Noone has built one since the initial prototype - or at least, no one outside black initiatives. That's it. It really should /not/ take 20 years. 20 months, is more like it, if you set sensible design goals.

IE: I want an electricity making machine. Anyone who utters the words "High temperature" "Hydrogen Production" or "enhanced proliferation resistance" will be summarily fired. "Reliability" "Safety" and "Simplicity" are our watchwords. .

Most research on advanced reactor types turn into exercises in extremely advanced materials science due to goals creep - Trying to make a reactor that can safely operate at a temperature of over 900 degrees celcius genuinely is a 20 year project. It is also fracking pointless - the supply of fuel for a thorium breeder is effectively infinite, maximizing thermodynamic efficiency at the cost of engineering difficulty and complexity is the kind of very special stupid that only ever infests smart people.