/sec. The technique has been adapted to existing ultrasonic grinding equipment located in a shielded glove-box facility, which had been used for less precise ultrasonic drilling operations. The technique is currently being used to obtain radial samples from fresh (unirradiated) mixed-oxide fuel pellets that have been subjected to low-burnup exposure in the transient test reactor (TREAT). The individual samples taken from each pellet undergo burnup analysis using standard gamma-counting techniques. The gamma emitters are the ¹⁴⁰Ba-¹⁴⁰La daughters formed during the radioactive decay of the activated fuel pellets. The analytical results, expressed as fiss/(g sec), obtained for discrete radial locations provide the necessary information on the radial power distribution within the individual pellet.