Insights on the instability and stabilizing techniques for natural circulation loops

• Insertion of an orifice stabilizes an unstable point near the lower threshold whereas it destabilizes a stable point near the upper threshold in SPNCS and TPNCS. • Insertion of an orifice destabilizes a stable point near the lower threshold, whereas it stabilizes an unstable point near the upper threshold for SCNCSs. • For SPNCSs, decreasing the L t /D ratio is found to decrease the unstable zone whereas for TPNCSs, decreasing the L t /D ratio increases the stable zone leading to stabilization of both. • Reducing the L t /D ratio enhances the flow rate and the heat transport capability while stabilizing single-phase and two-phase NCSs whereas it widens the unstable zone in supercritical loops. • Transmission of instability from an unstable loop to other series-coupled loops is possible with highly attenuated amplitude of oscillation. There is a generally held belief that the insertion of an orifice which is equivalent to increasing the L t /D ratio is always stabilizing SPNCSs. In this paper, it has been shown that the insertion of an orifice can stabilize or destabilize depending on whether the loop is operating near the lower or upper threshold of instability for single-phase loops. Besides, increasing the L t /D ratio increases the unstable zone in single-phase loops and, hence, is destabilizing. For two-phase loops, insertion of an orifice or increasing the L t /D ratio significantly shrinks the stable zone increasing the unstable zone as in single-phase loops. Thus for both single-phase and two-phase loops, reducing the L t /D is stabilizing. Contrary to this, for the supercritical loops L t /D ratio (or orificing) has a complex effect on instability. For example, increasing the L t /D or insertion of an orifice shrinks the unstable zone giving a stabilizing effect. Also, reducing the L t /D ratio is seen to shift both the lower and upper thresholds to higher powers and, in this sense, is stabilizing. However, it is also found to widen the unstable zone with a decrease in L t /D and, in this sense, is destabilizing. The paper also reviews the available stabilizing techniques to identify the techniques which do not significantly reduce the heat transport capability while stabilizing. For single-phase and two-phase loops, the best way to stabilize is the reduction of L t /D ratio as it stabilizes with enhancement in heat transport capability. Introduction of an orifice enhances the unstable zone in single-phase and two-phase loops whereas it has a mixed effect in supercritical loops. Increase in L t /D is found to reduce the flow and hence narrows down the pseudocritical region and hence the unstable region to stabilize supercritical loops. Reduction of L t /D ratio is found to stabilize supercritical loops at high inlet temperatures, whereas it widens the unstable region at low inlet temperatures, which is attributed to the widening of the pseudocritical region. The paper also examines the various requirements for maximizing the power of natural circulation based reactors. Apart from reducing the frictional force, enhancing the surface area density in the core has a significant influence on enhancing the reactor power and various options for the same has been identified in the paper.