(389d) Experimental Investigation of the Pebbles Residence Time in a Pebble Bed Reactor (PBR) Using Residence Time Distribution (RTD) Technique

Authors: 
Al-Dahhan, M., Missouri University of Science and Technology
The granular flow of pebbles in a pebble bed reactor (PBR) demonstrated the flow under the influence of gravity with long lasting frictional contacts. The basic physics governing it is not fully understood and hence the dynamic core of a PBR and non-idealities associated with pebbles flow inside the reactor core are of trivial significance from safety analysis and licensing, and thermal hydraulics point of view. The experimental setup mimics the slow flow of pebbles in the core under the influence of gravity. The pebble bed test reactor made of acrylic material (1 foot outer diameter with 11.95 inch inside diameter and 1 foot in height) is filled with ½� glass marbles. The main reason to choose these dimensions of test reactor is from experimental feasibility point of view. The residence time of pebbles in the core is an important parameter from various neutronic and safety related considerations. This can be controlled by controlling the exit flow rate of pebbles and control over radial position of returned pebbles. An exit controller is installed at the bottom opening in the cone. An exit flow rate of one pebble exiting every five seconds is used which can be set to the desired exit flow rate used in PBR (one pebble exiting every thirty sec or higher). The main reason to use this exit flow rate is to avoid experiments of prolonged duration. The glass marbles coming out of opening in extractor tube, which is operated by a solenoid operated slider, falls into a conveyor bin just below the reactor. From there, the glass marble is transferred back to the hopper at the top via adjustable speed conveyor (TipTrak from UNITRAK). Conveyor bin releases glass marbles in this hopper. Marbles are then transferred to the top of the reactor via inlet control mechanism which consists of straight and elbow sections of one pebble diameter tube. This inlet control mechanism also has three swivel joints. The inlet control mechanism is connected to a top plate (diameter matching with the reactor) having 17 holes. These holes are provided to return the pebble at 17 different radial positions in a non-violent manner. The vertical leg of conveyor belt is kept at a sufficient distance (~150 cm) away from the test reactor.

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 2â? in length, 1â? 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 Gattâ??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.

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