EFFECT OF SPACER AND GEOMETRICAL PARAMETERS OF NUCLEAR FUEL RODS ON TURBULENT MIXING RATE OF COOLANT IN SUBCHANNELS
The Advanced Heavy Water Reactor (AHWR) is a vertical pressure tube type Boiling Water Reactor (BWR). The purpose of use of spacer in nuclear fuel bundle is to maintain the required spacing so that its effect is to promote better flow-mixing between sub-channels. The requirements for spacers are thus challenging and so it is easy to understand why the spacer grid is at the heart of any successful fuel assembly design. The fuel bundle is separated into number of imaginary interacting flow channel called subchannels. The effect of spacer on Inter-subchannel mixing data has been obtained for usual BWR in which rods are set in square-square, rectangular-rectangular and square-rectangular subchannel array which is not applicable for AHWR, where rods are set in circular subchannel array. Subchannel mixing data obtained for BWR cannot be used for AHWR since it is a function of geometry and operating condition. The objective of present work is to establish effect of spacer and geometrical parameters of nuclear fuel rods on turbulent mixing rate in subchannels of AHWR rod bundle. Experiments have been carried out in a scaled test facility of AHWR rod bundle, developed at Bhabha Atomic Research Centre, Trombay, Maharashtra. The spacer was installed at 2963 mm (37 mm at the end of the mixing section) from the entry section in the test section. The experimental results (Blockage ratio 4%) have been compared with the case of  without spacer and finally new correlations have been developed between mixing number, combined Reynolds Number & gap-to-centroidal ratio (S/δ). The instrument was calibrated prior to each set of analysis with standard solution. For each test, the experiments have been repeated three times. The turbulent mixing rates  have been determined by solving equations (3) and (4). The uncertainty analysis has been carried out for the measurement of flow rate, concentration and height of the test section. The percentage error is found to be within ± 2.0%.