A rupture is unlikely as most pebble bed designs use helium as the coolent. It being a noble gas it won't react chemically, and unlike water cooled designs it d

A rupture is unlikely as most pebble bed designs use helium as the coolent. It being a noble gas it won't react chemically, and unlike water cooled designs it doesn't undergo phase change (liquidgas) so no massive pressure build up. And as there is no oxygen fire is unlikely.