Transient modelling of the flow and heat transfer in a once through helical coil steam generator tube for a Small Modular PWR

Abstract

With a shortage of electricity experienced worldwide, the modular nuclear reactor has become a viable solution to consider. The main focus when developing a modular reactor is for it to be compact and easily manufactured and transported. This necessitates the redesign of the once bulky steam generator (SG). New technological developments are therefore being looked into, including integrated once through (IOTSG) and once through helical coil steam generators (OTHSG). This allows for a more compact design than the conventional u-tube steam generators (UTSG), and leads to the SG being able to produce super-heated steam with associated higher generation efficiencies. Various mathematical models exist to simulate UTSGs, but limited work could be found for the new OTHSG designs. From the literature review it is evident that abundant work has been done on the modelling of single phase flow within tubes, but for two-phase flow boiling within pipes the research is more limited. The different flow regimes, cross-over points and bubble formations make the modelling even more challenging when transient conditions are considered. The aim of this study is to develop a transient model that can simulate the OTHSG from start-up through boiling and up to super-heated steam conditions. In order to develop a thorough understanding of the fundamentals, a customised transient homogeneous two-phase flow model is first developed using Engineering Equation Solver (EES), and the results compared with that of a model generated using the commercial software package Flownex. Flownex is then used to model more complex transients. It was shown that the helical coil of the SG can be simplified and represented by equivalent vertical pipes in parallel with an enhanced heat transfer coefficient applied to cater for the coil geometry. The steady state results obtained compare well with experiments done by Cinotti (2002) on the IRIS reactor’s OTHSG. When evaluating the Flownex result for the heat transfer rate of the OTHSG, including the coil enhancement factor, the error was 0.48%. The largest error was found to be that of the predicted secondary side pressure loss, namely 2.87%. A simulation of a cold start-up transient was successfully performed and the results also compare well with experimental data. Evidence of such a transient simulation could not be found elsewhere in literature.