Flow Instability

Unstable flow can take place in the form of flow oscillations or flow reversals. The flow oscillations are variations in flow due to the void formations or mechanical obstacles from design and manufacturing. The flow oscillation in one reactor coolant channel at times causes flow oscillations in the surrounding coolant channels due to the flow redistribution. The flow oscillations are undesirable for numerous reasons. At first, constant flow oscillations can cause undesirable forced mechanical vibration of the components. This can lead to malfunction of those components due to fatigue. Secondly, flow oscillations can cause system control problems of specific significance in liquid-cooled nuclear reactors since the coolant is also employed as the moderator. Thirdly, flow oscillations affect the local heat transfer characteristics and boiling. It has been found via testing that the critical heat flux (CHF) needed for departure from nucleate boiling (DNB) can be lowered by as much as 40% whenever flow is oscillating. This harshly decreases the thermal limit and the power density all along the length of the reactor core. Again, it has been found via testing which flow oscillations is not a significant trouble for some pressurized water reactors unless power is above 150% for the common flow conditions. Flow oscillations can be a trouble during natural circulation operations since of the low flow rates there.

Throughout the natural circulation, the steam bubbles developed during a flow oscillation might have sufficient effect to really cause complete flow reversal in the exaggerated channel.

Both the flow reversals and flow oscillations lead to a very unstable situation as the steam blankets formed on heated surfaces directly influence the capability to transfer heat away from those surfaces.