(193g) Design and Development of a Radioisotope-Based Technique for Investigating Pebble Flow Characteristics in Cold-Flow Experimental Pebble Bed Reactors

Challenges are posed to investigating the pebble flow dynamics using traditional optical velocimetry methods due to the dense and opaque characteristics of pebble-bed nuclear reactors. As a result, there are only a limited number of experimental studies on pebble flow dynamics in the literature. In this study, a radioisotope-based residence time distribution (RTD) measurement technique was developed to investigate pebble flow in a scaled-down cold-flow experimental pebble bed setup with one pebble at a time discharge rate and continuous pebble recirculation features using 6.0 cm graphite pebbles for the first time. The developed RTD technique uses a set of five collimated Sodium Iodide (NaI) detectors to track the photon counts of a radioactive tracer pebble as it flows down the bed and exits the reactor. The tracer pebble was prepared in-house by tagging one of the pebbles with a Co-60 radioisotope particle of 500 μm in diameter and of an initial activity of 500 μCi. The overall residence time was found to increase as the radius of the initial seeding position increased from the center towards the wall, but with a higher increase rate near the wall compared to the center region. The zonal residence time findings indicated a uniform radial profile of the residence time in the upper section of the bed, except near the wall. However, non-uniformity in the radial profile increased in the lower sections. The cone region had the highest difference in the zonal residence time, with a 200% increase in the tracer's residence time when seeded at the wall (r/R=0.88) compared to the center point (r/R=0.0). The designs of the experimental setup and the developed technique, including the radioisotope tracer pebble preparation, are described and presented, and the obtained results are analyzed and compared with the literature.