Investigation of condensation process inside inclined tube

Generation III+ reactor designs partially rely on passive safety systems. It aims to increase the plant safety standards and to reduce investment costs. Passive Decay Heat Removal Systems, such as the Emergency Condenser (EC) of the KERENA reactor design, plays an important role in the safety in nuclear power plants. As part of the emergency cooling chain, EC removes the decay heat from the reactor pressure vessel and transfers it to the flooding pool. For the successful design of the EC, the reliable prediction of the condensation heat transfer inside inclined pipes is one of the important factors.
One-dimensional (1D) codes, such as ATHLET, RELAP and TRACE, are widely used today by engineers to predict the thermal hydraulic behavior of the system in nuclear power plant. However, state-of-the-art 1D codes are mainly validated for active components, and the qualification of passive systems is still a remaining problem. The goal of this thesis therefore is to investigate the condensation phenomena in EC using current advanced 1D code ATHLET (Analysis of Thermal-hydraulics of Leaks and Transients). The performance of ATHLET for prediction of condensation in slightly inclined tube was assessed and the results showed that the standard models in ATHLET code have significant deficiencies on the prediction of the condensation heat transfer coefficients. Thus, the new empirical model has been derived using experimental data from COSMEA (COndenSation test rig for flow Morphology and hEAt transfer studies) tests, condensation experiments for flow morphology and heat transfer studies in a single slightly inclined tube conducted by HZDR (Helmholtz-Zentrum Dresden-Rossendorf) and data sourced from literature.
The new model was developed using Machine Learning – Regression Analysis methodology in MATLAB, which consists of upper liquid film condensation and bottom convective condensation. It was further implemented in ATHLET with Python programming language and the modified ATHLET code was used to calculate the COSMEA experiments. The post-calculation results were compared to experiments in three aspects: heat flux, condensation rate and void fraction along the whole pipe. The outcomes showed that the modified ATHLET code can be used to recalculate the relevant heat transfer values of experiments under different pressure and mass flow rate conditions.