459364 Experimental Investigation of the Pebbles Residence Time in a Pebble Bed Reactor (PBR) Using Residence Time Distribution (RTD) Technique
In the current study, investigations of overall pebbles residence time are carried out by implementing radioisotopes non-invasive flow visualization techniques such as residence time distribution (RTD) measurement technique. A dedicated RTD setup consisting of two collimated scintillation detectors was implemented around the continuous pebble re-circulation experimental setup. One detector located at the entrance of test reactor and the other one at the exit reactor. A lead collimator for scintillation detectors is fabricated using water-jet machining facility available with Missouri S&T. The slit in the collimator is 2” in length, 1” thick and has a width of 1 mm. The tracer position is identified by simultaneously monitoring photo-peak counts received by these detectors. In the current setup design, the detector collimators are having horizontal slits. When the radioactive tracer particle is in the plane of horizontal slit, maximum counts are recorded. This principle is used to record the time of entry and departure of tracer particle from which overall residence time of pebbles is calculated for different initial seeding positions of tracer particle. In addition, this techniques can provide further insights on non-idealities, i.e. stagnant zones, associated with pebbles flow in the core in a non-invasive manner. A stagnant/dead zone may exist in the pebble bed reactor near the transition from cylindrical to conical section at the pebbles outlet section for recirculation from the bottom of PBR as shown experimentally in Gatt’s study. The results show the characteristics of overall pebble residence time/transient number at different initial seeding positions of a tracer particle. It has been reported that the overall pebbles residence time/transient increases with change in dimensionless initial seeding position (r/R) from the center towards the wall (169 % increase is observed for r/R of 0.92 with respect to r/R of 0). Residence time experimental results provided benchmark data that could be used for assessment of commercial Computational Fluid Dynamics (CFD) codes such as Discrete Element Method (DEM) based simulation results. These validated computational methodologies can then be used to carry out high fidelity simulations and obtain predictions of actual scale PBR core dynamics.