The difference is in Chernobyl there were mechanical safety systems to stop a meltdown. current designs are such that physics prevents it from being able to melt down. While its possible to improperly maintain safety systems causing them to fail, physics doesn't need maintenance.
Do you have link?
I am curious what this design is.
If I just randomly search, it is just hundreds of designs, everyone has a 'theory' on some design that is safe and effective. Hard to filter out the 'crack-pot' from the actual physicists.
Do you have one in-particular.
(edit, also. I ask because Chernobyl was also deemed impossible by 'physics'. Technically it did not melt down first, the reactor didn't 'fail', something else fails, then melt down happens. The risk was never a nuclear explosion, it was release of nuclear material and radiation)
to quote wikipedia which expalins it better than i can;
"When the reactor temperature rises, the atoms in the fuel move rapidly, causing Doppler broadening. The fuel then experiences a wider range of neutron speeds. Uranium-238, which forms the bulk of the uranium, is much more likely to absorb fast or epithermal neutrons at higher temperatures. This reduces the number of neutrons available to cause fission, and reduces power. Doppler broadening therefore creates a negative feedback: as fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms. The pebble-bed reactor is designed so that this effect is relatively strong, inherent to the design, and does not depend on moving parts. If the rate of fission increases, temperature increase and Doppler broadening reduces the rate of fission. This negative feedback creates passive control of the reaction process."
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.
