Full scale water CHF testing of the CANFLEX bundle

CANFLEX is a 43-clement CANDU fuel bundle that uses two diameters of elements, 13.5mm for the inner eight, and 11.5mm for the remainder. The thermalhydraulic performance of the CANFLEX bundle has been improved over the current 37-element design by both the increased fuel subdivision and by the use of patented CHF enhancing buttons attached to the elements. To quantify the improved performance, a series of CHF and pressure drop measurements with an electrically heated assembly simulating a string of twelve aligned CANFLEX bundles in a 5.1% crept CANDU fuel channel, were undertaken at Stern Laboratories. The heated part of the string was nominally 6m long and was equipped with spacer planes, bearing pads, button planes and simulated end plates to mimic the geometry of a string of aligned CANFLEX bundles. The axial heat flux profile was a cosine skewed towards the outlet end of the fuel string and the radial profile simulated that for natural uranium fuel. The five downstream bundles were equipped with moveable internal thermocouples to measure the surface temperature and to detect CHIF. As one of the applications of CANFLEX is to alleviate the eroding operating margins due to reactor aging, the string was tested in a flow tube that simulated a 5.1 % crept pressure tube. Both single- and two-phase pressure drop, and CHF data were taken. The flow conditions covered the ranges from 6 to 11 MPa outlet pressure, 10 to 25 kg/s flow, and 200 to 290 deg C channel inlet temperature. For the same channel inlet conditions, both the single- and two-phase pressure drops were very similar to 37-element fuel. Thus, there should be no significant effect on overall reactor operation during transition refueling. For the parameter region around the normal reactor operating conditions, the channel dryout power for CANFLEX was at least 10% higher than that for the 37-element design, based on similar channel inlet conditions. The focus of this paper is on the experimental hardware and the procedures used to obtain high quality thermalhydraulic data. (author)