The Flow Accelerated Corrosion (FAC) of carbon steels is a form of corrosion that is accelerated by a flow of water or wet steam. It is responsible for numerous degradations on steam / water circuits of fossil and nuclear power plants (NPPs). FAC causes wall thinning on the inside surface of components and can lead to leaks or ruptures if wear is not detected in time. Due to the stakes of personnel safety and plant availability associated to the FAC phenomenon, EDF has developed a software called BRT-CICEROTM for the surveillance of the secondary piping system of its Pressurized Water Reactors (PWRs). The software enables the operator to calculate the wear rates taking into account all the influencing parameters such as : pipe isometrics, chromium content of the steel, chemical conditioning and operating parameters of the secondary circuit (temperature, pressure, etc.).
The use of BRT-CICEROTM, whose developments began in the early 90's, is mandatory for all NPPs of EDF. It enables to identify the most sensitive pipe elements to FAC and to calculate their remaining time to reach the minimum design thickness. BRT-CICEROTM helps then the operators to manage the risk of FAC on pipes and to build appropriate inspection programs prior to outages. The inspections consist in wall thickness measurements by ultrasonic and chromium content measurements by portative XRF devices qualified by EDF. The chemical history (pH, chemical agent contents and cycle durations) is input in BRT-CICEROTM at the plant level. The pipe lines are modeled according to isometrics and represented in 2D. Some characteristics are associated to the lines (temperature, pressure, steam quality, flow) and others to the individual pipe elements (diameter, nominal thickness, steel grade, minimum required thickness, manufacturing tolerances, chromium content, etc.).
The wall thickness loss and lifetime calculations are realized for each pipe element and for each cycle using approved scientific modules : chemistry, hydrodynamics, void fraction and thermodynamics. The operational feedback is taken into account using geometrical factors that are fitted statistically for non straight pipe elements : elbow, bend, tee, reduction, diaphragm, venturi. Different functionalities are available for handling and sorting the results : graphical visualizations, ranking of the pipe elements by margins, lifetime predictions, etc.
BRT-CICEROTM performs residual thickness calculations considering conservative values for the initial thickness and the chromium content. The input of inspection data (real thickness and chromium content) improves the predictions and decreases their conservatism.
The BRT-CICEROTM software is designed for :
  • optimizing the maintenance costs by performing the pipe inspections only where it is necessary ;
  • increasing the safety level of the pipes with conservative predictions : a study realized in 2005 demonstrated that out of 6000 wall thickness measurements made on the French fleet, only 11 were lower than the prediction of BRT-CICEROTM (conservatism rate of 99.8 %).

Demonstration of the conservatism of the software

In the framework of the French pressure vessel law issued on March 15th 2000, the software BRT-CICEROTM has been recognized by the French authority for the surveillance of FAC on the secondary pressure piping of EDF's NPPs. The software is available for licensing to other utilities and is already in use on Koeberg NPPs (South Africa), Daya Bay and Ling Ao NPPs (China). It takes advantage of the experience feedback of the French fleet, of the R&D improvements (especially from the laboratory tests conducted on EDF's CIROCO loop) and is frequently updated. Each year, the software takes into account new and user-defined chemical agents. The computation of the saturation pressures, enthalpy, density and dynamic viscosity of the steam / water fluid will be based on the IAPWS 1997 meeting proceedings. Other evolutions on interfaces for instance will also be available.