Preliminary analysis for the Swelling and Densification Phenomena of a Dual Cooled Fuel Rod

A dual cooled fuel rod consists of a coaxial inner and an outer cladding tube and annular pellets. Two coolant passages and two gaps between a pellet and cladding tubes are distinct features of a dual cooled fuel. There are three major concerns such as a different elongation of the inner/outer cladding tubes, a heat split and an irradiation elongation of the pellets. In the case of a heat split, swelling, densification, thermal expansion and creep of the pellets and a creep of the cladding tubes are the causes. Because there is one gap between a pellet and a cladding tube in a solid fuel, deformation direction of a pellet is always one way. But the deformation direction of an annular pellet is a concern because it could expand both inward and outward. In this regard, one gap could become a gap contact faster than another. If a gap is closed, the thermal resistance between a pellet and a cladding tube lessens. Then, heat split phenomenon happens. It could be a critical problem threatening the integrity of dual cooled fuels. It is necessary to study in which direction an annular pellet expands and to develop a method to maintain the difference of the gap conductance of the inner and outer gaps. In this paper, a quasi-static analysis method is established to predict the deformation direction of pellets during a swelling and densification