Analysis of metastable regimes in a parallel channel single phase natural circulation system with RELAP5/MOD3.2

The objective of the present study is to assess the capability of RELAP5/MOD3.2 computer code to predict the metastable regime (multiple steady states) in a parallel channel system under natural circulation. To examine the basic behavior of such a system, an analysis was carried out for natural circulation flows in vertical multiple channel systems with non-uniform heat inputs. A three parallel channel configuration connected to common inlet and outlet headers with a down-comer, was investigated using RELAP5/MOD3.2 computer code. The results showed the existence of a metastable regime. It was seen that there are two steady state points for the same heat flux ratio differing in flow direction. Further the power level at which flow reversal occurs had been investigated. It was found that, if the power is increased in the down-flowing channel keeping the power of the other two channels constant, then it will start flowing up-wards only after reaching a critical value of power. Similarly, if the power of an up-flowing channel is decreased, it will start flowing down only after reaching another critical value of power. It was observed that the two critical values of power are not the same. It was found that the flow reversal power level depends upon the operating procedure, that is, rate of power rise or power set back. A non-dimensional heat flux ratio (NH) has been defined to predict the flow reversal in parallel channel system. The reason for flow reversal has also been investigated. It was observed that the flow directions in the natural circulation loop follow the maximum density (averaged) difference among the channels.