THTR-300 https://de.wikipedia.org/wiki/Kernkraftwerk_THTR-300 . Not alumina pebbles, graphite pebbles with enclosed BISO particles (predecessor to TRISO from the original article). Whole containment is contaminated with graphite dust, very high beta radiation levels. They had problems with broken graphite pebbles all over the reactor because the pebbles were brittle, often got jammed and had to be unjammed by force. They even produced a radiation release to possibly "blow out" graphite dust from the gas pipes that had collected there.