STUDY ON CHEMICAL INTERACTION OF U_3Si_2 DISPERSOID AND ALUMINUM MATRIX IN PLATE FUEL ELEMENTS AND ITS INFLUENCE ON THE MECHANICAL PROPERTIES

摘要

Dispersion-type plate fuel elements are being fabricated with U_3Si_2 dispersoid (prepared by an innovative powder processing route) in aluminum matrix and clad in Al alloy for the modified core of the APSARA reactor by. a standard picture frame technique followed by hot roll bonding operation at the Bhabha Atomic Research Centre Metallic Fuels Division. The fabrication regime allows the fuel elements to be exposed at 500℃ for almost 5 h (total duration including hot roll bonding and blister test operation). Therefore, it is expected that during hot roll bonding and blister test operation, U_3Si_2 will chemically interact with aluminum and form an intermediate phase. Hence, the chemical interaction behavior of fuel dispersoid (U_3Si_2, prepared by powder metallurgy route) and matrix (aluminum) in plate fuel elements and its effect on mechanical properties is studied in the present paper. Therefore, a comparative study between an actual plate fuel element (i.e., U_3Si_2 dispersed in aluminum matrix and with Al alloy clad) and a sandwich plate with chemically inert material (i.e., Y_2O_3) as dispersed in aluminum matrix with Al alloy clad was carried out. The roll bonded samples were investigated through pull and peel tests, microhardness, tensile test, optical microscopy, scanning electron microscopy, electron probe microana-lysis, and X-ray diffraction for various metallurgical examinations. During the course of study, it was observed that U_3Si_2 dispersoids in actual plate fuel elements were enveloped by a different phase while the dispersoid of Y_2O_3 remained inert in the surrogate plate under a similar fabrication history. The study concludes that limited exposure of the actual fuel plate at 500℃ for 5 h results in improvement of bond strength mainly due to chemical interaction between fuel dispersoid and aluminum. The study also concludes that the tensile strength and ductility of the fuel plates did not show any adverse effects during dispersoid-matrix chemical interaction; however, the modulus of elasticity was found lower than the theoretically estimated value calculated by composite theory. The observations derived in the study are critical from the viewpoint that a decrease in the elastic modulus of the plate would adversely affect its flow-induced vibration properties during reactor operation. It may also be concluded that exposing the plate fuel elements at 500℃ for longer duration (i.e., 30 h) will result in excessive swelling because of the accelerated interaction between dispersoid and matrix, which will eventually deteriorate the desired properties.