Low pressure natural circulation systems fit the concept of passive safety systems as useful feature to bring a nuclear power plant to a safe and stable state during a postulated accident without active systems. The operation is characterized by low mass fluxes and passing the known unstable two-phase region from activation to a continuous heat removal in the stable two-phase region. The occurring mass flow oscillations and the condensation induced water hammer phenomenon limited the performance of heat removal. These effects are highly undesirable because they may effect destruction of entire apparatus. The GENEVA test facility as an open natural circulation system represents the containment cooling condenser of the KERENA^TM reactor concept in the main dimensions. Initiated by the steam supply, the mass flow results from induced heat flux and is limited only by the internal pressure drops: the local, the frictional, the acceleration and the gravitational pressure drop across the flow path. The influence of the flow resistance on its unstable two-phase region and their oscillatory behavior are experimentally studied in detail. Its variation is summarized in stability maps plotted in the parameter plan of the heat flux and the subcooling number at the inlet of the heated section. Thereby, the dynamical evolution of the boiling boundary indicates the initiation of two-phase flow oscillations and the condensation induced water hammer phenomenon. The increasing local pressure drop at the inlet of the heated section reduces the velocity amplitudes but a clear reduction of the pressure surges could not be determined although these should be more pronounced due to decreasing buoyancy and increasing back flow.

中文翻译：

---

节流对开放自然循环系统中两相流稳定性的影响

低压自然循环系统符合无源安全系统的概念，该概念是在假定无有源系统的事故期间使核电厂进入安全稳定状态的有用功能。该操作的特征在于低的质量通量，并使已知的不稳定的两相区域从激活过渡到稳定的两相区域中的连续除热。发生的质量流量振荡和冷凝引起的水锤现象限制了除热性能。这些效果是非常不希望的，因为它们可能会损坏整个设备。作为开放式自然循环系统的GENEVA测试设备代表了KERENA ^TM的安全壳冷却冷凝器反应堆概念的主要方面。由蒸汽供应引发的质量流是由感应热通量产生的，并且仅受内部压降限制：内部，整个流动路径上的摩擦，加速度，重力和压降。实验研究了流动阻力对其不稳定的两相区域及其振荡行为的影响。它的变化总结在热通量和加热段入口处的过冷量参数计划中绘制的稳定性图中。因此，沸腾边界的动态演化表明两相流振荡的开始和凝结引起的水锤现象。