Experimental investigation of effect of spacer on single phase turbulent mixing rate on simulated subchannel of Advanced Heavy Water Reactor

Abstract

The Advanced Heavy Water Reactor (AHWR) is a perpendicular pressure tube type, heavy water moderated and boiling light water cooled natural circulation based reactor. The fuel bundle of AHWR contains 54 fuel rods orderly in three concentric rings of 12, 18 and 24 fuel rods. This fuel bundle is separated into number of imaginary interacting flow channel called subchannels. 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 however not in case of 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.Spacer is used in the fuel rod bundle of a nuclear reactor to uphold appropriate gaps between the fuel pins ensuring enough heat transfer to the coolant. The coolant flow circulation in single and two phase flow situation is extremely vital for AHWR rod bundle to assure its safety and performance. Single phase flow situation exists in reactor rod bundle all through start-up condition and up to definite span of rod bundle when it is working at full power. However, being a natural circulation BWR, conversion from single phase to two phase flow situation occurs in reactor rod bundle with enhanced power. Prediction of thermal margin of the reactor has necessitated the purpose of inter-subchannel mixing of coolant between these subchannels. The inter-subchannel mixing consists of three independent phenomena; turbulent mixing, void drift and diversion cross flow.Still there are huge differences in prediction between the models and experimental data of turbulent mixing rate as of one subchannel array to another Sharma and Nayak (2015). This is for the reason that mixing phenomena are extremely dependent on geometry and operating conditions. It may possibly be noted that the subchannel geometry of AHWR rod bundle is entirely different from usual BWRs. The rods in AHWR bundle are arranged in circular subchannel array unlike conventional BWRs geometry in which rods are orderly in square-square, rectangular-rectangular and square-rectangular subchannel array. In the analysis of above, data obtained from usual BWRs cannot be used for AHWR. In addition, AHWR being a natural circulation BWR, the mass flux condition in the subchannels can be different depending on the power, which is altered from usual BWRs where the mass flux in the subchannel is more or less constant irrespective of power.The objective of present work is to establish effect of spacer on mixing rate in subchannels of AHWR rod bundle. Experiments were carried out in a scaled test facility of AHWR rod bundle. The spacer was installed at 2963mm (37mm at the end of the mixing section), 2926mm (74mm at the end of the mixing section) and 2889mm (111mm at the end of the mixing section) from the entry section in the test section respectively for three different positions. The present results (Blockage ratio 4%) were compared with Sharma and Nayak (2015) (without spacer) and finally new correlations were developed between mixing number and Reynolds Number and for average turbulent mixing rate and spacer position (each location of the spacer). It is found that the present correlations are applicable for prediction of turbulent mixing rate enhancement due to spacer at three different location. The facility simulates 1:1 geometry of 1/12th symmetrical division of AHWR rod bundle. Water was used as the operational fluid and the turbulent mixing tests were carried out at atmospheric condition with no heat addition. The turbulent mixing rate was experimentally measured for AHWR operating condition. The mass flow rate in actual rod bundle of AHWR varied from 0 to 4.7kg/s depending upon operating condition. Hence three subchannels in 1/12 sector, the mass flow rate can differ in the range from 0.0 to 0.12kg/s and in that order range of mean velocity is around 0–1.2m/s. Turbulent mixing rate and enhancement factor for three different positions 37, 74 and 111mm were experimentally determined by adding tracer fluid in one subchannel and measuring the concentration of that in former subchannels at the last part of the flow path.