Failure Analysis and Repair of Nuclear Thermal Shield

Flow-induced vibration occurs due to coolant flow through the annular passages between the reactor vessel and internal components such as the core barrel or thermal shield in some Pressurized Water Reactors. This complex phenomenon can cause excessive vibratory motions and fatigue failures. These have been several instances of thermal shield failures due to turbulent flow-induced vibrations. In this particular case, the flow-induced vibration resulted in failure of the thermal shield support structure. Excessive vibration of the thermal shield was sufficient to cause broken bolts in the bottom supports, to back dowel pins out of position, to produce excessive wear at motion limiter keys, and failure of the upper supports.

We performed an independent failure analysis on the thermal shield support structure. The purpose of this work was to provide a redesigned support system which would maintain integrity and functionality for the remaining expected life of the reactor. In order to understand the failure mechanism and quantify the failure modes, the initial investigation included a detailed review and evaluation of the existing data and inspection results for the thermal shield. The results of this investigation revealed details of the thermal shield deflections due to turbulent flow induced random vibration. Stress in the lower support block bolts, dowels and the upper flexures were analyzed. Fatigue and failure analyses of the fasteners, bottom blocks and flexures were performed. These calculations also included an evaluation of thermal and seismic load effects on the fatigue life of the support system.

 


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