Assessment of a Steam Generator Circulation Ratio by a Thermal Balance Based on External Wall Temperature Measurements

During operation, sludge steadily appears at a slow pace on the secondary side of nuclear power plant steam generators. This leads to clogging of the tube bundle support plates, and consequently to a change in the thermal-hydraulic flow conditions. The circulation ratio of a steam generator is defined as the ratio between the total flowrate circulating in the riser and the steam flowrate at the outlet of the steam generator. This is a good indicator of the hydraulic pressure losses in the circulation loop. In particular, the increase in hydraulic resistance due to the tube support plate clogging leads to a drop in this parameter. For this reason, in order to check that clogging does not reach too high a level, the circulation ratio is regularly evaluated on steam generators of French nuclear power plants, and then compared to established safety limits. The purpose of this paper is to present an accurate method to determine the circulation ratio of a steam generator based on temperature measurements taken around the wall of the steam generator. This method consists of carrying out a thermal balance of the flow circulating in the downcomer. In order to accomplish this, the temperature of the water circulating in the downcomer is evaluated using thermocouple belts put on the external wall of the appliance. However, additional hypotheses in the calculation method are considered in order to take into account for the heat transfer between hot water inside the downcomer and the sensors. The steam generator circulation loop and the clogging of the tube support plates are presented in §1. Then §2 and §3 describe in detail the method and the associated hypotheses as well as the required instrumentation. Finally, §4 presents an application of this method to real cases of clogged steam generators.