Control Rod Guide Tube Support Pin Cracking in PWRs
Cracking and failure of Control Rod Guide Tube (CRGT) Support Pins (commonly referred to as “Split Pins”) has been encountered at a number of pressurized water reactors (PWRs). The failures in this highly stressed pin have been attributed to stress corrosion cracking. Prior efforts to resolve this problem included improvements in the heat treatment and in the pin geometry. However, these changes have not sufficiently reduced the stress levels to prevent stress corrosion attack. Our engineering team established new design improvements that mitigate such failures.
Extensive finite element analysis was performed in which the split pin was subjected to preload, bending and thermal shock operating conditions. The stresses were high and the water contained sufficient oxygen to initiate the stress corrosion cracking mechanism. Thus, ways were sought to generate an initial compressive stress on the surface of the most highly stressed area (at the neck) of the split pin. Results of analysis confirmed the feasibility of quenching to induce a residual compressive stress field in the highly stressed fillet areas of sufficient intensity to protect the pin from intergranular stress corrosion attack. Quenching provides high resistance to stress corrosion cracking by reducing the operating surface stress levels below the critical level at which the stress corrosion cracking mechanism is initiated.