Techniques for Assuring the Integrity of Fuel Pins for Fast Reactors

Abstract

Abstract The integrity of the nuclear fuel pins is of paramount importance from the point of view of safe and efficient operation of nuclear reactors. The fissile fuel pellets are loaded in thin clad tubes and closed by welding of end plugs at both the ends. The integrity of the final fuel pin fabricated is ensured by employing a number of techniques, both destructive and non-destructive at different stages of fabrication. The specifications of the raw materials used, intermediate products and the final product are very carefully made by the designers in consultation with the manufacturers and quality control personnel. The starting feed materials used for fabricating the hardware viz .clad tubes, end plugs, spacer wire and spring are subjected to rigorous quality control checks so that the products meet the design criteria and withstand safely the operating conditions. The material, dimensions, physical, mechanical and chemical characteristics of the clad material are specified considering the time to be spent by the fuel pin in the reactor and the adverse conditions like radiation, temperature, pressure to which the fuel pin is subjected. A number of quality control checks are carried out to assure that the hardware meet the required parameters. Uranium - Plutonium mixed oxide (MOX), enriched Uranium dioxide fuel, Plutonium - Uranium Carbide (MC) pellets are commonly used as fuel for fast reactors. MOX fuel is made from a mixture of Uranium dioxide and Plutonium dioxide as the starting material. The specifications of initial powders, blended powder, green pellets and sintered pellets are carefully drawn so that the integrity of the fuel pin is not endangered in the reactor. A number of non-destructive evaluation techniques have been devised and adopted at Bhabha Atomic Research Centre during the manufacture of MOX and MC pellets for Prototype Fast Breeder Reactor (PFBR) and Fast Breeder Test Reactor (FBTR). The accepted fuel pellets are encapsulated in first end plug welded tubes after degassing. The integrity of the welding of the end plugs depends on the welding procedures adopted. The evaluation techniques should be chosen such that the quality of the welds is ensured. The strength, penetration, freedom from defects, leak tightness, dimensions and color are the major parameters checked. The welds are subjected to a number of checks before wire wrapping. The quality of wire wrap welds is also ensured by suitable methods. The fuel pins are subjected to metrological inspection(length, diameter, bow). Even though the fuel pins are fabricated as per the approved plans, the designers would still like to have a final check on the correct loading of the hardware and fuel pellets in the correct order. A number of non-destructive evaluation techniques have been developed specially for mixed oxide and mixed carbide fuel pins which ensures loading of pellets of required composition and hardware components in the correct order. Loading of pellets of wrong composition, absence of pellets or hardware and presence of wrong component affect the integrity of the fuel pin during operation. An attempt has been made in this presentation to bring out various factors affecting the integrity of the fuel pins in the reactor. The steps taken by different agencies, viz. designers, fabricators and quality control personnel are described. The techniques used for assuring the integrity of the fuel pins are presented. Special emphasis has been given to new non-destructive evaluation techniques developed for fuel pins containing plutonium.