The objective of the present work is to establish the effect of spacer and geometrical parameters of nuclear fuel rods on the turbulent mixing rate in subchannels of the advanced heavy water reactor (AHWR) rod bundle. Experiments on the AHWR rod bundle have been carried out in a scaled test facility developed at Bhabha Atomic Research Centre, Trombay, Maharashtra. In order to confirm the validity of the proposed method, experimental data on the turbulent mixing rate were obtained using a tracer technique under adiabatic conditions with 3.5-m vertical test channels, consisting of three subchannels. The spacer was installed at 2963 mm (37 mm at the end of the mixing section), 2926 mm (74 mm at the end of the mixing section), and 2889 mm (111 mm at the end of the mixing section) from the entry section in the test section, respectively, for three different positions. The experimental results (blockage ratio 4%) have been compared with the case without spacer and finally new correlations have been developed between average mixing number, combined Reynolds number, and gap-to-centroidal ratio (S/δ). The range of average Reynolds number covered was 0 to 6424. The correlation is applicable for a vertical pressure tube–type boiling water reactor (AHWR) with a reasonable accuracy. The instrument was calibrated prior to each set of analyses with standard solution. It predicts a reasonable mixing at a higher S/δ as compared to without spacer, which is the most improved feature of the correlation when compared with the existing ones. The uncertainty analysis has been carried out for the measurement of flow rate, concentration, and height of the test section. The proposed correlation may be applicable for the thermal-hydraulic design of an AHWR with an improved accuracy. A complete set of mixing data was obtained which can be used to calibrate thermal-hydraulic codes.