Computational analysis of gas–liquid flow in an idealised CANDU reactor header under conditions that are relevant to small-break loss-of-coolant and loss-of-flow accidents

Abstract

Extensive numerical simulations of isothermal air/water flow in an idealised model of the header/feeder system of a CANDU nuclear reactor were performed under conditions that are relevant to postulated loss-of-flow accidents (LOFA) and small-break loss-of-coolant accidents (SBLOCA). Following our recent work, in which the present CFD methodology was validated against in–house experimental results, we used the volume of fluid method to solve the system of multi-phase flow governing equations and the detached eddy simulation model to close this system in turbulent flow. We computed the steady-state liquid flow distribution to the different feeders for different combinations of inlet water and air mass flow rates and different mass extraction rates, namely, values of the ratio of the leak liquid mass flow rate and the inlet liquid mass flow rate, and found that, in general, this distribution tends to become more uniform as the inlet flow rate of either the liquid or the gas increases. A “free surface” separating air above from an air–water mixture below was observed in many cases, generally moving downward with decreasing inlet flow rates and increasing mass extraction ratio. The liquid distribution to non-leaking feeders was not affected significantly by a change in the location of the leak. Time histories of flow properties following a step change in boundary conditions showed that the change of flow rate through each feeder can be represented by a first-order model with a time constant of roughly 1 s. As part of the validation of the present CFD analysis, the computed local pressure loss coefficient for a leaking feeder was found to be in excellent agreement with previously published experimental results. The static pressure loss across the header, estimated from available semi-empirical two-phase flows models was, in most cases, found to be within 10% of the CFD solution.