Investigation on static and dynamic bifurcation in two-phase natural circulation

Full text of publication follows: Facing increasing concern of the next generation advanced nuclear reactor research and development, especially the onset of Generation IV reactor study, passive techniques has become key issues for the sake of nuclear power safety and efficiency, among which two-phase natural circulation takes a very important place. Based on the viewpoint of nonlinear dynamics, this paper presents a systematic investigation on static and dynamic instability of two-phase natural circulation, both theoretically and experimentally. Through the bifurcation concept and DERPAR algorithm, static bifurcation diagram of two-phase natural circulation, which demonstrates intrinsically nonlinear excursion characteristics of the system, is acquired, basing on homogenous and drift flux flow models. With the diagram, it is pointed out that there exists the bifurcation and hysteresis phenomena in certain conditions, which may lead to thermal-siphon instability induced flow-rate jump. Meanwhile, stability margin and the maximum capacity of heating load are defined on the basis of bifurcation analysis. Parametric analysis that is rather practical in such operations of residual heat removal etc. are presented as well. Moreover, a comparison between bifurcation analysis of homogeneous equilibrium model and a non-equilibrium one shows that subcooled boiling has somewhat a significant influence on the low outlet quality instabilities, which means that subcooling effect should not be ignored for instability consideration. Furthermore, a simplified dynamic instability analysis in a two-phase natural circulation loop is also delivered. To verify and investigate more detailed facts on static and dynamic bifurcation (instability) in two-phase natural circulation system, experiment on an eight-meter-high test loop is carried out. Such dynamical data as pressure and pressure drop of heating channel, rising section and downcomer, flowrate of the loop and temperatures along the loop, etc. are acquired. The static bifurcation (flow excursion) and hysteresis phenomena on low-pressure conditions, as well as related parametric effects as predicted in theoretical calculation are observed and verified. Meanwhile, through spectrum analysis, pseudo phase space analysis and pattern recognition techniques, several modes of flowrate oscillations, which correspond to complex bifurcation of the nonlinear system are thus identified. Finally, with the aid of flow pattern visualization, related explanations are given. (authors)