A simplified model with lumped parameters for mass, momentum and energy governing equations is usually used for thermal-hydraulic analysis during severe accident of a Nuclear Power Plant (NPP). In one of this kind of model, the flow in the flow path between two control volumes is usually simplified as one-dimensional pipe flow, and the extended expression of the Bernoulli Integral in the unsteady flow is used to solve the momentum governing equation correspondingly. It is noticed that the solution of the velocity in the flow field is very sensitive to the length of the streamline, so-called as inertia length introduced by the unsteady flow, corresponding to the inertia loss in the flow path.

Based on the theoretical model for the extended expression of the Bernoulli Integral in the unsteady flow, this paper shows the theoretical sensitivity analysis of the inertial length to the solution of the momentum governing equation firstly. According to the analysis, a sensitive study model for the inertia length was built by the thermal-hydraulic code, and the responses of the velocity, pressure and temperature versus different inertia lengths were studied. The results show that there is a slower time response of the fluid system states while the inertia length increases, and the thermal-hydraulic response is very sensitive to the inertia length of a flow path. Therefore, it is strongly recommended to choose the inertia length very carefully when dealing with the inertia response of the thermal-hydraulic system during severe accident analysis.