in a modern nuclear plant (pebble-bed designs), when it "goes down" all it does is stop generating power, nothing more
The Soviets masters also used to tout their reactors as "sure to the point of enabling us to build them on the Red Square". When Cherno blasted stuff away they were nowhere to be found, and poor suckers (civilians and liquidators) enjoyed the ride.
Even the modern EPR was also touted as absolutely sure, then experts mandated by the first customer (the Finns) discovered that the classic "sump clogging" problem may cause a major accident, as explained in "Nucleonics Week" (Volume 45, Number 11 - March 11, 2004). It was at least partially fixed, but for one discovered bug how many remain hidden?
As for your pebble thingie don't neglect criticism (page 41). Published by anti-nukers, yep, but please read the authors' pedigrees (page 4).
For informed people anyone claiming that a complex technical thingie (for example a nuclear powerplant or a piece of software) is "fully debugged and sure" is either a naive enthusiast or a liar.