[fennecfoxen]

"Walkaway safe" means that in an emergency/failure situation, it transitions from "working reactor" to "broken reactor" without threatening lives and property with danger (at least, not threatening once you've exited the reactor complex proper -- you might in fact need to walk away, especially if there are fires and things that caused the problem).

The alternative is a Fukushima-style transition from "working reactor" to "unapproachable death trap generating clouds of hydrogen gas which can explode the containment dome, sending toxic radioactive smoke from uranium fires into the atmosphere (while other material leaches into the ocean via the water table)". Even Fukishima's spent-fuel cooling ponds required water be added for for weeks and weeks on end.

If you'd like it to keep the reactor working while walking away from it, that's another matter which is keyworded something like "unattended operation" not "walk-away safe".