Primary water stress corrosion cracking (PWSCC) of Alloy 600 reactor pressure vessel closure head penetrations is very sensitive to a number of material and service parameters. Plant experience to date does indicate that time at elevated temperatures is a strong indicator of cracking potential. As a result, lowering temperatures can be an effective way to increase the time for crack initiation, to reduce the rate of crack propagation, to reduce the number of required closure head penetration inspections and to extend the useful life of an existing closure head or a replacement closure head. In this paper, the reactor coolant flow paths into and out of the upper head region of a pressurized water reactor are presented along with the field modifications performed to reduce the upper head region temperature. These modifications are known as an Upper Head Temperature Reduction (UHTR) program. In addition, a simplified model for assessing the impact of temperatures on the potential for crack initiation and crack growth of Alloy 600 closure head penetrations is presented. The impact of a UHTR program and the resulting reduced temperature on crack initiation, as measured by effective degradation years (EDYs), and on crack growth is also discussed. Actual UHTR implementation designs and recent Westinghouse UHTR field modification experiences are also presented.

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