Stress analysis of screws in the fuel channel fastener assembly

The function of fuel channel fastener assembly is to keep enough clearance between fuel channels, allowing the insertion of control rod and fixing the channel on the fuel bundle. The assembly device is not safety related component, however, in case of the screw breaking, it may cause loose parts, which might adversely affect the normal operation of inserting and pulling fuel assemblies, and/or the movement of the control rods. In this paper, the possible loading conditions applied to the fuel channel fastener assembly are considered to analyze the stress state in screw. In order to assess the improper positioning of fuel channel, explicit finite element procedures is employed to simulate the complex contact/impact behaviors occurring between the fastener assembly and the neighboring fuel channel or the fuel rack, in which the effects of dynamic impact on the screw and initial contact speed are the main concern. The analysis results reveal that the reduced neck close to the screw head has the highest stress. If the external loads drive the stress up to the yielding limit, crack initiation will occur on the screw neck and thereby, under the tensile loadings and reactor core environment, initiating intergranular stress corrosion cracking (IGSCC) on the screw