Critical Heat Flux and Heat Transfer Enhancement in Nuclear Fuel Bundles - A Review of the Past 50 Years

Abstract

The power output of CANDU® fuel bundles is limited by CHF occurrence. Because of this, there has been a long interest in optimizing the fuel bundle CHF (Critical Heat Flux) and CCP (Critical Channel Power, or power corresponding to the first occurrence of CHF in any fuel channel). An important reason for increasing the CCP is the need to regain operating margins in ageing CANDU® reactors. Ageing will result in pressure-tube diametric creep, a reduction of reactor inlet-header temperature, and an increase of the hydraulic resistance in parts of the flow circuits. These effects will all reduce the CCP with time but can be partially recovered by remedial action (chemical cleaning of parts of the circuit, pressure tube replacement). Increasing the CCP can complement these actions, and delay pressure tube replacement. In this presentation, various methods of increasing the CHF power of CANDU®-type fuel bundles are reviewed, e.g. increased bundle surface area, reduced hydraulic resistance, CHF-enhancing bundle appendages, and redefining CHF. The application of several of these CHF enhancement principles have been used in the design of the 43-element CANFLEX bundle. The following sections describe the most promising methods.