Reduction of Pressure-Tube to Calandria-Tube Contact Conductance to Enhance the Passive Safety of a CANDU-PHW Reactor

Abstract

One of the ways to enhance the passive safety of a CANDU-PHW (CANada Deuterium Uranium - Pressurized Heavy Water) reactor is to reduce the moderator subcooling requirements during a postulated loss-of-coolant accident (LOCA). The increased moderator temperatures would enhance the heat transfer from the moderator to the surrounding shield tank during a postulated accident. This reduction in subcooling requirements can be achieved by incorporating a wire screen in the fuel channel annulus, right next to the calandria tube. The technique has been demonstrated to significantly reduce the moderator subcool ing requirements so that the calandria tube was not forced into film boiling upon pressure-tube ballooning contact with 0 degrees C subcoding outside the calandria tube. An experiment was performed at AECL Research's Whiteshell Laboratories to investigate the changes in heat transfer characteristics between a pressure tube and a caiandria tube with a wire screen placed in the fuel-channel annulus. This paper describes the results of this experiment. Results from computer simulations performed to assess the effect of the wire screen on the performance of a CANDU fuel channel during selected LOCA scenarios are also presented.