Abstract
In the analysis of the loss of coolant accident (LOCA) in water reactors, the frictional resistance is treated in a one-dimensional sense using the quasi-steady state approximation for evaluation of the friction factor. The calculation has predicted one or more flow reversals. The present paper examines the limits of validity of the quasi-steady approximation by comparing with a new exact solution assuming no flow separation. Radial velocity distributions, mean flowrate and friction coefficients are compared for the specific case of a ramp change in pressure gradient including the limiting step. For low ramp rates with transient laminar flow in circular pipes the quasi-steady approximation is found valid; for high ramp rates the time to zero bulk flow is underestimated by 22.2%.
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