More info: [1] Most of the pebbles were removed through the fueling system, but broken pebbles and dust remain, all highly radioactive. Bits of broken pebbles are jammed into crevices in broken graphite. "The reflector bottom of the core cavity is funnel shaped, with an inclination of 30°, ending in the 0.5 m wide vertical fuel discharge tube. In that geometrical setting it could simply be assumed that all pebbles must have rolled into the discharge tube and that there was no need for an inspection. The reflector bottom, however, contains many slits for coolant penetration, 3.4 cm wide and in radial orientation. A piece of broken pebble could have stuck in such a slit, and, standing out, one or more pebbles might lay behind it guided by the slit like by a rail."
"An upper limit of 197 fuel pebbles, respectively fuel pebble equivalent in form of broken pieces, has been evaluated to be residual in the reactor vessels containing an upper limit amount of 98g of fissile material."
"AVR has most probably the strongest β-contamination, and in the worst form, of all nuclear installations world wide".
This is why complexity in the radioactive portion of the system is strongly undesirable. If anything goes wrong, dealing with it is extremely expensive, difficult, and dangerous.
It is often exhausting arguing with overly enthusiastic people claiming certain reactor designs aren't just safer but "inherently safe". All operations with massive energy releases are inherently unsafe. That's just thermodynamics at work, and the reason we still have fire brigades after millennia of using fire. What makes a difference with nuclear is the energy concentrations and the material hazard involved. The problem from your citation is a subtle and arguably minor engineering oversight which wouldn't be much of an issue to handle on say a coal plant. Nuclear however multiplies the consequences of any design defect.
I believe thats why certain molten salt reactors do away with any complexity in the reaction chambers - some designs are planned to operate with fuel dissolved in the salt. Criticality comes from the shape of the reaction chamber vessel. Worst kind of failure mode - fuel cell rupture/core meltdown are not really possible - simply because those are the normal operating procedure. Should anything go really wrong the molten salt/fuel mix should melt emergency reliefs and drain into non-criticality vessel.
I do not remember the nitty-gritty details and can't find it now, sorry.
Now imagine the emergency drain failure, due to a quake seismic shift, massive explosion nearby or a construction worker forgetting his helmet in. What's the plan B?
Rely on gravity and put the non-critical tank underneath the critical tank. Make sure to make the critical tank a much lower melting point than the non-critical tank. Burst diaphragms (well, low melting point plugs, like steam boilers have used for a few centuries now).
Possibly you're confusing "inherently safe" with "it can't break or become uneconomical". Also talking about inherently safe plants, usually they're talking physics, an inattentive enough electrician could probably find a way to put his hand across the output of an alternator, there's just nothing "nuclear" about those kind of accidents other than the worksite happening to be a plant.
Inherently safe just means you can't have a Chernobyl because water doesn't burn as well as charcoal. Its inherently very difficult to set PWR moderator on fire (water) compared to the Chernobyl experience (graphite, more or less purified coal). Although its against the spirit of the linked article, the PWR "near" my house inherently can't have a sodium leak because it doesn't use sodium as a coolant.
"An upper limit of 197 fuel pebbles, respectively fuel pebble equivalent in form of broken pieces, has been evaluated to be residual in the reactor vessels containing an upper limit amount of 98g of fissile material."
"AVR has most probably the strongest β-contamination, and in the worst form, of all nuclear installations world wide".
This is why complexity in the radioactive portion of the system is strongly undesirable. If anything goes wrong, dealing with it is extremely expensive, difficult, and dangerous.
[1] http://www.wmsym.org/archives/2000/pdf/36/36-5.pdf