Insights on the instability and stabilizing techniques for natural circulation loops

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.