The influence of flow obstructions on counter-current flow and flooding in vertical and horizontal tubes

In CANDU reactors, during a postulated loss of coolant accident (LOCA), the water coming from the inlet and outlet headers enters the fuel channels through the feeder pipes which consist of vertical and horizontal runs. In some feeders, orifices and/or venturi type flow obstructions are installed for flow adjustments and measurements. Steam produced in the feeders and/or in the fuel channels may flow in the direction opposite to that of the water, thereby creating vertical and horizontal counter-current two-phase flows in the feeder pipes. Under these conditions, the rate at which cooling water can enter the fuel channels may be limited by the flooding phenomena. Thus, knowledge of the flooding phenomena in a geometry similar to the header-feeder system in a CANDU reactor is of prime importance in the safety analysis of nuclear reactors in order to improve the prediction of the time required for the emergency cooling injection system to refill the fuel channels. This paper presents data on the effects that an orifice has on counter-current flow from the point of inception of flooding up to the zero penetration limit in vertical tubes and in test sections containing vertical and horizontal runs. The influence of an elbow between a vertical and a horizontal run is also investigated. (author) 11 refs., 9 figs