These photos show experimental results using the UCB X-ray Pebble Recirculation Experiment (X-PREX) to study the slot-flow of the reflector pebbles, confirming that pebbles spread at an approximately constant shallow angle emerging from the injection tube when plug flow is simulated experimentally using a downward-moving piston. The injection channel in this system contains a column of pebbles that is not jammed and will flow readily to the top of the heap as pebbles are removed from the bottom of the container. The polypropylene pebbles show an angle of repose of 20 degrees, corresponding to a friction coefficient of 0.37. This is assumed to be higher than that of graphite lubricated in salt, but additional confirmation measurements will be required. These tests also demonstrate pebble flow without bridging, even in a slot representing a significantly deformed divider plate that converges to 2.4 pebble diameters at its end at an angle of 1.5 degrees. No mechanism for pebble jamming has been identified for slots that maintain a vertical geometry.
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