Numerous works dedicated to flow instability [8,9,14,15] offer classifications of instability types that are generally consistent with regard to a sufficiently general subdivision into static and dynamic instabilities, but differ when a more detailed classification is made. In this case, thermal-hydraulic instability of the same physical nature is related by different authors to different types. Often, flow instability in coolant natural circulation systems is considered as an independent type, though, as one can see later, natural circulation does not determine the physical nature of instability, but rather affects the system of interconnected parameters (i.e., quantitatively influences the stability boundary due to changes of boundary conditions).