PSI - Issue 14

Sudheer Patri et al. / Procedia Structural Integrity 14 (2019) 688–695

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Sudheer patri et. al./ Structural Integrity Procedia 00 (2018) 000–000

1. Introduction Screw joints are frequently used for axial load transmission between various structural members of a system / mechanism. Even though design and operational aspects of screwed joints are well understood, their failure is reported occasionally either due to improper design, manufacturing or operation (V. Y. Yu et. al. (2005), R. Lacalle et. al. (2008)). Their failure may make the respective system in-operable, penalising with respect to safety and economics. Thus, the importance of screwed joints cannot be over emphasised. The premature failure of a structural screw joint, consisting of 6 nos. of screwed fasteners, in a meticulously designed complex mechanism has led to detailed investigations to identify the root cause and then to re-visit and improve the design features, transportation methodology and operational philosophy. The mechanism involved is Start up Neutron Detector Handling Mechanism (SNDHM), which is a carrier mechanism for high temperature neutron detectors. These detectors are used for measuring the neutron flux during first approach of criticality in Indian Fast Breeder Reactors (FBRs). The overall length of this long slender mechanism is 15 m and the maximum diameter is 200 mm. Non-functionality of this mechanism may delay the achievement of criticality of the reactor, penalising on economics. The SNDHM has been manufactured by an Indian industry and was delivered to Indira Gandhi centre for Atomic Research (IGCAR), Kalpakkam after successful completion of all performance tests. During installation of SNDHM in an experimental facility in IGCAR, failure of one of the screwed joints was observed. All the 6 nos. of M5 screws of the joint ruptured in tension. The failure investigations and the system improvements carried out are the main In a FBR, the first approach to criticality is done by loading fuel subassemblies in batches. Neutron flux measurement is carried out after completion of loading of each batch of fuel assemblies, until criticality is attained. Since the flux available in the core will be very less during this period, it is required to introduce the neutron detectors into the core for flux monitoring. After flux monitoring, these detectors are to be withdrawn from the core for facilitating fuel handling operation. SNDHM is a dedicated system used for raising / lowering the neutron detector assembly into / from the core. After commencement of normal operation, these detectors are withdrawn from the core. Fig. 1 shows the schematic arrangement and the vertical section of SNDHM. It consists of six subassemblies, viz., Neutron detector assembly, Mobile assembly, Outer stationary tube, Drive Screw-nut mechanism, outer housing and Drive motor. Neutron detector assembly consists of three detectors assembled one over the other. These detectors are protected from sodium by housing them in a SS thimble. Mobile assembly is a long pipe like structure, bolted to the Nut of screw-nut assembly at top end. The neutron detector assembly is fixed to it at the bottom end. Mobile assembly is guided by outer stationary tube at two elevations to ensure smooth travel. The outer stationary tube is guided at two elevations and supported by the surrounding reactor structures, which provide sufficient rigidity to it. Screw-nut assembly converts the rotary motion of motor output shaft into linear motion of mobile assembly. Rotation of nut is prevented by two guide columns. The drive screw is supported by two angular contact ball bearings, which transmit the load of mobile assembly to outer housing. Top portion of the mechanism is hermetically sealed. Stepped construction is used to prevent radiation streaming. The total travel of mobile assembly is approximately 3 m. The movement of mobile assembly is stopped at its top and bottom positions by reed switches which switches off the motor power supply. focus of the present paper. 2. Description of SNDHM