Simulation of Sludge Deposit onto a 900 MWe Steam Generator Tubesheet with the 3D Code GENEPI

Abstract

Heat transfer processes use fluids which are generally not pure and can react with transfer surfaces. These surfaces are subject to deposits which can be sediments harmful to heat transfer and to integrity of materials. For nuclear plant steam generators, sludge build-up accelerates secondary side corrosion by concentrating chemical species. A major safety problem involved with such a corrosion is the growing of circumferential cracks which are very difficult to detect and size with eddy current probes. With a view to understand and control this problem, it is necessary to develop a mathematical model for the prediction of sludge behavior in PWR steam generators. Based on hndamental principles, tlus work intends to use different models available in literature for the predction of the phenomenon leading to the accumulation of sludge particles at the bottom (the tubesheet) of a PWR. For that, a three-dimensional simulation of magnetite particulate fouling with the finite elements code GENEPI is performed on a 900 MWe steam generator. The use of GEMEPI code, orignally designed and qualified for the analysis of steam generators thermalhydraulics is done in two steps. First, the local thermalhydraulic conditions of the carrier phase are calculated with the classical conservation equations of mass, momentum and enthalpy for the steam/water mixture (homogeneous model). Then, they are used for the solving of a particle transport equation. The mass transfer processes, which have been taken into account, are gravitational settling, sticking probability and reentrainment describing respectively the transport of sludge particles to the tubesheet, the particle attachement to this sutface and the re-suspension of deposited particles from the tubesheet. A sink term characterizing the blowdown effect is also considered in the calculations. Deposition on the tubebundle surface area is not modelled. For this first approach, the simulation is made with a single particle size and density (dp = 10 pm, p, = 6000 kg/m3) and as for the suspension, a 5 ppm mass concentration at the bottom of the downcomer is initially imposed. The magnetite particle concentration in a 900 MWe steam generator and the extent of deposit build-up onto the tubesheet are obtained. To some extent, the code predictions are qualitatively correct; however, quantitative evaluation and validation depend on fbture developments of models and await appropriate experimental data.