After a period of several years of operation, steam generators can be affected by fouling and clogging. Fouling means that deposits of sludge accumulate on tubes or tube support plates (TSP). That results in a reduction of heat exchange capabilities and can be modelled by means of a fouling factor. Clogging is a reduction of flow free area due to an accumulation of sludge in the space between TSP and tubes. The increase of the clogging ratio results in an increase of the overall TSP pressure loss coefficient. The link between the clogging ratio and the overall TSP pressure loss coefficient is the most important aspect of our capability to accurately calculate the thermal-hydraulics of clogged steam generators. The aim of the paper is to detail the experimental approach chosen by EDF and AREVA NP to address the calculation uncertainties. The calculation method is classically based on the computation of a single-phase (liquid-only) pressure loss coefficient, which is multiplied by a two-phase flow factor. Both parameters are well documented and can be derived on the basis of state of the art methods such as IDEL’CIK diagrams and CHISHOLM formula. The experimental approach consists of a validation of the correlations by performing tests on a mock-up section with an upward flow throughout a vertical array of tubes. A mixture of water and vapour refrigerant R116 is used to represent two-phase flows. The tube bundle is composed of a 25 tubes array in a square arrangement. The overall height of the mock-up is 2 m. Eight test TSPs were manufactured, considering eight different clogging configurations: six plates with a typical clogging profile at six clogging ratios (0, 44%, 58%, 72%, 86%, 95%), and two plates with a clogging ratio of 72% associated with two different clogging profiles (large bending radius profile and rectangular profile). A series of tests were performed in 2009 in single-phase flow conditions. Two-phase flow tests with a mixture of liquid water and vapour refrigerant R116 will be performed in 2010. The paper illustrates the main results obtained during the single-phase tests performed in 2009.
During their commissioning, steam generators are clean, which means there is no fouling of the heat transfer surface of tubes and no clogging of the flow area on the secondary side. Then sludge appears steadily at a slow pace during operation. Sludge initiates a partial loss of cooling capacity which is modeled by a fouling factor and which mainly results in vapor pressure decrease. Sludge also initiates a reduction of the secondary side flow area, known as clogging. Four safety-related issues are dependant on clogging [1]: the secondary water mass balance, the thermohydraulics oscillations, the tube vibration risk and the resistance of internal structures. This paper focuses on the last of these issues. A numerical application, based on the modeling of a fictitious steam generator, is detailed in this presentation. The order of magnitude is an 8-times increase of the loads in normal operating conditions in case of a typical 60% clogging ratio of the upper tube support plate, and a 12-times increase in case of incidental depressurization transient. These theoretical results emphasize the need to take these loads properly into account in the checking of the mechanical behavior of the internal structure of the steam generators in operation in case of significant sludge deposits.
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.
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