Development of a fouling monitor for safety-related heat exchangers

Periodic safety reviews (PSRs) are performed to verify and/or improve the safety of operating nuclear power plants. When the PSR of Gori Units 3/4 in South Korea was performed, a few safety-related heat exchangers were pointed out as components in need of an ageing management program. This study shares the motivation with the necessity of the ageing management program. We developed a condition monitoring methodology for identifying one of the ageing mechanisms in the safety-related heat exchangers and named it the fouling monitor. The fouling monitor is composed of (1) the model for determining the test fouling resistance and the projected design fouling resistance at a test condition, and (2) the monitoring and detection of their deviation. The mathematical model for determining the projected design fouling resistance at a test condition needs to be considered carefully because the heat exchanger, and particularly in this study, the letdown heat exchanger, is normally tested under operating conditions that differ considerably from the design conditions. This need was met by developing independent physical and empirical correction methods of which consistency is cross-checked. Either or both correction method(s) convert the design fouling resistance into a projected fouling resistance, which is finally comparable with the test fouling resistance. The monitoring and detection method tracks the deviation of the test fouling resistance and the projected fouling resistance. Considering the stochastic characteristics of fouling phenomena, we adopted the sequential probability ratio test to determine appropriate scheduling for cost- and safety-optimized maintenance of the heat exchanger.