Steam generator thermal-fluid simulation using a coupled Flownex and RELAP5/mod4.0 approach

Abstract

The steam generator (SG) is one of the biggest structural components in a nuclear power plant (NPP), and can in many respects be seen as the heart of the system. It functions as a heat exchanger between the primary and secondary loops of a NPP. In addition to this the SG also serves as an important safety barrier since it separates the primary and secondary sides to avoid contamination of the secondary loop with radioactive material. Relap5 is a well-known thermal-fluid simulation program in the nuclear environment, used to simulate steady-state and transient scenarios. Relap5 is well suited to simulate the primary loop of a NPP. It has limitations in terms of high licensing cost and it is quite complex for a user to set up a model in the software. Flownex is another thermal-fluid network simulation package with the advantage that the user can set up a simulation with relative ease, and it can simulate detailed transients. A limitation of Flownex is the heat transfer correlations used for certain components. Flownex is also very powerful in its ability to simulate the secondary loop of the NPP. Flownex recently got the ability to couple with Relap5 and thereby gained access to its complex matrix of heat transfer correlations and Relap5’s detailed approach to solving the primary side of a NPP. The purpose of this study is to find the optimum position within the SG to couple Flownex with Relap5 in terms of accuracy and ease of obtaining a solution. Two Flownex models were developed as part of this study where Relap5 was implemented to calculate convection heat transfer coefficients. For the first model, Relap5 calculated only the convection heat transfer coefficients for the primary side of the SG and these were used as input for the Flownex model (FRHCP). For the second model Relap5 calculated the convection heat transfer coefficients for the secondary side of the SG as input for the Flownex model (FRHCS). The results of the two Flownex models were then compared to the results of a complete SG Relap5 homogeneous model as well as a Flownex model developed in an earlier study. The earlier Flownex model used a custom written C# script to calculate the convection heat transfer coefficients for the secondary side of the SG (FSHCS). The geometry used in this study was based on technical drawings of the SG installed at Koeberg NPP and was simplified to a one-dimensional model. Plant data was used to verify the accuracy of the models at 100%, 80% and 60% power output levels. From the study it was found that the FRHCS model over predicted the total heat transferred in the SG boiling region on average by 5% when compared to the measured data of Koeberg. The FRHCP model over-estimated the total heat transferred in the boiling region by 6% when compared to the measured data. The results obtained were mainly due to the over-estimation of the SG’s secondary side convection heat transfer coefficients in both models. The Relap5 homogeneous model that was used for comparison, under predicted the heat transferred in the SG boiling region on average by 0.5%. It was concluded that the FRHCS model offers a significant advantage in simplicity over using only a Relap5 homogeneous model for normal operational analysis. It also offers improvements in the heat transfer coefficient calculations compared to just using Flownex to simulate the steam generator where an over prediction of the heat transferred in the boiling region of the SG of about 10% can be expected. The new integrated model can be further expanded to simulate the complete system including the balance of plant components, the nuclear reactor and the auxiliaries, for an overall NPP analysis.