Thermalhydraulic Performance of CANFLEX Fuel

Abstract

CANFLEX is a 43-element CANDU fuel bundle, which is being jointly developed by AECL and KAERI, to facilitate the use of various advanced fuel cycles in CANDU reactors through the provision of enhanced operating margins. The design uses two element diameters (13.5 and 11.5 mm) to reduce maximum element ratings by 20%, yet maintaining the same bundle power and a uranium content very close to that of the standard 37-element design. The CANFLEX design also includes the use of critical- heat-flux (CHF) enhancing appendages, to increase the minimum CHF ratio or dryout margin of the bundle. The combination of these two features makes the CANFLEX bundle of interest for both advanced fuel cycles and also for use in existing CANDU reactors where it can compensate lost operating margin that is due to rising inlet header temperature and pressure tube creep. The major components of the thermalhydraulic testing are the evaluation of the CHF performance and the pressure drop with respect to the current 37-element bundle design. In addition, post dryout and drypatch mapping has been done to assist in the understanding of the performance of the CHF enhancing appendages, and to provide a baseline for future modifications to further improve the CHF enhancement. Both the CHF and the pressure drop measurements have been done using Refrigerant-134a, the first with an electrically heated 6-m-long simulation of a string of 12 CANFLEX fuel bundles, and the second with a string of 5 actual CANFLEX fuel bundles. The results clearly demonstrate the superior thermalhydraulic performance of the CANFLEX design compared with the current 37-element design, and based on this a demonstration irradiation of 24 bundles is planned in a CANDU 6 reactor for 1998. To allow full advantage to be taken of the improved performance, a water CHF test is planned before any full-core conversion to CANFLEX. This paper gives the results from the R-134a program showing an expected minimum critical channel- power improvement of 4% compared with the current 37-element bundles. It also shows that there is a similar or possibly slightly less of a reduction in the CHF performance because of pressure tube creep with a CANFLEX channel, as compared to a 37-element channel. In addition, examples of the detailed CHF, pressure drop and post dryout measurements are given.