Fluid flow in a diametrally expanded CANDU fuel channel – Part 1: Experimental study

The overall aim of this study is to develop and validate experimental and computational capabilities to provide high-resolution isothermal fluid velocity data for nuclear reactor investigations. The current work presents full-field velocity data in a 1:1 replica of a 37-element CANDU nuclear fuel bundle using the Magnetic Resonance Velocimetry (MRV) measurement technique. The diameter of the pressure tube is 6% larger than at beginning-of-life to simulate the long-term aging process in an actual CANDU nuclear reactor due to diametral creep at end-of-life conditions. In Part 1, the MRV technique is presented with a focus on measurement errors and the estimation of these errors. The three-dimensional, three-component mean velocity field inside the fuel channel, including the subchannel flow between the fuel elements, is provided at a resolution of 0.78 × 0.78 × 0.78 mm3. The MRV data is validated through bulk flow rate calculations and Laser Doppler Velocimetry measurements conducted at selected locations inside the flow system. Overall, the deviations in the full-field MRV data are low except for local differences in regions with high convective acceleration and high velocity. The measurement accuracy can be improved by applying newly developed MRV sequences that are insensitive to these effects. In this specific experiment, it is found that up to 35% of the coolant bypasses the fuel bundle in a 6% diametrally expanded pressure tube containing a single bundle. The data is used as a reference for a companion paper (Part 2), which presents a computational fluid dynamics approach using an implicit large eddy simulation. In conclusion, the presented experimental methodology provides accurate full-field mean velocity data in complex channel geometries. More realistic flow experiments in replicas of entire fuel channels can be investigated in detail with relatively little effort and in a short time.