Superphenix in France (1973-1998) and Monju in Japan (1994-1995, 2010-2016) have both had significant technical challenges. The Soviets built have some sodium reactors.

I used to be very for a PRISM style reactor like TerraPower is working for, especially with something like Integral Fast Reactor's on-site non-proliferation-safe pyroprocessing. But man, over the years, I just appreciate more and more how hard it is to build and maintain well. I'm both rooting for TerraPower, but also, it low key feels like an "if not when" situation, that this an incredibly energetic unsafe system to be dealing with, and it seems hard to imagine this being a safe long term cost effective solution. I hope the inspections are very very for real, very in depth, very detailed, given the scope of what is being built. It's not even a big reactor! But that much very high temperature sodium going around, right by a big nuclear reactor (smartly TerraPower has separate nuclear and energy "wings), is deeply concerning, and needs incredibly detailed inspections if this is to provide the lasting safe value it is purporting to deliver.