New method to assess tube support plate clogging phenomena in steam generators of nuclear power plants

Abstract

Pressurized water reactor steam generators (PWR SGs) are large components whose main function is to cool the fission reactor by extracting the thermal power conveyed by the primary coolant, and thus to produce steam for the turbine generator. Tube support plate (TSP) clogging phenomena may occur in the SG when iron oxide particles carried in the secondary feedwater get unavoidably deposited inside the SG structure, and specifically in two main regions: on the U-tube outer walls and on the quatrefoil sections of the plates that support the U-tubes. They may reduce its cooling efficiency and impact its dynamic behaviour, thus leading to possible safety issues. Methods based on local inspections of the SG have been designed and are already in use. They produce estimators of the local TSP clogging ratio, during the yearly outage of the plant for refuelling and maintenance. A new method to assess a global estimator of the TSP clogging ratio is presented. This method is based on a 1D physical model of the SG that reproduces the complex dynamics of the two-phase flow phenomena inside the SG. The model was developed in the Modelica language. This model is used to compute response curves of the SG characteristics to a particular transient that challenges the dynamics of the SG, when affected by TSP clogging. The estimator is obtained by comparing the computed response curves to real response curves measured on-site. The method is still under validation. However, first results show that it is able to give global estimators that are consistent with local ones. This new method is expected to improve the monitoring of possible TSP clogging phenomena by producing quantitative estimators with better accuracy, with a quarterly periodicity while the plant is in operation.