Cracking and Failure of Control Rod Guide Tube Support Pins
Cracking and failure of Control Rod Guide Tube (CCRGT) Support Pins (Split Pins) was encountered at a number of pressurized water reactors. The failures in this highly stressed location have been attributed to stress corrosion cracking (SCC). Prior efforts to resolve this issue included improvements in heat treatment and in the pin design. However, these changes did not significantly reduce the stress levels to prevent stress corrosion attack.
O’Donnell Consulting performed studies to establish design improvements to reduce possibilities of failures. Extensive finite element analysis was performed in which the split pin was subjected to preload, bending and thermal shock loading conditions.
Since the stresses were high and the water contained sufficient oxygen to initiate the SCC mechanism, various methods were sought to generate an initial compressive stress on the surface at the neck of the split pin (the most highly stressed area). The results confirmed the feasability of quenching to induce compressive stresses in this location.
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