In this paper, the investigation on the instability in parallel channel system is summarized systematically. This phenomenon in parallel channel system is very typical, interesting and challengeable. The experiment data of a twin-channel system is used as the validation. Two typical methods are adopted to simulate this phenomenon for deciding the instability boundary. One is the integral method, which is based on the model of Clausse and Lahey and developed by Lee and Pan and GUO; the other is the classical system analysis code: Relap5/MOD3.4. In the experiment the influences of inlet resistance, system pressure and nonuniform heating are obtained. The influences of system pressure and inlet resistance can be simulated by both methods. However, there are some differences between the results of two methods. And for the effects of nonuniform heating and asymmetric inlet resistances, which are very popular in the nuclear power system, the results of numerical methods cannot get a good numerical agreement with those of experiment. It should be noticed in the practical engineering design. Finally, the typical “Ledinegg” instability phenomenon may occur in the parallel channel system according to the numerical results. Sometimes it will induce the burnout before the parallel channel instability. Both methods predict the same tendency. And a detailed explanation is given. The slope of the pressure drop-mass flux curve is the key to avoid the flow excursion phenomenon in parallel channel system.

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