That Wikipedia page does not mention any breakthrough in passive stability, and unless somebody mentions it, the safe thing to assume is that any fast breeder has none. What means that it can basically blow like the Chernobyl reactor or worse due to failure of equipment.
Fast reactors usually have a strong negative temperature coefficient of reactivity. This is inherent to the temperature of the fuel and responds instantly. The Integral Fast Reactor, for instance, was designed to tolerate loss of coolant flow without even needing to insert the control rods to shutdown, with no core damage.
The main issue I have with fast reactors is the liquid sodium coolants typically used, very hazardous stuff if it was to leak. Molten salts are a nice alternative.
I'm no physicist, is this traveling wave reactor a "fast breeder" that you're talking about? From what I read, the stability comes from it not being a runaway reaction that needs active brakes. If something fails, the reaction will stop on its own rather than having a meltdown or explosion.
But I'm just parroting what I read in the past (not sure where), I don't do nuclear physics and can't say whether it's snake oil or real.
I'm not an expert. My knowledge of nuclear physics is limited to some liquid model theory at university (what is basically the simplest model you can find and completely useless), and some empiric data. I also just met this design. But I do know some general principles.
The Wiki page claims very briefly this is a fast breeder design where it says it uses fast neutrons, also, it doesn't mention moderation, that is the process that converts fast neutrons into slow ones.
Now, the thing that makes slow neutron reactions safer is that nothing happens unless the neutrons goes into the moderation medium, so if things deviate from the design, the reaction stops. Fast neutron reactions do not have this property, so any stability must be designed into it. That does not mean that you can't get some passive stability built into it, what it does mean is that it must be actively put into the project, and you must correctly account for any possible failure mode.
Thus, a good rule of thumb is that if somebody is talking about fast reactors and doesn't take 90% of the time talking about safety, then that somebody does not have a viable idea.
That Wikipedia page does not mention any breakthrough in passive stability, and unless somebody mentions it, the safe thing to assume is that any fast breeder has none. What means that it can basically blow like the Chernobyl reactor or worse due to failure of equipment.