The Advanced Heavy Water Reactor (AHWR) is a vertical pressure tube–type, heavy water–moderated and boiling light water–cooled natural circulation–based reactor. The fuel bundle of an AHWR contains 54 fuel rods arranged in three concentric rings of 12, 18, and 24 fuel rods. This fuel bundle is divided into a number of imaginary interacting flow passages called subchannels. The transition from single-phase to two-phase flow occurs in a reactor rod bundle with an increase in power. Two-phase flow regimes like bubbly, slug/churn, and annular flow are normally encountered in a reactor rod bundle. Prediction of the thermal margin of the reactor necessitates the determination of the turbulent-mixing rate of the coolant among these subchannels under these flow regimes. Thus, it is vital to evaluate turbulent mixing between the subchannels of an AHWR rod bundle.

In this paper, experiments were carried out to determine the two-phase turbulent-mixing rate in different flow regimes in the simulated subchannels of the reactor. The size of the rod and the pitch in the test were the same as those of an actual rod bundle in the prototype. Three subchannels are considered in 1/12th of the cross section of the rod bundle. Water and air were used as the working fluid, and the turbulent-mixing tests were carried out at atmospheric conditions without addition of heat. The void fraction was varied from 0 to 0.8 under various ranges of superficial liquid velocity. The turbulent-mixing rate was experimentally determined by adding tracer fluid in one subchannel and measuring its concentration in other subchannels at the end of the flow path. The test data were compared with existing models in the literature. It was found that none of the models could predict the measured turbulent-mixing rate in the rod bundle of the reactor.