Nondestructive characterization techniques for assuring the integrity of fuel pins for fast reactors

The integrity of nuclear fuel pins is assured by following a comprehensive quality control plan that is carefully made based on the inputs from the designers, fuel fabricators and utilities. It plays an important role in the safe and efficient operation of nuclear reactors. The fuel pellets are encapsulated in thin clad tubes after a series of quality control /process control checks and the welded fuel pins are subjected to a number of advanced non-destructive characterization techniques to assure the integrity of the fuel pins and to characterize the fuel materials inside the fuel pins. Uranium - Plutonium mixed oxide (MOX) fuel (made from a mixture of Depleted Uranium dioxide and Plutonium dioxide as the starting material), enriched Uranium dioxide fuel, Plutonium - Uranium mixed carbide (MC) pellets are used for fast reactors. A number of non-destructive evaluation techniques have been devised and adopted during the manufacture of the fuel pins at Bhabha Atomic Research Centre (BARC) for Prototype Fast Breeder Reactor (PFBR) and Fast Breeder Test Reactor (FBTR), Kalpakkam. The integrity of the welding of the end plugs is ensured by visual, X-radiography, ultrasonic testing and helium leak testing. During the manufacture of MOX fuel pins for PFBR, digital X-ray imaging technique with a flat panel detector and micro-focal radiography has been adopted at BARC. Ultrasonic evaluation technique has also been developed for the same purpose. Studies have been carried out to establish the correlation between digital X-ray imaging technique, and ultrasonic testing using destructive metallography of end plug welds. A number of non-destructive characterization techniques like X-Gamma autoradiography (XGAR), and Passive Gamma Scanning (PGS) have been developed specially for material characterization of fuel pellets, correct loading of pellets of required composition the fuel and hardware components and detection of PuO₂ agglomerates. An attempt has been made in this presentation to describe the various non -destructive characterization techniques followed for assuring the integrity of the fuel pins, characterize the fuel materials encapsulated, compares them and presents Indian experience during the manufacture of fuel pins for fast reactors.